Optical filter and optical system

Abstract

The present invention addresses the problem of providing an optical filter using a liquid crystal polarization interference element capable of suppressing variation in the wavelength at which maximum transmittance of light is exhibited even when the light is incident from an oblique direction, and to provide an optical system using the optical filter. The above problem is solved by a configuration in which a liquid crystal polarization interference element has two or more sets comprising a first liquid crystal layer and a second liquid crystal layer, the first liquid crystal layer and the second liquid crystal layer including a liquid crystal layer R in which a horizontally aligned rod-shaped liquid crystal compound is fixed and a liquid crystal layer D in which a vertically aligned discoid liquid crystal compound is fixed, the liquid crystal layer R and the liquid crystal layer D having parallel slow axes and equal in-plane retardation, and the in-plane slow axes of the first liquid crystal layer and the second liquid crystal layer intersecting.

Description

Optical Filters and Optical Systems

[0001] The present invention relates to an optical filter using a liquid crystal polarization interference element, and an optical system using this optical filter.

[0002] 2. Description of the Related Art Bandpass filters that transmit light in a specific wavelength range and block light of other wavelengths are used in various optical devices.

[0003] Known bandpass filters include polarization interference optical filters using dielectric multilayer films, optical filters combining polarizers and birefringent crystals, etc. Also known is a bandpass filter, as described in Patent Document 1, in which a Solk optical filter (folded Solk optical filter) is arranged between polarizers arranged in a crossed Nicol configuration, and the filter is formed by alternately laminating birefringent plates (λ / 2 retardation plates) of equal thickness, in which the angle between the direction of the transmission axis of the polarizer and the slow axis is +ρ, and birefringent plates in which the angle is −ρ.

[0004] Furthermore, Patent Document 1 proposes an optical filter (SOLK optical filter) that can realize a bandpass filter with a small number of parts, which has a structure in which two different types of polarization regions of crystals are periodically arranged, and in which the major axis of an index ellipsoid cut parallel to the interface between the two different types of polarization regions is different in the two different types of polarization regions.

[0005] Japanese Patent Application Laid-Open No. 2004-101577

[0006] However, conventional bandpass filters such as those described in Patent Document 1 have a problem in that the wavelength of light exhibiting maximum transmittance varies when light is incident from an oblique direction, resulting in a so-called wavelength shift. Among conventional bandpass filters, dichroic filters are particularly popular for applications in which light can be separated into two optical paths according to wavelength by transmitting only a specific wavelength range and reflecting other wavelengths. However, dichroic filters have the disadvantage that the transmitted and reflected wavelengths change when light is incident from an oblique direction.

[0007] The object of the present invention is to solve the problems of the conventional technology, and to provide an optical filter using a liquid crystal polarization interference element that, when used in a bandpass filter (dichroic filter), can suppress fluctuations in the wavelength of light that exhibits maximum transmittance when light is incident from an oblique direction, i.e., wavelength shift, and an optical system that uses this optical filter.

[0008] In order to solve this problem, the present invention has the following configuration: [1] An optical filter having a first polarizer, a liquid crystal polarization interference element, and a second polarizer in this order, wherein the first polarizer is an absorptive linear polarizer, and the second polarizer is a reflective linear polarizer, and the liquid crystal polarization interference element has two or more pairs of liquid crystal layers in the thickness direction, each pair consisting of a first liquid crystal layer and a second liquid crystal layer, wherein the first liquid crystal layer includes at least one liquid crystal layer R1 formed by fixing horizontally aligned rod-shaped liquid crystal compounds and at least one liquid crystal layer D1 formed by fixing vertically aligned discotic liquid crystal compounds, and the second liquid crystal layer includes at least one liquid crystal layer R2 formed by fixing horizontally aligned rod-shaped liquid crystal compounds and at least one liquid crystal layer D2 formed by fixing vertically aligned discotic liquid crystal compounds, and the in-plane slow axis of the liquid crystal layer R1 is parallel to the in-plane slow axis of the liquid crystal layer D1, An optical filter, wherein the in-plane slow axis of the liquid crystal layer R2 is parallel to the in-plane slow axis of the liquid crystal layer D2, the in-plane slow axis of the liquid crystal layer R1 is intersecting the in-plane slow axis of the liquid crystal layer R2, the sum of the in-plane retardations of the liquid crystal layer R1 is equal to the sum of the in-plane retardations of the liquid crystal layer D1, and the sum of the in-plane retardations of the liquid crystal layer R2 is equal to the sum of the in-plane retardations of the liquid crystal layer D2. [2] The optical filter according to [1], wherein the liquid crystal polarization interference element has an in-plane retardation of the first liquid crystal layer equal to the in-plane retardation of the second liquid crystal layer. [3] The liquid crystal polarization interference element is the optical filter according to [1] or [2], wherein the in-plane slow axes of all the liquid crystal layers R1 are parallel and the in-plane slow axes of all the liquid crystal layers R2 are parallel, and further, the in-plane retardation of all the first liquid crystal layers is equal and the in-plane retardation of all the second liquid crystal layers is equal. [4] The liquid crystal polarization interference element is the optical filter according to any of [1] to [3], wherein the liquid crystal layer pairs arranged on both sides in the thickness direction and the liquid crystal layer pair arranged in the center in the thickness direction have different in-plane slow axes of the liquid crystal layer R1 of the first liquid crystal layer and the liquid crystal layer R2 of the second liquid crystal layer, and further, the in-plane retardation of the first liquid crystal layer is different between the liquid crystal layer pairs arranged on both sides in the thickness direction and the liquid crystal layer pair arranged in the center in the thickness direction.[5] The liquid crystal polarization interference element is an optical filter according to any one of [1] to [4], wherein the liquid crystal layer pairs arranged on both sides in the thickness direction and the liquid crystal layer pair arranged in the center in the thickness direction have different in-plane slow axis directions of the liquid crystal layer R1 of the first liquid crystal layer and the liquid crystal layer R2 of the second liquid crystal layer. [6] An optical filter having a first polarizer, a liquid crystal polarization interference element, and a second polarizer in this order, wherein the first polarizer is an absorptive linear polarizer, and the second polarizer is a reflective linear polarizer, the liquid crystal polarization interference element has two or more pairs of liquid crystal layers in the thickness direction, each pair consisting of a first liquid crystal layer and a second liquid crystal layer, wherein the first liquid crystal layer or the second liquid crystal layer includes at least one liquid crystal layer R containing a rod-shaped liquid crystal compound, and the first liquid crystal layer or the second liquid crystal layer includes at least one liquid crystal layer D containing a discotic liquid crystal compound, the in-plane slow axis of the first liquid crystal layer intersects with the in-plane slow axis of the second liquid crystal layer, and the sum of the in-plane retardations of the first liquid crystal layer and the second liquid crystal layer is equal to each other. [7] The liquid crystal polarization interference element is an optical filter according to claim [6], wherein the liquid crystal layer pairs arranged on both sides in the thickness direction and the liquid crystal layer pair arranged in the center in the thickness direction have different in-plane slow axis directions of the first liquid crystal layer and the second liquid crystal layer. [8] The optical filter according to any one of [1] to [7], wherein the first polarizer and the second polarizer are arranged with their transmission axes perpendicular to each other. [9] The optical filter according to any one of [1] to [7], wherein the first polarizer and the second polarizer are arranged with their transmission axes parallel to each other.

[10] The optical filter according to any one of [1] to [9], wherein the reflective linear polarizer has a selective reflection layer including at least one cholesteric liquid crystal layer and a λ / 4 retardation plate.

[11] The optical filter according to

[10] , wherein the selective reflection layer has a cholesteric liquid crystal layer R formed using a rod-shaped liquid crystal compound and a cholesteric liquid crystal layer D formed using a discotic liquid crystal compound.

[12] The optical filter according to

[10] or

[11] , wherein the λ / 4 retardation plate is made of a reverse dispersion liquid crystal compound.

[13] The optical filter according to any one of

[10] to

[12] , wherein the λ / 4 retardation plate includes a C plate.

[14] The optical filter according to any one of [1] to [9], wherein the reflective linear polarizer is made of a dielectric multilayer film.

[15] The optical filter according to any one of [1] to [9], wherein the reflective linear polarizer is a wire grid polarizer.

[16] An optical system comprising a light source unit, the optical filter according to any one of [1] to

[15] , and a light receiving unit.

[17] The optical system according to

[16] , comprising a condensing lens.

[18] The optical system according to

[16] or

[17] , comprising a beam splitter.

[0009] According to the present invention, for example, in a bandpass filter (dichroic filter), it is possible to suppress the variation in the wavelength of light that exhibits the maximum transmittance when light is incident from an oblique direction, that is, the wavelength shift.

[0010] Fig. 1 is a diagram conceptually showing an example of an optical filter of the present invention. Fig. 2 is a graph for explaining the optical filter of the present invention. Fig. 3 is a graph for explaining the optical filter of the present invention. Fig. 4 is a conceptual diagram for explaining another example of a liquid crystal polarization interference element used in the optical filter of the present invention. Fig. 5 is a conceptual diagram for explaining another example of a liquid crystal polarization interference element used in the optical filter of the present invention. Fig. 6 is a conceptual diagram for explaining another example of a liquid crystal polarization interference element used in the optical filter of the present invention. Fig. 7 is a diagram conceptually showing an example of an optical system of the present invention. Fig. 8 is a diagram conceptually showing another example of an optical system of the present invention.

[0011] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The optical filter and optical system of the present invention will be described in detail below with reference to preferred embodiments shown in the accompanying drawings.

[0012] In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. In addition, all of the drawings shown below are conceptual diagrams for explaining the present invention, and the positional relationship, size, thickness, shape, etc. of each component may differ from the actual ones.

[0013] FIG. 1 conceptually illustrates an example of an optical filter according to the first embodiment of the present invention. The optical filter of the present invention comprises a first polarizer, a liquid crystal polarization interference element, and a second polarizer arranged in this order. A liquid crystal polarization interference element 16 is disposed between the first polarizer 12 and the second polarizer. The first polarizer and the second polarizer are disposed in a crossed Nicol configuration, with their transmission axes perpendicular to each other. The optical filter 10 shown in FIG. 1 is a bandpass filter (dichroic filter) that transmits light in a specific wavelength range and reflects light of other wavelengths. The optical filter according to the first embodiment of the present invention differs from the optical filter according to the second embodiment in the liquid crystal polarization interference element. This will be described in detail later.

[0014] The first polarizer 12 is an absorptive linear polarizer that transmits linearly polarized light in a predetermined direction and absorbs linearly polarized light in a direction perpendicular to the predetermined direction. In the optical system of the present invention, there are no limitations on the absorptive linear polarizer, and various known linear polarizers can be used, such as iodine-based polarizers, dye-based polarizers using dichroic dyes, and polyene-based polarizers.

[0015] In the present invention, the absorptive linear polarizer is not limited to the above-mentioned form, and various forms can be used as long as it limits the function of light to only one-way polarization.For example, when optical elements such as a light source (light source unit) and a light receiving element (light receiving unit) combined with the optical filter of the present invention have a polarizer, the form in which polarized light is originally emitted from the light source combined with the optical filter of the present invention, and the form in which the light receiving element (light receiving unit) combined with the optical filter of the present invention has one-way polarization sensitivity characteristics, etc., the polarizers of these optical elements, such as the light source and the light receiving element, are also considered to be polarizers constituting the optical filter of the present invention.In addition, examples of the form in which polarized light is originally emitted from the light source include a polarized light source and light reflected from a substrate at Brewster's angle.

[0016] On the other hand, the second polarizer 14 is a reflective linear polarizer that transmits linearly polarized light in a predetermined direction and reflects linearly polarized light in a direction perpendicular to the predetermined direction. In the optical filter 10 of the present invention, the first polarizer 12, which is an absorptive linear polarizer, is basically on the light incident side, and the second polarizer 14, which is a reflective linear polarizer, is on the light exit side.

[0017] In the present invention, the reflective linear polarizer is not limited, and various known ones can be used. Examples include a reflective linear polarizer having a selective reflection layer containing at least one cholesteric liquid crystal layer and a λ / 4 retardation plate, a film made of a dielectric multilayer film obtained by stretching layers containing two types of polymers as described in JP-A-2011-053705, and a wire grid polarizer as described in JP-A-2015-286656. Commercially available reflective linear polarizers can also be suitably used. Examples of commercially available reflective linear polarizers include a reflective polarizer (product name APF) manufactured by 3M, a wire grid polarizer manufactured by THORLABS, and a wire grid polarizer (product name WGF) manufactured by Asahi Kasei Corporation. Reflective linear polarizers having a selective reflection layer containing a cholesteric liquid crystal layer and a λ / 4 retardation plate will be described in detail below.

[0018] In the illustrated optical filter 10, a liquid crystal polarization interference element 16 is disposed between the first polarizer 12 and the second polarizer 14. Note that in FIG. 1 , the first polarizer 12 and the second polarizer 14 are spaced apart from the liquid crystal polarization interference element 16. However, the present invention is not limited to this, and the first polarizer 12 and the second polarizer 14 may be stacked in contact with the liquid crystal polarization interference element 16. Furthermore, when the first polarizer 12 and the second polarizer 14 are in contact with the liquid crystal polarization interference element 16, they may be adhered to each other, if necessary, with an adhesive that is transparent to transmitted light, such as an OCA (Optical Clear Adhesive) or an acrylic pressure-sensitive adhesive.

[0019] The liquid crystal polarization interference element 16 is an optical element that acts as a λ / 2 retardation plate for light in a specific wavelength range (specific wavelength) and does not act as a retardation plate (retardation layer) for other light. As described above, the first polarizer 12 and the second polarizer 14 are arranged in a crossed Nicol configuration with their transmission axes orthogonal to each other. Of the light incident on the optical filter 10, only linearly polarized light in a direction corresponding to the transmission axis is transmitted through the first polarizer 12. Of this linearly polarized light, light in the specific wavelength range has its polarization direction rotated by 90° by the liquid crystal polarization interference element 16 and enters the second polarizer 14. The second polarizer 14 is arranged in a crossed Nicol configuration with the first polarizer 12. Therefore, the light in the specific wavelength range, whose polarization direction has been rotated by 90° by the liquid crystal polarization interference element 16, is transmitted through the second polarizer 14 and exits the optical filter 10.

[0020] In contrast, light outside the specific wavelength range, for which the liquid crystal polarization interference element 16 does not function as a retardation plate, enters the second polarizer 14 with its polarization direction intact, i.e., linear polarization whose polarization direction is the same as the transmission axis direction of the first polarizer 12. As described above, the second polarizer 14 is a reflective linear polarizer. Therefore, it has a reflection axis perpendicular to the transmission axis. The first polarizer 12 and the second polarizer 14 are arranged in a crossed Nicol configuration. That is, the second polarizer 14, which is a reflective linear polarizer, has a reflection axis in the same direction as the transmission axis of the first polarizer 12. Therefore, light outside the specific wavelength range, for which the liquid crystal polarization interference element 16 does not function as a retardation plate, is reflected by the second polarizer 14, which is a reflective linear polarizer, with its polarization direction identical to the transmission axis of the first polarizer 12, passes through the liquid crystal polarization interference element 16 as is, and enters the first polarizer 12. As described above, the polarization direction of the linearly polarized light reflected by the second polarizer 14 coincides with the transmission axis of the first polarizer 12. Therefore, the linearly polarized light reflected by the second polarizer 14 passes through the first polarizer 12 as is and exits the optical filter 10.

[0021] That is, the optical filter 10 of the present invention can extract, as transmitted light, light in a specific wavelength range for which the liquid crystal polarization interference element 16 acts as a λ / 2 retardation plate, out of the incident light. Furthermore, the optical filter 10 of the present invention can extract, as reflected light reflected by the optical filter 10 (second polarizer 14), light outside the specific wavelength range for which the liquid crystal polarization interference element 16 does not act as a retardation plate. That is, the optical filter of the present invention functions as a dichroic filter that extracts light in other wavelength ranges as reflected light, while functioning as a bandpass filter that extracts light in a predetermined narrowband wavelength as transmitted light.

[0022] In the above example, the first polarizer 12 and the second polarizer 14 are arranged in a crossed Nicol state, in which their transmission axes are orthogonal to each other, but the optical filter of the present invention is not limited to this. That is, in the optical filter of the present invention, the first polarizer 12 and the second polarizer 14 may be arranged in a parallel Nicol state, in which their transmission axes are parallel to each other. When the first polarizer 12 and the second polarizer 14 are arranged in a parallel Nicol state, the optical filter 10 has the opposite effect to when both polarizers are arranged in a crossed Nicol state.

[0023] When the first polarizer 12 and the second polarizer 14 are arranged in a parallel Nicol configuration, the optical filter 10 functions as follows. Similarly, in this case, of the linearly polarized light transmitted through the first polarizer 12, light in a specific wavelength range for which the liquid crystal polarization interference element 16 acts as a λ / 2 retardation plate has its polarization direction changed by 90° and enters the second polarizer 14. Light with other wavelengths for which the liquid crystal polarization interference element 16 does not act as a retardation plate enters the second polarizer 14 while maintaining the polarization direction of the transmission axis of the first polarizer 12. In this example, the first polarizer 12 and the second polarizer 14 are arranged in a parallel Nicol configuration. Therefore, light in a specific wavelength range for which the liquid crystal polarization interference element 16 acts as a λ / 2 retardation plate, whose polarization direction has been changed by 90°, is reflected by the second polarizer 14, has its polarization direction restored by the liquid crystal polarization interference element 16, and enters the first polarizer 12. The polarization direction of this light coincides with the transmission axis of the first polarizer 12, and therefore it is transmitted through the first polarizer 12. On the other hand, the polarization direction of light outside the specific wavelength range in which the liquid crystal polarization interference element 16 does not act as a retardation plate coincides with the transmission axis of the first polarizer 12, and therefore it is transmitted through the second polarizer 14, which is arranged in parallel Nicols with the first polarizer 12, and then emitted. That is, in this case, the optical filter 10 acts as a dichroic filter that emits light in the specific wavelength range in which the liquid crystal polarization interference element 16 acts as a retardation plate as reflected light and transmits light of other wavelengths.

[0024] As described above, various configurations are available for the first polarizer and the second polarizer that constitute the photo-optical filter of the present invention. Here, the following description will be given using a representative example in which the first polarizer 12 and the second polarizer 14 are arranged with their transmission axes in a crossed Nicol configuration. However, the configuration described below also applies to the above-described configuration in which the first polarizer 12 and the second polarizer 14 are arranged in a parallel Nicol configuration, unless otherwise specified.

[0025] 1, the liquid crystal polarization interference element 16 is disposed between the first polarizer 12 and the second polarizer 14. The liquid crystal polarization interference element 16 is an optical element that acts as a λ / 2 retardation plate for light in a specific wavelength range (specific wavelength) and does not act as a retardation plate for other light.

[0026] The liquid crystal polarization interference element 16 has a configuration in which two or more liquid crystal layer sets 26 are stacked in the thickness direction, with each set consisting of a first liquid crystal layer 20 and a second liquid crystal layer 24. Therefore, the total number of stacked first liquid crystal layers 20 and second liquid crystal layers 24 is an even number. In other words, the liquid crystal polarization interference element 16 has a configuration in which the same first liquid crystal layers 20 and second liquid crystal layers 24 are stacked alternately.

[0027] In the liquid crystal polarization interference element 16 used in the optical filter of the first embodiment of the present invention, the first liquid crystal layer 20 has a rod-shaped liquid crystal layer 20R1 and a discotic liquid crystal layer 20D1. The rod-shaped liquid crystal layer 20R1 corresponds to the liquid crystal layer R1 in the present invention, and the discotic liquid crystal layer 20D1 corresponds to the liquid crystal layer D1 in the present invention. In addition, in the liquid crystal polarization interference element 16 used in the optical filter of the first embodiment of the present invention, the second liquid crystal layer 24 has a rod-shaped liquid crystal layer 24R2 and a discotic liquid crystal layer 24D2. The rod-shaped liquid crystal layer 24R2 corresponds to the liquid crystal layer R2 in the present invention, and the discotic liquid crystal layer 24D2 corresponds to the liquid crystal layer D2 in the present invention.

[0028] Both the rod-shaped liquid crystal layer 20R1 and the rod-shaped liquid crystal layer 24R2 are liquid crystal layers formed by horizontally aligning and fixing rod-shaped liquid crystal compounds 18R. Both the discotic liquid crystal layer 20D1 and the discotic liquid crystal layer 24D2 are liquid crystal layers formed by vertically aligning and fixing discotic liquid crystal compounds 18D. In the following description, when it is not necessary to distinguish between the rod-shaped liquid crystal layer 20R1 and the rod-shaped liquid crystal layer 24R2, they will also be collectively referred to as "rod-shaped liquid crystal layers." In the following description, when it is not necessary to distinguish between the discotic liquid crystal layer 20D1 and the discotic liquid crystal layer 24D2, they will also be collectively referred to as "disctic liquid crystal layers."

[0029] In the present invention, the boundaries between the rod-shaped liquid crystal layer and the discotic liquid crystal layer in the first liquid crystal layer 20 and the second liquid crystal layer 24 can be detected by observation using a scanning electron microscope (SEM), or by cutting the liquid crystal polarization interference element 16 obliquely and analyzing the liquid crystal compounds on the surface of the cross section. The method of cutting the liquid crystal polarization interference element 16 obliquely and analyzing each liquid crystal layer is described in detail in "Depth-Dependent Determination of Molecular Orientation for WV-Film" by Yohei Takahashi et al. (FMC8-3, IDW'04, pp. 651-654).

[0030] In the first liquid crystal layer 20, the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 is parallel to the in-plane slow axis of the discotic liquid crystal layer 20D1. In the second liquid crystal layer 24, the in-plane slow axis of the rod-shaped liquid crystal layer 24R2 is parallel to the in-plane slow axis of the discotic liquid crystal layer 24D2. In the present invention, "parallel" does not only mean completely parallel, but also includes a case where the angle between the two axes is greater than 0° and less than or equal to 10°.

[0031] The in-plane slow axes of the rod-shaped liquid crystal layer and the discotic liquid crystal layer can be detected by the above-mentioned method of observation using an SEM and the method of obliquely cutting and analyzing the liquid crystal polarization interference element 16 .

[0032] Furthermore, in the liquid crystal polarization interference element 16, the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 of the first liquid crystal layer 20 intersects with the in-plane slow axis of the rod-shaped liquid crystal layer 24R2 of the second liquid crystal layer 24. Specifically, the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 and the in-plane slow axis of the rod-shaped liquid crystal layer 24R2 are tilted in opposite directions at the same angle with respect to a certain reference line. More specifically, the direction of the in-plane slow axis of the liquid crystal layer is, for example, defined as 0° with the direction of the reference line being positive (+) counterclockwise and negative (−) clockwise. For example, if the angle between the reference line and the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 is φ°, the angle between the reference line and the in-plane slow axis of the rod-shaped liquid crystal layer 24R2 is −φ°. That is, the angle formed between the reference line and the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 and the angle formed between the reference line and the in-plane slow axis of the rod-shaped liquid crystal layer 24R2 have the same absolute value.

[0033] The optical filter 10 in the illustrated example is configured, for example, such that, with the transmission axis of the first polarizer 12 as a reference line, the angle formed between the transmission axis of the first polarizer 12 and the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 in the first liquid crystal layer 20 of the liquid crystal polarization interference element 16 is "φ [°]," and the angle formed between the transmission axis of the first polarizer 12 and the in-plane slow axis of the rod-shaped liquid crystal layer 24R2 in the second liquid crystal layer 24 of the liquid crystal polarization interference element 16 is "-φ [°]." In other words, the optical filter 10 is configured such that the transmission axis (reference line) of the first polarizer 12 coincides with the bisector of the angle formed between the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 in the liquid crystal polarization interference element 16 and the in-plane slow axis of the rod-shaped liquid crystal layer 24R2.

[0034] In the optical filter 10 of the present invention, the reference line is not limited to the transmission axis of the first polarizer 12. As an example, in the optical filter 10 of the present invention, the reference line may be any one of the absorption axis of the first polarizer 12, the transmission axis of the second polarizer 14, and the absorption axis of the second polarizer 14.

[0035] 1 , in a liquid crystal polarization interference element 16 in which the same first liquid crystal layers 20 and second liquid crystal layers 24 are alternately stacked, the absolute value [°] of the angle between the reference line and the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 and the in-plane slow axis of the rod-shaped liquid crystal layer 24R2 can be calculated using the following formula depending on the rotation angle (optical rotation angle) of linearly polarized light intended by the liquid crystal polarization interference element 16 and the number of first liquid crystal layers 20 and second liquid crystal layers 24 included in the liquid crystal polarization interference element 16. That is, since the rotation angle of linearly polarized light intended by the liquid crystal polarization interference element 16 is usually 90°, the absolute value of the angle between the reference line and the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 and the in-plane slow axis of the rod-shaped liquid crystal layer 24R2 can be calculated using the following formula: 90 ÷ (number of layers of the first liquid crystal layer 20 and the second liquid crystal layer 24) ÷ 2

[0036] For example, if the number of layers in the first liquid crystal layer 20 and the second liquid crystal layer 24 is eight, i.e., if there are four liquid crystal layer sets 26, then "90 ÷ 8 ÷ 2 = 5.625." Therefore, in the above example, the angle between the transmission axis of the first polarizer 12 and the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 of the first liquid crystal layer 20 is 5.625°, and the angle between the transmission axis of the first polarizer 12 and the in-plane slow axis of the rod-shaped liquid crystal layer 24R2 of the second liquid crystal layer 24 is −5.625°.

[0037] The number of first liquid crystal layers 20 and second liquid crystal layers 24 that the liquid crystal polarization interference element 16 has, i.e., the number of liquid crystal layer pairs 26, can be detected by the above-mentioned SEM observation method and by cutting the liquid crystal polarization interference element 16 at an angle.

[0038] In the liquid crystal polarization interference element 16, the absolute values ​​of the angle between the reference line and the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 and the angle between the reference line and the in-plane slow axis of the rod-shaped liquid crystal layer 24R2 are not limited to being perfectly equal, and may have an error of ±10° or less. However, it is preferable that this error is small, and it is most preferable that the absolute values ​​of the angle between the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 and the in-plane slow axis of the rod-shaped liquid crystal layer 24R2 are equal.

[0039] In the liquid crystal polarization interference element 16, the rod-shaped liquid crystal layer 20R1 and the discotic liquid crystal layer 20D1 of the first liquid crystal layer 20 have the same in-plane retardation (Re). Also, in the liquid crystal polarization interference element 16, the rod-shaped liquid crystal layer 24R2 and the discotic liquid crystal layer 24D2 of the second liquid crystal layer 24 have the same in-plane retardation. In the illustrated example, the first liquid crystal layer 20 and the second liquid crystal layer 24 each include one rod-shaped liquid crystal layer and one discotic liquid crystal layer. However, as will be described later, the present invention is not limited to this, and the first liquid crystal layer and the second liquid crystal layer may each include multiple rod-shaped liquid crystal layers and multiple discotic liquid crystal layers. In this case, in the first liquid crystal layer 20 and the second liquid crystal layer 24, the sum of the in-plane retardations of the multiple rod-shaped liquid crystal layers is set to be equal to the sum of the in-plane retardations of the multiple discotic liquid crystal layers. In this case, it is preferable to increase the number of liquid crystal layers by dividing each liquid crystal layer into smaller regions (rod-shaped liquid crystal layers) composed of rod-shaped liquid crystal compound 18R and regions (discotic liquid crystal layers) composed of discotic liquid crystal compound 18D, thereby reducing the difference between the front (normal) retardation and the polar retardation for a wider range of oblique directions.

[0040] Furthermore, in the liquid crystal polarization interference element 16, it is preferable that the in-plane retardation of the first liquid crystal layer 20 and the in-plane retardation of the second liquid crystal layer 24 are equal to each other.

[0041] In the present invention, the term "equal in-plane retardation" does not necessarily mean that the in-plane retardation is completely identical, and may have an error of 10% or less. However, it is preferable that this difference is small, and it is preferable that the in-plane retardation of the rod-shaped liquid crystal layer and the discotic liquid crystal layer in the first liquid crystal layer 20 and the second liquid crystal layer 24, and the in-plane retardation of the first liquid crystal layer 20 and the second liquid crystal layer 24, are completely identical.

[0042] The liquid crystal polarization interference element is not limited to a configuration in which the in-plane retardation of the first liquid crystal layer is equal to the in-plane retardation of the second liquid crystal layer. For example, the in-plane retardation of the first liquid crystal layer may be different from the in-plane retardation of the second liquid crystal layer, provided that the sum of the in-plane retardations of the first and second liquid crystal layers satisfies the desired retardation value of the present invention. For example, the desired retardation value of the present invention may be a half-wave retardation value.

[0043] In the liquid crystal polarization interference element 16, the measurement wavelength for in-plane retardation is determined, for example, as follows: Two polarizers are arranged in a crossed Nicol configuration, and the liquid crystal polarization interference element 16 is placed between them. In this case, the transmission axis or absorption axis of one of the polarizers arranged in a crossed Nicol configuration is aligned with the bisector of the angle formed by the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 and the in-plane slow axis of the rod-shaped liquid crystal layer 24R2. In this state, the transmittance is measured at each wavelength, and the wavelength with the highest transmittance is determined as the measurement wavelength for in-plane retardation.

[0044] The in-plane retardation of each liquid crystal layer may be measured by a known method, such as a method using an AxoScan manufactured by Axometrics. Alternatively, the in-plane retardation of each liquid crystal layer may be calculated using Δnd, where Δn is the birefringence of the liquid crystal compound 18 constituting the liquid crystal layer, and d is the thickness of the liquid crystal layer.

[0045] In the liquid crystal polarization interference element 16, the in-plane retardation of the rod-shaped liquid crystal layer and the discotic liquid crystal layer is preferably half the wavelength of light at which the liquid crystal polarization interference element 16 is expected to function as a λ / 2 retarder. That is, the in-plane retardation of the first liquid crystal layer 20 and the second liquid crystal layer 24 is preferably half the wavelength of light at which the liquid crystal polarization interference element 16 is expected to function as a λ / 2 retarder. For example, when the wavelength of light at which the liquid crystal polarization interference element 16 is expected to mainly function as a λ / 2 retarder is 550 nm, the in-plane retardation of the rod-shaped liquid crystal layer and the discotic liquid crystal layer is preferably 137.5 nm. Therefore, in this case, the in-plane retardation of the first liquid crystal layer 20 and the second liquid crystal layer 24 is preferably 275 nm.

[0046] With this configuration, the liquid crystal polarization interference element 16 acts as a λ / 2 retarder for light in a specific wavelength range. As described above, the liquid crystal polarization interference element 16 has a rod-shaped liquid crystal layer made of rod-shaped liquid crystal compound 18R and a discotic liquid crystal layer made of discotic liquid crystal compound 18D, and is configured by alternately stacking first and second liquid crystal layers 20 and 24 whose in-plane slow axes have opposite directions relative to the reference line and whose absolute values ​​of the in-plane slow axes are equal relative to the reference line. That is, the liquid crystal polarization interference element 16 has a rod-shaped liquid crystal layer and a discotic liquid crystal layer, and is configured by alternately stacking first and second liquid crystal layers 20 and 24 whose in-plane slow axes have angles of "φ" and "-φ" relative to the reference line. Light passing through such a liquid crystal polarization interference element 16 is alternately and repeatedly influenced by the in-plane slow axis having an angle of "φ" with respect to the reference line and the in-plane slow axis having an angle of "-φ" with respect to the reference line. For example, when the absolute value of the angle with respect to the reference line is 5.625°, the light passing through the liquid crystal polarization interference element 16 is rotated by the in-plane slow axis having an angle of 5.625° with respect to the reference line, and then rotated by the in-plane slow axis having an angle of -5.625° with respect to the reference line. Therefore, the in-plane retardation of the first liquid crystal layer 20 and the second liquid crystal layer 24 is set as described above according to the wavelength of light for which the liquid crystal polarization interference element 16 is intended to function as a λ / 2 retarder. Furthermore, the angle of the in-plane slow axis in the first liquid crystal layer 20 and the second liquid crystal layer 24 is adjusted according to the number of layers of the first liquid crystal layer 20 and the second liquid crystal layer 24. This makes it possible to form a liquid crystal polarization interference element 16 that acts as a λ / 2 phase difference plate for light in a specific wavelength range and does not act as a phase difference plate for other light, that is, does not sense in-plane retardation.

[0047] Therefore, by arranging this liquid crystal polarization interference element between two polarizers arranged in a crossed Nicol configuration, with the transmission axis or absorption axis of one polarizer coinciding with the bisector of the angle formed by the in-plane slow axes of the first liquid crystal layer 20 and the second liquid crystal layer 24, as described above, only light in a specific wavelength range among the linearly polarized light transmitted through one polarizer can be rotated (optically rotated) by λ / 2 and emitted from the other polarizer. On the other hand, the liquid crystal polarization interference element does not function as a retardation plate for wavelengths other than the specific wavelength range. In other words, the liquid crystal polarization interference element does not experience in-plane retardation for wavelengths other than the specific wavelength range. Therefore, as described above, polarized light of a wavelength other than the specific wavelength range that passes through the first polarizer 12 reaches the second polarizer 14 as it is, hits the reflection axis of the second polarizer 14 parallel to the reflection axis of the second polarizer 14, is reflected by the second polarizer, returns to the first polarizer 12 as it is, and returns to the incident side through the transmission axis of the first polarizer 12. That is, only light outside a specific wavelength range can be reflected and emitted toward the first polarizer 12. Therefore, the optical filter of the present invention can provide a bandpass filter, known as a dichroic filter, which transmits specific wavelengths and reflects the rest. By similarly arranging a liquid crystal polarization interference element between two polarizers arranged in a parallel Nicol configuration, a dichroic filter can be obtained, which, contrary to the case where the polarizers are arranged in a crossed Nicol configuration as described above, emits light in a specific wavelength range in which the liquid crystal polarization interference element acts as a half-wave retardation plate as reflected light and emits light in other wavelength ranges in which the liquid crystal polarization interference element does not act as a retardation plate as transmitted light.

[0048] As described above, conventional bandpass filters have a problem in that, when light is incident from an oblique direction, the wavelength of light showing maximum transmittance varies, as conceptually shown in the upper part of Fig. 2, resulting in a so-called wavelength shift. Furthermore, when light is incident from an oblique direction on a dichroic filter that transmits light in a specific wavelength range and reflects light in other wavelength ranges, the transmitted light undergoes a wavelength shift as shown in the upper part of Fig. 2, and the reflected light also undergoes a similar wavelength shift as shown in the lower part of Fig. 2. In contrast, in the liquid crystal polarization interference element 16, the first liquid crystal layer 20 and the second liquid crystal layer 24 both include a rod-shaped liquid crystal layer made of rod-shaped liquid crystal compound 18R and a discotic liquid crystal layer made of discotic liquid crystal compound 18D, whose in-plane slow axes are parallel to each other, and the in-plane retardation of the rod-shaped liquid crystal layer is equal to that of the discotic liquid crystal layer. Therefore, the retardation in the thickness direction (Rth) of the rod-shaped liquid crystal layer can be offset by the retardation in the thickness direction of the discotic liquid crystal layer in the first liquid crystal layer 20 and the second liquid crystal layer 24. As a result, by using the liquid crystal polarization interference element 16 as a bandpass filter, it is possible to suppress wavelength shift, which is a change in the wavelength of light showing maximum transmittance, even when light is incident from an oblique direction.

[0049] In the present invention, there are no limitations on the thickness of the first liquid crystal layer 20 and the second liquid crystal layer 24, and thicknesses that provide the desired in-plane retardation may be appropriately set depending on the rod-shaped liquid crystal compound 18R and discotic liquid crystal compound 18D used. The first liquid crystal layer 20 and the second liquid crystal layer 24 are typically formed using the same liquid crystal compound. As described above, the first liquid crystal layer 20 and the second liquid crystal layer 24 have the same in-plane retardation. Therefore, the first liquid crystal layer 20 and the second liquid crystal layer 24 typically have the same thickness.

[0050] Furthermore, there are no limitations on the thicknesses of the rod-shaped liquid crystal layer 20R1 and the discotic liquid crystal layer 20D1 in the first liquid crystal layer 20 and the rod-shaped liquid crystal layer 24R2 and the discotic liquid crystal layer 24D2 in the second liquid crystal layer 24. That is, the thicknesses of the rod-shaped liquid crystal layer and the discotic liquid crystal layer in the first liquid crystal layer 20 and the second liquid crystal layer 24 may be appropriately set so as to obtain the desired in-plane retardation depending on the liquid crystal compound used. Here, it is preferable that the rod-shaped liquid crystal compound 18R forming the rod-shaped liquid crystal layer and the discotic liquid crystal compound 18D forming the discotic liquid crystal layer have similar Δn values, and more preferably, the same Δn values. Therefore, it is preferable that the thicknesses of the rod-shaped liquid crystal layer and the discotic liquid crystal layer in the first liquid crystal layer 20 and the second liquid crystal layer 24 are the same.

[0051] In the present invention, the thicknesses of the first liquid crystal layer 20 and the second liquid crystal layer 24 correspond to the thicknesses of the rod-shaped liquid crystal layer and the discotic liquid crystal layer described above. The first liquid crystal layer 20 and the second liquid crystal layer 24 are usually formed using the same liquid crystal compound. As described above, the in-plane retardation of the first liquid crystal layer 20 and the second liquid crystal layer 24 is the same. Therefore, the thicknesses of the first liquid crystal layer 20 and the second liquid crystal layer 24 are usually the same.

[0052] The thickness of the first liquid crystal layer 20 and the second liquid crystal layer 24 is preferably 1 to 5 μm, more preferably 1 to 3 μm. Therefore, the thickness of the rod-shaped liquid crystal layer and the discotic liquid crystal layer in the first liquid crystal layer 20 and the second liquid crystal layer 24 is preferably 0.5 to 2.5 μm, more preferably 0.5 to 1.5 μm.

[0053] There are no other limitations on the total number of stacked first and second liquid crystal layers 20 and 24, as long as there are two or more liquid crystal layer sets 26, i.e., four or more layers, and the number is an even number. The total number of stacked first and second liquid crystal layers 20 and 24 is preferably 6 to 30 layers, more preferably 6 to 20 layers, and even more preferably 6 to 10 layers.

[0054] In the liquid crystal polarization interference element used in the optical filter of the present invention, the greater the total number of layers of the first liquid crystal layer 20 and the second liquid crystal layer 24, i.e., the greater the number of liquid crystal layer pairs 26, the narrower the wavelength range in which the liquid crystal polarization interference element 16 functions as a λ / 2 retarder. Therefore, in the liquid crystal polarization interference element, the greater the total number of layers of the first liquid crystal layer 20 and the second liquid crystal layer 24, the narrower the half-width of the wavelength range of transmitted light. In other words, the greater the total number N of layers of the first liquid crystal layer 20 and the second liquid crystal layer 24, the narrower the bandpass filter with a narrower transmission wavelength range the optical filter 10 can be. Therefore, the total number N of layers of the first liquid crystal layer 20 and the second liquid crystal layer 24, i.e., the number of liquid crystal layer pairs 26, can be selected appropriately depending on the transmission wavelength range required for the optical filter 10. If a wide band is desired, a smaller number of layers is selected, and if a narrow band is required, a larger number of layers is selected.

[0055] The liquid crystal polarization interference element 16 having such a first liquid crystal layer 20 and a second liquid crystal layer 24 may be fabricated by a known method, such as a coating method using a liquid crystal composition for forming a rod-shaped liquid crystal layer and a discotic liquid crystal layer.

[0056] First, an alignment film oriented in one direction is formed on an appropriately selected support. The alignment film may be a known alignment film, such as a rubbed film made of an organic compound such as a polymer, an obliquely evaporated film of an inorganic compound, a film having microgrooves, a film formed by accumulating LB (Langmuir-Blodgett) films made of organic compounds such as ω-tricosanoic acid, dioctadecylmethylammonium chloride, and methyl stearate by the Langmuir-Blodgett method, or a film formed by applying an alignment film-forming coating liquid containing a photoalignment material to the surface of a support, drying the coating, and exposing the coating film using a polarizer such as a wire grid polarizer.

[0057] Separately, a liquid crystal composition for forming a rod-shaped liquid crystal layer containing rod-shaped liquid crystal compound 18R and a liquid crystal composition for forming a discotic liquid crystal layer containing discotic liquid crystal compound 18D are prepared. The solvent for preparing the compositions is not limited and can be selected appropriately depending on the purpose, but organic solvents are preferred. The organic solvent is not limited and can be selected appropriately depending on the purpose, and examples include ketones, alkyl halides, amides, sulfoxides, heterocyclic compounds, hydrocarbons, esters, and ethers. These may be used alone or in combination of two or more. Among these, ketones are preferred when environmental impact is taken into consideration.

[0058] After preparing the liquid crystal composition, the liquid crystal composition is applied to an alignment film to form a discotic liquid crystal layer, aligning the discotic liquid crystal compound 18D, followed by drying and, if necessary, curing the composition by UV irradiation or the like to form discotic liquid crystal layer 20D1. Next, a liquid crystal composition for forming a rod-shaped liquid crystal compound is applied to the discotic liquid crystal layer 20D1 to align the rod-shaped liquid crystal compound 18R, followed by drying and, if necessary, curing the composition by UV irradiation or the like to form rod-shaped liquid crystal layer 20R1, thereby forming the first liquid crystal layer 20. When a liquid crystal layer is formed on top of the liquid crystal layer by a coating method, the upper liquid crystal layer follows the alignment of the liquid crystal compound on the surface of the lower liquid crystal layer. The alignment directions of the liquid crystal compounds in the discotic liquid crystal layer 20D1 and the rod-shaped liquid crystal layer 20R1 are the same, i.e., their in-plane slow axes are parallel.

[0059] Similarly, a liquid crystal composition for forming a discotic liquid crystal layer is applied to the alignment film to form a discotic liquid crystal layer, and a liquid crystal composition for forming rod-shaped liquid crystal compounds is applied thereon to form a rod-shaped liquid crystal layer. The laminate of these two liquid crystal layers is peeled off from the alignment film, and the laminate is laminated on the previously formed first liquid crystal layer 20 (rod-shaped liquid crystal layer 20R1), and is attached using an OCA or the like.

[0060] In this case, the laminate is laminated on the first liquid crystal layer 20 so that the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 of the first liquid crystal layer 20 and the in-plane slow axis of the rod-shaped liquid crystal layer in the laminate to be laminated form a predetermined angle. For example, as described above, if the angle formed by the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 of the first liquid crystal layer 20 and the reference line is 5.625° and the angle formed by the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 of the second liquid crystal layer 24 and the reference line is −5.625°, the laminate is laminated on the first liquid crystal layer 20 so that the angle formed by the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 of the first liquid crystal layer 20 and the in-plane slow axis of the rod-shaped liquid crystal layer in the laminate becomes 11.25°. This forms a liquid crystal layer set 26 in which the first liquid crystal layer 20 having the rod-shaped liquid crystal layer 20R1 and the discotic liquid crystal layer 20D1 and the second liquid crystal layer 24 having the rod-shaped liquid crystal layer 24R2 and the discotic liquid crystal layer 24D2 are stacked.

[0061] Similarly, a liquid crystal composition for forming a discotic liquid crystal layer is applied to the alignment film to form a discotic liquid crystal layer, and a liquid crystal composition for forming rod-shaped liquid crystal compounds is applied thereon to form a rod-shaped liquid crystal layer. The two-layer liquid crystal layer stack is then peeled off from the alignment film, and, as before, the rod-shaped liquid crystal layer is laminated and attached to a second liquid crystal layer 24 (rod-shaped liquid crystal layer 24R2) with the in-plane slow axis angle of the rod-shaped liquid crystal layer aligned. By repeating this laminate stacking process for the number of first and second liquid crystal layers 20 and 24 to be laminated, i.e., the number of liquid crystal layer pairs 26 to be laminated, a liquid crystal polarization interference element 16 as shown in FIG. 1 can be fabricated.

[0062] After the liquid crystal polarization interference element 16 is fabricated in this manner, the first polarizer 12 and the second polarizer 14 are arranged in a crossed Nicol or parallel Nicol configuration on either side of the liquid crystal polarization interference element 16 so that the bisector of the angle formed by the in-plane slow axis of the rod-shaped liquid crystal layer 20R1 of the first liquid crystal layer 20 and the in-plane slow axis of the rod-shaped liquid crystal layer 24R2 of the second liquid crystal layer 24 coincides with, for example, the transmission axis of the first polarizer 12. This allows the fabrication of an optical filter 10 (bandpass filter (dichroic filter)) as shown in FIG.

[0063] In addition, in the liquid crystal polarization interference element, the method for producing the first and second liquid crystal layers is not limited to this method. For example, in the liquid crystal polarization interference element, the first and second liquid crystal layers may be formed by a coating method and directly laminated. Alternatively, in the liquid crystal polarization interference element, sheet-like first and second liquid crystal layers may be prepared, alternately laminated, and bonded with an optical bonding layer that is transparent to transmitted light, such as OCA, an acrylic pressure-sensitive adhesive, an adhesive, or a polymer layer. In this case, from the viewpoint of improving transmittance, it is preferable that the refractive index of the optical bonding layer is close to the refractive index of the liquid crystal. Specifically, the difference between the refractive index of the optical bonding layer and the refractive index of the liquid crystal is preferably 0.3 or less. Furthermore, it is preferable that the refractive index of the optical bonding layer be a value between the two birefringences possessed by the liquid crystal, because the difference in refractive index from either of the two refractive indices is small. Furthermore, in terms of the transmittance of transmitted light passing through the liquid crystal polarization interference element, the first and second liquid crystal layers are directly laminated by a coating method without an adhesive layer or the like.

[0064] In the liquid crystal polarization interference element 16, the rod-shaped liquid crystal compound 18R is not limited, and various known liquid crystal compounds can be used. Preferred rod-shaped liquid crystal compounds include azomethines, azoxy compounds, cyanobiphenyls, cyanophenyl esters, benzoates, cyclohexanecarboxylic acid phenyl esters, cyanophenylcyclohexanes, cyano-substituted phenylpyrimidines, alkoxy-substituted phenylpyrimidines, phenyldioxanes, tolanes, and alkenylcyclohexylbenzonitriles. Not only the above-mentioned low-molecular-weight liquid crystal molecules, but also polymeric liquid crystal molecules can be used.

[0065] It is more preferable to fix the alignment of the rod-shaped liquid crystal compound by polymerization. Examples of polymerizable rod-shaped liquid crystal compounds include those described in Makromol. Chem. , Vol. 190, p. 2255 (1989), Advanced Materials Vol. 5, p. 107 (1993), U.S. Patent Nos. 4,683,327, 5,622,648, 5,770,107, WO 95 / 22586, 95 / 24455, 97 / 00600, 98 / 23580, 98 / 52905, JP-A-1-272551, JP-A-6-16616, JP-A-7-110469, JP-A-11-80081, and compounds described in Japanese Patent Application No. 2001-64627 can be used. Furthermore, as the rod-shaped liquid crystal compound, for example, those described in JP-A-11-513019 and JP-A-2007-279688 can also be preferably used.

[0066] The discotic liquid crystal compound 18D is also not limited, and various known compounds can be used. As the discotic liquid crystal compound 18D, for example, compounds described in JP-A-2007-108732 and JP-A-2010-244038 can be preferably used. It is preferable to fix the alignment of the discotic liquid crystal compound 18D by polymerization.

[0067] In the liquid crystal polarization interference element shown in Fig. 1, the first liquid crystal layer 20 and the second liquid crystal layer 24 each include one rod-shaped liquid crystal layer and one discotic liquid crystal layer. However, the present invention is not limited to this. For example, the first liquid crystal layer 20 and the second liquid crystal layer 24 may include multiple rod-shaped liquid crystal layers and multiple discotic liquid crystal layers, such as a configuration of rod-shaped liquid crystal layer / discotic liquid crystal layer / rod-shaped liquid crystal layer / discotic liquid crystal layer.

[0068] Thus, when the first liquid crystal layer 20 and the second liquid crystal layer 24 each include a plurality of rod-shaped liquid crystal layers and discotic liquid crystal layers, there is no limit to the number of each layer. Optically, the greater the number of rod-shaped liquid crystal layers and discotic liquid crystal layers constituting the first liquid crystal layer 20 and the second liquid crystal layer 24, the more precisely the discotic liquid crystals can complement each other (optical compensation) in smaller regions. As a result, the thickness direction can be made more uniform with less bias, thereby reducing wavelength shift when light is incident from an oblique direction.

[0069] As described above, in the liquid crystal polarization interference element used in the optical filter of the present invention, when the first liquid crystal layer 20 and the second liquid crystal layer 24 each include a plurality of rod-shaped liquid crystal layers and discotic liquid crystal layers, the sum of the in-plane retardations of the rod-shaped liquid crystal layers is equal to the sum of the in-plane retardations of the discotic liquid crystal layers. Here, as long as the sum of the in-plane retardations of the rod-shaped liquid crystal layers is equal to the sum of the in-plane retardations of the discotic liquid crystal layers, the first liquid crystal layer 20 and the second liquid crystal layer 24 may have different numbers of rod-shaped liquid crystal layers and discotic liquid crystal layers. However, it is preferable that the number of rod-shaped liquid crystal layers and the number of discotic liquid crystal layers are equal to each other in the first liquid crystal layer 20 and the second liquid crystal layer 24.

[0070] Furthermore, in the liquid crystal polarization interference element, it is preferable that the in-plane retardation of the first liquid crystal layer 20 is equal to that of the second liquid crystal layer 24. In this case, as long as the in-plane retardation of the first liquid crystal layer 20 is equal to that of the second liquid crystal layer 24, the number of rod-shaped liquid crystal layers 20R1 and the number of discotic liquid crystal layers 20D1 in the first liquid crystal layer 20 may be different from the number of rod-shaped liquid crystal layers 24R2 and the number of discotic liquid crystal layers 24D2 in the second liquid crystal layer 24. However, it is preferable that the number of rod-shaped liquid crystal layers 20R1 and the number of discotic liquid crystal layers 20D1 in the first liquid crystal layer 20 is equal to the number of rod-shaped liquid crystal layers 24R2 and the number of discotic liquid crystal layers 24D2 in the second liquid crystal layer 24. Furthermore, the number of rod-shaped liquid crystal layers may be different from the number of discotic liquid crystal layers as long as the sum of the in-plane retardations of the rod-shaped liquid crystal layers is equal to the sum of the in-plane retardations of the discotic liquid crystal layers in the first liquid crystal layer 20 and the second liquid crystal layer 24. However, it is preferable that the number of rod-shaped liquid crystal layers is the same as the number of discotic liquid crystal layers in the first liquid crystal layer 20 and the second liquid crystal layer 24.

[0071] The liquid crystal polarization interference element 16 shown in Fig. 1 is formed by alternately stacking the same first liquid crystal layers 20 and the same second liquid crystal layers 24. That is, in the liquid crystal polarization interference element 16, all the first liquid crystal layers 20 are the same, and all the second liquid crystal layers 24 are the same. Therefore, in the liquid crystal polarization interference element 16 shown in Fig. 1, all the first liquid crystal layers 20 have the same in-plane retardation, and the in-plane slow axes of the rod-shaped liquid crystal layers and the discotic liquid crystal layers are parallel. Similarly, in the liquid crystal polarization interference element 16 shown in Fig. 1, all the second liquid crystal layers 24 have the same in-plane retardation, and the in-plane slow axes of the rod-shaped liquid crystal layers and the discotic liquid crystal layers are parallel.

[0072] However, the liquid crystal polarization interference element is not limited thereto, and may have a first liquid crystal layer having different in-plane retardations and in-plane slow axes that are not parallel to each other, or a second liquid crystal layer having different in-plane retardations and in-plane slow axes that are not parallel to each other. That is, in the liquid crystal polarization interference element used in the optical filter of the present invention, in the first and second liquid crystal layers, the in-plane slow axes of the rod-shaped liquid crystal layer and the discotic liquid crystal layer are parallel, the in-plane retardation (total) of the rod-shaped liquid crystal etc. and the discotic liquid crystal layer are equal, and further, the in-plane slow axes of the rod-shaped liquid crystal layers of the first and second liquid crystal layers intersect, and in addition, as long as the in-plane retardations of the first and second liquid crystal layers are equal, the liquid crystal layer pairs may differ in in-plane retardation, as well as the angle formed by the in-plane slow axis of the rod-shaped liquid crystal layer of the first liquid crystal layer and the reference line and the angle formed by the in-plane slow axis of the rod-shaped liquid crystal layer of the second liquid crystal layer and the reference line.

[0073] As an example, the in-plane retardation of the first liquid crystal layer and the second liquid crystal layer of the liquid crystal layer set on both sides in the thickness direction is made larger than that of the liquid crystal layer set at the center in the thickness direction, and the absolute values ​​of the angle between the in-plane slow axis of the rod-shaped liquid crystal layer of the first liquid crystal layer and the reference line and the angle between the in-plane slow axis of the rod-shaped liquid crystal layer of the second liquid crystal layer and the reference line are made smaller.As will be shown in the examples later, as an example, when the liquid crystal polarization interference element has eight first liquid crystal layers and second liquid crystal layers, that is, when it has four liquid crystal layer sets, in the first liquid crystal layer set, the in-plane retardation of the first liquid crystal layer (first layer) is Re1, the angle between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line is φ1, the in-plane retardation of the second liquid crystal layer (second layer) is Re1, and the angle between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line is −φ1, In a second liquid crystal layer set, the in-plane retardation of the first liquid crystal layer (third layer) is Re2 smaller than Re1, the angle formed between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line is φ2 larger than φ1, the in-plane retardation of the second liquid crystal layer (fourth layer) is Re2 smaller than Re1, and the angle formed between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line is −φ2 smaller than −φ1, i.e., having a larger absolute value; In a third liquid crystal layer set, the in-plane retardation of the first liquid crystal layer (fifth layer) is Re2, the angle formed between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line is φ2, and the in-plane retardation of the second liquid crystal layer (sixth layer) is Re2, and the angle formed between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line is −φ2; In the fourth liquid crystal layer pair, the in-plane retardation of the first liquid crystal layer (seventh layer) is Re1, the angle between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line is φ1, and the in-plane retardation of the second liquid crystal layer (eighth layer) is Re1, and the angle between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line is −φ1.

[0074] 3, a bandpass filter generates unnecessary transmission wavelength bands called side lobes at positions shorter and longer than the target transmission wavelength band, as indicated by arrows S in the figure. In contrast, as described above, in a liquid crystal polarization interference element, the side lobes can be reduced when used as a bandpass filter by increasing the in-plane retardation of the first and second liquid crystal layers of the liquid crystal layer pairs on both sides in the thickness direction compared to the liquid crystal layer of the liquid crystal layer pair at the center in the thickness direction and by reducing the absolute value of the angle between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line. In other words, in a liquid crystal polarization interference element, the in-plane retardation of the first and second liquid crystal layers of the liquid crystal layer pairs on both sides in the thickness direction is made larger than that of the liquid crystal layer of the liquid crystal layer pair in the center of the thickness direction, and the angle formed by the in-plane slow axis of the rod-shaped liquid crystal layer of the first liquid crystal layer and the in-plane slow axis of the rod-shaped liquid crystal layer of the second liquid crystal layer is made smaller, thereby reducing the side lobes when used as a bandpass filter.

[0075] In the above example, the in-plane retardation of the first and second liquid crystal layers of the liquid crystal layer pairs on both sides in the thickness direction is made larger than that of the liquid crystal layer of the liquid crystal layer pair at the center in the thickness direction, and the angle formed by the in-plane slow axis of the rod-shaped liquid crystal layer of the first liquid crystal layer and the in-plane slow axis of the rod-shaped liquid crystal layer of the second liquid crystal layer is made smaller, thereby reducing the side lobes when used in a band pass filter. However, in the present invention, the method for reducing the side lobes when used in a band pass filter is not limited to this.

[0076] Specifically, in a liquid crystal polarization interference element, the absolute value of the angle formed between the in-plane slow axis of the rod-shaped liquid crystal layer of each of the liquid crystal layer pairs on both sides in the thickness direction and the reference line is smaller than that of the liquid crystal layer pair at the center in the thickness direction, thereby reducing side lobes when used as a bandpass filter. In other words, in a liquid crystal polarization interference element used in the optical filter of the present invention, the angle formed between the in-plane slow axis of the rod-shaped liquid crystal layer of the first liquid crystal layer and the in-plane slow axis of the rod-shaped liquid crystal layer of the second liquid crystal layer of each of the liquid crystal layer pairs on both sides in the thickness direction is smaller than that of the liquid crystal layer of the liquid crystal layer pair at the center in the thickness direction, thereby reducing side lobes when used as a bandpass filter. In this case, it is preferable that the in-plane retardation of the liquid crystal layer pair at the center in the thickness direction is equal to that of the liquid crystal layer pair on both sides in the thickness direction.

[0077] In the first aspect of the present invention, the in-plane slow axis of the rod-shaped liquid crystal layer R1 in the first liquid crystal layer is parallel to the in-plane slow axis of the discotic liquid crystal layer D1. In the second liquid crystal layer, the in-plane slow axis of the rod-shaped liquid crystal layer R2 in the second liquid crystal layer is parallel to the in-plane slow axis of the discotic liquid crystal layer D2. Therefore, in this aspect, the in-plane slow axis of the rod-shaped liquid crystal layer in the first liquid crystal layer is the in-plane slow axis of the first liquid crystal layer. In addition, the in-plane slow axis of the rod-shaped liquid crystal layer in the second liquid crystal layer is the in-plane slow axis of the second liquid crystal layer.

[0078] In addition, in a second aspect described later, in a liquid crystal polarization interference element, the absolute value of the angle formed between the in-plane slow axes of the first and second liquid crystal layers of the liquid crystal layer pairs on both sides in the thickness direction and the reference line is smaller than that of the liquid crystal layer of the liquid crystal layer pair at the center in the thickness direction, thereby reducing the side lobes when used as a bandpass filter. In other words, in a liquid crystal polarization interference element, the angle formed between the in-plane slow axes of the first and second liquid crystal layers of the liquid crystal layer pairs on both sides in the thickness direction is smaller than that of the liquid crystal layer of the liquid crystal layer pair at the center in the thickness direction, thereby reducing the side lobes when used as a bandpass filter. In this case, it is preferable that the in-plane retardation of the liquid crystal layer pair at the center in the thickness direction is equal to that of the liquid crystal layer pairs on both sides in the thickness direction.

[0079] That is, as will be shown later in the examples, in this side lobe reduction method, as an example, when the liquid crystal polarization interference element has eight first and second liquid crystal layers, that is, when it has four liquid crystal layer pairs, in the first liquid crystal layer pair, the angle formed between the in-plane slow axis of the rod-shaped liquid crystal layer of the first liquid crystal layer (first layer) or the in-plane slow axis of the first liquid crystal layer and the reference line is defined as φ1, and the angle formed between the in-plane slow axis of the rod-shaped liquid crystal layer of the second liquid crystal layer (second layer) or the in-plane slow axis of the second liquid crystal layer and the reference line is defined as −φ1, In the second liquid crystal layer pair, the angle formed by the in-plane slow axis of the rod-shaped liquid crystal layer of the first liquid crystal layer (third layer) or the in-plane slow axis of the first liquid crystal layer and the reference line is set to φ2 larger than φ1, and the angle formed by the in-plane slow axis of the rod-shaped liquid crystal layer of the second liquid crystal layer (fourth layer) or the in-plane slow axis of the second liquid crystal layer and the reference line is set to -φ2 smaller than -φ1, i.e., having a larger absolute value; In the third liquid crystal layer pair, the angle formed by the in-plane slow axis of the rod-shaped liquid crystal layer of the first liquid crystal layer (fifth layer) or the in-plane slow axis of the first liquid crystal layer and the reference line is set to φ2, and the angle formed by the in-plane slow axis of the rod-shaped liquid crystal layer of the second liquid crystal layer (sixth layer) or the in-plane slow axis of the second liquid crystal layer and the reference line is set to -φ2; In the fourth liquid crystal layer pair, the angle formed by the in-plane slow axis of the rod-shaped liquid crystal layer of the first liquid crystal layer (seventh layer) or the in-plane slow axis of the first liquid crystal layer and the reference line is φ1, and the angle formed by the in-plane slow axis of the rod-shaped liquid crystal layer of the second liquid crystal layer (eighth layer) or the in-plane slow axis of the first liquid crystal layer and the reference line is −φ1.

[0080] The in-plane retardation of the first and second liquid crystal layers may be adjusted by changing the thicknesses of the first and second liquid crystal layers. Alternatively, the in-plane retardation may be adjusted by changing the liquid crystal compound used. Furthermore, in the case of the above-described manufacturing method, the angle between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line may be adjusted by adjusting the angle of the in-plane slow axis of the rod-shaped liquid crystal layer when stacked.

[0081] In such a configuration in which the in-plane retardation of the first and second liquid crystal layers in the liquid crystal layer sets on both sides in the thickness direction is larger than that of the liquid crystal layer in the central liquid crystal layer set in the thickness direction, and the absolute value of the angle between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line is smaller, there is no restriction on the number of central liquid crystal layer sets in which the in-plane retardation of the liquid crystal layer is larger and the absolute value of the angle between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line is smaller than that on both sides, i.e., the way in which the liquid crystal layer sets on both sides and the central part are divided, and it may be set appropriately depending on the number of liquid crystal layers (liquid crystal layer sets) that the liquid crystal polarization interference element has. Furthermore, there are no limitations on the in-plane retardation of the first and second liquid crystal layers in the liquid crystal layer pairs at both ends in the thickness direction, the angle between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line, and the in-plane retardation of the first and second liquid crystal layers in the liquid crystal layer pair at the center in the thickness direction, the angle between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line. That is, these angles can be set, for example, by simulation, to optimal in-plane retardation and angle that allow the liquid crystal polarization interference element to function as a λ / 2 retarder and reduce side lobes. Note that it is preferable to control the change in the in-plane retardation of the first and second liquid crystal layers and the change in the angle between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line from both ends in the stacking direction (thickness direction) to the center as smoothly and precisely as possible.

[0082] In the first embodiment of the optical filter of the present invention described above, the liquid crystal polarization interference element has two or more liquid crystal layer pairs in the thickness direction, each pair consisting of a first liquid crystal layer and a second liquid crystal layer; the first liquid crystal layer includes at least one liquid crystal layer R1 formed by fixing a horizontally aligned rod-shaped liquid crystal compound and at least one liquid crystal layer D1 formed by fixing a vertically aligned discotic liquid crystal compound; the second liquid crystal layer includes at least one liquid crystal layer R2 formed by fixing a horizontally aligned rod-shaped liquid crystal compound and at least one liquid crystal layer D2 formed by fixing a vertically aligned discotic liquid crystal compound; the in-plane slow axis of the liquid crystal layer R1 is parallel to the in-plane slow axis of the liquid crystal layer D1, the in-plane slow axis of the liquid crystal layer R2 is parallel to the in-plane slow axis of the liquid crystal layer D2; the in-plane slow axis of the liquid crystal layer R1 intersects with the in-plane slow axis of the liquid crystal layer R2; The total in-plane retardation of the liquid crystal layer R1 is equal to the total in-plane retardation of the liquid crystal layer D1, and the total in-plane retardation of the liquid crystal layer R2 is equal to the total in-plane retardation of the liquid crystal layer D2.

[0083] In contrast, in a second aspect of the optical filter of the present invention, the liquid crystal polarization interference element has two or more liquid crystal layer pairs, each consisting of a first liquid crystal layer and a second liquid crystal layer, in the thickness direction, wherein the first or second liquid crystal layer includes at least one liquid crystal layer R (rod-shaped liquid crystal layer) containing a rod-shaped liquid crystal compound, and the first or second liquid crystal layer includes at least one liquid crystal layer D (disctic liquid crystal layer) containing a discotic liquid crystal compound, the in-plane slow axis of the first liquid crystal layer intersects with the in-plane slow axis of the second liquid crystal layer, and the sum of the in-plane retardations of the first liquid crystal layer and the second liquid crystal layer is equal. In the liquid crystal polarization interference element of the optical filter of the second aspect of the present invention, since the liquid crystal layer pair includes a discotic liquid crystal layer and a rod-shaped liquid crystal layer, when used as a bandpass filter in combination with a polarizer, for example, as shown in FIG. 1 , it is possible to suppress a change in the wavelength of light showing maximum transmittance when light is incident from an oblique direction, i.e., a wavelength shift. The optical filter according to the second aspect of the present invention has the same configuration as the optical filter according to the first aspect, except for the liquid crystal polarization interference element, i.e., the first polarizer 12 and the second polarizer 14 are the same as those in the optical filter according to the first aspect.

[0084] In the second embodiment of the optical filter of the present invention, the liquid crystal polarization interference element basically follows the description of the first embodiment of the optical filter of the present invention with respect to various components such as rod-shaped liquid crystal compounds and discotic liquid crystal compounds, and the composition forming the liquid crystal layer, etc. Furthermore, the direction (angle) of the in-plane slow axis in each liquid crystal layer pair, the in-plane retardation of each liquid crystal layer, the number of liquid crystal layer pairs, etc. may follow the description of the first embodiment or may be different.

[0085] The liquid crystal polarization interference element according to the second aspect of the optical filter of the present invention can have various configurations different from those of the liquid crystal polarization interference element according to the first aspect of the optical filter of the present invention. In the following description, the "liquid crystal polarization interference element according to the second aspect of the optical filter of the present invention" will also be referred to as the "liquid crystal polarization interference element according to the second aspect of the present invention" for convenience.

[0086] As described above, in the liquid crystal polarization interference element of the second embodiment of the present invention, the first or second liquid crystal layer contains at least one rod-shaped liquid crystal layer, and the first or second liquid crystal layer contains at least one discotic liquid crystal layer. Therefore, the liquid crystal polarization interference element of the second embodiment of the present invention may have two or more liquid crystal layer sets 54, each set consisting of a first liquid crystal layer 50 having only a rod-shaped liquid crystal layer composed of rod-shaped liquid crystal compound 18R and a second liquid crystal layer 52 having only a discotic liquid crystal layer composed of discotic liquid crystal compound 18D, as conceptually shown in Fig. 4 . Alternatively, the liquid crystal polarization interference element of the second embodiment of the present invention may have two or more liquid crystal layer sets 54, each set consisting of a first liquid crystal layer (second liquid crystal layer) having only a rod-shaped liquid crystal layer and a second liquid crystal layer (first liquid crystal layer) having both a rod-shaped liquid crystal layer and a discotic liquid crystal layer. Alternatively, the liquid crystal polarization interference element of the second aspect of the present invention may have two or more liquid crystal layer sets 54, each set consisting of a first liquid crystal layer (second liquid crystal layer) having only a discotic liquid crystal layer and a second liquid crystal layer (first liquid crystal layer) having a rod-shaped liquid crystal layer and a discotic liquid crystal layer. Furthermore, similar to the first aspect described above, the liquid crystal polarization interference element of the second aspect of the present invention may have two or more liquid crystal layer sets, each set consisting of a first liquid crystal layer having a rod-shaped liquid crystal layer and a discotic liquid crystal layer and a second liquid crystal layer having a rod-shaped liquid crystal layer and a discotic liquid crystal layer. Regardless of the configuration, in the interference element of the second aspect of the present invention, the in-plane slow axis of the first liquid crystal layer intersects with the in-plane slow axis of the second liquid crystal layer, and the sum of the in-plane retardations of the first liquid crystal layer and the second liquid crystal layer is equal to each other.

[0087] In the liquid crystal polarization interference element according to the second aspect of the present invention, the in-plane retardation of the rod-shaped liquid crystal layer may be different from that of the discotic liquid crystal layer in at least one of the first and second liquid crystal layers, as long as the sum of the in-plane retardation of the first and second liquid crystal layers is equal to that of the second liquid crystal layer. As an example, consider a configuration having a plurality of liquid crystal layer sets 70, each of which is conceptually shown in FIG. 5 , and includes a first liquid crystal layer 60 having a rod-shaped liquid crystal layer 56R made of rod-shaped liquid crystal compound 18R and a discotic liquid crystal layer 56D made of discotic liquid crystal layer 18D, and a second liquid crystal layer 68 having a rod-shaped liquid crystal layer 62R made of rod-shaped liquid crystal compound 18R and a discotic liquid crystal layer 62D made of discotic liquid crystal layer 18D. In this case, as long as the sum of the in-plane retardations of the first liquid crystal layer 60 and the second liquid crystal layer 68 is equal to each other, the rod-shaped liquid crystal layer may be made thicker than the discotic liquid crystal layer, as shown in Fig. 5, so that the in-plane retardation of the rod-shaped liquid crystal layer is greater than that of the discotic liquid crystal layer in both the first liquid crystal layer 60 and the second liquid crystal layer 68. Alternatively, in the liquid crystal polarization interference element of the second aspect of the present invention, when both the first and second liquid crystal layers include a rod-shaped liquid crystal layer and a discotic liquid crystal layer, as long as the sum of the in-plane retardations of the first and second liquid crystal layers is equal to each other, the in-plane retardation of the discotic liquid crystal layer in both the first and second liquid crystal layers may be greater than that of the rod-shaped liquid crystal layer. Furthermore, in the liquid crystal polarization interference element of the second aspect of the present invention, when the first liquid crystal layer and the second liquid crystal layer both have a rod-shaped liquid crystal layer and a discotic liquid crystal layer, as long as the sum of the in-plane retardations of the first liquid crystal layer and the second liquid crystal layer is equal, the first liquid crystal layer (second liquid crystal layer) may have a larger in-plane retardation of the rod-shaped liquid crystal layer, and the second liquid crystal layer (first liquid crystal layer) may have a larger in-plane retardation of the discotic liquid crystal layer.

[0088] In this configuration, as long as the sum of the in-plane retardation of the first liquid crystal layer and the in-plane retardation of the second liquid crystal layer are equal, the in-plane slow axis of the first liquid crystal layer and the in-plane slow axis of the second liquid crystal layer intersect with each other in the liquid crystal polarization interference element of the second aspect of the present invention, regardless of the configuration.

[0089] That is, the interference element of the second aspect of the present invention has a plurality of liquid crystal layer pairs each consisting of a first liquid crystal layer and a second liquid crystal layer, wherein the first liquid crystal layer and / or the second liquid crystal layer has a rod-shaped liquid crystal layer and the first liquid crystal layer and / or the second liquid crystal layer has a discotic liquid crystal layer, the in-plane slow axis of the first liquid crystal layer intersects with the in-plane slow axis of the second liquid crystal layer, and further, various configurations are available as long as the sum of the in-plane retardations of the first liquid crystal layer and the sum of the in-plane retardations of the second liquid crystal layer are equal.

[0090] In the above examples, when the first and second liquid crystal layers each include a rod-shaped liquid crystal layer and a discotic liquid crystal layer, each of the first and second liquid crystal layers includes one rod-shaped liquid crystal layer and one discotic liquid crystal layer. However, in the present invention, whether the liquid crystal polarization interference element is the optical filter of the first or second embodiment, at least one of the first and second liquid crystal layers may include a plurality of rod-shaped liquid crystal layers and / or a plurality of discotic liquid crystal layers. As an example, as conceptually shown in Figure 6, the liquid crystal polarization interference element may be configured to include two or more liquid crystal layer sets 78, each set including a first liquid crystal layer 74 having two alternating rod-shaped liquid crystal layers 80R and two alternating discotic liquid crystal layers 80D, and a second liquid crystal layer 76 having two alternating rod-shaped liquid crystal layers 82R and two alternating discotic liquid crystal layers 82D.

[0091] In a configuration in which at least one of the first and second liquid crystal layers has a plurality of rod-shaped liquid crystal layers and discotic liquid crystal layers, the number of at least one of the rod-shaped liquid crystal layers and discotic liquid crystal layers may be different between the first and second liquid crystal layers. Furthermore, in a liquid crystal polarization interference element, either the first or second liquid crystal layer may have two or more layers of at least one of rod-shaped liquid crystal layers and discotic liquid crystal layers, and the other may have one rod-shaped liquid crystal layer and one discotic liquid crystal layer. Alternatively, in a liquid crystal polarization interference element used in the optical filter of the second aspect of the present invention, either the first or second liquid crystal layer may have two or more layers of at least one of rod-shaped liquid crystal layers and discotic liquid crystal layers, and the other may have only rod-shaped liquid crystal layers or only discotic liquid crystal layers.

[0092] In the liquid crystal polarization interference element in the optical filter of the first embodiment and the optical filter of the second embodiment of the present invention, the rod-shaped liquid crystal layer and the discotic liquid crystal layer of the first liquid crystal layer and the second liquid crystal layer may contain an infrared absorbing dye. By containing the infrared absorbing dye in the rod-shaped liquid crystal layer and the discotic liquid crystal layer, the liquid crystal wavelength dispersion in the liquid crystal layer can be made to be a strong forward dispersion. As a result, the wavelength range of light over which the liquid crystal polarization interference element acts as a λ / 2 wave plate can be narrowed. In other words, by adding an infrared absorbing dye to the rod-shaped liquid crystal layer and the discotic liquid crystal layer to make the liquid crystal wavelength dispersion in the liquid crystal layer a strong forward dispersion, a bandpass filter with a narrower transmission wavelength range can be obtained.

[0093] As the infrared absorbing dye, various infrared absorbing dyes can be used that can reduce the difference in refractive index between the x and y directions by being oriented in the same direction as the liquid crystal compound. There are no particular limitations on the infrared absorbing dye, as long as it absorbs infrared light (e.g., light with a wavelength of 700 to 900 m). Among these, dichroic dyes are preferred. A dichroic dye refers to a dye that exhibits different absorbance in the long axis direction and the short axis direction of the molecule. Examples of infrared absorbing dyes that can be used include diketopyrrolopyrrole dyes, diimmonium dyes, phthalocyanine dyes, naphthalocyanine dyes, azo dyes, polymethine dyes, anthraquinone dyes, pyrylium dyes, squarylium dyes, triphenylmethane dyes, cyanine dyes, and aminium dyes. Metal complex dyes and boron complex dyes can also be used as the infrared absorbing dye. Infrared absorbing dyes are described in detail in WO 2019 / 044859.

[0094] There is no limitation on the amount of the infrared absorbing dye added to the rod-shaped liquid crystal layer and the discotic liquid crystal layer, and it may be set appropriately depending on the width of the transmission wavelength range required for the bandpass filter.

[0095] Furthermore, in the liquid crystal polarization interference element in the optical filter of the first aspect and the optical filter of the second aspect of the present invention, the rod-shaped liquid crystal layer and the discotic liquid crystal layer of the first liquid crystal layer and the second liquid crystal layer may contain a liquid crystal elastomer. The rod-shaped liquid crystal layer and the discotic liquid crystal layer containing a liquid crystal elastomer may be formed using a liquid crystal elastomer, or a liquid crystal layer formed from a normal liquid crystal compound other than an elastomer may contain a liquid crystal elastomer.

[0096] In this way, by including a liquid crystal elastomer in the rod-shaped liquid crystal layer and the discotic liquid crystal layer, the first liquid crystal layer and the second liquid crystal layer can be made elastic, and the thickness of the liquid crystal layer can be changed by stretching or shrinking the optical filter in the plane direction. By changing the thickness of the liquid crystal layer, the in-plane retardation of the liquid crystal layer can be changed. As a result, in a bandpass filter, it is possible to change the wavelength range of light transmitted through the optical filter. In other words, by including a liquid crystal elastomer in the rod-shaped liquid crystal layer and the discotic liquid crystal layer, the wavelength range can be changed by stretching and shrinking the liquid crystal layer, i.e., the optical filter, and active wavelength control is possible in the bandpass filter.

[0097] There are no limitations on the liquid crystal elastomer, and various known ones can be used. As an example of the liquid crystal elastomer, a liquid crystal elastomer prepared from a liquid crystal monomer, a crosslinker, and a plasticizer, as described in JP 2020-131638 A, can be used. This provides the liquid crystal elastomer with mechanical properties and rubber elasticity, enabling it to deform in response to the external force required for active wavelength control.

[0098] When the rod-shaped liquid crystal layer and the discotic liquid crystal layer are formed from a normal liquid crystal compound that is not an elastomer and a liquid crystal elastomer is added to impart elasticity, there is no restriction on the amount of liquid crystal elastomer added, and it may be set appropriately depending on the required elasticity, i.e., the control range of the transmission wavelength range.

[0099] The optical filter of the present invention comprises a first polarizer, a liquid crystal polarization interference element, and a second polarizer arranged in this order. In the optical filter (optical filter 10) of the present invention shown in FIG. 1 , the first polarizer 12 and the second polarizer 14 sandwiching the liquid crystal polarization interference element in the thickness direction are arranged with their transmission axes perpendicular to each other (crossed Nicols), thereby transmitting light in a specific wavelength range where the liquid crystal polarization interference element acts as a λ / 2 retardation plate and reflecting light of other wavelengths. Furthermore, the first polarizer 12 and the second polarizer 14 sandwiching the liquid crystal polarization interference element in the thickness direction are arranged with their transmission axes parallel to each other (parallel Nicols), thereby reflecting light in a specific wavelength range where the liquid crystal polarization interference element acts as a λ / 2 retardation plate and transmitting light of other wavelengths. However, the optical filter of the present invention using a liquid crystal polarization interference element is not limited thereto, and various configurations can be used. For example, in the optical filter of the present invention, the liquid crystal polarization interference element may be configured such that the angle of the in-plane slow axis with respect to the reference line increases successively in the alternate stacking direction of the first liquid crystal layer and the second liquid crystal layer, as in Example 30 described later. When using such a liquid crystal polarization interference element, when the optical filter (bandpass filter (dichroic filter)) of the present invention transmits light of a specific wavelength through the second polarizer and emits it, it may be preferable that the polarizers sandwiching the liquid crystal polarization interference element in the thickness direction are arranged with their transmission axes parallel to each other, i.e., in a parallel Nicol state.

[0100] As in Example 30 described later, a liquid crystal polarization interference element having a configuration in which the angle of the in-plane slow axis gradually increases in the stacking direction does not act as a retardation plate for light in a specific wavelength range, but acts as a λ / 2 retardation plate for light of other wavelengths, contrary to the above-mentioned liquid crystal polarization interference element. Therefore, in the optical filter of the present invention using this liquid crystal polarization interference element, in order to transmit light in a specific wavelength range through the second polarizer and emit it, it is necessary to arrange the first polarizer, which is an absorptive linear polarizer, and the second polarizer, which is a reflective linear polarizer, in a parallel Nicol state with their transmission axes parallel.

[0101] As with the optical filter 10 shown in Figure 1, in this optical filter, only linearly polarized light in a direction corresponding to the transmission axis of the first polarizer is transmitted through the first polarizer. Of this linearly polarized light, light in a specific wavelength range is transmitted through the liquid crystal polarization interference element in its original polarization direction and enters the second polarizer, because the liquid crystal polarization interference element does not function as a retardation plate. Here, the second polarizer and the first polarizer are arranged in a parallel Nicol configuration. Therefore, light of a specific wavelength that remains linearly polarized after passing through the first polarizer 12 is transmitted through the second polarizer 14 and exits.

[0102] On the other hand, light outside the specific wavelength range, because the liquid crystal polarization interference element acts as a ½ phase retarder, has its polarization direction changed by 90° and enters the second polarizer 14. As described above, the second polarizer is a reflective linear polarizer. Therefore, it has a reflection axis perpendicular to the transmission axis. Furthermore, the first polarizer and the second polarizer are arranged in a parallel Nicol configuration, with their transmission axes parallel. That is, the second polarizer, which is a reflective linear polarizer, has a reflection axis perpendicular to the transmission axis of the first polarizer 12. Therefore, light outside the specific wavelength range for which the liquid crystal polarization interference element acts as a λ / 2 phase retarder has its polarization direction changed by 90° with respect to the transmission axis of the first polarizer, is reflected by the second polarizer, which is a reflective linear polarizer, and then has its polarization direction changed by 90° again by the liquid crystal polarization interference element before entering the first polarizer 12. As described above, the polarization direction of the linearly polarized light reflected by the second polarizer is perpendicular to the transmission axis of the first polarizer 12. Therefore, the linearly polarized light reflected by the second polarizer 14 is transmitted through the first polarizer 12 and emitted.

[0103] That is, the optical filter of the present invention uses a liquid crystal polarization interference element configured such that the angle of the in-plane slow axis increases sequentially in the stacking direction, and by arranging the first polarizer and the second polarizer in parallel Nicols, light of a specific wavelength range in which the liquid crystal polarization interference element 16 does not act as a retardation plate can be transmitted through the optical filter and emitted as transmitted light. On the other hand, by arranging the first polarizer and the second polarizer in parallel Nicols, light of a specific wavelength range in which the liquid crystal polarization interference element 16 acts as a λ / 2 retardation plate can be reflected by the optical filter (second polarizer) and emitted as reflected light. That is, the optical filter of the present invention configured as such also functions as a dichroic filter, which extracts light of a predetermined narrowband wavelength as transmitted light, while also functioning as a bandpass filter which extracts light of other wavelengths as reflected light.

[0104] In the optical filter of the present invention, which uses a liquid crystal polarization interference element having a configuration in which the angle of the in-plane slow axis increases sequentially in the stacking direction, the first polarizer and the second polarizer may be arranged in a crossed Nicol state, as a matter of course. As in the previous example, when the first polarizer and the second polarizer are arranged in a crossed Nicol state, an optical filter having the opposite effect to that when both polarizers are arranged in a parallel Nicol state can be realized.

[0105] In an optical filter using this liquid crystal polarization interference element, when the first polarizer and the second polarizer are arranged in a crossed Nicol configuration, the optical filter functions as follows. Similarly, in this case, of the linearly polarized light transmitted through the first polarizer, light in a specific wavelength range for which the liquid crystal polarization interference element does not function as a retardation plate is incident on the second polarizer while remaining linearly polarized in the direction of the transmission axis of the first polarizer. Furthermore, light in a wavelength range other than the specific wavelength range for which the liquid crystal polarization interference element functions as a λ / 2 retardation plate has its polarization direction changed by 90° and is incident on the second polarizer. Here, in this example, the first polarizer and the second polarizer are arranged in a crossed Nicol configuration. Therefore, the second polarizer, which is a reflective linear polarizer, has a reflection axis in the same direction as the transmission axis of the first polarizer. Therefore, light in the specific wavelength range for which the liquid crystal polarization interference element does not function as a retardation plate is reflected by the second polarizer, passes through the liquid crystal polarization interference element as is, and is incident on the first polarizer. Since the polarization direction of this light is aligned with the transmission axis of the first polarizer 12, it passes through the first polarizer and exits. On the other hand, light outside the specific wavelength range for which the liquid crystal polarization interference element acts as a λ / 2 retardation plate has its polarization direction changed by 90° by the liquid crystal polarization interference element and enters the second polarizer. As described above, in this example, the first polarizer and the second polarizer are arranged in a crossed Nicol configuration. Therefore, light outside the specific wavelength range, whose polarization direction has been changed by 90°, passes through the second polarizer and exits. That is, in this case, the optical filter acts as a dichroic filter that reflects and emits light in the specific wavelength range for which the liquid crystal polarization interference element does not act as a retardation plate, and transmits light of other wavelengths.

[0106] When the optical filter of the present invention is used as a bandpass filter, it is preferable to set the transmission axes of the first polarizer and the second polarizer at an appropriate angle in order to preferably obtain the desired bandpass characteristics. In particular, when the optical filter of the present invention is used as a bandpass filter, by appropriately adjusting the angles of the transmission axes of the polarizers sandwiching the liquid crystal polarization interference element in the thickness direction, it is possible to reduce the magnitude of the side lobes generated at wavelengths on both sides (longer and shorter wavelength sides) of the main bandpass wavelength, and to adjust the magnitude of the side lobes on the longer and shorter wavelength sides to be equal.

[0107] Furthermore, in the optical filter of the present invention, the shape of the bandpass filter curve can be adjusted by appropriately adjusting the size and angle of the slow axis of the liquid crystal polarization interference element. For example, it can be made into a gentle shape like a Gaussian function or a steep shape like a rectangular function. These can be adjusted to the desired shape depending on the application of the bandpass filter, and can be preferably used for, for example, long-pass and short-pass applications.

[0108] In the optical filter of the present invention, a retarder may be provided between the first polarizer and the liquid crystal polarization interference element, or between the second polarizer, the first polarizer, and the liquid crystal polarization interference element. That is, in the optical filter of the present invention, a retarder may be provided on one or both sides between the liquid crystal polarization interference element and the polarizer. This retarder has the effect of maintaining the orthogonal relationship of the polarization directions of the linear polarizers arranged in a crossed Nicol configuration not only in the frontal direction but also in the oblique direction off-axis of the polarizer. As a result, when the optical filter of the present invention is used as a bandpass filter, excellent bandpass characteristics similar to those obtained in the frontal direction can be obtained in the oblique direction. The in-plane slow axis of the retarder is preferably parallel to the absorption axis of either the first polarizer or the second polarizer arranged in a crossed Nicol configuration. This allows the polarization state to be compensated so as to maintain the orthogonal relationship of the polarization directions in the oblique direction without affecting the frontal direction. Examples of the retardation plate include a positive C plate formed by vertically aligning rod-shaped liquid crystals and a positive A plate formed by horizontally aligning rod-shaped liquid crystals, a negative C plate formed by discotic liquid crystals and a negative A plate formed by discotic liquid crystals, or a combination thereof. Furthermore, a biaxial refractive index B plate (with an Nz factor of 0.1 to 0.9) can also be used as the retardation plate.

[0109] As described above, in the optical filter of the present invention, a reflective linear polarizer having a selective reflection layer including at least one cholesteric liquid crystal layer and a λ / 4 retardation plate can be used as the second polarizer, i.e., the reflective linear polarizer. This reflective linear polarizer will be described below.

[0110] First, the cholesteric liquid crystal layer used in the reflective linear polarizer will be described. The cholesteric liquid crystal layer refers to a liquid crystal layer in which a liquid crystal compound is cholesterically oriented. Note that cholesteric orientation refers to the orientation of a liquid crystal compound in a cholesteric liquid crystal phase, and the cholesteric liquid crystal layer may be any layer in which the cholesteric orientation is maintained. Typically, a polymerizable liquid crystal compound having a polymerizable group is oriented in a cholesteric liquid crystal phase by adding a chiral agent or the like, and then polymerized and cured by ultraviolet irradiation, heating, or the like to form a layer with no fluidity.

[0111] The cholesteric liquid crystal layer is preferably a layer that has been changed to a state in which its orientation does not change due to an external field, external force, etc. Note that it is sufficient for the cholesteric liquid crystal layer to retain the optical properties of the cholesteric liquid crystal phase within the layer, and the liquid crystal compound in the cholesteric liquid crystal layer may no longer exhibit liquid crystallinity. For example, a polymerizable liquid crystal compound may be polymerized by a curing reaction and no longer exhibit liquid crystallinity.

[0112] The central wavelength λ of the reflected light of a cholesteric liquid crystal layer depends on the helical pitch number P (= helical period) of the helical structure in the cholesteric liquid crystal phase and is expressed by the relationship λ = n × P, where n is the average refractive index of the cholesteric liquid crystal layer. The central wavelength of the reflected light of a cholesteric liquid crystal layer can be calculated as follows: When the transmission spectrum of a cholesteric liquid crystal layer is measured from the normal direction of the cholesteric liquid crystal layer using a spectrophotometer, a spectrum having a peak where the transmittance decreases in a region near the central wavelength of the reflected light is obtained. That is, a reflection spectrum having a peak where the reflectance increases in a region near the central wavelength of the reflected light is obtained. Of the two wavelengths at which the transmittance is half the value of the largest peak, the value of the shorter wavelength is λ1 (nm) and the value of the longer wavelength is λ2 (nm). The central wavelength λ of the reflected light can be calculated using the following formula: λ = (λ1 + λ2) / 2 Note that, when the reflection spectrum of the outermost cholesteric liquid crystal layer is determined by the same procedure, λ1 calculated by the above procedure becomes λmin, which is the half-value wavelength on the short wavelength side. Furthermore, when the reflection spectrum of the outermost cholesteric liquid crystal layer is determined by the same procedure, λ2 becomes λmax, which is the half-value wavelength on the long wavelength side. For more specific procedures, please refer to the description of the examples below.

[0113] The helical pitch of a cholesteric liquid crystal phase varies depending on the type of chiral agent used together with the liquid crystal compound and its concentration, and a cholesteric liquid crystal phase with the desired pitch can be obtained by adjusting one or more of the above. Regarding the method for measuring the helical direction and helical pitch, the methods described in "Introduction to Liquid Crystal Chemistry Experiments," edited by the Japanese Liquid Crystal Society, published by Sigma Publishing in 2007, page 46, and "Liquid Crystal Handbook," published by the Liquid Crystal Handbook Editorial Committee, Maruzen, page 196, can be used. Using the above-described method, λmax and λmin, as well as P, as described in the following formulas (1) and (2), can be measured, and nmax and nmin can be determined. Formula (1): nmax = λmax / P Formula (2): nmin = λmin / P

[0114] For a cholesteric liquid crystal layer, nmax calculated by the above formula (1) is often 1.50 to 2.00, and may be 1.60 to 1.90. For a cholesteric liquid crystal layer, nmin calculated by the above formula (2) is often 1.35 to 1.75, and may be 1.45 to 1.65. Note that nmax is a larger value than nmin.

[0115] The liquid crystal compound contained in the cholesteric liquid crystal layer is not particularly limited, and examples thereof include rod-shaped liquid crystal compounds and discotic liquid crystal compounds. Examples of the rod-shaped liquid crystal compound include known rod-shaped liquid crystal compounds, and polymerizable rod-shaped liquid crystal compounds having a polymerizable group are preferred. Examples of the rod-shaped liquid crystal compound are not particularly limited, and examples thereof include those described in claim 1 of JP-A-11-513019 or paragraphs

[0026] to

[0098] of JP-A-2005-289980.

[0116] It is also preferable to use a liquid crystal compound with a high refractive index anisotropy Δn (high Δn) as the rod-shaped liquid crystal compound. Here, Δn is the difference between the refractive index in the slow axis direction and the refractive index in the fast axis direction. When a rod-shaped liquid crystal compound has high Δn characteristics, a high reflectance can be obtained even with a small number of turns in the helical structure of the cholesteric liquid crystal phase, thereby achieving the desired reflection characteristics even with a thin film thickness. By reducing the film thickness, the magnitude of the phase difference generated for incident light obliquely tilted from the normal direction of the cholesteric liquid crystal layer can be reduced. Liquid crystal compounds with a high refractive index anisotropy Δn are not particularly limited, but the compounds exemplified in paragraphs

[0014] to

[0029] of WO 2019 / 182129 and compounds represented by the following general formula (I) can be preferably used.

[0117]

[0118] In general formula (I), P 1 and P 2 each independently represents a hydrogen atom, —CN, —NCS, or a polymerizable group. 1 and Sp 2 each independently represents a single bond or a divalent linking group.1 and Sp 2 does not represent a divalent linking group containing at least one group selected from the group consisting of an aromatic hydrocarbon ring group, an aromatic heterocyclic group, and an aliphatic hydrocarbon ring group. 1 , Z 2 and Z 3 each independently represents a single bond, —O—, —S—, —CHR—, —CHRCHR—, —OCHR—, —CHRO—, —SO—, or —SO ２ -, -COO-, -OCO-, -CO-S-, -S-CO-, -O-CO-O-, -CO-NR-, -NR-CO-, -SCHR-, -CHRS-, -SO-CHR-, -CHR-SO-, -SO2-CHR-, - CHR-SO2-, -CF2O-, -OCF2-, -CF2S-, -SCF2-, -OCHRCHRO-, -SCHRCHRS-, -SO-CHRCHR-SO-, -SO2-CHRCHR-SO2-, -CH=C H-COO-, -CH=CH-OCO-, -COO-CH=CH-, -OCO-CH=CH-, -COO-CHRCHR-, -OCO-CHRCHR-, -CHRCHR-COO-, -CHRCHR-OCO-, -COO-CHR-, -OCO-CHR-, -CHR-COO-, -CHR-OCO-, -CR=CR-, -CR=N-, -N=CR-, -N=N-, -CR=N-N=CR-, -CF=CF- or C≡C-. R represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms. When multiple Rs are present, they may be the same or different. Z １ and Z ２ When there are a plurality of Z, they may be the same or different. ３ may be the same or different, provided that Sp 2 Z connected to 3 represents a single bond. 1 and X 2 Each of X independently represents a single bond or S-. 1 and X 2 may be the same or different. 1 and multiple Xs 2At least one of them represents -S-. In general formula (I), k represents an integer of 2 to 4. In general formula (I), m and n each independently represent an integer of 0 to 3. Multiple m's may be the same or different. In general formula (I), A 1 , A 2 , A 3 and A 4 each independently represents a group represented by any one of the following general formulae (B-1) to (B-7), or a group formed by linking two to three groups represented by any one of the following general formulae (B-1) to (B-7). 2 and A 3 may be the same or different. 1 and A 4 When there are a plurality of each, they may be the same or different.

[0119]

[0120] In general formulas (B-1) to (B-7), W 1 ~W 18 are each independently CR 1 or N, R 1 represents a hydrogen atom or the following substituent L. In general formulae (B-1) to (B-7), Y 1 ~Y 6 are each independently NR 2 , O or S, R 2 represents a hydrogen atom or the following substituent L. In general formulae (B-1) to (B-7), G 1 ~G 4 are each independently CR 3 R 4 , N.R. 5 , O or S, R 3 ~R 5 Each of the groups independently represents a hydrogen atom or the following substituent L. In general formulae (B-1) to (B-7), M 1 and M 2 are each independently CR 6 or N, R 6 represents a hydrogen atom or the following substituent L. In formulae (B-1) to (B-7), * represents a bonding position.

[0121] The substituent L is an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, an alkylamino group having 1 to 10 carbon atoms, an alkylthio group having 1 to 10 carbon atoms, an alkanoyl group having 1 to 10 carbon atoms, an alkanoyloxy group having 1 to 10 carbon atoms, an alkanoylamino group having 1 to 10 carbon atoms, an alkanoylthio group having 1 to 10 carbon atoms, an alkyloxycarbonyl group having 2 to 10 carbon atoms, an alkylaminocarbonyl group having 2 to 10 carbon atoms, an alkylthiocarbonyl group having 2 to 10 carbon atoms, a hydroxy group, an amino group, a mercapto group, a carboxy group, a sulfo group, an amido group, a cyano group, a nitro group, a halogen atom, or a polymerizable group. However, when the above groups described as the substituent L have -CH2-, groups in which at least one of the -CH2- groups contained in the above groups is replaced with -O-, -CO-, -CH=CH-, or C≡C- are also included in the substituent L. Furthermore, when the above group described as the substituent L has a hydrogen atom, a group in which at least one of the hydrogen atoms contained in the above group is replaced with at least one selected from the group consisting of a fluorine atom and a polymerizable group is also included in the substituent L.

[0122] Refractive index anisotropy Δn of liquid crystal compound ５５０ The refractive index anisotropy at a wavelength of 550 nm is preferably 0.12 or more, more preferably 0.16 or more, even more preferably 0.20 or more, and most preferably 0.25 or more. ５５０ The upper limit of the refractive index anisotropy at a wavelength of 550 nm is preferably 0.90 or less, more preferably 0.70 or less, and most preferably 0.50 or less, from the viewpoint of suppressing interfacial reflection.

[0123] Examples of discotic liquid crystal compounds include known discotic liquid crystal compounds, and preferred examples include polymerizable discotic liquid crystal compounds having a polymerizable group. Examples of discotic liquid crystal compounds are not particularly limited, but for example, the discotic liquid crystal compounds described in paragraphs

[0020] to

[0122] of JP-A-2007-108732 can be suitably used. Furthermore, it is also preferable to use a liquid crystal compound with a high refractive index anisotropy Δn (high Δn). Here, Δn is the difference between the refractive index in the slow axis direction and the refractive index in the fast axis direction. When a discotic liquid crystal compound has high Δn characteristics, high reflectance can be obtained even with a small number of turns in the helical structure of the cholesteric liquid crystal phase, thereby achieving desired reflection characteristics even with a thin film thickness. Thinning the film can reduce the magnitude of the retardation caused by incident light obliquely tilted from the normal direction of the cholesteric liquid crystal layer. As the discotic compound having a high Δn, for example, the discotic liquid crystal compound described in paragraphs

[0012] to

[0108] of JP-A No. 2010-244038 can be suitably used. The refractive index anisotropy Δn550 (refractive index anisotropy at a wavelength of 550 nm) of the liquid crystal compound is preferably 0.12 or more, more preferably 0.16 or more, even more preferably 0.20 or more, and most preferably 0.25 or more. Δn ５５０ The upper limit of the refractive index anisotropy at a wavelength of 550 nm is preferably 0.90 or less, more preferably 0.70 or less, and most preferably 0.50 or less, from the viewpoint of suppressing interfacial reflection.

[0124] The polymerizable group that the liquid crystal compound may have is not particularly limited, but is preferably a functional group capable of addition polymerization reaction, more preferably a polymerizable ethylenically unsaturated group or a ring-polymerizable group, and even more preferably a (meth)acryloyl group, a vinyl group, a styryl group, or an allyl group. The number of polymerizable groups that the liquid crystal compound has is not particularly limited, but is preferably 2 or more. The upper limit is not particularly limited, but is often 10 or less.

[0125] The cholesteric liquid crystal layer is preferably a layer formed using a liquid crystal compound having a polymerizable group, and more preferably a layer in which the orientation state of the liquid crystal compound having a polymerizable group is fixed. The "fixed" state is the most typical and preferred state in which the orientation of the liquid crystal compound is maintained. While not limited thereto, specifically, it is more preferable that the layer has no fluidity and can stably maintain the fixed orientation state without causing any change in the orientation state due to an external field or external force, usually within a temperature range of 0 to 50°C, or under more severe conditions, within a temperature range of −30 to 70°C.

[0126] The selective reflection layer preferably has two or more cholesteric liquid crystal layers. When the selective reflection layer has two or more cholesteric liquid crystal layers, it preferably includes a cholesteric liquid crystal layer R formed using a rod-shaped liquid crystal compound and a cholesteric liquid crystal layer D formed using a discotic liquid crystal compound.

[0127] The cholesteric liquid crystal layer R is preferably a liquid crystal layer formed using liquid crystal compounds that are substantially rod-shaped liquid crystal compounds. The phrase "substantially consisting of rod-shaped liquid crystal compounds" means that the rod-shaped liquid crystal compounds account for 95% by mass or more of the liquid crystal compounds contained in the cholesteric liquid crystal layer R. Examples of rod-shaped liquid crystal compounds contained in the cholesteric liquid crystal layer R are as described above.

[0128] The cholesteric liquid crystal layer D is preferably a liquid crystal layer formed using a liquid crystal compound that is substantially a discotic liquid crystal compound. The phrase "substantially consisting of a discotic liquid crystal compound" means that the discotic liquid crystal compound accounts for 95% by mass or more of the liquid crystal compounds contained in the cholesteric liquid crystal layer D. Examples of the discotic liquid crystal compound contained in the cholesteric liquid crystal layer D are as described above.

[0129] The thickness of the cholesteric liquid crystal layer R is preferably 0.1 μm or more, more preferably 0.2 μm or more, and even more preferably 0.3 μm or more. The thickness of the cholesteric liquid crystal layer R is preferably 10.0 μm or less, more preferably 7.0 μm or less, and even more preferably 5.0 μm or less. The thickness of the cholesteric liquid crystal layer R can be measured by preparing a cross-section of the optical laminate and observing it with a scanning electron microscope. The thickness of the cholesteric liquid crystal layer R is a value obtained by averaging the thicknesses of the cholesteric liquid crystal layer R at any five points on the cross-section of the optical laminate. When the cross-section of the optical laminate is observed with a scanning electron microscope, the region of the cholesteric liquid crystal layer R and the region of the cholesteric liquid crystal layer D, which will be described later, can be distinguished by the difference in contrast of the observed image. Furthermore, the cholesteric liquid crystal layer R and the cholesteric liquid crystal layer D can also be distinguished by using composition analysis in the film thickness direction by time-of-flight secondary ion mass spectrometry (TOF-SIMS).

[0130] The Rth of the cholesteric liquid crystal layer R is preferably 8 to 800 nm, more preferably 16 to 560 nm, and even more preferably 24 to 400 nm at a wavelength of 550 nm. The Rth of the cholesteric liquid crystal layer R may be measured by taking out only the cholesteric liquid crystal layer R from the optical laminate, or may be measured by measuring the Rth of a layer prepared under the same conditions as when the cholesteric liquid crystal layer R was prepared.

[0131] The thickness of the cholesteric liquid crystal layer D is preferably 0.1 μm or more, more preferably 0.2 μm or more, and even more preferably 0.3 μm or more. The thickness of the cholesteric liquid crystal layer D is preferably 10.0 μm or less, more preferably 7.0 μm or less, and even more preferably 5.0 μm or less. The thickness of the cholesteric liquid crystal layer D can be measured in the same manner as the thickness of the cholesteric liquid crystal layer R described above.

[0132] The Rth of the cholesteric liquid crystal layer D is preferably −8 to −800 nm, more preferably −16 to −560 nm, and even more preferably −24 to −400 nm at a wavelength of 550 nm. The Rth of the cholesteric liquid crystal layer D may be measured by removing only the cholesteric liquid crystal layer D from the selective reflection layer, or may be measured by measuring the Rth of a layer prepared under the same conditions as those for preparing the cholesteric liquid crystal layer D.

[0133] The helical pitch number P of the cholesteric liquid crystal phase of the cholesteric liquid crystal layer R or the cholesteric liquid crystal layer D may vary in the film thickness direction. In the following description, the state in which the helical pitch number P varies in the film thickness direction is referred to as a pitch gradient, and a layer in which the helical pitch number P varies in the film thickness direction is referred to as a pitch gradient layer. The pitch gradient layer can be produced by a known method, and can be produced by referring to, for example, JP 2020-060627 A.

[0134] In a pitch gradient layer, the helical pitch number P varies in the film thickness direction, allowing it to reflect light in multiple wavelength ranges. When the outermost cholesteric liquid crystal layer is a pitch gradient layer, a region of the cholesteric liquid crystal layer 2.0 μm from the surface of the outermost cholesteric liquid crystal layer on the first adjacent layer side is obtained, and cross-sectional SEM observation is performed. The helical pitch is measured from the bright and dark striped pattern that appears in the observed image. Here, the distance between light and dark, bright and dark, in the striped pattern corresponds to one full rotation of the helix. Furthermore, the reflection spectrum of the outermost cholesteric liquid crystal layer is obtained. Next, the cholesteric liquid crystal is modeled using Berreman's 4x4 matrix method, and the reflection spectrum is calculated based on the helical pitch obtained by the above method and the tentative nmax and nmin. A combination of nmax and nmin that matches the calculated reflection spectrum and the measured reflection spectrum is searched for, and the nmax and nmin with the highest degree of match are determined.

[0135] The number of cholesteric liquid crystal layers in the selective reflection layer may be 3 or more, 4 or more, 6 or more, or 8 or more. The number of cholesteric liquid crystal layers in the reflective polarizer is preferably 120 or less, more preferably 80 or less, and even more preferably 40 or less.

[0136] The central wavelengths λ of the reflected light of the cholesteric liquid crystal layers in the selective reflection layer may be different from each other, or some or all of them may be the same. When cholesteric liquid crystal layers with different central wavelengths λ of reflected light are stacked, the entire reflective polarizer can reflect light in the wavelength range corresponding to the central wavelength λ of each cholesteric liquid crystal layer.

[0137] Furthermore, when the selective reflection layer has three or more cholesteric liquid crystal layers, it is preferable to alternately laminate the above-mentioned cholesteric liquid crystal layer R and the above-mentioned cholesteric liquid crystal layer D. This is because the cholesteric liquid crystal layer R often has a positive Rth, whereas the cholesteric liquid crystal layer D often has a negative Rth, and therefore there is an effect that the Rths of the two layers cancel each other out. Details of this will be explained below.

[0138] In the selective reflection layer having n layers of cholesteric liquid crystal layers, the reflection layers are arranged in order from the light source side to the L １ , L ２ , L ３ , ..., L ｎ (n is an integer of 4 or more), the reflective layer L １ to the reflective layer L ｉ The sum of Rth of each layer up to (i is an integer equal to or less than n) is SRth ｉ Specifically, SRth ｉ is expressed as follows: SRth １ = Rth １ SRth ２ = Rth １ +Rth ２ ... SRth ｉ = Rth １ +Rth ２ +...+Rth ｉ ... SRth ｎ = Rth １ +Rth ２ +...+Rth ｉ +...+Rth ｎ All these SRth ｉ (SRth １ ~SRth ｎThe absolute values ​​of Rth of each layer in the above formula are preferably 0.3 μm or less, more preferably 0.2 μm or less, and even more preferably 0.1 μm or less. ｉ is calculated by the above-described formula for calculating Rth. ｉ It is believed that by setting the value of the thickness of the reflective layer to the above preferred range, the phase difference that occurs when light passes through each reflective layer can be reduced.

[0139] When the selective reflection layer contains two or more cholesteric liquid crystal layers, a layer other than the cholesteric liquid crystal layer may be contained between the two cholesteric liquid crystal layers. The layer other than the cholesteric liquid crystal layer is not particularly limited, and examples thereof include an adhesion layer (e.g., an adhesive layer, a pressure-sensitive adhesive layer, etc.), a refractive index adjustment layer, a resin film, a positive C plate, and an alignment layer.

[0140] Furthermore, when the selective reflection layer includes a cholesteric liquid crystal layer R and a cholesteric liquid crystal layer D, and when the selective reflection layer includes a cholesteric liquid crystal layer R and a cholesteric liquid crystal layer D, it is preferable to arrange the liquid crystal compounds (rod-shaped liquid crystal compounds or discotic liquid crystal compounds) so that the alignment direction (slow axis direction) of the liquid crystal compounds changes continuously at the interface in order to reduce the difference in refractive index. For example, when the cholesteric liquid crystal layer R is formed on the cholesteric liquid crystal layer D, the above-mentioned arrangement can be achieved by directly applying a coating liquid containing a rod-shaped liquid crystal compound onto the cholesteric liquid crystal layer D, and aligning the slow axis direction continuously at the interface due to the alignment restricting force of the discotic liquid crystal compound contained in the cholesteric liquid crystal layer D.

[0141] The thickness of the selective reflection layer is preferably 0.2 μm or more, more preferably 0.4 μm or more, and even more preferably 0.6 μm or more, and is preferably 20.0 μm or less, more preferably 14.0 μm or less, and even more preferably 10.0 μm or less.

[0142] As described above, a reflective linear polarizer using a cholesteric liquid crystal layer has a λ / 4 retardation plate. Hereinafter, the λ / 4 retardation plate will be described. In the following description, the λ / 4 retardation plate used in a reflective linear polarizer will also be simply referred to as a retardation plate. In a reflective linear polarizer, a retardation plate used in combination with a cholesteric liquid crystal layer refers to a layer having in-plane retardation (Re). It is preferable that the retardation plate exhibits an Re of λ / 4 (or an odd multiple thereof) at a predetermined wavelength λnm. The Re of the retardation plate at a wavelength of 550 nm may have an error of about 25 nm around the ideal value (137.5 nm), and is, for example, preferably 110 to 160 nm, more preferably 120 to 150 nm.

[0143] The refractive index nx1 in the in-plane slow axis direction of the retardation plate and the refractive index ny1 in the in-plane fast axis direction are calculated from the reflectance by obtaining the reflection spectrum when linearly polarized light is incident on the retardation plate using a spectrophotometer. Specifically, they are measured by the method described in the examples below.

[0144] The refractive index nx1 in the in-plane slow axis direction is often 1.40 to 1.80, and may be 1.50 to 1.70. The refractive index ny1 in the in-plane fast axis direction is often 1.35 to 1.75, and may be 1.45 to 1.65. Note that nx1 is a larger value than ny1.

[0145] It is more preferable that the retarder exhibits the characteristics of a λ / 4 retarder at each wavelength across the visible light region, and such a retarder is particularly called a broadband λ / 4 retarder. The broadband λ / 4 retarder preferably has an in-plane retardation (Re(λ)) at a wavelength of λ nm that satisfies the following formulas (A) and (B): Formula (A) Re(450) / Re(550)<1.00 Formula (B) Re(650) / Re(550)≧1.00 Re(450) represents the in-plane retardation of the λ / 4 retarder at a wavelength of 450 nm, Re(550) represents the in-plane retardation of the λ / 4 retarder at a wavelength of 550 nm, and Re(650) represents the in-plane retardation of the λ / 4 retarder at a wavelength of 650 nm.

[0146] The retardation plate may be composed of a single layer retardation plate, or may be composed of two or more layer retardation plates laminated by lamination, sequential formation, or other methods. The retardation plate referred to here is a layer exhibiting optical anisotropy. Examples of the retardation plate include layers in which at least two of nx, ny, and nz are different. Note that nx represents the refractive index in the direction perpendicular to the thickness direction of the retardation plate (in-plane direction) and in the direction giving the maximum refractive index (slow axis direction). ny represents the refractive index in the in-plane direction of the retardation plate and in the direction perpendicular to the nx direction (fast axis direction). nz represents the refractive index in the thickness direction of the retardation plate.

[0147] The material constituting the retardation plate is not particularly limited, and examples thereof include liquid crystal compounds and polymers. A liquid crystal compound can form a retardation plate by orienting a liquid crystal material to exhibit refractive index anisotropy. A polymer can form a retardation plate by stretching a polymer film obtained by casting or coating to exhibit refractive index anisotropy. The retardation plate used in the present invention is preferably a layer formed using a liquid crystal compound, and more preferably a layer formed using a liquid crystal compound having a polymerizable group. That is, the retardation plate of the present invention preferably contains a liquid crystal compound. The liquid crystal compound used in the retardation plate is preferably a liquid crystal compound having a polymerizable group.

[0148] The liquid crystal compound may be a liquid crystal compound exhibiting either normal wavelength dispersion or reverse wavelength dispersion. When a retardation plate exhibiting the characteristics of a wideband λ / 4 plate as a single film is used, a liquid crystal compound exhibiting reverse wavelength dispersion is preferred, and a liquid crystal compound having two or more polymerizable groups and exhibiting reverse wavelength dispersion is more preferred.

[0149] In this specification, the term "liquid crystal compound exhibiting reverse wavelength dispersion" refers to a compound that satisfies the relationship between the above-mentioned formulas (A) and (B) when the in-plane retardation (Re) value at a specific wavelength (visible light range) of an optically anisotropic layer prepared using this compound is measured.

[0150] In addition, in this specification, the term "liquid crystal compound exhibiting forward wavelength dispersion" refers to a compound that satisfies the relationships of the following formulas (C) and (D) when the in-plane retardation (Re) value at a specific wavelength (visible light range) of a retardation plate prepared using this compound is measured: Formula (C) Re(450) / Re(550)≧1.00 Formula (D) Re(650) / Re(550)<1.00

[0151] As described above, the retardation plate is preferably a layer formed using a liquid crystal compound having a polymerizable group, and more preferably a layer formed by fixing the alignment state of a liquid crystal compound having a polymerizable group.

[0152] The orientation state that a liquid crystal compound having a polymerizable group can assume is not particularly limited, and examples thereof include homogeneous orientation, homeotropic orientation, twisted orientation, cholesteric orientation, hybrid orientation (an orientation in which the tilt angle of the liquid crystal compound changes continuously from one surface to the other), and tilted orientation (an orientation in which the tilt angle of the liquid crystal compound is constant from one surface to the other). Twisted orientation refers to an orientation state in which the liquid crystal compound is twisted around the thickness direction as the axis of rotation, and when the liquid crystal compound is twisted and has a predetermined tilt angle (tilt angle greater than 0°), it corresponds to twisted hybrid orientation. In this specification, twisted orientation refers to an embodiment in which the twist angle of the liquid crystal compound is less than 360°, and the above-mentioned cholesteric orientation refers to an embodiment in which the twist angle of the liquid crystal compound is 360° or more.

[0153] A retardation plate formed using a liquid crystal compound may have a plurality of regions along the thickness direction in which the liquid crystal compound has different alignment states. For example, the retardation plate may have a region along the thickness direction in which the liquid crystal compound is fixed in a homogeneously aligned state and a region in which the liquid crystal compound is fixed in a twisted aligned state.

[0154] The thickness of the retardation plate is not particularly limited, but is preferably 0.1 to 10.0 μm, more preferably 0.5 to 5.0 μm.

[0155] Specific examples of the configuration of a wideband λ / 4 retarder include those configured with a single-layer retarder, such as retarders using a liquid crystal compound exhibiting reverse wavelength dispersion as disclosed in WO 2019 / 160016, JP 2020-173460, and WO 2021 / 157694, and retarders having a plurality of regions along the thickness direction in which the alignment state of the liquid crystal compound is different as disclosed in WO 2022 / 030308 and JP 2022-184691. Examples of a laminate of two or more retardation plates include a configuration in which a λ / 4 retardation plate and a λ / 2 retardation plate are combined, as disclosed in JP-A-2001-108825, JP-A-2001-91741, WO 2013 / 137464, etc., and a configuration in which a retardation plate having a twisted orientation, as disclosed in JP-A-2001-21720, JP-A-2014-209219, WO 2022 / 255105, etc., is combined with another retardation plate.

[0156] Furthermore, the retardation layer may further include another retardation plate such as a positive C plate or a negative C plate in order to compensate for a change in retardation caused by incident light in an oblique direction.

[0157] Such an optical filter of the present invention can be used for any wavelength, i.e., for any electromagnetic wave such as ultraviolet light, visible light, infrared light, terahertz waves, and millimeter waves.

[0158] The optical system of the present invention includes a light source unit, the optical filter of the present invention, and a light receiving unit. In such an optical system of the present invention, for example, by using the optical filter of the present invention as a bandpass filter, an optical system with little loss of received light can be realized.

[0159] In the optical system of the present invention, there is no limitation on the light source unit, and various known light source units (light source, light-emitting element) that can emit light of a desired wavelength can be used, such as an LED (Light Emitting Diode), an organic EL (OLED organic electro-luminescence) element, a fluorescent lamp, a halogen lamp, a laser light source, a plasma light source, etc. In addition, in the optical system of the present invention, there is no limitation on the light receiving unit, and various known light receiving units (light receiving element, image sensor) can be used, such as a CCD sensor, a photomultiplier, a CMOS sensor, a photodiode, etc., as long as they can receive light of a target wavelength and measure the light.

[0160] FIG. 7 conceptually illustrates an example of an optical system of the present invention in combination with a condensing lens. The optical system shown in FIG. 7 includes a light source unit 90, a condensing lens 92, an optical filter 94 of the present invention, a light receiving unit 96a for transmitted light, and a light receiving unit 96b for reflected light. Various known condensing lenses can be used. In this optical system, divergent light emitted by the light source 90 is collected by the condensing lens 92. In this optical system, an optical filter 94 (bandpass filter (dichroic filter)) of the present invention is disposed at the divergent or condensing portion of the light. In the illustrated example, the optical filter 94 of the present invention is disposed at the condensing portion of the light. Furthermore, as shown in FIG. 7 , in this optical system, the optical filter 94 is disposed at an angle with respect to the optical axis of the condensing lens 92. The light receiving unit 96a for transmitted light is disposed on the optical axis of the condensing lens 92. On the other hand, the light receiving unit 96b for reflected light is disposed on the optical path of the light reflected by the tilted optical filter 94.

[0161] As an example, the optical filter 94 shown in the figure has a first polarizer and a second polarizer arranged in a crossed Nicol configuration with their transmission axes perpendicular to each other. This optical filter 94 also uses the liquid crystal polarization interference element 16 shown in FIG. 1 as the liquid crystal polarization interference element. Accordingly, in this case, light emitted from the light source 90, collected by the condensing lens 92, and incident on the optical filter 94, light within a specific wavelength range passes through the optical filter 94 and enters the light-receiving unit 96a for transmitted light, where it is photometered. On the other hand, light outside the specific wavelength range that is collected by the condensing lens 92 and incident on the optical filter 94 is reflected by the optical filter 94 and enters the light-receiving unit 96b for reflected light, where it is photometered. The first and second polarizers of the optical filter 94 may also be arranged in a parallel Nicol configuration with their transmission axes parallel to each other. In this case, as described above, of the light that is collected by the collecting lens 92 and incident on the optical filter 94, light in a specific wavelength range is reflected by the optical filter 94 and photometered by the light-receiving unit 96b for reflected light. Also, in this optical system, of the light that is collected by the collecting lens 92 and incident on the optical filter 94, light outside the specific wavelength range is transmitted through the optical filter 94 and photometered by the light-receiving unit 96a for transmitted light.

[0162] The optical system shown in FIG. 7 can also utilize an optical filter using a liquid crystal polarization interference element having a configuration in which the angle of the in-plane slow axis increases sequentially in the stacking direction, as described above in Example 30. In this optical system, when the first and second polarizers of the optical filter are arranged in a parallel Nicol configuration, light in a specific wavelength range, which is collected by the collecting lens 92 and incident on the optical filter 94, passes through the optical filter and enters the light-receiving unit 96a for transmitted light, where it is photometered. In addition, in this optical system, light outside the specific wavelength range, which is collected by the collecting lens 92 and incident on the optical filter 94, is reflected by the optical filter 94 and enters the light-receiving unit 96b for reflected light, where it is photometered. On the other hand, in this optical system, when the first and second polarizers of the optical filter are arranged in a crossed Nicol configuration, light in a specific wavelength range, which is collected by the collecting lens 92 and incident on the optical filter 94, is reflected by the optical filter 94 and enters the light-receiving unit 96b for reflected light, where it is photometered. In addition, in this optical system, light outside the specific wavelength range that is collected by the collecting lens 92 and incident on the optical filter 94 passes through the optical filter 94 and enters the light receiving unit 96a for transmitted light, where it is measured.

[0163] As described above, the optical filter of the present invention exhibits bandpass performance over the same wavelength for both light incident from the front and from oblique directions, allowing light of desired wavelengths over a wide angular range to be collected at the light receiving section. The optical system of the present invention can thereby achieve high light receiving efficiency and minimal light receiving loss. The optical system of the present invention using a condensing lens can be used, for example, in an imaging system. This imaging system achieves high light receiving efficiency when the condensing lens collects light from various directions from the light source unit 90, which is the object to be imaged, and then highly collects the light at the light receiving unit 96a, which is an imaging element. Furthermore, because the optical filter of the present invention exhibits bandpass performance over a wide angle, a thin, compact optical system can be realized by using a condensing lens with a high numerical aperture. Furthermore, the optical system of the present invention can also be used in systems that utilize optical fibers. In this case, a system that achieves high efficiency similar to that described above can be constructed by providing an optical fiber output terminal on the light source unit 90 side and an optical fiber input terminal on the light receiving unit 96a side. Specifically, the light from the optical fiber output terminal and / or the light collected by the lens from that light both contain a mixture of light from various angles. Therefore, the optical system (optical filter) of the present invention can be used to efficiently select only the desired wavelengths from light of any angle and integrate them into the sensor.

[0164] As shown in FIG. 7 , not only the condensing lens 92 but also the light transmitted through the lens contains light at various angles. In contrast, as described above, the liquid crystal polarization interference element in the optical filter of the present invention exhibits very little wavelength shift due to oblique incidence of light. That is, since the optical filter of the present invention has little angle dependency, it functions as a bandpass filter and reflector that acts on the same wavelength range for light incident at various angles, regardless of the angle of transmission and the angle of reflection. In other words, the optical filter of the present invention functions as a bandpass filter (dichroic filter) that acts appropriately on a specific wavelength for light incident at various angles through the lens, as well as for diverging and condensing light. The angle dependency of the optical filter of the present invention is also similar to that of the optical system shown in FIG. 8 below.

[0165] FIG. 8 conceptually illustrates an example of an optical system incorporating a beam splitter according to the present invention. The optical system shown in FIG. 8 includes a light source 90, a beam splitter 98, an optical filter 94 according to the present invention, a light receiving unit 96a for transmitted light, and a light receiving unit 96b for reflected light. Various known beam splitters can be used. In this optical system, the beam splitter 98 splits the linearly traveling light emitted by the light source 90 into multiple (two in the figure) different angular directions. Furthermore, in this optical system, an optical filter 94 (bandpass filter (dichroic filter)) according to the present invention is disposed in the region after the splitting of the light. Furthermore, as shown in FIG. 8, in this optical system, the light split by the beam splitter 98 and transmitted through the optical filter 94 is received by the light receiving unit 96a for transmitted light corresponding to each light and photometrically measured. Furthermore, the light split by the beam splitter 98 and reflected by the optical filter 94 is received by the light receiving section 96b for reflected light corresponding to each light and photometry is performed.

[0166] As an example, in the illustrated optical filter 94, the first and second polarizers are arranged in a crossed Nicol configuration, with their transmission axes orthogonal to each other. Furthermore, this optical filter 94 uses the liquid crystal polarization interference element 16 shown in FIG. 1 as the liquid crystal polarization interference element. Accordingly, in this case, of the light emitted from the light source 90, split by the beam splitter 98, and incident on the optical filter 94, light within a specific wavelength range passes through the optical filter 94 and enters the light-receiving unit 96a for transmitted light corresponding to each light, where it is photometered. On the other hand, light outside the specific wavelength range that is split by the beam splitter 98 and incident on the optical filter 94 is reflected by the optical filter 94 and enters the light-receiving unit 96b for reflected light corresponding to each light, where it is photometered. The first and second polarizers of the optical filter 94 may also be arranged in a parallel Nicol configuration, with their transmission axes parallel to each other. In this case, as described above, light in a specific wavelength range that is split by the beam splitter 98 and incident on the optical filter 94 is reflected by the optical filter 94 and photometered by the light-receiving unit 96b for reflected light that corresponds to each light. On the other hand, light outside the specific wavelength range that is split by the beam splitter 98 and incident on the optical filter 94 passes through the optical filter 94 and photometered by the light-receiving unit 96a for transmitted light that corresponds to each light.

[0167] 8 can also utilize an optical filter using a liquid crystal polarization interference element having a configuration in which the angle of the in-plane slow axis increases sequentially in the stacking direction, as shown in Example 30 described above. In this optical system, when the first polarizer and second polarizer of the optical filter are arranged in parallel Nicols, light emitted from the light source 90, split by the beam splitter 98, and incident on the optical filter 94 is split into light of a specific wavelength range that passes through the optical filter 94 and is incident on a light receiving unit 96a for transmitted light corresponding to each light, where it is photometered. In addition, in this optical system, light of wavelengths other than the specific wavelength range that is split by the beam splitter 98 and incident on the optical filter 94 is reflected by the optical filter 94 and is incident on a light receiving unit 96b for reflected light corresponding to each light, where it is photometered. On the other hand, in this optical system, when the first polarizer and second polarizer of the optical filter are arranged in a crossed Nicol configuration, light of a specific wavelength out of the light split by the beam splitter 98 and incident on the optical filter 94 is reflected by the optical filter 94 and incident on the light receiving unit 96b for reflected light corresponding to each light, where it is photometered. Also, in this optical system, light outside the specific wavelength range that is split by the beam splitter 98 and incident on the optical filter 94 is transmitted through the optical filter 94 and incident on the light receiving unit 96a for transmitted light corresponding to each light, where it is photometered.

[0168] The optical filter 94 of the present invention exhibits bandpass performance for the same wavelength for split light traveling in different angular directions. As a result, high light receiving efficiency can be achieved in each of the multiple light receiving units 96 that receive the split light. The optical system of the present invention using the beam splitter 98 in this manner can be applied to various optical systems such as sensors and lasers. Furthermore, in systems using optical fibers, the system of the present invention can be used when switching the connection destination depending on the destination of the optical signal after splitting a straight-traveling light beam into beams traveling in different angles using a beam splitter.

[0169] The optical filter and optical system of the present invention have been described in detail above, but the present invention is not limited to the above examples, and various improvements and modifications may be made without departing from the spirit of the present invention.

[0170] The features of the present invention will be explained in more detail below with reference to examples. The materials, reagents, amounts used, amounts of substances, ratios, treatment details, and treatment procedures shown in the following examples can be appropriately changed without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below.

[0171] Comparative Example 1 (Formation of Alignment Film) A glass substrate was prepared as a support. The following alignment film-forming coating liquid was applied to the support by spin coating. The support on which the coating film of the alignment film-forming coating liquid had been formed was dried on a hot plate at 60°C for 60 seconds to form alignment film P-1, which was a photo-alignment film.

[0172] Coating liquid for forming alignment film ----------------------------------- 1.00 parts by mass of the following photoalignment material - Water 16.00 parts by mass - Butoxyethanol 42.00 parts by mass - Propylene glycol monomethyl ether 42.00 parts by mass ---------------------------------------------------

[0173] Material for photo alignment

[0174] (Exposure of Alignment Film) Next, using an ultraviolet exposure device, the alignment film P-1 was irradiated with ultraviolet light that had been linearly polarized by a wire grid polarizer (ProFlux PPL02, manufactured by Moxtek) that was installed so that the angle of the absorption axis was φ1 (=0°). The ultraviolet light had an illuminance of 4.5 mW / cm 2 , cumulative irradiation dose 300 mJ / cm 2 The angle of the absorption axis is the angle with respect to the longitudinal direction of the substrate, and the counterclockwise direction is taken as positive.

[0175] (Formation of Rod-Shaped Liquid Crystal Layer) The following composition B-1 was prepared as a liquid crystal composition for forming a rod-shaped liquid crystal layer.

[0176] Composition B-1 --------------------------------------------------- Rod-like liquid crystal compound L-1 shown below: 100.00 parts by mass Polymerization initiator (Irgacure (registered trademark) 907, manufactured by BASF) 3.00 parts by mass Photosensitizer (KAYACURE DETX-S, manufactured by Nippon Kayaku) 1.00 part by mass Leveling agent T-1 shown below: 0.08 parts by mass Methyl ethyl ketone 2000.00 parts by mass

[0177] Rod-shaped liquid crystal compound L-1

[0178] Leveling agent T-1

[0179] The rod-shaped liquid crystal layer was formed by applying composition B-1 onto the alignment film P-1. That is, composition B-1 was first applied onto the alignment film P-1, followed by heating and then ultraviolet curing to form a rod-shaped liquid crystal layer, which is a liquid crystal fixed layer. More specifically, the rod-shaped liquid crystal layer was formed by applying composition B-1 onto the alignment film P-1 to obtain a coating film, heating this coating film to 80°C on a hot plate, and then irradiating ultraviolet light of 365 nm wavelength at 300 mJ / cm using a high-pressure mercury lamp in a nitrogen atmosphere at 80°C. 2 The coating film was irradiated with an irradiation dose of 1000 .mu.m to fix the orientation of the rod-like liquid crystal compound.

[0180] Eight similar rod-shaped liquid crystal layers were formed. The eight rod-shaped liquid crystal layers were confirmed to have the following optical properties using an AxoScan (manufactured by Axometrics): thickness: 1.72 μm, Δn: 0.16, in-plane retardation: 275 nm.

[0181] The formed rod-shaped liquid crystal layers were peeled off from the alignment film and bonded using an adhesive (SK Dyne 2057, manufactured by Soken Chemical & Engineering Co., Ltd.) to fabricate a liquid crystal polarization interference element having eight rod-shaped liquid crystal layers. In this process, the in-plane slow axes of the rod-shaped liquid crystal layers in the odd-numbered layers (first liquid crystal layers) were crossed with the in-plane slow axes of the rod-shaped liquid crystal layers in the even-numbered layers (second liquid crystal layers). Specifically, the angle bisecting the crossing angle formed by the in-plane slow axes of the odd-numbered and even-numbered layers was used as the reference (reference line), and the angle θ of the in-plane slow axes of the odd-numbered layers was positive (+) and the angle θ of the in-plane slow axes of the even-numbered layers was negative (-), with the counterclockwise direction being positive (+) and the clockwise direction being negative (-). That is, the fabricated liquid crystal polarization interference element had four liquid crystal layer pairs, each consisting of a first liquid crystal layer and a second liquid crystal layer.

[0182] [Preparation of Absorptive Linear Polarizer] An absorptive linear polarizer was prepared by the following procedure.

[0183] (Preparation of Cellulose Acylate Film 1) <Preparation of Core Layer Cellulose Acylate Dope> The following composition was charged into a mixing tank and stirred to dissolve each component, thereby preparing a cellulose acetate solution to be used as the core layer cellulose acylate dope. ------------------------------------------------ Core Layer Cellulose Acylate Dope------------------------------------------------ - Cellulose acetate having an acetyl substitution degree of 2.88: 100 parts by mass - Polyester compound B described in the examples of JP2015-227955A: 12 parts by mass - Compound F below: 2 parts by mass - Methylene chloride (first solvent): 430 parts by mass - Methanol (second solvent): 64 parts by mass

[0184] Compound F

[0185] <Preparation of Outer Layer Cellulose Acylate Dope> 10 parts by mass of the following matting agent solution was added to 90 parts by mass of the prepared core layer cellulose acylate dope to prepare a cellulose acetate solution to be used as the outer layer cellulose acylate dope.

[0186] Matting agent solution - Silica particles with an average particle size of 20 nm (AEROSIL R972, manufactured by Nippon Aerosil Co., Ltd.) 2 parts by mass Methylene chloride (first solvent) 76 parts by mass Methanol (second solvent) 11 parts by mass The above-mentioned cellulose acylate dope for the core layer 1 part by mass

[0187] <Preparation of Cellulose Acylate Film 1> The prepared core layer cellulose acylate dope and outer layer cellulose acylate dope were filtered through a filter paper with an average pore size of 34 μm and a sintered metal optical filter with an average pore size of 10 μm. Then, the core layer cellulose acylate dope and the outer layer cellulose acylate dope on both sides were simultaneously cast onto a drum at 20°C from a casting nozzle (band caster). The film was then peeled off while the solvent content was approximately 20% by mass, and both ends of the film in the width direction were fixed with tenter clips. The film was then stretched in the transverse direction at a stretch ratio of 1.1 times and dried. The film was then further dried by transporting it between the rolls of a heat treatment device to prepare an optical film with a thickness of 40 μm, designated as Cellulose Acylate Film 1. The Re of the obtained Cellulose Acylate Film 1 was 0 nm.

[0188] (Formation of Photo-Alignment Layer PA1) The coating liquid S-PA-1 for forming an alignment layer, which will be described later, was continuously applied onto the cellulose acylate film 1 using a wire bar. The support on which the coating film was formed was dried with hot air at 140°C for 120 seconds. Subsequently, the coating film was irradiated with polarized ultraviolet light (10 mJ / cm 2 A photo-alignment layer PA1 was formed by irradiating the substrate with light (using an ultra-high pressure mercury lamp). The film thickness was 0.3 μm.

[0189] ------------------------------------------------------------------ (Coating liquid for forming alignment layer S-PA-1) -------------------------------------------------- Polymer M-PA-1 (shown below) 100.00 parts by mass Acid generator PAG-1 (shown below) 5.00 parts by mass Acid generator CPI-110TF (shown below) 0.005 parts by mass Xylene 1220.00 parts by mass Methyl isobutyl ketone 122.00 parts by mass ------------------------------------------------------------------

[0190] Polymer M-PA-1

[0191] Acid generator PAG-1

[0192] Acid generator CPI-110TF

[0193] (Formation of Optically Absorbent Anisotropic Layer P1) On the obtained photo-alignment layer PA1, the following coating solution S-P-1 for forming an optically absorbent anisotropic layer was continuously applied using a wire bar to form a coating layer. The formed coating layer was then heated at 140°C for 30 seconds and cooled to room temperature (23°C). Next, it was heated at 90°C for 60 seconds and cooled again to room temperature. Thereafter, an LED lamp (center wavelength 365 nm) was used to apply the coating solution S-P-1 to form an optically absorbent anisotropic layer. The coating layer was then heated at 140°C for 30 seconds and cooled to room temperature again. The coating layer was then heated at 90°C for 60 seconds and cooled again to room temperature. After that, an LED lamp (center wavelength 365 nm) was used to apply the coating solution S-P-1 to form an optically absorbent anisotropic layer. The coating layer was then heated at 90°C for 60 seconds and cooled again to room temperature. The coating layer was then heated at an illuminance of 200 mW / cm. ２ The optically absorptive anisotropic layer P1 was formed on the photo-alignment layer PA1 by irradiating the layer for 2 seconds under the irradiation conditions of 1.6 μm.

[0194] 0.25 parts by mass of dichroic substance D-1 below 0.36 parts by mass of dichroic substance D-2 below 0.59 parts by mass of dichroic substance D-3 below 2.21 parts by mass of polymer liquid crystal compound M-P-1 below 1.36 parts by mass of low molecular weight liquid crystal compound M-1 below 0.200 parts by mass of polymerization initiator IRGACURE OXE-02 (manufactured by BASF) 0.026 parts by mass of surfactant F-3 below Cyclopentanone 46.00 parts by mass Tetrahydrofuran 46.00 parts by mass Benzyl alcohol 3.00 parts by mass ----------------------------------------------------------------------------------

[0195] Dichroic substance D-1 Dichroic substance D-2 Dichroic substance D-3

[0196] Polymer liquid crystal compound M-P-1

[0197] Low molecular liquid crystal compound M-1

[0198] Surfactant F-3

[0199] In this manner, an absorptive linear polarizer was produced.

[0200] (Preparation of Reflective Linear Polarizer) A reflective linear polarizer was prepared as follows.

[0201] [Preparation of Coating Liquid for Reflective Layer] [Coating Liquid R-1 for Reflective Layer] The composition shown below was stirred and dissolved in a container kept at 70° C. to prepare Coating Liquid R-1 for Reflective Layer, where R represents a coating liquid using a rod-like liquid crystal compound.

[0202] -------------------------------------------------- Coating liquid R-1 for reflective layer -------------------------------------------------- Methyl ethyl ketone 120.9 parts by mass Cyclohexanone 21.3 parts by mass Mixture X of the following rod-shaped liquid crystal compounds 100.0 parts by mass Photopolymerization initiator B 1.00 part by mass Chiral agent A 4.18 parts by mass Surfactant F1 0.1 part by mass ----------------------------------- Mixture X of Rod-shaped Liquid Crystal Compounds

[0203] In the above mixture X, the numerical values ​​are in mass %. R is a group bonded via an oxygen atom. Furthermore, the average molar absorption coefficient of the above rod-shaped liquid crystal compound in the wavelength range of 300 to 400 nm was 140 / mol cm.

[0204] Chiral agent A

[0205] Surfactant F1

[0206] Photopolymerization initiator B

[0207] The chiral agent A is a chiral agent whose helical twisting power (HTP) is reduced by light.

[0208] [Reflective Layer Coating Solution R-2] This was prepared in the same manner as Reflective Layer Coating Solution R-1, except that the amount of chiral agent A added was changed as shown in Table 1 below.

[0209]

[0210] [Reflective layer coating solution D-1] The composition shown below was stirred and dissolved in a container kept at 50° C. to prepare a reflective layer coating solution D-1, where D represents a coating solution using a discotic liquid crystal compound.

[0211] -------------------------------- Coating liquid D-1 for reflective layer ---------------------------------- 80 parts by mass of discotic liquid crystal compound (A) below 20 parts by mass of discotic liquid crystal compound (B) below 10 parts by mass of polymerizable monomer E1 below 0.3 parts by mass of surfactant F2 below 3 parts by mass of photopolymerization initiator (Irgacure 907, manufactured by BASF) 5.45 parts by mass of the above chiral agent A 290 parts by mass of methyl ethyl ketone 50 parts by mass of cyclohexanone --------------------------------

[0212] Discotic Liquid Crystal Compound (A) Discotic Liquid Crystal Compound (B)

[0213] Polymerizable monomer E1

[0214] Surfactant F2

[0215] [Reflective Layer Coating Solutions D-2 and D-3] These were prepared in the same manner as Reflective Layer Coating Solution D-1, except that the amount of chiral agent A added was changed as shown in Table 2 below.

[0216]

[0217] [Preparation of Reflective Circular Polarizer 1 (Selective Reflection Layer)] A 100 μm thick PET film (A4265, manufactured by Toyobo Co., Ltd.) was prepared as a temporary support, and the PET surface on which the easy-adhesive layer was not formed was subjected to a rubbing treatment. The prepared reflective layer coating solution R-1 was applied using a wire bar coater, and then dried at 110°C for 72 seconds. Thereafter, the reflective circular polarizer was subjected to a rubbing treatment at 100°C under a low-oxygen atmosphere (100 ppm or less) with an illuminance of 80 mW / cm. 2 , irradiation amount 500mJ / cm 2The coating was then cured by irradiating it with light from a metal halide lamp at a discharge rate of 150 W min / m to form a first blue light reflective layer (first cholesteric liquid crystal layer) made of a cholesteric liquid crystal layer. The light was irradiated from the cholesteric liquid crystal layer side. The coating thickness was adjusted so that the film thickness of the first blue light reflective layer after curing would be 2.6 μm. Next, the surface of the first blue light reflective layer was irradiated with light from a metal halide lamp at a discharge rate of 150 W min / m. 2 After the corona treatment, the reflective layer coating liquid D-1 was applied onto the corona treated surface using a wire bar coater.

[0218] Subsequently, the coating film was dried at 70°C for 2 minutes, and after the solvent was evaporated, it was heat-aged at 115°C for 3 minutes to obtain a uniformly oriented state. Thereafter, the coating film was kept at 45°C and irradiated with ultraviolet light (300 mJ / cm) using a metal halide lamp in a nitrogen atmosphere. 2 ) and cured to form a second blue light reflective layer (second cholesteric liquid crystal layer) on the first blue light reflective layer. Light was irradiated from the cholesteric liquid crystal layer side. At this time, the coating thickness was adjusted so that the film thickness of the second blue light reflective layer after curing was 2.0 μm.

[0219] Next, the reflective layer coating solution D-2 was applied onto the second blue light reflective layer using a wire bar coater. Subsequently, the coating film was dried at 70°C for 2 minutes, and after the solvent was evaporated, it was heat-aged at 115°C for 3 minutes to obtain a uniformly oriented state. Thereafter, this coating film was kept at 45°C and irradiated with ultraviolet light (300 mJ / cm) using a metal halide lamp under a nitrogen atmosphere. 2 ) and cured to form a green light reflective layer (third cholesteric liquid crystal layer) on the second blue light reflective layer. Light was irradiated from the cholesteric liquid crystal layer side. At this time, the coating thickness was adjusted so that the film thickness of the green light reflective layer after curing was 2.7 μm.

[0220] Next, the reflective layer coating solution R-2 was applied onto the green light reflective layer using a wire bar coater, and then dried at 110°C for 72 seconds. Thereafter, the coating solution was dried in a low-oxygen atmosphere (100 ppm or less) at 100°C with an illuminance of 80 mW and an irradiation dose of 500 mJ / cm. 2The coating was cured by irradiating it with light from a metal halide lamp (1000 W / m²), thereby forming a red light reflective layer (fourth cholesteric liquid crystal layer) on the green light reflective layer. The light was irradiated from the cholesteric liquid crystal layer side. The coating thickness was adjusted so that the red light reflective layer after curing had a thickness of 3.4 μm.

[0221] Next, the red light reflecting layer surface was subjected to a discharge of 150 W·min / m 2 After corona treatment at 70°C, the reflective layer coating solution D-3 was applied to the corona-treated surface using a wire bar coater. Subsequently, the coating film was dried at 70°C for 2 minutes, and after the solvent was evaporated, it was heat-aged at 115°C for 3 minutes to obtain a uniform alignment state. Thereafter, this coating film was kept at 45°C and irradiated with ultraviolet light (300 mJ / cm) using a metal halide lamp under a nitrogen atmosphere. 2 ) and cured to form a yellow light reflective layer (fifth cholesteric liquid crystal layer) on the red light reflective layer. Light was irradiated from the cholesteric liquid crystal layer side. At this time, the coating thickness was adjusted so that the film thickness of the yellow light reflective layer after curing was 3.4 μm.

[0222] By the above procedure, a reflective circular polarizer 1 (selective reflection layer) having the first to fifth cholesteric liquid crystal layers in this order was obtained.

[0223] Table 4 shows the central reflection wavelength and film thickness of each cholesteric liquid crystal layer of the produced reflective circular polarizer 1. Here, the central reflection wavelength shown in Table 3 corresponds to the central wavelength of the reflected light of the above-mentioned cholesteric liquid crystal layer. The central reflection wavelength (central wavelength of the reflected light) was confirmed by creating a film in which each cholesteric liquid crystal layer was coated in a single layer. The film thickness was confirmed using an SEM.

[0224]

[0225] In this manner, a reflective circular polarizer 1 (selective reflection layer) was produced.

[0226] [Preparation of λ / 4 Retardation Plate] A λ / 4 retardation plate was prepared by the following procedure.

[0227] [Preparation of Retardation Plate 1] Reverse wavelength dispersion retardation plate 1 was prepared with reference to the method described in paragraphs 0151 to 0163 of JP 2020-084070 A. Retardation plate 1 had Re = 146 nm and Rth = 73 nm.

[0228] [Preparation of Positive C Plate 1] Positive C Plate 1 was prepared by adjusting the film thickness with reference to the method described in paragraphs 0132 to 0134 of JP 2016-053709 A. However, the support was changed from a polyethylene terephthalate film (PET film) to a triacetyl cellulose film (TAC film). Positive C Plate 1 had Re = 0.1 nm and Rth = -80 nm.

[0229] The prepared retardation plate 1 and a positive C plate 1 were laminated together to prepare a λ / 4 retardation plate.

[0230] [Preparation of Reflective Linear Polarizer] The prepared λ / 4 retardation plate and reflective circular polarizer 1 (selective reflection layer) were laminated in this order to prepare a reflective linear polarizer. The transmission axis of the reflective linear polarizer was set at an angle shifted by 45° from the slow axis of retardation plate 1.

[0231] [Preparation of Bandpass Filter] A bandpass filter was prepared by arranging the prepared absorptive linear polarizer, liquid crystal polarization interference element, and reflective linear polarizer in this order. The linear polarizers were arranged in a crossed Nicol configuration, with their transmission axes perpendicular to each other. The surface of the reflective linear polarizer facing the liquid crystal polarization interference element was the surface of the retardation layer 1. The transmission axes of the absorptive linear polarizer and the reflective linear polarizer were perpendicular to each other. The bandpass filter was prepared by aligning the transmission axis of one polarizer with the line bisecting the crossing angle formed by the in-plane slow axes of the odd-numbered and even-numbered layers when the rod-like liquid crystal layers were stacked. The transmittance of the prepared bandpass filter was measured. Incident light was incident from the absorptive polarizer side. The wavelength (center wavelength) and half-width showing the maximum transmittance, as well as the wavelength shift and side lobe, were measured using a spectroradiometer "SR-3" manufactured by Topcon Technohouse Corporation. Reflectance was also measured in the same manner as transmittance. The wavelength shift (absolute value) was measured when light was incident at a polar angle of 60° relative to when it was incident at a polar angle of 90°. Incident light from a polar angle of 60° was incident from two directions, at azimuth angles of 0° and 90°, and the average value was used as the measured value. The transmittance measurement revealed that the fabricated bandpass filter had a center wavelength of 550 nm, a half-width of 120 nm, a wavelength shift of 90 nm, and a side lobe of 10%. The size of the side lobe is the ratio of the side lobe transmittance to the transmittance of the center wavelength. Furthermore, the reflectance measurement showed opposite characteristics to the transmittance, with non-transmitted light (i.e., components whose polarization was not changed by the liquid crystal polarization interference element) being reflected by the reflective linear polarizer on the exit side and emitted from the entrance side.

[0232] Example 1 The following composition D-1 was prepared as a liquid crystal composition for forming a discotic liquid crystal layer. Composition D-1 ---------------------------------------------------------------- Discotic liquid crystal compound L-2 (shown below) 80.00 parts by mass Discotic liquid crystal compound L-3 (shown below) 20.00 parts by mass Polymerization initiator (Irgacure (registered trademark) 907, manufactured by BASF) 5.00 parts by mass Megafac F444 (manufactured by DIC) 0.50 parts by mass Methyl ethyl ketone 300.00 parts by mass

[0233] Discotic liquid crystal compound L-2 Discotic liquid crystal compound L-3

[0234] The prepared composition D-1 was applied to the same alignment film P-1 as in Comparative Example 1, followed by heating and UV curing to form a discotic liquid crystal layer (thickness 0.86 μm) that was a liquid crystal fixed layer containing a discotic liquid crystal compound. Next, the same composition B-1 as in Comparative Example 1 was applied to the same alignment film P-1 as in Comparative Example 1, followed by heating and UV curing to form a rod-shaped liquid crystal layer (thickness 0.86 μm) that was a liquid crystal fixed layer containing a rod-shaped liquid crystal compound. The temperature, heating, and curing conditions during the formation of the discotic liquid crystal layer and the rod-shaped liquid crystal layer were the same as those for the formation of the rod-shaped liquid crystal layer in Comparative Example.

[0235] Eight liquid crystal layers each having the same discotic liquid crystal layer and rod-shaped liquid crystal layer were formed. Hereinafter, for convenience, these liquid crystal layers each having the discotic liquid crystal layer and rod-shaped liquid crystal layer will also be referred to as "unit layer." The eight unit layers thus fabricated were confirmed to have the following optical properties using an AxoScan (manufactured by Axometrics). Rod-shaped liquid crystal layer: Thickness: 0.86 μm, Δn: 0.16, in-plane retardation: 137.5 nm. Discotic liquid crystal layer: Thickness: 0.86 μm, Δn: 0.16, in-plane retardation: 137.5 nm.

[0236] The formed unit layers were peeled from the alignment film and bonded together using an adhesive (SK Dyne 2057, manufactured by Soken Chemical & Engineering Co., Ltd.). Eight layers were stacked to fabricate a liquid crystal polarization interference element. In this process, the in-plane slow axes of the rod-shaped liquid crystal layers of the odd-numbered layers (first liquid crystal layers) of the stacked unit layers were aligned so that they intersected with the in-plane slow axes of the rod-shaped liquid crystal layers of the even-numbered layers (second liquid crystal layers). Specifically, the angle bisecting the crossing angle formed by the in-plane slow axes of both layers was used as the reference line (reference line), with the counterclockwise direction being positive (+) and the clockwise direction being negative (-). The eight unit layers were stacked and bonded together so that the angle θ of the in-plane slow axes of the rod-shaped liquid crystal layers of the odd-numbered layers was 5.625° and the angle θ of the in-plane slow axes of the rod-shaped liquid crystal layers of the even-numbered layers was −5.625°. That is, the fabricated liquid crystal polarization interference element had four liquid crystal layer pairs, each consisting of a first liquid crystal layer and a second liquid crystal layer. As described above, when a liquid crystal layer is formed on top of another liquid crystal layer by coating, the orientation of the liquid crystal compound in the upper liquid crystal layer follows the orientation of the liquid crystal compound in the lower layer. Therefore, in this example, the angle θ of the in-plane slow axis of the rod-shaped liquid crystal layer is 5.625° for odd-numbered layers and −5.625° for even-numbered layers, and the angle θ of the in-plane slow axis of the discotic liquid crystal layer is 5.625° for odd-numbered layers and −5.625° for even-numbered layers.

[0237] [Fabrication of Bandpass Filter] Using the fabricated liquid crystal polarization interference element, a bandpass filter having an absorptive linear polarizer, a liquid crystal polarization interference element, and a reflective linear polarizer was fabricated in the same manner as in Comparative Example 1. The center wavelength, half width, wavelength shift, and side lobe of the fabricated bandpass filter were measured in the same manner as in Comparative Example 1.

[0238] As a result, the fabricated bandpass filter had a center wavelength of 550 nm, a half-width of 120 nm, a wavelength shift of less than 5 nm, and a side lobe of 10%. Furthermore, when the reflectance was measured, the characteristics were the opposite of the transmittance, and non-transmitted light (i.e., components whose polarization was not changed by the liquid crystal polarization interference element) was reflected by the reflective linear polarizer on the output side and emitted from the input side. The wavelength shift of the bandpass filter of Comparative Example 1, in which the first and second liquid crystal layers were formed only with rod-shaped liquid crystal compounds, was 90 nm. Thus, by having the first and second liquid crystal layers comprise rod-shaped liquid crystal layers and discotic liquid crystals, the wavelength shift of the bandpass filter when light is incident obliquely can be significantly suppressed.

[0239] [Example 2] In Example 1, the thickness of the unit layers and the angle of the in-plane slow axis when stacking the unit layers were adjusted as shown in the table below, and eight unit layers were stacked to prepare a liquid crystal polarization interference element. As described above, the unit layers are liquid crystal layers having rod-shaped liquid crystal layers and discotic liquid crystal layers. That is, in this example as well, there are four liquid crystal layer pairs each consisting of a first liquid crystal layer (odd-numbered layer) and a second liquid crystal layer (even-numbered layer).

[0240]

[0241] A bandpass filter was fabricated using the prepared liquid crystal polarization interference element in the same manner as in Example 1 (i.e., Comparative Example 1). The center wavelength, half-width, wavelength shift, and side lobes of the fabricated bandpass filter were measured in the same manner as in Comparative Example 1. The results showed that the center wavelength of the fabricated bandpass filter was 550 nm, the half-width was 120 nm, the wavelength shift was less than 5 nm, and the side lobes were 3% or less. As described above, the side lobes of the bandpass filter of Example 1 were 10%. Thus, the side lobes of the bandpass filter can be reduced by increasing the in-plane retardation of the liquid crystal layer pairs on both sides in the thickness direction compared to the liquid crystal layer of the central liquid crystal layer pair in the thickness direction and reducing the absolute value of the angle between the in-plane slow axis of the rod-shaped liquid crystal layer and the reference line. Furthermore, the reflectance measurement showed characteristics opposite to those of the transmittance, with non-transmitted light (i.e., components whose polarization is not changed by the liquid crystal polarization interference element) being reflected by the reflective linear polarizer on the exit side and exiting from the entrance side.

[0242] Example 3 A bandpass filter was fabricated in the same manner as in Example 1, except that the reflective linear polarizer in Example 1 was replaced with a broadband dielectric multilayer film (manufactured by 3M, product name: APF). As a result, the fabricated bandpass filter had a center wavelength of 550 nm, a half-width of 120 nm, a wavelength shift of less than 5 nm, and a side lobe of 10%. Furthermore, when the reflectance was measured, it was found to have characteristics opposite to those of the transmittance, and non-transmitted light (i.e., components whose polarization was not changed by the liquid crystal polarization interference element) was reflected by the reflective linear polarizer on the exit side and emitted from the entrance side. This demonstrated that the wavelength shift of the bandpass filter when light was incident obliquely could be significantly suppressed.

[0243] [Example 4] A bandpass filter was fabricated in the same manner as in Example 1, except that the reflective linear polarizer in Example 1 was replaced with a wire grid polarizer manufactured by THORLABS. As a result, the fabricated bandpass filter had a center wavelength of 550 nm, a half-width of 120 nm, a wavelength shift of less than 5 nm, and a side lobe of 10%. In addition, when the reflectance was measured, it was found to have characteristics opposite to those of the transmittance, and non-transmitted light (i.e., components whose polarization was not changed by the liquid crystal polarization interference element) was reflected by the reflective linear polarizer on the output side and emitted from the input side. This demonstrated that the wavelength shift of the bandpass filter when light was incident obliquely could be significantly suppressed.

[0244] Example 10 A liquid crystal polarization interference element was fabricated by changing the number of unit layers from 8 to 12 in Example 1 and changing the in-plane slow axis angles of the liquid crystal layers as follows. As described above, the unit layers are liquid crystal layers having rod-shaped liquid crystal layers and discotic liquid crystal layers. In this example, the number of unit layers is 12, the in-plane slow axis angles θ of the rod-shaped liquid crystal layers are 3.75° for odd-numbered layers and −3.75° for even-numbered layers, and the in-plane slow axis angles θ of the discotic liquid crystal layers are 3.75° for odd-numbered layers and −3.75° for even-numbered layers. That is, in this example, there are six liquid crystal layer pairs, each consisting of a first liquid crystal layer (odd-numbered layer) and a second liquid crystal layer (even-numbered layer).

[0245] A bandpass filter was fabricated using this liquid crystal polarization interference element in the same manner as in Example 1. Furthermore, the center wavelength, half-width, wavelength shift, and side lobe of the fabricated bandpass filter were measured in the same manner as in Comparative Example 1. As a result, the center wavelength of the fabricated bandpass filter was 550 nm, the half-width was 80 nm, the wavelength shift was less than 5 nm, and the side lobe was 10%. Furthermore, the reflectance was measured, and the results showed characteristics opposite to those of the transmittance, with non-transmitted light (i.e., components whose polarization was not changed by the liquid crystal polarization interference element) being reflected by the reflective linear polarizer on the exit side and emitted from the entrance side.

[0246] Example 11 In Example 1, a liquid crystal polarization interference element was fabricated by configuring odd-numbered unit layers (first liquid crystal layers) solely from rod-shaped liquid crystal layers and even-numbered unit layers (second liquid crystal layers) solely from discotic liquid crystal layers, with the following characteristics: That is, in this example, the unit layers are liquid crystal layer pairs, and the liquid crystal layer pairs have a configuration as conceptually shown in FIG. 4 . In this example, the number of unit layers is 4; the in-plane slow axis angle θ of the rod-shaped liquid crystal layers of the odd-numbered unit layers (first liquid crystal layers) is 5.625°; the thickness is 1.72 μm, Δn is 0.16, and the in-plane retardation is 275 nm; and the in-plane slow axis angle θ of the discotic liquid crystal layers of the even-numbered unit layers (second liquid crystal layers) is −5.625°; the thickness is 1.72 μm, Δn is 0.16, and the in-plane retardation is 275 nm. That is, in this example as well, there are four liquid crystal layer pairs each consisting of a first liquid crystal layer (odd-numbered layer) and a second liquid crystal layer (even-numbered layer).

[0247] A bandpass filter was fabricated using this liquid crystal polarization interference element in the same manner as in Example 1. Furthermore, the center wavelength, half-width, wavelength shift, and side lobe of the fabricated bandpass filter were measured in the same manner as in Comparative Example 1. As a result, the center wavelength of the fabricated bandpass filter was 550 nm, the half-width was 120 nm, the wavelength shift was less than 10 nm, and the side lobe was 10%. Furthermore, the reflectance was measured, and the results showed characteristics opposite to those of the transmittance, with non-transmitted light (i.e., components whose polarization was not changed by the liquid crystal polarization interference element) being reflected by the reflective linear polarizer on the exit side and emitted from the entrance side.

[0248] [Example 12] In Example 1, each unit layer was composed of a laminate of one rod-shaped liquid crystal layer and one discotic liquid crystal layer having the same in-plane retardation. In contrast, in this example, a liquid crystal polarization interference element was fabricated by adjusting the thicknesses of the rod-shaped liquid crystal layer and the discotic liquid crystal layer in the unit layer so that the in-plane retardation was as follows: Rod-shaped liquid crystal layer: Thickness: 1.29 μm, Δn: 0.16, in-plane retardation: 206.2 nm Discotic liquid crystal layer: Thickness: 0.43 μm, Δn: 0.16, in-plane retardation: 68.8 nm That is, in this example, the liquid crystal layer set has a configuration conceptually shown in FIG. 5. Furthermore, in this example, the number of unit layers is also eight, and there are four liquid crystal layer sets each consisting of a first liquid crystal layer (odd-numbered layer) and a second liquid crystal layer (even-numbered layer).

[0249] A bandpass filter was fabricated using this liquid crystal polarization interference element in the same manner as in Example 1. Furthermore, the center wavelength, half-width, wavelength shift, and side lobe of the fabricated bandpass filter were measured in the same manner as in Comparative Example 1. As a result, the center wavelength of the fabricated bandpass filter was 550 nm, the half-width was 120 nm, the wavelength shift was less than 10 nm, and the side lobe was 10%. Furthermore, the reflectance was measured, and the results showed characteristics opposite to those of the transmittance, with non-transmitted light (i.e., components whose polarization was not changed by the liquid crystal polarization interference element) being reflected by the reflective linear polarizer on the exit side and emitted from the entrance side.

[0250] [Example 13] In Example 1, each of the odd-numbered and even-numbered unit layers was composed of one rod-shaped liquid crystal layer and one discotic liquid crystal layer. In contrast, in this example, each of the odd-numbered and even-numbered unit layers was modified to have a total of four layers, with two rod-shaped and two discotic liquid crystal layers alternately stacked. That is, in this example, the liquid crystal layer pairs had a configuration conceptually shown in FIG. 6 . The properties of each layer were as follows: Rod-shaped liquid crystal layer: Thickness: 0.43 μm, Δn: 0.16, in-plane retardation: 68.8 nm Discotic liquid crystal layer: Thickness: 0.43 μm, Δn: 0.16, in-plane retardation: 68.8 nm Eight of these unit layers were stacked to fabricate a liquid crystal polarization interference element in the same manner as in Example 1. Therefore, this example also had four liquid crystal layer pairs, each consisting of a first liquid crystal layer (odd-numbered layer) and a second liquid crystal layer (even-numbered layer).

[0251] Using this liquid crystal polarization interference element, a bandpass filter was produced in the same manner as in Example 1. Furthermore, the center wavelength, half-width, wavelength shift, and side lobe of the produced bandpass filter were measured in the same manner as in Comparative Example 1. As a result, the produced bandpass filter had a center wavelength of 550 nm, a half-width of 120 nm, a wavelength shift of less than 3 nm, and a side lobe of 10%. Furthermore, the reflectance was measured, and the results showed characteristics opposite to those of the transmittance, with non-transmitted light (i.e., components whose polarization is not changed by the liquid crystal polarization interference element) being reflected by the reflective linear polarizer on the output side and emitted from the input side.

[0252] Example 14 In Example 1, the thicknesses of the rod-shaped liquid crystal layer and the discotic liquid crystal layer in the unit layer were changed as follows: Rod-shaped liquid crystal layer: Thickness: 2.87 μm, Δn: 0.135, in-plane retardation: 387.5 nm Discotic liquid crystal layer: Thickness: 2.87 μm, Δn: 0.135, in-plane retardation: 387.5 nm

[0253] Note that both Δn: 0.135 of the rod-shaped liquid crystal layer and Δn: 0.135 of the discotic liquid crystal are values ​​at a wavelength of 1550 nm. The Δn: 0.15 of the rod-shaped liquid crystal layer and Δn: 0.15 of the discotic liquid crystal in Example 1 are values ​​at a wavelength of 550 nm, and the difference in Δn value from this example is due to the normal dispersion characteristic of Δn of the liquid crystal (the property that Δn becomes smaller as the wavelength becomes longer).

[0254] Using these unit layers, a liquid crystal polarization interference element was formed in the same manner as in Example 1. That is, in this example, the number of unit layers was also eight, and there were four liquid crystal layer pairs each consisting of a first liquid crystal layer (odd-numbered layer) and a second liquid crystal layer (even-numbered layer). Furthermore, using this liquid crystal polarization interference element, a bandpass filter was fabricated in the same manner as in Example 1. The center wavelength, half-width, wavelength shift, and side lobe of the fabricated bandpass filter were measured in the same manner as in Comparative Example 1. As a result, the fabricated bandpass filter had a center wavelength of 1550 nm, a half-width of 340 nm, a wavelength shift of less than 14 nm, and a side lobe of 10%. Furthermore, when the reflectance was measured, it was found to have the opposite characteristics to the transmittance, and the light that was not transmitted (i.e., the component whose polarization was not changed by the liquid crystal polarization interference element) was reflected by the reflective linear polarizer on the output side and emitted from the input side.

[0255] Comparative Example 14 A liquid crystal polarization interference element was prepared in the same manner as in Example 14, except that the discotic liquid crystal layer was replaced with a rod-shaped liquid crystal layer. Eight of these unit layers were stacked to prepare a liquid crystal polarization interference element in the same manner as in Example 1. Therefore, this example also had four liquid crystal layer pairs, each consisting of a first liquid crystal layer (odd-numbered layer) and a second liquid crystal layer (even-numbered layer). A bandpass filter was prepared using this liquid crystal polarization interference element in the same manner as in Example 1. The center wavelength, half-width, wavelength shift, and side lobes of the prepared bandpass filter were measured in the same manner as in Comparative Example 1. The results showed that the center wavelength of the prepared bandpass filter was 1550 nm, the half-width was 340 nm, the wavelength shift was less than 250 nm, and the side lobes were 10%. Furthermore, the reflectance was measured and found to have the opposite characteristics to the transmittance. The non-transmitted light (i.e., the component whose polarization was not changed by the liquid crystal polarization interference element) was reflected by the reflective linear polarizer on the exit side and emitted from the entrance side. From this result, it can be seen that even in Example 14, which has a different central wavelength (1550 nm) from Example 1, the wavelength shift is greatly improved as compared with Comparative Example 14.

[0256] Example 20 In Example 1, a retardation plate was placed between the first polarizer (absorptive linear polarizer) of the polarizers arranged in crossed Nicols and the liquid crystal polarization interference element. The retardation plate was as shown below. Adjacent to the first polarizer, a positive C plate (thickness direction retardation Rth: -90 nm) formed by vertical alignment of rod-shaped liquid crystals and a positive A plate (in-plane direction retardation Re: 140 nm) formed by horizontal alignment of rod-shaped liquid crystals were placed in this order and laminated. In this case, the in-plane slow axis of the positive A plate was placed parallel to the absorption axis of one of the polarizers. This retardation plate has the effect of maintaining the orthogonal relationship of the polarization directions of the linear polarizers arranged in crossed Nicols not only in the front direction but also in the oblique direction. In this way, a bandpass filter was produced.

[0257] The center wavelength, half-width, wavelength shift, and side lobe of the fabricated bandpass filter were measured in the same manner as in Comparative Example 1. As a result, the center wavelength of the fabricated bandpass filter was 550 nm, the half-width was 120 nm, the wavelength shift was less than 3 nm, and the side lobe was 10%. Furthermore, the reflectance was measured and found to have the opposite characteristics to the transmittance, with the non-transmitted light (i.e., the component whose polarization is not changed by the liquid crystal polarization interference element) being reflected by the reflective linear polarizer on the exit side and exiting from the entrance side.

[0258] For the bandpass filters of each of the above-mentioned Examples, bandpass filters were fabricated in the same manner, except that the linear polarizers were arranged in a parallel Nicol configuration with parallel transmission axes, and the characteristics were measured. The transmission axis of the linear polarizer was aligned with the transmission axis of the linear polarizer on the light incident side. As a result, for each bandpass filter, the center wavelength of the reflected light was 550 nm, and except that the wavelengths of the transmitted light and the reflected light were reversed, the same characteristics as those of the corresponding Examples were obtained for light of each wavelength.

[0259] Example 30: A liquid crystal polarization interference element was prepared by arranging eight unit layers in the same manner as in Example 1 with the in-plane slow axes as shown in the table below. Furthermore, the polarizer arrangement was changed from crossed Nicols (orthogonal) to parallel Nicols (parallel) to prepare a bandpass filter. That is, this example also had four liquid crystal layer pairs, each consisting of a first liquid crystal layer (odd-numbered layer) and a second liquid crystal layer (even-numbered layer). This unit layer arrangement corresponds to a bandpass filter prepared by disposing a Solk optical filter (Van Solk optical filter) between polarizers arranged in parallel Nicols, with birefringent plates (λ / 2 retardation plates) of equal thickness stacked between the polarizer's transmission axis and the slow axis at angles ρ, 3ρ, 5ρ, ....

[0260]

[0261] The center wavelength, half-width, wavelength shift, and side lobe of the fabricated bandpass filter were measured in the same manner as in Comparative Example 1. As a result, the center wavelength of the fabricated bandpass filter was 550 nm, the half-width was 120 nm, the wavelength shift was less than 5 nm, and the side lobe was 10%. In addition, the reflectance was measured, and the characteristics were opposite to those of the transmittance, and the non-transmitted light (i.e., the component whose polarization is not changed by the liquid crystal layer) was reflected by the reflective polarizer on the output side and emitted from the input side. In addition, the reflectance was measured, and the characteristics were opposite to those of the transmittance, and the non-transmitted light (i.e., the component whose polarization is changed by the liquid crystal polarization interference element) was reflected by the reflective linear polarizer on the output side and emitted from the input side.

[0262] A bandpass filter was fabricated and its characteristics were measured in the same manner as in Example 30, except that the linear polarizers were arranged in a crossed Nicol configuration with their transmission axes perpendicular to each other. As a result, with this bandpass filter, the center wavelength of the reflected light was 550 nm, and the wavelengths of the transmitted light and the reflected light were reversed, but other than this, the same characteristics as in Example 30 were obtained for light of each wavelength.

[0263] The optical performance of the optical system of the present invention, which uses the optical filter of the present invention as a bandpass filter, was evaluated by optical simulation using "Lighting Simulator CAD" manufactured by Camerium.

[0264] Example 40: Using the optical filter of Example 1 as a bandpass filter, an optical system was constructed having a light source unit, a condenser lens, a bandpass filter, a light receiving unit for transmitted light, and a light receiving unit for reflected light, as shown in FIG. 7 . The performance of the intensity of light passing through this optical system was evaluated. The wavelength of light emitted from the light source unit was 550 nm, the focal length of the condenser lens was 50 mm, the distance between the light source and the condenser lens was 100 mm, and the distance between the condenser lens and the light receiving unit was 100 mm. Comparative Example 40: Using the optical filter of Comparative Example 1 as a bandpass filter, an optical system similar to that of Example 40 was constructed, and performance evaluation was similarly performed.

[0265] [Example 41] Using the optical filter of Example 1 as a bandpass filter, an optical system was constructed having a light source unit, a beam splitter, a bandpass filter, two light receiving units for transmitted light, and two light receiving units for reflected light, as shown in Figure 8. For this optical system, the performance of the intensity of light passing through the system was evaluated. The wavelength of light emitted from the light source unit was 550 nm, and the distance between the two light receiving units was 30 mm. [Comparative Example 41] Using the optical filter of Comparative Example 1 as a bandpass filter, an optical system similar to that of Example 41 was constructed, and performance evaluation was similarly performed.

[0266] As a result of the evaluation of the optical systems described above, in all of Example 40 and Example 41, the intensity of light received by the light receiving unit of the optical system was 20 times or more higher than the results of the corresponding Comparative Examples 40 and 41. This shows the effect of light over a wide angular range entering the light receiving unit, since the bandpass performance of the bandpass filter, i.e., the optical filter of the present invention, does not change even if the angle of the light ray incident on the light receiving unit is large. This demonstrates that an optical system with little light receiving loss can be obtained according to the present invention.

[0267] [Example 50] In Example 1, the angle of the in-plane slow axis when laminating the unit layers was adjusted as shown in the table below, and eight unit layers were laminated to prepare a liquid crystal polarization interference element. As described above, the unit layer is a liquid crystal layer having a rod-shaped liquid crystal layer and a discotic liquid crystal layer. That is, in this example, there are also four liquid crystal layer pairs each consisting of a first liquid crystal layer (odd-numbered layer) and a second liquid crystal layer (even-numbered layer).

[0268]

[0269] A bandpass filter was fabricated using the fabricated liquid crystal polarization interference element in the same manner as in Example 1 (i.e., Comparative Example 1). The center wavelength, half-width, wavelength shift, and side lobes of the fabricated bandpass filter were measured in the same manner as in Comparative Example 1. The results showed that the fabricated bandpass filter had a center wavelength of 550 nm, a half-width of 120 nm, a wavelength shift of less than 5 nm, and a side lobe of 3% or less. As described above, the side lobes of the bandpass filter of Example 1 were 10%. Thus, the side lobes of the bandpass filter can be reduced by reducing the absolute value of the angle between the in-plane slow axis and the reference line of the liquid crystal layers of the liquid crystal layer pairs on both sides of the thickness direction compared to the liquid crystal layer of the central liquid crystal layer pair in the thickness direction. Furthermore, the reflectance measurement showed characteristics opposite to those of the transmittance, with non-transmitted light (i.e., components whose polarization is not changed by the liquid crystal polarization interference element) reflected by the reflective linear polarizer on the exit side and exiting from the entrance side. These results clearly demonstrate the effectiveness of the present invention.

[0270] It can be suitably used as a bandpass filter (dichroic filter) in various optical devices.

[0271] 10 Optical filter 12 First polarizer 14 Second polarizer 16 Liquid crystal polarization interference element 18R Rod-shaped liquid crystal compound 18D Discotic liquid crystal compound 20, 50, 60, 74 First liquid crystal layer 20R1, 24R2, 56R, 62R, 80R, 82R Rod-shaped liquid crystal layer 20D1, 24D2, 56D, 62D, 80D, 82D Discotic liquid crystal layer 24, 52, 68, 76 Second liquid crystal layer 26, 54, 70, 78 Liquid crystal layer set 90 Light source unit 93 Condenser lens 94, 94a, 94b Optical filter 96, 96a Light receiving unit 98 Beam splitter

Claims

1. An optical filter having, in this order, a first polarizer, a liquid crystal polarization interference element, and a second polarizer, wherein the first polarizer is an absorptive linear polarizer, and the second polarizer is a reflective linear polarizer, the liquid crystal polarization interference element has two or more pairs of liquid crystal layers in the thickness direction, each pair consisting of a first liquid crystal layer and a second liquid crystal layer, the first liquid crystal layer including at least one liquid crystal layer R1 formed by fixing horizontally aligned rod-shaped liquid crystal compounds and at least one liquid crystal layer D1 formed by fixing vertically aligned discotic liquid crystal compounds, the second liquid crystal layer including at least one liquid crystal layer R2 formed by fixing horizontally aligned rod-shaped liquid crystal compounds and at least one liquid crystal layer D2 formed by fixing vertically aligned discotic liquid crystal compounds, the in-plane slow axis of the liquid crystal layer R1 and the in-plane slow axis of the liquid crystal layer D1 being parallel to each other, an in-plane slow axis of the liquid crystal layer R2 and an in-plane slow axis of the liquid crystal layer D2 are parallel to each other; an in-plane slow axis of the liquid crystal layer R1 and an in-plane slow axis of the liquid crystal layer R2 intersect with each other; a sum of in-plane retardations of the liquid crystal layer R1 and the liquid crystal layer D1 is equal; and a sum of in-plane retardations of the liquid crystal layer R2 and the liquid crystal layer D2 is equal to each other.

2. The optical filter according to claim 1, wherein the liquid crystal polarization interference element has an in-plane retardation of the first liquid crystal layer equal to an in-plane retardation of the second liquid crystal layer.

3. The optical filter according to claim 1, wherein the liquid crystal polarization interference element is such that the in-plane slow axes of all of the liquid crystal layers R1 are parallel, and the in-plane slow axes of all of the liquid crystal layers R2 are parallel, and further, the in-plane retardation of all of the first liquid crystal layers is equal, and the in-plane retardation of all of the second liquid crystal layers is equal.

4. The optical filter of claim 1, wherein the liquid crystal polarization interference element is such that the orientation of the in-plane slow axis of the liquid crystal layer R1 of the first liquid crystal layer and the orientation of the in-plane slow axis of the liquid crystal layer R2 of the second liquid crystal layer are different between the liquid crystal layer pairs arranged on both sides in the thickness direction and the liquid crystal layer pair arranged in the center in the thickness direction, and further, the in-plane retardation of the first liquid crystal layer is different between the liquid crystal layer pairs arranged on both sides in the thickness direction and the liquid crystal layer pair arranged in the center in the thickness direction.

5. The optical filter of claim 1, wherein the liquid crystal polarization interference element has a liquid crystal layer pair arranged on both sides in the thickness direction and a liquid crystal layer pair arranged in the center in the thickness direction, the liquid crystal layer R1 of the first liquid crystal layer and the liquid crystal layer R2 of the second liquid crystal layer having different in-plane slow axis directions.

6. An optical filter having a first polarizer, a liquid crystal polarization interference element, and a second polarizer in this order, wherein the first polarizer is an absorptive linear polarizer, and the second polarizer is a reflective linear polarizer, the liquid crystal polarization interference element has two or more pairs of liquid crystal layers in the thickness direction, each pair consisting of a first liquid crystal layer and a second liquid crystal layer, wherein the first liquid crystal layer or the second liquid crystal layer includes at least one liquid crystal layer R containing a rod-shaped liquid crystal compound, and the first liquid crystal layer or the second liquid crystal layer includes at least one liquid crystal layer D containing a discotic liquid crystal compound, the in-plane slow axis of the first liquid crystal layer intersects with the in-plane slow axis of the second liquid crystal layer, and the sum of the in-plane retardations of the first liquid crystal layer and the second liquid crystal layer is equal.

7. The optical filter of claim 6, wherein the liquid crystal polarization interference element has a liquid crystal layer pair arranged on both sides in the thickness direction and a liquid crystal layer pair arranged in the center in the thickness direction, the liquid crystal layer pair having a different in-plane slow axis direction of the first liquid crystal layer and the second liquid crystal layer.

8. The optical filter according to claim 1 or 6, wherein the first polarizer and the second polarizer are arranged with their transmission axes perpendicular to each other.

9. The optical filter according to claim 1 or 6, wherein the first polarizer and the second polarizer are arranged with their transmission axes parallel to each other.

10. The optical filter according to claim 1 or 6, wherein the reflective linear polarizer has a selective reflection layer containing at least one cholesteric liquid crystal layer, and a λ / 4 phase difference plate.

11. The optical filter according to claim 10, wherein the selective reflection layer comprises a cholesteric liquid crystal layer R formed using a rod-shaped liquid crystal compound and a cholesteric liquid crystal layer D formed using a discotic liquid crystal compound.

12. The optical filter according to claim 10, wherein the λ / 4 retardation plate is made of a reverse dispersion liquid crystal compound.

13. The optical filter of claim 10, wherein the λ / 4 retardation plate comprises a C-plate.

14. The optical filter according to claim 1 or 6, wherein the reflective linear polarizer is made of a dielectric multilayer film.

15. The optical filter according to claim 1 or 6, wherein the reflective linear polarizer is a wire grid polarizer.

16. An optical system comprising a light source unit, an optical filter according to claim 1 or 6, and a light receiving unit.

17. The optical system of claim 16, comprising a focusing lens.

18. The optical system of claim 16, comprising a beam splitter.

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