Optical filter and optical system

Abstract

The present invention addresses the problem of providing an optical filter and an optical system using a liquid crystal polarization interference element which, when used for a band-pass filter or the like, can inhibit fluctuation in the wavelength of light exhibiting a maximum transmittance even when the light is incident from an oblique direction. The problem is solved by the configuration in which the liquid crystal polarization interference element includes two or more sets each made up of 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 obtained by fixing a horizontally arrayed rod-shaped liquid crystal compound and a liquid crystal layer D obtained by fixing a vertically arrayed discoid liquid crystal compound. The liquid crystal layer R and the liquid crystal layer D have parallel slow axes and equal in-plane retardation. Furthermore, the in-plane slow axes of the first liquid crystal layer and the second liquid crystal layer intersect each other.

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 when light is incident from an oblique direction, the wavelength of light that exhibits maximum transmittance fluctuates, resulting in a so-called wavelength shift.

[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, 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, 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 that transmits linearly polarized light in a predetermined direction, and the second polarizer is an absorptive linear polarizer that transmits linearly polarized light parallel to or perpendicular to the predetermined direction, and the liquid crystal polarization interference element has two or more pairs of liquid crystal layers, each pair consisting of a first liquid crystal layer and a second liquid crystal layer, in a thickness direction, wherein the first liquid crystal layer includes at least one liquid crystal layer R1 formed by fixing horizontally aligned rod-like 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-like 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 the optical filter according to any one of [1] to [4], wherein the liquid crystal layer R1 and the liquid crystal layer D1 of the first liquid crystal layer, and the liquid crystal layer R2 and the liquid crystal layer D2 of the second liquid crystal layer, contain an infrared absorbing dye. [6] The liquid crystal polarization interference element is the optical filter according to any one of [1] to [5], wherein the liquid crystal layer R1 and the liquid crystal layer D1 of the first liquid crystal layer, and the liquid crystal layer R2 and the liquid crystal layer D2 of the second liquid crystal layer contain a liquid crystal elastomer. [7] 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 that transmits linearly polarized light in a predetermined direction, and the second polarizer is an absorptive linear polarizer that transmits linearly polarized light parallel to or perpendicular to the predetermined direction, and the liquid crystal polarization interference element has two or more pairs of liquid crystal layers, each pair consisting of a first liquid crystal layer and a second liquid crystal layer, in the thickness direction, 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. [8] The optical filter according to [7], wherein the liquid crystal layer R and the liquid crystal layer D of the liquid crystal polarization interference element contain an infrared absorbing dye. [9] The optical filter according to [7] or [8], wherein the liquid crystal layer R and the liquid crystal layer D of the liquid crystal polarization interference element contain a liquid crystal elastomer.

[10] The optical filter according to any one of [1] to [6], wherein a retardation layer is provided between at least one of the first polarizer and the second polarizer and the liquid crystal polarization interference element, and the in-plane slow axis of the retardation layer is parallel to the absorption axis of either the first polarizer or the second polarizer.

[11] The optical filter according to any one of [7] to [9], wherein a retardation layer is provided between at least one of the first polarizer and the second polarizer and the liquid crystal polarization interference element, and the in-plane slow axis of the retardation layer is parallel to the absorption axis of either the first polarizer or the second polarizer.

[12] The liquid crystal polarization interference element is an optical filter according to any one of [1] to [6] and

[10] , wherein the liquid crystal layer pairs arranged on both sides 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.

[13] The liquid crystal polarization interference element is an optical filter according to any one of [7] to [9] and

[11] , wherein the liquid crystal layer pairs arranged on both sides in the thickness direction have different in-plane slow axes of the first liquid crystal layer and the liquid crystal layer pair arranged in the thickness direction.

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

[13] , and a light receiving unit.

[15] The optical system according to

[14] , comprising a condensing lens.

[16] The optical system according to

[14] or

[15] , comprising a beam splitter.

[17] The optical system according to any one of

[14] to

[16] , which has a light-guiding element.

[18] The optical system according to any one of

[14] to

[17] , in which the optical filter and the light-receiving unit are adjacent to each other.

[19] The optical system according to

[18] , which has a plurality of optical filters with different central wavelengths of transmitted light.

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

[0010] FIG. 1 is a diagram conceptually illustrating 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 illustrating an example of an optical system of the present invention. FIG. 8 is a diagram conceptually illustrating another example of an optical system of the present invention. FIG. 9 is a diagram conceptually illustrating another example of an optical system of the present invention. FIG. 10 is a diagram conceptually illustrating another example of an optical system of the present invention. FIG. 11 is a diagram conceptually illustrating 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 a first embodiment of the optical filter 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. The optical filter 10 shown in FIG. 1 is a bandpass filter (narrow-band filter) that transmits light in a specific wavelength range and blocks light of other wavelengths, and includes a first polarizer 12, a second polarizer 14, and a liquid crystal polarization interference element 16. The liquid crystal polarization interference element 16 is disposed between the first polarizer 12 and the second polarizer. The optical filter of the first embodiment and the optical filter of the second embodiment of the present invention have the same configuration except for the liquid crystal polarization interference element. This will be described in detail later.

[0014] The first polarizer 12 and the second polarizer 14 are absorptive linear polarizers that transmit linearly polarized light in a predetermined direction. In this example, the first polarizer 12 and the second polarizer 14 are arranged in a crossed Nicol configuration with their transmission axes perpendicular to each other. There are no limitations on the first polarizer 12 and the second polarizer 14, and various known absorptive linear polarizers can be used, such as iodine-based polarizers, dye-based polarizers using dichroic dyes, and polyene-based polarizers. In the following description, unless otherwise specified, the term "polarizer" refers to an absorptive linear polarizer.

[0015] 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.

[0016] In addition, in the optical filter of the present invention, the polarizer is not limited to the above-mentioned form, and various polarizers can be used as long as the function of light is limited to 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 polarized light and reflected light from a substrate at Brewster's angle.

[0017] 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, in this example, the first polarizer 12 and the second polarizer 14 are polarizers 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 of the first polarizer 12 is transmitted through the first polarizer 12. The linearly polarized light transmitted through the first polarizer 12 then enters the liquid crystal polarization diffraction element 16. In the liquid crystal polarization diffraction element 16, the polarization direction of light in the specific wavelength range that has transmitted through the first polarizer 12 is rotated by 90° by the liquid crystal polarization interference element 16 and transmitted through the liquid crystal polarization diffraction element 16. On the other hand, light other than the light in the specific wavelength range is transmitted through the liquid crystal polarization interference element 16 in its original polarization direction because the liquid crystal polarization interference element 16 does not function as a retardation plate. The linearly polarized light transmitted through the liquid crystal polarization interference element 16 then enters the second polarizer 14. In this example, the first polarizer 12 and the second polarizer 14 are arranged in a crossed Nicol configuration with their transmission axes perpendicular to each other. Therefore, light in a specific wavelength range, whose polarization direction has been rotated 90° by the liquid crystal polarization interference element 16, passes through the second polarizer 14, which is arranged in a crossed Nicol configuration with the first polarizer 12, and is emitted. On the other hand, light outside the specific wavelength range, for which the liquid crystal polarization interference element 16 does not function as a retardation plate, remains linearly polarized in the direction of the transmission axis of the first polarizer 12, and is therefore blocked (absorbed) by the second polarizer 14, which is arranged in a crossed Nicol configuration with the first polarizer 12. In this way, the optical filter 10 of the present invention, in which the first polarizer 12 and the second polarizer 14 are arranged in a crossed Nicol configuration, functions as a bandpass filter that transmits only light in a specific wavelength range and blocks other light.

[0018] 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.

[0019] When the first polarizer 12 and the second polarizer 14 are arranged in a parallel Nicol state, the optical filter 10 functions as follows. Similarly, in this example, 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 is transmitted through the liquid crystal polarization interference element 16. In contrast, light other than the specific wavelength range for which the liquid crystal polarization interference element 16 does not act as a retardation plate is transmitted through the liquid crystal polarization interference element 16 as linearly polarized light in the polarization direction of the transmission axis of the first polarizer 12. The linearly polarized light transmitted through the liquid crystal polarization interference element 16 then enters the second polarizer 14. In this example, the first polarizer 12 and the second polarizer 14 are arranged in a parallel Nicol state with their transmission axes parallel to each other. Therefore, linearly polarized light in a specific wavelength range, the polarization direction of which has been shifted by 90° when the liquid crystal polarization interference element 16 acts as a λ / 2 retardation plate, is blocked (absorbed) by the second polarizer 14 arranged in parallel Nicols with the first polarizer 12. On the other hand, light outside the specific wavelength range, when the liquid crystal polarization interference element 16 does not act as a λ / 2 retardation plate, remains linearly polarized in the direction of the transmission axis of the first polarizer 12, and is incident on the second polarizer 14 arranged in parallel Nicols with the first polarizer 12, passes through the second polarizer, and is emitted. That is, the optical filter 10 of the present invention, in which the first polarizer 12 and the second polarizer 14 are arranged in parallel Nicols, functions as a wavelength-selective filter that blocks only light in a specific wavelength range and transmits and emits light other than that.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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."

[0026] 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).

[0027] 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°.

[0028] 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 .

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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

[0033] 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°.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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" relative to the reference line and the in-plane slow axis having an angle of "-φ" relative to the reference line. For example, when the absolute value of the angle relative 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° relative to the reference line, and then rotated by the in-plane slow axis having an angle of -5.625° relative 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 depending on the wavelength of light for which the liquid crystal polarization interference element 16 is intended to function as a λ / 2 retarder. Furthermore, the angles of the in-plane slow axes of the first liquid crystal layer 20 and the second liquid crystal layer 24 are adjusted depending on the number of layers of the first liquid crystal layer 20 and the second liquid crystal layer 24 in the liquid crystal polarization interference element 16. 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.

[0044] Therefore, by arranging this liquid crystal polarization interference element between two polarizers arranged in crossed Nicols so that the transmission axis or absorption axis of one polarizer coincides 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, it is possible to obtain a bandpass filter that rotates (optical rotation) only light of a specific wavelength range, of the linearly polarized light transmitted through one polarizer, by λ / 2 and emits it from the other polarizer, as described above. Furthermore, in the optical filter 10 shown in Figure 1, by similarly arranging a liquid crystal polarization interference element between two polarizers arranged in parallel Nicols, it is possible to obtain a wavelength selection filter that, of the linearly polarized light transmitted through one polarizer, only light outside the specific wavelength range, of which the liquid crystal polarization interference element does not rotate the polarization direction, is emitted from the other polarizer, as described above.

[0045] As described above, conventional bandpass filters suffer from a problem of wavelength shift, i.e., when light is incident from an oblique direction, the wavelength of light exhibiting maximum transmittance fluctuates, as conceptually shown in FIG. 2 . In contrast, the liquid crystal polarization interference element 16 used in the optical filter of the present invention has a first liquid crystal layer 20 and a second liquid crystal layer 24 each including a rod-shaped liquid crystal layer composed of rod-shaped liquid crystal compound 18R and a discotic liquid crystal layer composed of discotic liquid crystal compound 18D, both of which have parallel in-plane slow axes, and the in-plane retardation of the rod-shaped liquid crystal layer is equal to the in-plane retardation of the discotic liquid crystal layer. Therefore, in the first liquid crystal layer 20 and the second liquid crystal layer 24, the thickness direction retardation (Rth) of the rod-shaped liquid crystal layer can be offset by the thickness direction retardation of the discotic liquid crystal layer. As a result, by using the liquid crystal polarization interference element 16 as a bandpass filter, wavelength shift, i.e., a change in the wavelength of light exhibiting maximum transmittance, can be suppressed even when light is incident from an oblique direction.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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, the narrower the wavelength range over which the liquid crystal polarization interference element 16 acts as a λ / 2 retarder. That is, in the liquid crystal polarization interference element used in the optical filter of the present invention, the greater the number of liquid crystal layer pairs 26, the narrower the wavelength range over which the liquid crystal polarization interference element 16 acts 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. That is, in the case of a bandpass filter in which polarizers are arranged in a crossed Nicol configuration, 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 that transmits wavelengths. 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 sets 26, can be selected appropriately depending on the width of the transmission wavelength range (light-blocking wavelength range) required for the optical filter 10, with a smaller number of layers selected when a wide band is preferred and a larger number of layers selected when a narrow band is required.

[0052] 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.

[0053] 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 a microgroove, 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 using a polarizer such as a wire grid polarizer.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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 with the liquid crystal polarization interference element 16 sandwiched therebetween 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 makes it possible to fabricate the optical filter 10 (bandpass filter, wavelength selection filter) as shown in FIG.

[0060] In the liquid crystal polarization interference element used in the optical filter of the present invention, 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 perspective of improving transmittance, it is preferable that the refractive index of the optical bonding layer be 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 this reduces the difference in refractive index from either of the two refractive indices. 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 preferably directly laminated by a coating method without an adhesive layer or the like.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] In the liquid crystal polarization interference element used in the optical filter of the present invention, 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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 the liquid crystal polarization interference element used in the optical filter of the present invention, 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 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 bandpass filter.

[0072] 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.

[0073] 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.

[0074] 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 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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 described above, 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.

[0081] In the second embodiment of the optical filter of the present invention, the liquid crystal polarization interference element basically follows the descriptions of the first embodiment in terms of various components such as rod-shaped liquid crystal compounds and discotic liquid crystal compounds, and the composition forming the liquid crystal layer, etc. In addition, 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 descriptions of the first embodiment or may be different.

[0082] 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.

[0083] 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 liquid crystal polarization 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.

[0084] 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.

[0085] 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.

[0086] That is, the liquid crystal polarization 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, 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, as long as the sum of the in-plane retardations of the first liquid crystal layer and the second liquid crystal layer are equal, various configurations are available.

[0087] 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 either the first or second embodiment of the liquid crystal polarization interference element, 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 include two or more liquid crystal layer sets 78, each of which includes 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.

[0088] 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 the liquid crystal polarization interference element 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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 blocking 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 blocking 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 such a liquid crystal polarization interference element is used, in the optical filter (bandpass filter) of the present invention, when light of a specific wavelength is transmitted through the second polarizer and emitted, 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.

[0097] 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.

[0098] As with the optical filter 10 shown in Fig. 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, the light in the specific wavelength range, which is linearly polarized light that has passed through the first polarizer 12, passes through the second polarizer 14 and is emitted.

[0099] On the other hand, the liquid crystal polarization interference element acts as a half-wave retarder, so that light outside the specific wavelength range has its polarization direction changed by 90° and enters the second polarizer 14. Here, the first polarizer and the second polarizer are arranged in a parallel Nicol state with their transmission axes parallel. Therefore, after passing through the first polarizer 12, light outside the specific wavelength range, whose polarization direction has been changed by 90° by the liquid crystal polarization interference element, is blocked (absorbed) by the second polarizer arranged in a parallel Nicol state with the first polarizer 12.

[0100] As described above, the optical filter of the present invention, which uses a liquid crystal polarization interference element configured such that the angle of the in-plane slow axis increases sequentially in the stacking direction, arranges the first polarizer and the second polarizer in parallel Nicols, so that, of the incident light, light in a specific wavelength range in which the liquid crystal polarization interference element 16 does not function as a retardation plate is transmitted through the optical filter and exits as transmitted light, while blocking other light. In other words, the optical filter of the present invention having this configuration functions as a bandpass filter that transmits light in a specific wavelength range and blocks other light.

[0101] 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.

[0102] 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 that has passed 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 does not change its polarization direction in the liquid crystal polarization interference element, and enters the second polarizer as linearly polarized light in the direction of the transmission axis of the first polarizer. In contrast, light outside 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° by the liquid crystal polarization interference element and enters 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 has a transmission axis that is perpendicular to the transmission axis of the first polarizer. Therefore, light in a specific wavelength range for which the liquid crystal polarization interference element does not function as a retardation plate and is linearly polarized in the direction of the transmission axis of the first polarizer is blocked (absorbed) by the second polarizer. In contrast, after passing through the first polarizer in which the liquid crystal polarization interference element acts as a λ / 2 retardation plate, light outside the specific wavelength range, whose polarization direction has been changed by 90° by the liquid crystal polarization interference element, passes through the second polarizer and is emitted. In other words, in this case, the optical filter of the present invention having this configuration functions as a wavelength selection filter, blocking light in the specific wavelength range in which the liquid crystal polarization interference element does not act as a retardation plate and transmitting light other than that.

[0103] 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.

[0104] 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.

[0105] In the optical filter of the present invention, a retardation layer 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 retardation layer can be provided on one or both sides between the liquid crystal polarization interference element and the polarizer. This retardation layer has the effect of maintaining the orthogonal relationship of the polarization directions of linear polarizers arranged in a crossed Nicol configuration not only in a frontal view but also in an 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 in a frontal view can be obtained even in an oblique view. The in-plane slow axis of the retardation layer 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 an oblique direction without affecting the frontal view. Examples of the retardation layer include a positive C plate formed by vertically aligning rod-like liquid crystals and a positive A plate formed by horizontally aligning rod-like 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 B plate (with an Nz factor of 0.1 to 0.9) that is a biaxial refractive index material can also be used as the retardation layer.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] 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, and a light-receiving unit 96. Various known condensing lenses can be used. In this optical system, divergent light emitted by the light source 90 is condensed by the condensing lens 92 and received by the light-receiving unit 96 for photometry. In such an optical system, an optical filter 94 (bandpass filter) of the present invention is disposed in either the divergent or condensing portion of the light. In the illustrated example, the optical filter 94 of the present invention is disposed in the condensing portion of the light. As described above, the optical filter of the present invention exhibits bandpass performance for the same wavelength for both light coming from the front and light coming from oblique directions, allowing light of desired wavelengths over a wide angular range to be collected at the light-receiving unit. This allows the optical system of the present invention to achieve high light-receiving efficiency and minimize light-receiving loss. An optical system of the present invention using a condensing lens in this manner can be used, for example, in an imaging system. This imaging system achieves high light-receiving efficiency when a condenser lens condenses light emitted from the light source unit 90, which is the object to be imaged, onto the light-receiving unit 96, which is an imaging element. Furthermore, because the optical filter of the present invention has wide-angle bandpass performance, a thin, compact optical system can be realized by using a condenser 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, an optical fiber output terminal can be provided on the light source unit 90 side and an optical fiber input terminal on the light-receiving unit 96 side, thereby achieving a system with high efficiency similar to that described above. Specifically, the light from the optical fiber output terminal and / or the light condensed by the lens contains 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 wavelength from light of any angle and integrate it into a sensor.

[0110] 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 at which the light is transmitted and reflected. In other words, the optical filter of the present invention functions as a bandpass filter that acts appropriately on a specific wavelength for light incident at various angles after passing through the lens, as well as for diverging light and condensing light. The angle dependency of the optical filter of the present invention is also true for each of the optical systems described below.

[0111] 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, and a light receiving unit 96. 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 illustrated example) different angular directions. The split light is received by corresponding light receiving units 96 for photometry. In this optical system, an optical filter 94 (bandpass filter) according to the present invention is disposed in the region after the split light. The optical filter 94 according to 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 all of the multiple light receiving units 96 that receive the split light. This optical system using a beam splitter 98 according to the present invention can be applied to various optical systems, such as sensors and lasers. In addition, in systems using optical fibers, the system of the present invention can be used when a straight light beam is split into different angles using a beam splitter, and then the connection destination is switched depending on the destination of the optical signal.

[0112] FIG. 9 conceptually illustrates an example of combining a light-guiding element (light guide plate) with an optical system of the present invention. The optical system shown in FIG. 9 includes a light source unit 90, a light-guiding element 100, an optical filter 94 of the present invention, and a light-receiving unit 96. Various known light-guiding elements can be used. In this optical system, light emitted from the light source unit 90 in various angular directions is incident on one end of the light-guiding element 100, and the mixed light propagating through the light-guiding element 100 is emitted from the other end of the light-guiding element 100 and received by the light-receiving unit 96 for photometry. In this optical system, an optical filter 94 (bandpass filter) of the present invention is disposed at the exit position of the propagating light from the light-guiding element 100. The optical filter 94 of the present invention exhibits bandpass performance at the same wavelength for propagating light traveling in different angular directions. As a result, high light-receiving efficiency is achieved for the propagating light emitted from the light-guiding element. The optical system of the present invention using such a light guide element can be used in a sensing system using a light guide element, a display system such as an AR system, and an optical communication system using a waveguide.

[0113] In the optical system of the present invention, an optical filter 94 of the present invention and a light receiving unit 96 may be provided adjacent to each other, as conceptually shown in FIG. 10 . In the optical system shown in FIG. 10 , divergent light emitted from a light source unit 90 reaches the optical filter 94 (bandpass filter) of the present invention from various angular directions. Here, the optical filter 94 of the present invention has wide-angle bandpass performance, allowing light of a desired wavelength to be introduced into the adjacent light receiving unit over a wide angle. As a result, high light receiving efficiency is achieved in the light receiving unit 96. In an optical system in which the light source unit 90 emits divergent light, utilizing an optical system of the present invention that uses the optical filter 94 of the present invention allows the optical filter 94 of the present invention to exhibit the desired bandpass performance over a wide angle, which is effective in reducing the thickness of the system.

[0114] Furthermore, in an optical system of the present invention in which an optical filter of the present invention and a light receiving unit are adjacent to each other, multiple optical filters with different center wavelengths of transmitted light may be arranged. FIG. 11 shows an example. The optical system shown in FIG. 11 is similar to the example shown in FIG. 10 in that the optical filter (bandpass filter) of the present invention achieves wide-angle bandpass performance and high light receiving efficiency for the divergent light emitted by the light source unit 90. Here, the example shown in FIG. 11 includes optical filters 94a and 94b of the present invention with different center wavelengths of transmitted light, and multiple light receiving units 96 corresponding to each optical filter. This allows the optical system shown in FIG. 11 to simultaneously receive light of different wavelengths. Specifically, the optical system of the present invention shown in FIG. 11 can be used as a multispectral sensor. For example, the optical system of the present invention can realize an optical system that senses diffused light from the skin, containing health information, at different wavelengths with high light receiving efficiency using a thin optical system. While the example shown in FIG. 11 uses two optical filters with different center wavelengths of transmitted light, the optical system of the present invention is not limited thereto. For example, the optical system of the present invention can realize a thin and compact optical system capable of highly efficient measurement of multispectral, hyperspectral, and other wavelengths by increasing the number of optical filters with different central wavelengths of transmitted light, i.e., the number of wavelengths to be measured. Furthermore, the optical filter of the present invention may be formed by patterning multiple liquid crystal polarization interference elements with different central wavelengths of transmitted light in a plane, thereby creating an optical filter that can handle multiple wavelengths with a single element. In this case, a pattern with different retardation values ​​is formed in different locations, which can be achieved by varying either the film thickness or the birefringence of the liquid crystal layer in a plane. The patterning may be either discrete or continuous.

[0115] 7 to 11 are examples in which the optical filter 94 of the present invention is used as a bandpass filter. That is, the optical filter 94 is, as an example, an example in which a first polarizer and a second polarizer are arranged in a crossed Nicol configuration in the first embodiment of the optical filter of the present invention. Therefore, in this case, of the light emitted by the light source 90, light in a specific wavelength range in which the liquid crystal polarization interference element acts as a λ / 2 retardation layer is transmitted through the optical filter 94 and enters the light receiving element 96 for photometry, while the other light is blocked by the optical filter 94.

[0116] Furthermore, the optical filter according to the first aspect of the present invention, in which the first polarizer and the second polarizer are arranged in a parallel Nicol state, can also be used in the optical systems shown in Figures 7 to 11. When the first polarizer and the second polarizer are arranged in a parallel Nicol state, the function of the optical filter is reversed, as described above. Therefore, in this case, in the optical systems shown in Figures 7 to 11, of the light emitted by the light source 90, light in a specific wavelength range in which the liquid crystal polarization interference element acts as a λ / 2 retardation layer is blocked by the optical filter 94, and light outside this specific wavelength range is transmitted through the optical filter 94 and enters the light receiving element 96, where it is measured.

[0117] On the other hand, in the optical filter of the present invention having a liquid crystal polarization interference element configured such that the angle of the in-plane slow axis increases sequentially in the stacking direction as in Example 30, when the first polarizer and the second polarizer are arranged in parallel Nicols, the optical filter functions as a bandpass filter. Therefore, when this optical filter is used as a bandpass filter in the optical filter 94 in the optical system shown in Figures 7 to 11, light emitted by the light source 90, of a specific wavelength range in which the liquid crystal polarization interference element does not act as a λ / 2 retardation layer, passes through the optical filter 94 and enters the light receiving element 96 to be measured, while light outside this specific wavelength range is blocked by the optical filter 94.

[0118] Furthermore, as in Example 30, an optical filter using a liquid crystal polarization interference element configured such that the angle of the in-plane slow axis increases sequentially in the stacking direction, and in which the first and second polarizers are arranged in a crossed Nicol configuration, can also be used in the optical systems shown in Figures 7 to 11. When the first and second polarizers are arranged in a crossed Nicol configuration, the function of the optical filter is reversed, as described above. Therefore, in this case, of the light emitted by the light source 90, light in a specific wavelength range in which the liquid crystal polarization interference element does not act as a λ / 2 retardation layer is blocked by the optical filter 94, and light other than the light in this specific wavelength range passes through the optical filter 94 and enters the light receiving element 96, where it is measured.

[0119] 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.

[0120] 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.

[0121] 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.

[0122] 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 ---------------------------------------------------

[0123] Material for photo alignment

[0124] (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.

[0125] (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.

[0126] 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

[0127] Rod-shaped liquid crystal compound L-1

[0128] Leveling agent T-1

[0129] 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.

[0130] 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.

[0131] 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.

[0132] An optical filter was fabricated by placing the fabricated liquid crystal polarization interference element between polarizers arranged in parallel Nicols. The optical filter was fabricated by aligning the transmission axis of one of the polarizers 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-shaped liquid crystal layers were stacked. The wavelength (center wavelength) and half-width indicating the minimum transmittance of the non-transmitting portion, as well as the wavelength shift and side lobes, were measured for the fabricated optical filter using a spectroradiometer "SR-3" manufactured by Topcon Technohouse. In the present invention, the "non-transmitting portion" refers to a wavelength range in which the transmittance is reduced by rotating the polarization direction due to polarization interference. In other words, in the present invention, the "non-transmitting portion" refers to a wavelength range in which the liquid crystal polarization interference element acts as a half-phase retardation layer. The wavelength shift (absolute value) was measured when light was incident at a polar angle of 60° relative to when light was incident at a polar angle of 90°. The 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 taken as the measured value. As a result, the manufactured optical filter had a central wavelength of 550 nm in the non-transmitting portion, 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 difference between the transmittance (baseline) of the transmitting portion and the transmittance of the side lobe to the difference between the transmittance (baseline) of the transmitting portion and the transmittance at the center of the non-transmitting portion.

[0133] 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

[0134] Discotic liquid crystal compound L-2 Discotic liquid crystal compound L-3

[0135] 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.

[0136] 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.

[0137] 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.

[0138] The prepared liquid crystal polarization interference element was placed between polarizers arranged in parallel Nicols to prepare an optical filter. The optical filter was prepared by aligning the transmission axis of one of the polarizers with the line that bisects the crossing angle formed by the in-plane slow axes of the odd-numbered and even-numbered layers when the laminate was stacked. The center wavelength, half-width, wavelength shift, and side lobe of the non-transmitting portion of the prepared optical filter were measured in the same manner as in Comparative Example 1.

[0139] As a result, the non-transmitting portion of the fabricated optical filter had a central wavelength of 550 nm, a half-width of 120 nm, a wavelength shift of less than 5 nm, and a side lobe of 10%. The wavelength shift of the optical filter of Comparative Example 1, in which the first and second liquid crystal layers were formed only from 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 optical filter when light is incident obliquely can be significantly suppressed.

[0140] [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).

[0141]

[0142] An optical 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 optical filter were measured in the same manner as in Comparative Example 1. As a result, the center wavelength of the non-transmitting portion of the fabricated optical filter was 550 nm, the half-width was 120 nm, the wavelength shift was less than 5 nm, and the side lobe was 3% or less. As described above, the side lobe of the optical filter of Example 1 was 10%. In this way, the side lobe of the optical 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.

[0143] Example 3: An optical filter was fabricated in the same manner as in Example 2, using a liquid crystal polarization interference element in which an infrared-absorbing dye was added to the rod-shaped liquid crystal layer and discotic liquid crystal layer of each unit layer, based on optical simulations to evaluate the optical performance of a birefringent medium laminate. For the optical simulation, "Optical Waves in Layered Media 2nd Edition, by Pochi Yeh, published by Wiley-Interscience (March 3, 2005)" was used. The infrared-absorbing dye exhibited dichroic absorption in the near-infrared and was aligned as a guest dye in the host liquid crystal compound. In this example, the Δn(450) / Δn(650) ratio in each liquid crystal layer was 1.4.

[0144] The central wavelength, half-width, wavelength shift, and side lobes of the non-transmitting portion of the prepared optical filter were measured in the same manner as in Comparative Example 1. As a result, the central wavelength of the non-transmitting portion of the prepared optical filter was 550 nm, the half-width was 60 nm, the wavelength shift was less than 5 nm, and the side lobes were 3% or less. As described above, the central wavelength of the transmitted light of the optical filter of Example 2 was 550 nm, and the half-width was 120 nm. In this way, by adding an infrared absorbing dye to the rod-shaped liquid crystal layer and the discotic liquid crystal layer to achieve strong forward dispersion, the half-width of the non-transmitting portion was narrowed, and an optical filter with a narrower wavelength range of the non-transmitting portion was obtained.

[0145] [Example 4] Using the same optical simulation as in Example 3, a liquid crystal polarization interference element was fabricated using a liquid crystal elastomer as the liquid crystal compound forming the liquid crystal layer in Example 2, and an optical filter was fabricated in the same manner as in Example 2. The liquid crystal elastomer used was prepared from a liquid crystal monomer, a crosslinker, and a plasticizer, as described in JP 2020-131638 A. The liquid crystal polarization interference element of the fabricated optical filter could be stretched by 20%.

[0146] The central wavelength, half-width, wavelength shift, and side lobe of the non-transmitting portion of the prepared optical filter were measured in the same manner as in Comparative Example 1. As a result, when the liquid crystal polarization interference element was not stretched, the central wavelength of the non-transmitting portion of the prepared optical filter was 550 nm, the half-width was 60 nm, the wavelength shift was less than 5 nm, and the side lobe was 3% or less. In this optical filter, by stretching the liquid crystal polarization interference element by 20%, it is possible to control the central wavelength of the non-transmitting portion by 50 nm, and the central wavelength of the non-transmitting portion when stretched by 10% was 525 nm, and the central wavelength of the non-transmitting portion when stretched by 20% was 500 nm. In this way, since the rod-shaped liquid crystal layer and the discotic liquid crystal layer contain a liquid crystal elastomer, the wavelength range can be changed by stretching and shrinking the liquid crystal polarization interference element, and active wavelength control is possible in the optical filter.

[0147] 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).

[0148] Using this liquid crystal polarization interference element, an optical filter was produced in the same manner as in Example 1. The central wavelength, half-width, wavelength shift, and side lobe of the non-transmitting portion of the produced optical filter were measured in the same manner as in Comparative Example 1. As a result, the central wavelength of the non-transmitting portion of the produced optical filter was 550 nm, the half-width was 80 nm, the wavelength shift was less than 5 nm, and the side lobe was 10%.

[0149] 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).

[0150] Using this liquid crystal polarization interference element, an optical filter was produced in the same manner as in Example 1. The central wavelength, half-width, wavelength shift, and side lobe of the non-transmitting portion of the produced optical filter were measured in the same manner as in Comparative Example 1. As a result, the central wavelength of the non-transmitting portion of the produced optical filter was 550 nm, the half-width was 120 nm, the wavelength shift was less than 10 nm, and the side lobe was 10%.

[0151] [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).

[0152] Using this liquid crystal polarization interference element, an optical filter was produced in the same manner as in Example 1. The central wavelength, half-width, wavelength shift, and side lobe of the non-transmitting portion of the produced optical filter were measured in the same manner as in Comparative Example 1. As a result, the central wavelength of the non-transmitting portion of the produced optical filter was 550 nm, the half-width was 120 nm, the wavelength shift was less than 10 nm, and the side lobe was 10%.

[0153] [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).

[0154] Using this liquid crystal polarization interference element, an optical filter was produced in the same manner as in Example 1. The center wavelength, half-width, wavelength shift, and side lobe of the non-transmitting portion of the produced optical filter were measured in the same manner as in Comparative Example 1. As a result, the center wavelength of the non-transmitting portion of the produced optical filter was 550 nm, the half-width was 120 nm, the wavelength shift was less than 3 nm, and the side lobe was 10%.

[0155] 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

[0156] 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).

[0157] Using this unit layer, 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). Using this liquid crystal polarization interference element, an optical filter was fabricated in the same manner as in Example 1. Furthermore, the center wavelength, half-width, wavelength shift, and side lobe of the non-transmitting portion of the fabricated optical filter were measured in the same manner as in Comparative Example 1. As a result, the center wavelength of the non-transmitting portion of the fabricated optical filter was 1550 nm, the half-width was 340 nm, the wavelength shift was less than 14 nm, and the side lobe was 10%.

[0158] 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 such 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). An optical 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 lobe of the non-transmitting portion of the prepared optical filter were measured in the same manner as in Comparative Example 1. The results showed that the center wavelength of the non-transmitting portion of the prepared optical filter was 1550 nm, the half-width was 340 nm, the wavelength shift was less than 250 nm, and the side lobe was 10%. These results demonstrate that even in Example 14, which has a different center wavelength (1550 nm) from Example 1, the wavelength shift was significantly improved, as compared to Comparative Example 14.

[0159] Example 30: A liquid crystal polarization interference element was fabricated using the same eight unit layers as in Example 1, arranged with the in-plane slow axes as shown in the table below. Furthermore, an optical filter was fabricated by changing the polarizer arrangement from parallel Nicols (parallel) to crossed Nicols (orthogonal). 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 an optical filter in which a Solk optical filter (Van Solk optical filter) is arranged between polarizers arranged in crossed Nicols, and birefringent plates (λ / 2 retardation plates) of equal thickness are stacked, with the angles between the polarizer transmission axis and the slow axis being ρ, 3ρ, 5ρ, ...

[0160]

[0161] The center wavelength, half-width, wavelength shift, and side lobe of the non-transmitting portion of the produced optical filter were measured in the same manner as in Comparative Example 1. As a result, the center wavelength of the non-transmitting portion of the produced optical filter was 550 nm, the half-width was 120 nm, the wavelength shift was less than 5 nm, and the side lobe was 10%.

[0162] Example 31 In Example 30, an optical filter was produced by disposing a retardation layer between the first polarizer of the polarizers arranged in a crossed Nicol configuration and the liquid crystal polarization interference element. The retardation layer was as shown below. Adjacent to the first polarizer, a positive C plate (thickness direction retardation Rth of -90 nm) formed by vertical alignment of rod-shaped liquid crystals and a positive A plate (in-plane direction retardation Re of 140 nm) formed by horizontal alignment of rod-shaped liquid crystals were disposed in this order and laminated. In this case, the in-plane slow axis of the positive A plate was arranged parallel to the absorption axis of one of the polarizers. This retardation layer 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 front direction but also in the oblique direction.

[0163] The center wavelength, half-width, wavelength shift, and side lobe of the non-transmitting portion of the produced optical filter were measured in the same manner as in Comparative Example 1. As a result, the center wavelength of the non-transmitting portion of the produced optical filter was 550 nm, the half-width was 120 nm, the wavelength shift was less than 3 nm, and the side lobe was 10%.

[0164] The optical performance of the optical system of the present invention using the optical filter of the present invention was evaluated by optical simulation using "Lighting Simulator CAD" manufactured by Camerium.

[0165] [Example 40] Using the optical filter of Example 1, an optical system having a light source unit, a condenser lens, an optical filter, and a light receiving unit as shown in Figure 7 was constructed. 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, 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, an optical system similar to that of Example 40 was constructed, and performance evaluation was similarly performed.

[0166] [Example 41] Using the optical filter of Example 1, an optical system having a light source unit, a beam splitter, an optical filter, and two light receiving units as shown in Figure 8 was constructed. 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, an optical system similar to that of Example 41 was constructed, and performance evaluation was similarly performed.

[0167] [Example 42] Using the optical filter of Example 1, an optical system having a light source unit, a light guide element, an optical filter, and a light receiving unit as shown in Figure 9 was configured. 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 set to 550 nm. [Comparative Example 42] Using the optical filter of Comparative Example 1, an optical system similar to that of Example 42 was configured, and performance evaluation was similarly performed.

[0168] [Example 43] Using the optical filter of Example 1, an optical system was constructed having a light source unit, an optical filter, and a light receiving unit, with the optical filter and the light receiving unit adjacent to each other, as shown in Figure 10. 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. [Comparative Example 43] Using the optical filter of Comparative Example 1, an optical system similar to that of Example 43 was constructed, and performance evaluation was similarly performed.

[0169] Example 44 As shown in FIG. 11 , an optical system was constructed having a light source unit, two optical filters (first and second), and two light-receiving units (first and second), with the optical filters and light-receiving units adjacent to each other. Furthermore, these adjacent components were arranged in a direction that did not overlap with the path of light from the light source unit. For this optical system, the performance of the intensity of light passing through the system was evaluated. The first optical filter was the optical filter of Example 1 (with a non-transmitting portion having a central wavelength of 550 nm), and the second optical filter was the optical filter of Example 14 (with a non-transmitting portion having a central wavelength of 1550 nm). The light source unit used a light source that continuously emitted light with wavelengths from 550 to 1550 nm. Furthermore, the first light-receiving unit was evaluated for the intensity of light with a wavelength of 550 nm, and the second light-receiving unit was evaluated for the intensity of light with a wavelength of 1550 nm. Comparative Example 44 An optical system similar to that of Example 44 was constructed using the optical filter of Comparative Example 1 (center wavelength of the non-transmitting portion: 550 nm) as the first optical filter and the optical filter of Comparative Example 14 (center wavelength of the non-transmitting portion: 1550 nm) as the second optical filter, and performance evaluation was similarly performed.

[0170] As a result of the evaluation of the optical systems described above, in all of Examples 40 to 44, the intensity of light passing through the optical system was 20 times or more greater than the results of the corresponding Comparative Examples 40 to 44. This shows the effect of light entering the light receiving section over a wide range of angles, since the wavelength characteristics of the optical filter of the present invention do not change in the case of the Examples, which are optical systems of the present invention, even if the angle of the light ray incident on the light receiving section is large. This demonstrates that the present invention provides an optical system with low loss that can select only the required wavelength.

[0171] [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).

[0172]

[0173] An optical 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 optical filter were measured in the same manner as in Comparative Example 1. The results showed that the fabricated optical 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 optical 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 in the thickness direction compared to the liquid crystal layer of the central liquid crystal layer pair in the thickness direction. From these results, the effects of the present invention are clear.

[0174] It can be suitably used as a bandpass filter or the like in various optical devices.

[0175] 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 Light receiving unit 98 Beam splitter 100 Light guide element

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 that transmits linearly polarized light in a predetermined direction, and the second polarizer is an absorptive linear polarizer that transmits linearly polarized light parallel to or perpendicular to the predetermined direction, 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 comprises at least one liquid crystal layer R1 formed by fixing horizontally aligned rod-like 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 comprises at least one liquid crystal layer R2 formed by fixing horizontally aligned rod-like liquid crystal compounds and at least one liquid crystal layer D2 formed by fixing vertically aligned discotic liquid crystal compounds, an in-plane slow axis of the liquid crystal layer R1 and an in-plane slow axis of the liquid crystal layer D1 are parallel; 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; an in-plane slow axis of the liquid crystal layer R1 and an in-plane slow axis of the liquid crystal layer R2 intersect; 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.

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 includes the liquid crystal layer R1 and the liquid crystal layer D1 of the first liquid crystal layer, and the liquid crystal layer R2 and the liquid crystal layer D2 of the second liquid crystal layer, each containing an infrared absorbing dye.

6. The optical filter of claim 1, wherein the liquid crystal polarization interference element includes liquid crystal layers R1 and D1 of the first liquid crystal layer, and liquid crystal layers R2 and D2 of the second liquid crystal layer, each containing a liquid crystal elastomer.

7. 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 that transmits linearly polarized light in a predetermined direction, and the second polarizer is an absorptive linear polarizer that transmits linearly polarized light parallel to or perpendicular to the predetermined direction, 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 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.

8. The optical filter according to claim 7, wherein the liquid crystal polarization interference element includes the liquid crystal layer R and the liquid crystal layer D containing an infrared absorbing dye.

9. The optical filter according to claim 7, wherein the liquid crystal polarization interference element includes the liquid crystal layer R and the liquid crystal layer D each containing a liquid crystal elastomer.

10. The optical filter according to claim 1, further comprising a retardation layer between at least one of the first polarizer and the second polarizer and the liquid crystal polarization interference element, and the in-plane slow axis of the retardation layer is parallel to the absorption axis of either the first polarizer or the second polarizer.

11. The optical filter according to claim 7, further comprising a retardation layer between the liquid crystal polarization interference element and at least one of the first polarizer and the second polarizer, and the in-plane slow axis of the retardation layer is parallel to the absorption axis of either the first polarizer or the second polarizer.

12. The optical filter of claim 1, wherein the liquid crystal polarization interference element has a different in-plane slow axis direction of the liquid crystal layer R1 of the first liquid crystal layer and a different in-plane slow axis direction of the liquid crystal layer R2 of the second liquid crystal layer 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.

13. The optical filter of claim 7, wherein the liquid crystal polarization interference element has a different in-plane slow axis direction of the first liquid crystal layer and a different in-plane slow axis direction of the second liquid crystal layer 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.

14. An optical system comprising a light source unit, an optical filter according to any one of claims 1 to 13, and a light receiving unit.

15. The optical system of claim 14, comprising a focusing lens.

16. The optical system of claim 14, comprising a beam splitter.

17. The optical system of claim 14, comprising a light-directing element.

18. The optical system of claim 14, wherein the optical filter and the light receiving section are adjacent to each other.

19. The optical system according to claim 18, comprising a plurality of said optical filters, the central wavelengths of the transmitted light of which are different.

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