Optical system

Abstract

The present invention addresses the problem of providing an optical system which uses a new filter used for a band-pass filter or the like and which can improve light receiving efficiency. The problem is solved by this optical system having a light source part, a filter, and a light receiving part, wherein the filter has at least three sets of liquid crystal layers comprising a first liquid crystal layer formed by twist-orienting and fixing a liquid crystal compound and a second liquid crystal layer the liquid crystal compound twist direction of which is opposite to that of the first liquid crystal layer, the alignment directions of the liquid crystal compounds at an interface between the first liquid crystal layer and the second liquid crystal layer are parallel, and the twist angles of the liquid crystal compounds in the first liquid crystal layer and the second liquid crystal layer are equal.

Description

Optical System

[0001] The present invention relates to an optical system having a light source section, a filter using a liquid crystal polarization interference element, and a light receiving section.

[0002] 2. Description of the Related Art Optical systems using 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 a polarization interference filter using a dielectric multilayer film, a filter combining a polarizing element and a birefringent crystal, etc. Also known is a bandpass filter, as described in Patent Document 1, in which, between polarizers arranged in a crossed Nicol configuration, birefringent plates (λ / 2 plates) of equal thickness and in which the angle between the direction of the transmission axis of the polarizer and the slow axis is +ρ and a birefringent plate in which the angle is −ρ are alternately stacked.

[0004] Furthermore, Patent Document 1 proposes an optical filter (bandpass filter) made of crystal as an optical filter with a small number of parts, which has a structure in which two different types of polarization regions are periodically arranged in the crystal, 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] Optical systems using these optical filters are used in applications such as sensing and communications. For example, in sensing applications, optical systems using optical filters are used in, for example, spectrum detectors and multispectral cameras, in communications applications, optical branching, optical combining, optical switches, amplifiers, and resonators, and in display applications, optical systems using optical filters are used in, for example, wavelength selection elements used in AR (Augumented Reality) displays.

[0006] Japanese Patent Application Laid-Open No. 2004-101577

[0007] As described above, optical systems using bandpass filters are known in a variety of configurations. However, conventional bandpass filters such as those described in Patent Document 1 have a problem in that the wavelength of light exhibiting maximum transmittance varies when light is incident from an oblique direction, resulting in a so-called wavelength shift. In particular, among conventional bandpass filters, bandpass filters known as dichroic filters are often used to separate light into two optical paths according to wavelength by transmitting only light in a specific wavelength range and reflecting other wavelengths. However, they have a drawback in that the transmitted and reflected wavelengths change when light is incident from an oblique direction.

[0008] The object of the present invention is to provide an optical system with high light receiving efficiency that uses a novel filter that is different from any of the above and can be used as a bandpass filter, etc., and in particular a novel filter that suppresses changes in the transmission wavelength or even the reflection wavelength when light is incident at an oblique angle.

[0009] To solve this problem, the present invention has the following configuration. [1] An optical system including a light source unit, a filter, and a light-receiving unit, wherein the filter includes three or more liquid crystal layer pairs in the thickness direction, each pair consisting of a first liquid crystal layer formed by fixing a liquid crystal compound that is twisted in the thickness direction, and a second liquid crystal layer formed by fixing a liquid crystal compound that is twisted in the thickness direction, the second liquid crystal layer having the liquid crystal compound twisted in the thickness direction, the twist direction of the liquid crystal compound being opposite to the twist direction of the liquid crystal compound in the first liquid crystal layer, wherein in each liquid crystal layer pair, the alignment direction of the liquid crystal compound on the surface of the first liquid crystal layer facing the second liquid crystal layer is parallel to the alignment direction of the liquid crystal compound on the surface of the second liquid crystal layer facing the first liquid crystal layer, and the optical system includes a liquid crystal polarization interference element in which the twist angle of the liquid crystal compound in the first liquid crystal layer is equal to the twist angle of the liquid crystal compound in the second liquid crystal layer. [2] The optical system according to [1], wherein the filter includes a first polarizer, a liquid crystal polarization interference element, and a second polarizer arranged in this order. [3] The optical system according to [2], wherein the transmission axis of the first polarizer and the transmission axis of the second polarizer are orthogonal to each other. [4] The optical system according to [2], wherein the transmission axis of the first polarizer and the transmission axis of the second polarizer are parallel to each other. [5] The optical system according to [3] or [4], wherein the first polarizer and the second polarizer are linear absorptive polarizers. [6] The optical system according to [3] or [4], wherein the first polarizer is a linear absorptive polarizer and the second polarizer is a linear reflective polarizer. [7] The optical system according to [6], wherein the reflective linear polarizer has a selective reflection layer including at least one cholesteric liquid crystal layer and a λ / 4 retarder. [8] The optical system according to [7], wherein the selective reflection layer has a cholesteric liquid crystal layer R formed using a rod-shaped liquid crystal compound and a cholesteric liquid crystal layer D formed using a discotic liquid crystal compound. [9] The optical system according to [7] or [8], wherein the λ / 4 retarder is made of a reverse dispersion liquid crystal compound.

[10] The optical system according to any one of [7] to [9], wherein the λ / 4 retardation plate includes a C-plate.

[11] The optical system according to [6], wherein the reflective linear polarizer is made of a dielectric multilayer film.

[12] The optical system according to [6], wherein the reflective linear polarizer is a wire grid polarizer.

[13] When the total number of stacked layers of the first liquid crystal layer and the second liquid crystal layer is N and the twist angle of the liquid crystal compound in the first liquid crystal layer and the second liquid crystal layer is ±φ [°], the following formula is used: 0.9 × (129.05 × N. -0.961 )≦|φ|≦1.1×(129.05×N -0.961 ) is satisfied.

[14] The optical system according to any one of [1] to

[13] , wherein the filter has a retardation plate between one or both of the polarizers and the liquid crystal polarization interference element, and the in-plane slow axis of the retardation plate is parallel to the absorption axis of either of the polarizers.

[15] The optical system according to any one of [1] to

[14] , wherein the optical system has a condenser lens.

[16] The optical system according to any one of [1] to

[15] , wherein the optical system has a beam splitter.

[17] The optical system according to any one of [1] to

[16] , wherein the optical system has a light-guiding element.

[18] The optical system according to any one of [1] to

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

[19] The optical system according to

[18] , wherein the optical system has a plurality of filters whose transmitted light has different center wavelengths.

[0010] According to the present invention, an optical system is provided which uses a novel filter that can be used as a bandpass filter or the like, and which has high light receiving efficiency with little wavelength shift when light is incident obliquely.

[0011] FIG. 1 is a diagram conceptually showing an example of a filter used in the optical system of the present invention. FIG. 2 is a graph for explaining a filter used in the optical system of the present invention. FIG. 3 is a graph for explaining a filter used in the optical system of the present invention. FIG. 4 is a diagram conceptually showing another example of a filter used in the optical system of the present invention. FIG. 5 is a graph for explaining the filter shown in FIG. 4. FIG. 6 is a diagram conceptually showing an example of an optical system of the present invention. FIG. 7 is a diagram conceptually showing another example of an optical system of the present invention. FIG. 8 is a diagram conceptually showing another example of an optical system of the present invention. FIG. 9 is a diagram conceptually showing another example of an optical system of the present invention. FIG. 10 is a diagram conceptually showing another example of an optical system of the present invention. FIG. 11 is a diagram conceptually showing another example of an optical system of the present invention. FIG. 12 is a diagram conceptually showing another example of an optical system of the present invention.

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The optical system of the present invention will now be described in detail with reference to preferred embodiments shown in the accompanying drawings.

[0013] 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.

[0014] The optical system of the present invention includes a light source unit, a filter, and a light receiving unit. The filter includes a liquid crystal polarization interference element, which will be described later, and preferably includes a first polarizer (first polarizer), a liquid crystal polarization interference element, and a second polarizer (second polarizer), in this order. First, the filter used in the optical system of the present invention will be described.

[0015] An example of a filter used in the optical system of the present invention is conceptually shown in Figure 1. The filter 10 shown in Figure 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 14.

[0016] In the illustrated filter 10, the first polarizer 12 and the second polarizer 14 are provided as a preferred embodiment. That is, the filter used in the optical system of the present invention may be composed of only the liquid crystal polarization interference element 16 in the illustrated filter 10, or may have only one polarizer.

[0017] The first polarizer 12 and the second polarizer 14 are polarizers (polarizing plates) that transmit linearly polarized light in a predetermined direction and are arranged in a crossed Nicol configuration with their transmission axes perpendicular to each other. In this example, the first polarizer 12 and the second polarizer 14 are both absorptive linear polarizers (absorptive linear polarizers) that have absorption axes perpendicular to their transmission axes. There are no limitations on the first polarizer 12 and the second polarizer 14, and various known linear polarizers can be used, such as iodine-based polarizers, dye-based polarizers using dichroic dyes, and polyene-based polarizers.

[0018] In the illustrated 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 laminated in contact with the liquid crystal polarization interference element 16. Furthermore, when the first polarizer 12 and the second polarizer 14 are in contact with the liquid crystal polarization interference element 16, they may be adhered to each other, if necessary, with an adhesive that is transparent to transmitted light, such as an OCA (Optical Clear Adhesive) or an acrylic pressure-sensitive adhesive.

[0019] The liquid crystal polarization interference element 16 is an optical element that acts as a λ / 2 retardation plate (½ wavelength layer) for light in a specific wavelength range (specific wavelength) but does not act as a retardation plate for other light. As described above, the first polarizer 12 and the second polarizer 14 are absorptive linear polarizers arranged in a crossed Nicol configuration with their transmission axes perpendicular to each other. Therefore, of the light incident on the filter 10, only linearly polarized light in a specific direction is transmitted through the first polarizer 12. Of this linearly polarized light, light of a specific wavelength has its polarization direction rotated by 90° by the liquid crystal polarization interference element 16 and is incident on and transmitted through the second polarizer 14 arranged in a crossed Nicol configuration with the first polarizer 12. In contrast, light outside the specific wavelength range is blocked (absorbed) by the liquid crystal polarization interference element 16 because the liquid crystal polarization interference element 16 does not act as a retardation plate and is incident on the second polarizer 14 arranged in a crossed Nicol configuration with the first polarizer 12. Due to this optical effect, the filter 10 functions as a bandpass filter that transmits only light in a specific wavelength range and blocks other light.

[0020] In the present invention, the polarizer is not limited to the above-mentioned form, and various forms can be used as long as the function of the polarizer is limited to one-way polarization. For example, when optical elements such as a light source unit (light source) and a light receiving unit (light receiving element) used in the optical system of the present invention have polarizers, the form in which polarized light is originally emitted from the light source unit used in the optical system of the present invention, and the form in which the light receiving unit used in the optical system of the present invention has one-way polarization sensitivity characteristics, etc., the polarizers of these optical elements, such as the light source unit and the light receiving unit, are also considered to be polarizers constituting the filter used in the present invention. Examples of the form in which polarized light is originally emitted from the light source unit include a polarized light source and light reflected from a substrate at a Brewster angle.

[0021] In the above example, the first polarizer 12 and the second polarizer 14, both of which are absorptive linear polarizers, are arranged in a crossed Nicol state, but the present invention is not limited to this, and various configurations can be used. One example is a configuration in which the first polarizer 12 is an absorptive linear polarizer similar to the above, the second polarizer 14 is a reflective linear polarizer (reflective linear polarizer) having a reflection axis in a direction perpendicular to the absorption axis, and both linear polarizers are arranged in a crossed Nicol state.

[0022] Even in this configuration, of the light incident on the filter 10, only linearly polarized light in a specific direction is transmitted through the first polarizer 12. As described above, 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, but does not act as a retardation plate for other light. Furthermore, the first polarizer 12 and the second polarizer 14 are arranged in a crossed Nicol configuration. Therefore, as before, the polarization direction of light of a specific wavelength that is transmitted through the first polarizer 12 is rotated by 90° by the liquid crystal polarization interference element 16, and the light is then incident on and transmitted through the second polarizer 14, which is a reflective linear polarizer.

[0023] In contrast, light outside the specific wavelength range, for which the liquid crystal polarization interference element 16 does not function as a retarder, is incident on the second polarizer 14 with its polarization direction unchanged, i.e., as linearly polarized light whose polarization direction is the transmission axis of the first polarizer 12. As described above, in this example, the second polarizer 14 is a reflective linear polarizer. Therefore, it has a reflection axis perpendicular to the transmission axis. Furthermore, the first polarizer 12 and the second polarizer 14 are arranged in a crossed Nicol configuration with their transmission axes perpendicular to each other. That is, the second polarizer 14, which is a reflective linear polarizer, has a reflection axis in the same direction as the transmission axis of the first polarizer 12. Therefore, light outside the specific wavelength range, for which the liquid crystal polarization interference element 16 does not function as a retarder, is reflected by the second polarizer 14, which is a reflective linear polarizer, with its polarization direction identical to the transmission axis of the first polarizer 12, passes through the liquid crystal polarization interference element 16 as is, and is incident on the first polarizer 12. As described above, the polarization direction of the linearly polarized light reflected by the second polarizer 14 coincides with the transmission axis of the first polarizer 12. Therefore, the linearly polarized light reflected by the second polarizer 14 passes through the first polarizer 12 as is and is extracted.

[0024] That is, in the optical system of the present invention, by using a reflective linear polarizer as the second polarizer 14s of the filter 10, i.e., the polarizer on the output side, light of a wavelength at which the liquid crystal polarization interference element 16 acts as a λ / 2 retardation plate, among light incident on the filter 10, can be extracted as transmitted light that has passed through the filter 10, and light of a wavelength at which the liquid crystal polarization interference element 16 does not act as a retardation plate can be extracted as reflected light that has been reflected by the filter 10 (second polarizer 14). In other words, in the present invention, by using a reflective linear polarizer as the second polarizer 14, the filter 10 can function not only as a bandpass filter but also as a wavelength separation element (wavelength separation filter).

[0025] In the optical system of the present invention, the reflective linear polarizer is not limited, and any known reflective linear polarizer can be used, such as a combination of a selective reflection layer containing a cholesteric liquid crystal layer and a λ / 4 retardation plate, a stretched dielectric multilayer film, or a wire grid polarizer.

[0026] As is well known, a cholesteric liquid crystal layer is a liquid crystal phase formed by fixing a cholesterically oriented cholesteric liquid crystal compound. The liquid crystal compound may be a rod-shaped liquid crystal compound, a discotic liquid crystal compound, or a combination of both. Cholesteric liquid crystal phases are known to exhibit selective reflectivity at specific wavelengths. In a typical cholesteric liquid crystal phase, the central wavelength of selective reflection (selective reflection central wavelength) λ depends on the helical pitch of the cholesteric liquid crystal phase and follows the relationship between the average refractive index n of the cholesteric liquid crystal phase and the helical pitch: λ = n × helical pitch. Therefore, the selective reflection central wavelength can be adjusted by adjusting the helical pitch. The longer the helical pitch, the longer the selective reflection central wavelength of the cholesteric liquid crystal phase. The helical pitch is one pitch (helical period) of the helical structure of the cholesteric liquid crystal phase. In other words, the helical pitch is one turn of the helix, i.e., the length of the helical axis direction in which the director of the liquid crystal compound constituting the cholesteric liquid crystal phase rotates 360°. Furthermore, cholesteric liquid crystal phases exhibit selective reflection for either left- or right-handed circularly polarized light at a specific wavelength. Whether the reflected light is right-handed or left-handed circularly polarized light depends on the helical twist direction (sense) of the cholesteric liquid crystal phase. When the helical twist direction of the cholesteric liquid crystal layer is right-handed, the cholesteric liquid crystal phase selectively reflects right-handed circularly polarized light, and when the helical twist direction is left-handed, the cholesteric liquid crystal phase reflects left-handed circularly polarized light. The direction of rotation of the cholesteric liquid crystal phase can be adjusted by the type of liquid crystal compound forming the cholesteric liquid crystal layer and / or the type of chiral agent added.

[0027] When the reflective linear polarizer serving as the second polarizer 12 is constructed using a selective reflection layer including a cholesteric liquid crystal layer and a λ / 4 retardation plate, the slow axis of the λ / 4 retardation plate is set so that the linearly polarized light transmitted through the first polarizer 12 becomes circularly polarized light in the rotation direction selectively reflected by the cholesteric liquid crystal layer. This allows the linearly polarized light transmitted through the liquid crystal polarization interference element 16 in the transmission axis direction of the first polarizer 12 to be converted by the λ / 4 retardation plate into circularly polarized light in the rotation direction selectively reflected by the cholesteric liquid crystal layer, and this circularly polarized light can be reflected by the cholesteric liquid crystal layer. The circularly polarized light reflected by the cholesteric liquid crystal layer returns to the original linearly polarized light in the transmission axis direction of the first polarizer 12 by transmitting again through the λ / 4 retardation plate. Therefore, as described above, the light passes through the liquid crystal polarization interference element 16 and then the first polarizer 12, and is emitted as reflected light by the filter 10.

[0028] The selective reflection layer constituting the reflective linear polarizer may have multiple cholesteric liquid crystal layers. For example, the selective reflection layer may have a cholesteric liquid crystal layer R formed using a rod-shaped liquid crystal compound and a cholesteric liquid crystal layer D formed using a discotic liquid crystal compound. With such a configuration, the cholesteric liquid crystal layer R containing the rod-shaped liquid crystal compound has a positive Rth, while the cholesteric liquid crystal layer D containing the discotic liquid crystal compound has a negative Rth. Therefore, the Rths of the layers cancel each other out, thereby suppressing wavelength shifts in the reflection of light incident from oblique directions. The selective reflection layer may also have multiple cholesteric liquid crystal layers that selectively reflect different wavelengths of light. For example, the selective reflection layer may have a cholesteric liquid crystal layer that selectively reflects red light, a cholesteric liquid crystal layer that selectively reflects green light, and a cholesteric liquid crystal layer that selectively reflects blue light. With such a configuration, the optical system can perform wavelength separation by reflection in response to light of various wavelengths.

[0029] In the present invention, there are no limitations on the λ / 4 retardation plate, and various known λ / 4 retardation plates (λ / 4 plate, 1 / 4 wavelength plate) having a phase difference of approximately 1 / 4 wavelength at a specific wavelength can be used. A preferred example of the λ / 4 retardation plate is a λ / 4 retardation plate having a phase difference of 100 to 180 nm at a wavelength of 550 nm, and a more preferred example is a λ / 4 retardation plate having a phase difference of 120 to 160 nm. Examples of the λ / 4 retardation plate used in the present invention include various known λ / 4 retardation plates, such as a stretched polycarbonate film, a stretched norbornene-based polymer film, a transparent film containing and oriented inorganic particles having birefringence such as strontium carbonate, a thin film formed by obliquely depositing an inorganic dielectric on a substrate, a film in which a polymerizable liquid crystal compound is uniaxially oriented and fixed in orientation, and a film in which a liquid crystal compound is uniaxially oriented and fixed in orientation.

[0030] The λ / 4 retarder is preferably made of a reverse dispersion liquid crystal compound. That is, the λ / 4 retarder preferably has reverse dispersion (reverse wavelength dispersion). Reverse dispersion refers to a phenomenon in which, when the in-plane retardation (Re) value is measured at a specific wavelength (visible light range), the Re value increases as the measured wavelength increases. It is preferable that Re(450) / Re(550)<1.00 and Re(650) / Re(550)>1.00 are satisfied. By forming the λ / 4 retarder from a reverse dispersion liquid crystal compound, the effective in-plane retardation can be more preferably λ / 4 retardation in response to a wide wavelength range.

[0031] Furthermore, the λ / 4 retardation plate preferably includes a C plate (positive C plate). A positive C plate is a retardation plate having an Re of substantially zero and a negative Rth (thickness retardation). A positive C plate can be obtained, for example, by vertically aligning a rod-shaped liquid crystalline compound. When a layer formed by immobilizing a rod-shaped liquid crystalline compound is used as the λ / 4 retardation plate, the λ / 4 retardation plate has a positive Rth. In this case, when light is incident on the λ / 4 retardation plate from an oblique direction, the Rth acts to change the polarization state of the transmitted light, which may reduce the degree of polarization of the transmitted light. In contrast, when the λ / 4 retardation plate includes a positive C plate, the change in the polarization state of obliquely incident light is suppressed, allowing the λ / 4 retardation plate to properly act on the incident light. The positive C plate may be disposed adjacent to the λ / 4 retardation plate or within the λ / 4 retardation plate.

[0032] The reflective linear polarizer is not limited to those using a cholesteric liquid crystal layer, and various known ones can be used. Examples include a film made of a dielectric multilayer film formed by stretching a layer containing two types of polymers, as described in JP 2011-053705 A, and a wire grid polarizer, as described in JP 2015-286656 A. Commercially available reflective linear polarizers can also be suitably used. Examples of commercially available reflective linear polarizers include a reflective polarizer (product name APF) manufactured by 3M, a wire grid polarizer manufactured by THORLABS, and a wire grid polarizer (product name WGF) manufactured by Asahi Kasei Corporation.

[0033] In all of the above examples, in the filter 10 having the first polarizer 12, the liquid crystal polarization interference element 16, and the second polarizer 14, the first polarizer 12 and the second polarizer 14 are arranged in a crossed Nicol configuration, in which their transmission axes are perpendicular to each other. However, the present invention is not limited to this. That is, in the optical system of the present invention, the filter 10 may be arranged in a so-called parallel Nicol configuration, in which the first polarizer 12 and the second polarizer 14 are arranged in a parallel Nicol configuration, 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 configuration, a filter 10 having the opposite effect to that when both polarizers are arranged in a crossed Nicol configuration can be realized.

[0034] For example, if the first polarizer 12 and the second polarizer 14 are both absorptive linear polarizers and both polarizers are arranged in a parallel Nicol configuration, the filter 10 functions as follows. Even in this case, of the linearly polarized light transmitted through the first polarizer 12, light with a wavelength for which the liquid crystal polarization interference element 16 acts as a λ / 2 retardation plate has its polarization direction changed by 90° and enters the second polarizer 14. Other light with wavelengths for which the liquid crystal polarization interference element 16 does not act as a retardation plate enters the second polarizer 14 with the polarization direction of the transmission axis of the first polarizer 12 unchanged. In this example, the first polarizer 12 and the second polarizer 14 are arranged in a parallel Nicol configuration. Therefore, light with a wavelength for which the liquid crystal polarization interference element 16 acts as a λ / 2 retardation plate and whose polarization direction has been changed is absorbed by the second polarizer 14. On the other hand, light in a wavelength range in which the liquid crystal polarization interference element 16 does not act as a retardation plate and whose polarization direction coincides with the transmission axis of the first polarizer 12 is transmitted through the second polarizer 14 and emitted. That is, in this case, the filter 10 does not act as a bandpass filter, but acts as a wavelength selection filter that removes light of wavelengths in which the liquid crystal polarization interference element 16 acts as a retardation plate from the incident light.

[0035] On the other hand, when the first polarizer 12 is an absorptive linear polarizer and the second polarizer 14 is a reflective linear polarizer, and both polarizers are arranged in a parallel Nicol configuration, the filter 10 functions as follows. Similarly, in this case, of the linearly polarized light transmitted through the first polarizer 12, light with a wavelength for which the liquid crystal polarization interference element 16 acts as a λ / 2 retardation plate has its polarization direction changed by 90° and enters the second polarizer 14. Other light with wavelengths for which the liquid crystal polarization interference element 16 does not act as a retardation plate enters the second polarizer 14 while maintaining the polarization direction of the transmission axis of the first polarizer 12. Here, in this example, the first polarizer 12 and the second polarizer 14 are arranged in a parallel Nicol configuration. Therefore, light with a wavelength for which the liquid crystal polarization interference element 16 acts as a λ / 2 retardation plate and whose polarization direction has been changed by 90° is reflected by the second polarizer 14, has its polarization direction restored by the liquid crystal polarization interference element 16, and enters the first polarizer 12. The polarization direction of this light is aligned with the transmission axis of the first polarizer 12, and so it is transmitted through the first polarizer 12. In contrast, light in a wavelength range in which the liquid crystal polarization interference element 16 does not function as a retardation plate and whose polarization direction is aligned with the transmission axis of the first polarizer 12 is transmitted through the second polarizer 14 and emitted. That is, in this case, the filter 10 functions not only as a bandpass filter that emits light of wavelengths for which the liquid crystal polarization interference element 16 functions as a retardation plate as reflected light, but also as a wavelength separation element (wavelength separation filter) that transmits light of other wavelengths.

[0036] As described above, various configurations are available for the first polarizer and the second polarizer that constitute the filter in the optical system 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 absorptive linear polarizers with their transmission axes arranged in a crossed Nicol configuration. However, unless otherwise specified, the following configuration also applies to the above-described configuration in which the second polarizer is a reflective linear polarizer and / or the first polarizer 12 and the second polarizer 14 are arranged in a parallel Nicol configuration.

[0037] 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.

[0038] The liquid crystal polarization interference element 16 is formed by stacking an even number of liquid crystal layers, each of which has a liquid crystal compound 18 twisted in the thickness direction. The liquid crystal compound 18 is a rod-shaped liquid crystal compound. Specifically, the liquid crystal polarization interference element 16, i.e., the filter used in the optical system of the present invention, is formed by alternately stacking first liquid crystal layers 20, each of which has a liquid crystal compound 18 twisted in the thickness direction, and second liquid crystal layers 24, each of which has a liquid crystal compound 18 twisted in the thickness direction, and in which the twist direction of the liquid crystal compound 18 is opposite to that of the first liquid crystal layer 20. The liquid crystal polarization interference element 16 has a configuration in which three or more liquid crystal layer pairs 26 are stacked in the thickness direction, each pair consisting of a first liquid crystal layer 20 and a second liquid crystal layer 24. Therefore, the total number of first liquid crystal layers 20 and second liquid crystal layers 24 is an even number.

[0039] In one liquid crystal layer set 26, the alignment direction of the liquid crystal compound 18 on the surface of the first liquid crystal layer 20 facing the second liquid crystal layer 24 is parallel to the alignment direction of the liquid crystal compound on the surface of the second liquid crystal layer 24 facing the first liquid crystal layer 20. That is, in one liquid crystal layer set 26, the alignment directions of the liquid crystal compound 18 at the interface between the first liquid crystal layer 20 and the second liquid crystal layer 24 are parallel.

[0040] In one liquid crystal layer set 26, the alignment direction of the liquid crystal compound 18 on the surface of the first liquid crystal layer 20 facing the second liquid crystal layer 24 and the alignment direction of the liquid crystal compound 18 on the surface of the second liquid crystal layer 24 facing the first liquid crystal layer 20 can be detected by obliquely cutting the liquid crystal polarization interference element 16 and analyzing the alignment direction of the liquid crystal on the cross-sectional surface. This method 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).

[0041] Furthermore, in one liquid crystal layer set 26, the twist angle of the liquid crystal compound 18 in the thickness direction in the first liquid crystal layer 20 is equal to the twist angle of the liquid crystal compound 18 in the thickness direction in the second liquid crystal layer 24. As described above, the twist directions of the liquid crystal compound 18 in the thickness direction are opposite to those in the first liquid crystal layer 20 and the second liquid crystal layer 24. That is, for example, when the twist angle of the liquid crystal compound 18 in the first liquid crystal layer 20 is φ [°], the twist angle of the liquid crystal compound 18 in the first liquid crystal layer 20 is −φ [°]. Therefore, in one liquid crystal layer set 26, the liquid crystal compound 18 is twisted in the thickness direction to a certain angle in the first liquid crystal layer 20, and then twists back to its original angle in the second liquid crystal layer 24. For example, if the twist angle of the liquid crystal compound 18 in the thickness direction is 30°, the liquid crystal compound 18 will twist from 0° to 30° in the first liquid crystal layer 20, and then twist from 30° back to 0° in the second liquid crystal layer 24.

[0042] In this example, the twist angle of the liquid crystal compound is, for example, positive (+) when it is clockwise and negative (-) when it is counterclockwise, with the direction of the transmission axis of the first polarizer 12 being 0°. That is, the absolute values ​​of the twist angles of the first liquid crystal layer 20 and the second liquid crystal layer 24 are equal.

[0043] As described above, the liquid crystal polarization interference element 16 has the liquid crystal compound 18 (rod-shaped liquid crystal compound) twisted in the thickness direction, and further has the first liquid crystal layer 20 and the second liquid crystal layer 24 alternately stacked in the thickness direction, with the liquid crystal compound 18 oriented parallel to one another at the interface, the liquid crystal compound 18 twisted in opposite directions, and the absolute value of the twist angle being equal. That is, light passing through the liquid crystal polarization interference element 16 is repeatedly influenced by a slow axis rotating by a predetermined angle in one direction and a slow axis rotating by a predetermined angle in the opposite direction. For example, if the absolute value of the twist angle of the liquid crystal compound 18 is 30°, the light passing through the liquid crystal polarization interference element 16 is repeatedly influenced by a slow axis rotating from 0° to 30° and a slow axis rotating from 30° to 0°. Therefore, in the liquid crystal polarization interference element 16, by setting the Δnd of the first liquid crystal layer 20 and the second liquid crystal layer 24 according to the wavelength range transmitted through the filter 10, and further adjusting the twist angle of the liquid crystal compound in the first liquid crystal layer 20 and the second liquid crystal layer 24 according to the total number of layers of the first liquid crystal layer 20 and the second liquid crystal layer 24, it is possible to form a liquid crystal polarization interference element 16 that acts as a λ / 2 retarder for light in a specific wavelength range and does not act as a retarder for other light, i.e., does not experience retardation. Moreover, even when light is incident from an oblique direction, such a liquid crystal polarization interference element 16 exhibits small fluctuations in the wavelength range in which it acts as a retarder (λ / 2 retarder) and the other wavelength ranges in which it does not act as a retarder, i.e., so-called wavelength shift. In other words, such a liquid crystal polarization interference element 16 exhibits small changes in filter characteristics when light is incident obliquely. Therefore, the optical system of the present invention can appropriately transmit or even reflect light in the desired wavelength range, even when light is incident obliquely.

[0044] The number of liquid crystal layer pairs 26 that the liquid crystal polarization interference element 16 has can be detected by cutting the liquid crystal polarization interference element 16 obliquely and analyzing the orientation direction of the liquid crystal on the surface of the cross section. This method is described in detail in the above-mentioned publication by Yohei Takahashi et al. Furthermore, the change in the twist direction of the liquid crystal can be determined based on the difference in components in the depth direction of the element, as it is due to the difference in the chiral agent, using a time-of-flight secondary ion mass spectrometry (TOF-SIMS) device or the like. An example of a TOF-SIMS device is the "TOF.SIMS5" manufactured by ION-TOF.

[0045] In Δnd of the first liquid crystal layer 20 and the second liquid crystal layer 24 constituting the liquid crystal polarization interference element 16, Δn is the birefringence of the liquid crystal compound 18 constituting the first liquid crystal layer 20 and the second liquid crystal layer 24. Furthermore, d is the thickness of the first liquid crystal layer 20 and the second liquid crystal layer 24. Δn can also be measured using an AxoScan manufactured by Axometrics, Inc. In the present invention, Δnd of the first liquid crystal layer 20 and the second liquid crystal layer 24 is equal. As described above, the liquid crystal polarization interference element 16 acts as a λ / 2 retardation plate only for light in a specific wavelength range. Accordingly, Δnd of the first liquid crystal layer 20 and the second liquid crystal layer 24 is set to a wavelength at which the liquid crystal polarization interference element 16 is expected to act as a λ / 2 retardation plate, i.e., half the center wavelength (half wavelength) of the wavelength range expected to pass through the filter 10. For example, assuming that the wavelength at which the liquid crystal polarization interference element 16 acts as a λ / 2 retarder, i.e., the central wavelength of the wavelength range transmitted by the filter 10, is 550 nm, Δnd of the first liquid crystal layer 20 and the second liquid crystal layer 24 is set to 275 nm. Note that Δnd of the first liquid crystal layer 20 and the second liquid crystal layer 24 may have an error of about ±10% with respect to half the central wavelength of the wavelength range transmitted by the filter 10.

[0046] Meanwhile, the twist angle of the liquid crystal compound 18 in the first liquid crystal layer 20 and the second liquid crystal layer 24 constituting the liquid crystal polarization interference element 16 is set by simulation to be an optimal twist angle at which the liquid crystal polarization interference element 16 acts as a λ / 2 retardation plate, depending on the center wavelength of the wavelength range expected to be transmitted through the filter 10 and the total number N of layers of the first liquid crystal layer 20 and the second liquid crystal layer 24. This simulation can be performed using a general optical simulation means, or it can be calculated using LCD Master 1D (manufactured by Shintech Co., Ltd., Ver. 9.8.0.0).

[0047] Here, according to simulations by the inventors, the twist angle φ of the liquid crystal compound 18 in the first liquid crystal layer 20 and the second liquid crystal layer 24 with respect to the total number of layers N of the first liquid crystal layer 20 and the second liquid crystal layer 24 is as follows: when the number of stacked layers N is 2 (one liquid crystal layer set), the twist angle φ is 63.6°; when the number of stacked layers N is 4 (two liquid crystal layer sets), the twist angle φ is 35.5°; when the number of stacked layers N is 6 (three liquid crystal layer sets), the twist angle φ is 23.6°; when the number of stacked layers N is 8 (four liquid crystal layer sets), the twist angle φ is 17.7°; when the number of stacked layers N is 10 (five liquid crystal layer sets), the twist angle φ is 14.1°; and when the number of stacked layers N is 12 (six liquid crystal layer sets), the twist angle φ is 11.8°. When the number of stacked layers N is 14 (7 liquid crystal layer sets), the optimum twist angle φ is 10.1°, and when the number of stacked layers N is 16 (8 liquid crystal layer sets), the optimum twist angle φ is 8.8°.

[0048] As conceptually shown in FIG. 2, fitting this result (solid line) with an approximate curve (dashed line) gives the following equation: φ=129.05×N -0.961 Accordingly, in the present invention, the twist angle ±φ [°] of the liquid crystal compound 18 in the first liquid crystal layer 20 and the second liquid crystal layer 24, which corresponds to the total number N of layers of the first liquid crystal layer 20 and the second liquid crystal layer 24, is given by 0.9×(129.05×N -0.961 )≦|φ|≦1.1×(129.05×N -0.961 ) and |φ|=129.05×N -0.961 It is more preferable to set it as follows.

[0049] The absolute values ​​of the twist angles of the liquid crystal compound 18 in the first liquid crystal layer 20 and the second liquid crystal layer 24 are not limited to being the same, and may have an error of ±10% or less of the absolute value of the twist angle. However, it is preferable that this difference is small, and it is most preferable that the absolute values ​​of the twist angles of the liquid crystal compound 18 in the first liquid crystal layer 20 and the second liquid crystal layer 24 are the same.

[0050] The twist angle of the liquid crystal compound 18 in the first liquid crystal layer 20 and the second liquid crystal layer 24 constituting the liquid crystal polarization interference element 16 can be detected by obliquely cutting the liquid crystal polarization interference element 16 and analyzing the alignment direction of the liquid crystal on the surface of the cross section. This method is described in detail in the above-mentioned document by Yohei Takahashi et al. The twist angle of the liquid crystal compound 18 can also be measured using AxoScan (manufactured by Axometrics) by a separation measurement means that assumes a model with input parameters.

[0051] There is no limitation on the thickness d of the first liquid crystal layer 20 and the second liquid crystal layer 24, and the thickness may be appropriately set depending on the liquid crystal compound 18 used so that Δnd is half the central wavelength of the wavelength range transmitted by the filter 10. The thickness d of the first liquid crystal layer 20 and the second liquid crystal layer 24 is preferably 1 to 5 μm, and more preferably 1 to 3 μm. The first liquid crystal layer 20 and the second liquid crystal layer 24 are usually formed using the same liquid crystal compound 18. The Δnd of the first liquid crystal layer 20 and the second liquid crystal layer 24 are also equal. Therefore, the thicknesses of the first liquid crystal layer 20 and the second liquid crystal layer 24 are equal.

[0052] There are no other limitations on the total number N of stacked first and second liquid crystal layers 20 and 24, as long as there are three or more liquid crystal layer sets 26, i.e., six or more layers, and the number is an even number. The total number N 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.

[0053] In the filter 10 used in the optical system of the present invention, the greater the total number N of layers of the first and second liquid crystal layers 20 and 24, i.e., the greater the number of liquid crystal layer pairs 26, the narrower the wavelength range in which the liquid crystal polarization interference element 16 functions as a λ / 2 retarder. Therefore, in the filter 10, the greater the total number N of layers of the first and second liquid crystal layers 20 and 24, the narrower the half-width of the wavelength range of transmitted light. In other words, the greater the total number N of layers of the first and second liquid crystal layers 20 and 24, the narrower the filter 10 can be used as a bandpass filter (such as a wavelength separation element) with a narrower transmission wavelength range. Therefore, the total number N of layers of the first and second liquid crystal layers 20 and 24, i.e., the number of liquid crystal layer pairs 26, can be selected appropriately depending on the transmission wavelength range required for the filter 10. If a wide bandwidth is desired, a smaller number of layers is selected, and if a narrow bandwidth is required, a larger number of layers is selected.

[0054] Such a liquid crystal polarization interference element 16 may be fabricated by a known method, for example, by a coating method using a liquid crystal composition for forming the first liquid crystal layer 20 and the second liquid crystal layer 24.

[0055] First, an alignment film oriented in one direction is formed on an appropriately selected support. The alignment film may be a known alignment film, such as a rubbed film made of an organic compound such as a polymer, an obliquely evaporated film of an inorganic compound, a film having microgrooves, a film formed by accumulating LB (Langmuir-Blodgett) films made of organic compounds such as ω-tricosanoic acid, dioctadecylmethylammonium chloride, and methyl stearate by the Langmuir-Blodgett method, or a film formed by applying an alignment film-forming coating liquid containing a photoalignment material to the surface of a support, drying the coating, and exposing the coating film using a polarizer such as a wire grid polarizer.

[0056] Meanwhile, a composition (liquid crystal composition) for forming the first liquid crystal layer 20 and a composition for forming the second liquid crystal layer 24 are prepared, each containing a liquid crystal compound and a chiral agent capable of inducing a twisted alignment of the liquid crystal compound in the thickness direction. The first liquid crystal layer 20 and the second liquid crystal layer 24 have opposite twist directions of the liquid crystal compound 18 in the thickness direction, but the twist direction of the liquid crystal compound in the thickness direction can be selected by selecting the chiral agent. Furthermore, the twist angle of the liquid crystal compound 18 in the thickness direction can be adjusted by adjusting the amount of chiral agent added.

[0057] The solvent for preparing the composition is not limited and can be appropriately selected depending on the purpose, but organic solvents are preferred.The organic solvent is not limited and can be appropriately selected depending on the purpose, and examples thereof 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 considering the burden on the environment.

[0058] A composition for forming the first liquid crystal layer 20 is applied to the surface of the formed alignment film to align the liquid crystal compound 18, and then dried, and if necessary, the composition is hardened by irradiation with ultraviolet light or the like to form the first liquid crystal layer 20.

[0059] Next, a composition for forming the second liquid crystal layer 24 is applied to the surface of the formed first liquid crystal layer 20, dried, and optionally cured by ultraviolet irradiation or the like to form the second liquid crystal layer 24, thereby forming a first liquid crystal layer set. When a liquid crystal layer is formed on top of the first 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. Therefore, at the interface between the first liquid crystal layer 20 and the second liquid crystal layer 24, the alignment direction of the liquid crystal compound 18 in the first liquid crystal layer 20 and the alignment direction of the liquid crystal compound 18 in the second liquid crystal layer 24 are parallel (match).

[0060] Next, a composition for forming the first liquid crystal layer 20 is applied to the surface of the formed second liquid crystal layer 24, dried, and optionally cured by ultraviolet irradiation or the like to form the first liquid crystal layer 20. In the liquid crystal polarization interference element 16 constituting the filter 10 used in the optical system of the present invention, the twist of the liquid crystal compound 18 in the thickness direction in the first liquid crystal layer 20 and the twist of the liquid crystal compound 18 in the thickness direction in the second liquid crystal layer have the same twist angle but opposite twist directions. Therefore, if the orientation angle of the liquid crystal compound 18 at the interface between the first liquid crystal layer 20 formed on the surface of the alignment film and the alignment film is set to 0°, the orientation angle of the liquid crystal compound 18 on the upper surface of the second liquid crystal layer 24 also returns to 0°. Furthermore, as described above, when a liquid crystal layer is formed on top of another liquid crystal layer by a coating method, the upper liquid crystal layer follows the orientation of the liquid crystal compound on the surface of the lower liquid crystal layer. Therefore, at the interface between the second liquid crystal layer 24 and the first liquid crystal layer 20, the alignment direction of the liquid crystal compound 18 in the second liquid crystal layer 24 and the alignment direction of the liquid crystal compound 18 in the first liquid crystal layer 20 are parallel at 0°.

[0061] Next, a second liquid crystal layer 24 is similarly formed on the surface of the formed first liquid crystal layer 20, then a first liquid crystal layer 20 is similarly formed on the surface of the formed second liquid crystal layer 24, and then a second liquid crystal layer 24 is similarly formed on the surface of the formed first liquid crystal layer 20. This process is repeated the same number of times as the number of liquid crystal layers to be formed, i.e., the number of liquid crystal layer pairs to be formed, to produce a liquid crystal polarization interference element 16. Furthermore, for example, the alignment direction of the liquid crystal compound 18 in the first formed first liquid crystal layer 20 is aligned with the transmission axis of the first polarizer 12 (angle 0°), and further, the second polarizer 14 and the first polarizer 12 are arranged in crossed Nicols (or parallel Nicols) and sandwich the liquid crystal polarization interference element 16 in the thickness direction (stacking direction), thereby producing a filter 10 as shown in FIG.

[0062] The refractive indexes of a liquid crystal layer made of rod-shaped liquid crystal compounds (rod-shaped liquid crystal layer) and a liquid crystal layer made of discotic liquid crystal compounds (disk-shaped liquid crystal layer) each have a large and small refractive index (birefringence). Here, the larger and smaller refractive indices of the rod-shaped liquid crystal layer are designated nc1 and nc2, respectively, and the larger and smaller refractive indices of the discotic liquid crystal layer are designated nd1 and nd2, respectively. From the viewpoint of suppressing unwanted reflected light, it is preferable that the values ​​of nc1 and nd2 are close to each other, and that the values ​​of nc2 and nd2 are close to each other. Specifically, the difference between these values ​​is preferably 0.05 or less. For example, values ​​such as nc1 = 1.71, nc2 = 1.55, nd1 = 1.67, and nd2 = 1.51 are preferable. Furthermore, these refractive indices can be measured optically by peeling off the liquid crystal layer. For example, after processing so that the specular reflectance from the rear surface of the liquid crystal layer becomes 0, the direction of incidence of linearly polarized light when measuring the reflection spectrum with a spectrophotometer is set parallel to the axis of each refractive index to be measured, and the angle dependency of the reflectance obtained from the measurement can be fitted to a calculation formula to determine the reflectance. An example of a spectrophotometer is an ultraviolet-visible-near-infrared spectrophotometer V-750 manufactured by JASCO Corporation.

[0063] In the liquid crystal polarization interference element 16 of the filter 10 used in the optical system of the present invention, the first liquid crystal layer 20 and the second liquid crystal layer 24 are not limited to those formed by a coating method and directly laminated as described above. That is, the liquid crystal polarization interference element 16 may be formed by alternately laminating the first and second liquid crystal layers 20 and 24 in sheet form and bonding them with an optical bonding layer transparent to transmitted light, such as OCA, an acrylic pressure-sensitive adhesive, an adhesive, or a polymer layer. In this case, it is preferable that the refractive index of the optical bonding layer be close to the refractive index of the liquid crystal in order to improve transmittance. Specifically, the difference in refractive index 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 of the liquid crystal, since this minimizes the difference in refractive index from either of the two refractive indices. However, in terms of transmittance of transmitted light, the first liquid crystal layer 20 and the second liquid crystal layer 24 are directly laminated by a coating method without an adhesive layer or the like.

[0064] In the filter 10 (liquid crystal polarization interference element 16) used in the optical system of the present invention, the liquid crystal compound 18 (rod-shaped liquid crystal compound) is not limited, and various known liquid crystal compounds can be used. Preferred rod-shaped liquid crystal compounds include azomethines, azoxys, cyanobiphenyls, cyanophenyl esters, benzoates, cyclohexanecarboxylic acid phenyl esters, cyanophenylcyclohexanes, cyano-substituted phenylpyrimidines, alkoxy-substituted phenylpyrimidines, phenyldioxanes, tolanes, and alkenylcyclohexylbenzonitriles. In addition to the above-mentioned low-molecular-weight liquid crystal molecules, polymeric liquid crystal molecules can also be used in the present invention.

[0065] The orientation of the rod-shaped liquid crystal compound is preferably fixed 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. Pat. Nos. 4,683,327, 5,622,648, 5,770,107, WO 95 / 22586, 95 / 24455, 97 / 00600, 98 / 23580, 98 / 52905, JP-A-1-272551, JP-A-6-16616, JP-A-7-110469, JP-A-11-80081, and compounds described in Japanese Patent Application No. 2001-64627 can be used. Furthermore, as the rod-shaped liquid crystal compound, for example, those described in JP-A-11-513019 and JP-A-2007-279688 can also be preferably used.

[0066] As described above, the chiral agent has the function of inducing a twisted alignment of the liquid crystal compound in the thickness direction. The twist direction or helical pitch of the helical induced by the chiral agent varies depending on the compound, so the chiral agent can be selected according to the purpose.

[0067] The chiral agent is not particularly limited, and known compounds, isosorbide, isomannide derivatives, and the like can be used. Examples of known compounds include those described in "Liquid Crystal Device Handbook, Chapter 3, Section 4-3, Chiral Agents for TN (Twisted Nematic) and STN (Super Twisted Nematic)," page 199, edited by the 142nd Committee of the Japan Society for the Promotion of Science, 1989." Isosorbide specifically refers to a chiral agent having an isosorbide structure. Chiral agents that undergo back-isomerization, dimerization, or isomerization and dimerization upon irradiation with light, resulting in a decrease in helical twisting power (HTP), can also be suitably used.

[0068] Chiral agents generally contain an asymmetric carbon atom, but axially asymmetric or planarly asymmetric compounds without an asymmetric carbon atom can also be used as chiral agents. Examples of axially asymmetric or planarly asymmetric compounds include binaphthyl, helicene, paracyclophane, and derivatives thereof. The chiral agent may have a polymerizable group. When both the chiral agent and the liquid crystal compound have a polymerizable group, a polymer having a repeating unit derived from the polymerizable liquid crystal compound and a repeating unit derived from the chiral agent can be formed by polymerization of the polymerizable chiral agent and the polymerizable liquid crystal compound. In this embodiment, the polymerizable group of the polymerizable chiral agent is preferably the same type of group as the polymerizable group of the polymerizable liquid crystal compound. Therefore, the polymerizable group of the chiral agent is preferably an unsaturated polymerizable group, an epoxy group, or an aziridinyl group, more preferably an unsaturated polymerizable group, and even more preferably an ethylenically unsaturated polymerizable group. The chiral agent may also be a liquid crystal compound.

[0069] When the chiral agent has a photoisomerizable group, it is possible to form a pattern of a desired reflection wavelength corresponding to the emission wavelength by irradiating the chiral agent with actinic rays or the like through a photomask after coating and alignment. The photoisomerizable group is preferably an isomerization site of a compound exhibiting photochromic properties, an azo group, an azoxy group, or a cinnamoyl group. Specific compounds that can be used include those described in JP-A-2002-080478, JP-A-2002-080851, JP-A-2002-179668, JP-A-2002-179669, JP-A-2002-179670, JP-A-2002-179681, JP-A-2002-179682, JP-A-2002-338575, JP-A-2002-338668, JP-A-2003-313189, and JP-A-2003-313292.

[0070] The twist angle of the liquid crystal compound 18 in the thickness direction varies depending on the amount of chiral dopant added. Therefore, by selecting the chiral dopant and appropriately setting the amount added, the twist direction and twist angle of the liquid crystal compound 18 in the first liquid crystal layer 20 and the second liquid crystal layer 24 can be set arbitrarily.

[0071] In addition to the liquid crystal compound and the chiral agent, the composition for forming the first liquid crystal layer 20 and the second liquid crystal layer 24 may contain, if necessary, a polymerization initiator, a leveling agent, a crosslinking agent, a surfactant, etc.

[0072] In the filter 10 shown in FIG. 1 , all of the first liquid crystal layers 20 are the same, and all of the second liquid crystal layers 24 are the same. That is, in the filter 10 shown in FIG. 1 , all of the first liquid crystal layers 20 have the same Δnd and the same twist angle of the liquid crystal compound 18, and all of the second liquid crystal layers 24 have the same Δnd and the same twist angle of the liquid crystal compound 18. However, the present invention is not limited to this, and the liquid crystal layers may have a distribution of Δnd and the twist angle of the liquid crystal compound 18 in the thickness direction. That is, in the filter used in the optical system of the present invention, as long as the first and second liquid crystal layers have the same Δnd, the twist directions of the liquid crystal compound 18 are opposite, and the twist angles (absolute values ​​of the twist angles) are the same, the liquid crystal layer pairs may have different Δnd and twist angles of the liquid crystal compound 18.

[0073] As an example, a configuration is exemplified in which the liquid crystal layer pair at the center in the thickness direction (stacking direction) and the liquid crystal layer pairs on both sides in the thickness direction have different Δnd and twist angles of the liquid crystal compound 18. Specifically, compared to the liquid crystal layer of the liquid crystal layer pair at the center in the thickness direction, the Δnd of the liquid crystal layer of the liquid crystal layer pairs on both sides in the thickness direction may be made larger and the twist angle of the liquid crystal compound 18 may be made smaller. As will be shown later in the Examples, as an example, when a filter (liquid crystal polarization interference element) has eight liquid crystal layers, i.e., four liquid crystal layer sets, in the first liquid crystal layer set, Δnd of the first liquid crystal layer (first layer) is set to Δnd1, the twist angle of the liquid crystal compound is set to φ1, Δnd of the second liquid crystal layer (second layer) is set to Δnd1, and the twist angle of the liquid crystal compound is set to -φ1, in the second liquid crystal layer set, Δnd of the first liquid crystal layer (third layer) is set to Δnd2 smaller than Δnd1, the twist angle of the liquid crystal compound is set to φ2 larger than φ1, Δnd of the second liquid crystal layer (fourth layer) is set to Δnd2, and the twist angle of the liquid crystal compound is set to -φ2, In the third liquid crystal layer set, the Δnd of the first liquid crystal layer (fifth layer) is Δnd2, the twist angle of the liquid crystal compound is φ2, the Δnd of the second liquid crystal layer (sixth layer) is Δnd2, and the twist angle of the liquid crystal compound is −φ2; and in the fourth liquid crystal layer set, the Δnd of the first liquid crystal layer (seventh layer) is Δnd1, the twist angle of the liquid crystal compound is φ1, the Δnd of the second liquid crystal layer (eighth layer) is Δnd1, and the twist angle of the liquid crystal compound is −φ1.

[0074] 3, a bandpass filter (wavelength separation element, etc.) generates 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, these side lobes can be reduced by increasing the Δnd of the liquid crystal layers of the liquid crystal layer pairs on both sides in the thickness direction and reducing the twist angle of liquid crystal compound 18 compared to the liquid crystal layer of the central liquid crystal layer pair in the thickness direction.

[0075] The Δnd of the liquid crystal layer can be adjusted by, for example, changing the thickness of the liquid crystal layer, but it can also be adjusted by changing the liquid crystal compound used. The twist angle of the liquid crystal compound can be adjusted by changing the type and / or amount of the chiral dopant added.

[0076] In this configuration in which the Δnd of the liquid crystal layers of the liquid crystal layer pairs on both sides in the thickness direction is larger and the twist angle of the liquid crystal compound 18 is smaller than that of the liquid crystal layer of the central liquid crystal layer pair in the thickness direction, the number of central liquid crystal layers having a larger Δnd of the liquid crystal layer and a smaller twist angle of the liquid crystal compound 18 than that of the liquid crystal layer of the both sides, i.e., the division of the liquid crystal layer pairs on both sides and the central, is not limited and may be set appropriately depending on the number of liquid crystal layers (liquid crystal layer pairs) that the filter has. Furthermore, the Δnd of the liquid crystal layers of the liquid crystal layer pairs on both sides in the thickness direction and the twist angle of the liquid crystal compound 18 of the liquid crystal layer pair on both sides in the thickness direction, as well as the Δnd of the liquid crystal layer of the central liquid crystal layer pair in the thickness direction and the twist angle of the liquid crystal compound 18 may be set by simulation to optimal Δnd and twist angle that allow the liquid crystal polarization interference element to function as a λ / 2 retarder and reduce side lobes. It is preferable to control the change in the twist angle of the liquid crystal compound 18 from both sides toward the center in the stacking direction (thickness direction) and the thickness-wise distribution of Δnd of the liquid crystal layer of the liquid crystal layer set as smoothly and precisely as possible.

[0077] 1, the liquid crystal compound 18 in each liquid crystal layer is a rod-shaped liquid crystal compound, and the liquid crystal layer is composed solely of a rod-shaped liquid crystal compound, but the present invention is not limited to this. That is, in a filter used in the optical system of the present invention, the liquid crystal layer may contain a discotic liquid crystal compound in addition to the liquid crystal compound 18, which is a rod-shaped liquid crystal compound, as in the first liquid crystal layer 32 and the second liquid crystal layer 34 of the filter 30 conceptually shown in FIG.

[0078] Specifically, in the liquid crystal polarization interference element constituting the filter used in the optical system of the present invention, the first or second liquid crystal layer may include at least one liquid crystal layer containing a rod-shaped liquid crystal compound (rod-shaped liquid crystal layer), and the first or second liquid crystal layer may include at least one liquid crystal layer containing a discotic liquid crystal compound (disctic liquid crystal layer). In this case, both the first and second liquid crystal layers may contain a rod-shaped liquid crystal layer and a discotic liquid crystal layer. Alternatively, only one of the first and second liquid crystal layers may contain a rod-shaped liquid crystal layer and a discotic liquid crystal layer, and the other may contain only a rod-shaped liquid crystal layer or only a discotic liquid crystal layer. Alternatively, one of the first and second liquid crystal layers may contain only a rod-shaped liquid crystal layer, and the other may contain only a discotic liquid crystal layer. By having a rod-shaped liquid crystal layer and a discotic liquid crystal layer in the liquid crystal polarization interference element constituting the filter, wavelength shift when light is incident from an oblique direction can be further reduced. Therefore, in an optical system that handles light rays from multiple angles, such as from the front and obliquely, it is possible to efficiently select only the desired wavelength regardless of the angle from which light is incident, which is preferable in terms of overall high light receiving efficiency.

[0079] In the following description, the liquid crystal compound 18 is also referred to as a rod-shaped liquid crystal compound 18 in order to clearly distinguish it from the discotic liquid crystal compound 40. In addition, in the filter 30 shown in Fig. 4, the same components are denoted by the same reference numerals, and the following description will mainly focus on the different components.

[0080] 4, the first liquid crystal layer 32 and the second liquid crystal layer 34 are also formed by fixing rod-shaped liquid crystal compounds 18 and discotic liquid crystal compounds 40 that are twisted in the thickness direction. Also in the filter 30, the twist directions of the liquid crystal compounds are opposite to each other in the first liquid crystal layer 32 and the second liquid crystal layer 34, but the twist angles of the liquid crystal compounds are the same. That is, the total twist angles of the rod-shaped liquid crystal compounds 18 and the discotic liquid crystal compounds 40 in the first liquid crystal layer 32 and the second liquid crystal layer 34 are in the relationship of "φ" and "-φ", as in the previous example. Furthermore, also in the filter 30, the orientation directions of the liquid crystal compounds are parallel at the interface between the first liquid crystal layer 32 and the second liquid crystal layer 34.

[0081] In the filter 30 shown in FIG. 4 , the first liquid crystal layer 32 first has rod-shaped liquid crystal compounds 18 twisted in the thickness direction, from bottom to top in the figure, followed by discotic liquid crystal compounds 40 twisted in the thickness direction. The second liquid crystal layer 34 on top of that has, conversely, the discotic liquid crystal compounds 40 twisted in the thickness direction, from bottom to top in the figure, followed by rod-shaped liquid crystal compounds 18 twisted in the thickness direction. The twisted orientations of the liquid crystal compounds in the first liquid crystal layer 32 and the second liquid crystal layer 34 are opposite to each other. The filter 30 also has a liquid crystal polarization interference element 46 in which such first liquid crystal layers 32 and second liquid crystal layers 34 are alternately stacked, and the liquid crystal polarization interference element 46 has three or more liquid crystal layer pairs, each consisting of a first liquid crystal layer 32 and a second liquid crystal layer 34. In the example shown in Figure 4, in the liquid crystal layer set 36, the first liquid crystal layer 32 is "rod-shaped liquid crystal compound / discotic liquid crystal compound" and the second liquid crystal layer 34 is "discotic liquid crystal compound / rod-shaped liquid crystal compound" in the thickness direction from bottom to top in the figure, but the present invention is not limited to this. For example, in a filter used in the optical system of the present invention, the first liquid crystal layer may be "rod-shaped liquid crystal compound / discotic liquid crystal compound" and the second liquid crystal layer may be "rod-shaped liquid crystal compound / discotic liquid crystal compound" in the thickness direction from bottom to top in the figure. Furthermore, the number, order, and thickness of the regions composed of rod-shaped liquid crystal compound 18 and the regions composed of discotic liquid crystal compound 40 may be changed as appropriate, provided that the sum of Δnd and the twist angles of the liquid crystal compounds in each liquid crystal layer remains unchanged.

[0082] 5, in a bandpass filter (such as a wavelength separation element), when light is incident from an oblique direction, a wavelength shift occurs in which the transmission wavelength range moves toward shorter wavelengths. In contrast, by having the first liquid crystal layer 32 and the second liquid crystal layer 34 include a region made of rod-shaped liquid crystal compound 18 and a region made of discotic liquid crystal compound 40, the retardation (Rth) in the thickness direction of the first liquid crystal layer 32 and the second liquid crystal layer 34 can be reduced, and the wavelength shift (coloring) caused by light incident from an oblique direction can be more effectively suppressed.

[0083] When the first liquid crystal layer 32 and the second liquid crystal layer 34 are composed of a region consisting of rod-shaped liquid crystal compound 18 and a region consisting of discotic liquid crystal compound 40, there is no limitation on the ratio of the thickness of the region consisting of rod-shaped liquid crystal compound 18 to the region consisting of discotic liquid crystal compound 40. Here, when the first liquid crystal layer 32 and the second liquid crystal layer 34 are composed of a region consisting of rod-shaped liquid crystal compound 18 and a region consisting of discotic liquid crystal compound 40, it is preferable that the Δnd of the liquid crystal layer be shared equally between the region consisting of rod-shaped liquid crystal compound 18 and the region consisting of discotic liquid crystal compound 40, depending on the Δn of the liquid crystal compound used. Furthermore, it is preferable that the Δn of rod-shaped liquid crystal compound 18 and the Δn of discotic liquid crystal compound 40 are the same from the viewpoint of reducing interfacial reflection, but different Δn values ​​may also be used.

[0084] The liquid crystal polarization interference element 46, which is composed of a liquid crystal layer having a region consisting of the rod-shaped liquid crystal compound 18 and a region consisting of the discotic liquid crystal compound 40, can be formed by a coating method using a composition for forming the region consisting of the rod-shaped liquid crystal compound 18 in the first liquid crystal layer 32, a composition for forming the region consisting of the discotic liquid crystal compound 40 in the first liquid crystal layer 32, a composition for forming the region consisting of the discotic liquid crystal compound 40 in the second liquid crystal layer 34, and a composition for forming the region consisting of the rod-shaped liquid crystal compound 18 in the second liquid crystal layer 34, as described above. When a region consisting of the discotic liquid crystal compound 40 is formed on a region consisting of the rod-shaped liquid crystal compound 18, the liquid crystal compound in the region formed on the top follows the alignment direction (longitudinal direction) of the liquid crystal compound in the region below, as described above. When a region consisting of the rod-shaped liquid crystal compound 18 is formed on a region consisting of the discotic liquid crystal compound 40, the liquid crystal compound in the region formed on the top follows the alignment direction (longitudinal direction) of the liquid crystal compound in the region below, as described above. Therefore, in a liquid crystal layer having a region made of rod-shaped liquid crystal compounds 18 and a region made of discotic liquid crystal compounds 40, the liquid crystal compounds are twisted and oriented continuously in the thickness direction within a single liquid crystal layer, and the orientation directions of the liquid crystal compounds at the interface between the first liquid crystal layer 32 and the second liquid crystal layer 34 are parallel.

[0085] As mentioned above, the present invention is not limited to direct lamination of liquid crystal layers (regions) by coating, but may also involve laminating sheet-like liquid crystal layers and attaching them with OCA or the like.

[0086] In the present invention, when the first liquid crystal layer 32 and the second liquid crystal layer 34 have a region made of the discotic liquid crystal compound 40, there is no limitation on the discotic liquid crystal compound to be used, and various known compounds can be used. As the discotic liquid crystal compound, for example, compounds described in JP-A-2007-108732 and JP-A-2010-244038 can be preferably used. When a discotic liquid crystal compound is used in the liquid crystal layer, the discotic liquid crystal compound 40 stands up in the thickness direction in the liquid crystal layer as shown in FIG. 4 , and the optical axis derived from the liquid crystal compound is defined as an axis perpendicular to the disc surface, i.e., a so-called fast axis.

[0087] Although the first liquid crystal layer 32 and the second liquid crystal layer 34 shown in FIG. 4 each have one region composed of rod-shaped liquid crystal compound 18 and one region composed of discotic liquid crystal compound 40, the present invention is not limited thereto. That is, in the present invention, when the first liquid crystal layer and the second liquid crystal layer each have a region composed of rod-shaped liquid crystal compound and a region composed of discotic liquid crystal compound, each liquid crystal layer may have multiple regions composed of rod-shaped liquid crystal compound and / or multiple regions composed of discotic liquid crystal compound. In this case, it is preferable to increase the number of layers by dividing each liquid crystal layer into smaller regions composed of rod-shaped liquid crystal compound and discotic liquid crystal compound. This reduces the difference between the front (normal) retardation and the polar angle retardation within a wide azimuthal angle range.

[0088] The twist angle and twist direction of the liquid crystal compound in the first liquid crystal layer 32 and the second liquid crystal layer 34 constituting the liquid crystal polarization interference element 46 can be detected by obliquely cutting the liquid crystal polarization interference element 46 and analyzing the alignment direction of the liquid crystal on the surface of the cross section. This method is described in detail in the above-mentioned document by Yohei Takahashi et al.

[0089] In the filter (such as a bandpass filter) used in the optical system of the present invention, the transmission axes of the crossed Nicol (parallel Nicol) polarizers arranged on either side of the liquid crystal polarization interference element 46 are preferably set at an appropriate angle to obtain the desired bandpass characteristics. As a preferred example, the angle of the transmission axis can be adjusted to reduce the magnitude of side lobes occurring at wavelengths on both sides (longer and shorter wavelength sides) of the main bandpass wavelength and to equalize the magnitude of the side lobes on the longer and shorter wavelength sides.

[0090] Furthermore, in the filter used in the optical system of the present invention, a retarder may be provided between one or both sides of the crossed-Nicol polarizers arranged on either side of the liquid crystal polarization interference element 46 and the liquid crystal polarization interference element 46. This retarder has the effect of maintaining the orthogonal relationship of the polarization directions of the linear polarizers arranged in the crossed-Nicol configuration not only in the frontal direction but also in the oblique direction off-axis of the polarizers. This allows for the same excellent bandpass characteristics to be obtained in the oblique direction as in the frontal direction. By aligning the in-plane slow axis of the retarder parallel to the absorption axis of one of the pair of crossed-Nicol polarizers, the polarization state can be compensated to maintain the orthogonal relationship of the polarization directions in the oblique direction without affecting the frontal direction. Examples of retarders include a positive C plate formed by vertically oriented rod-shaped liquid crystals, a positive A plate formed by horizontally oriented rod-shaped liquid crystals, a negative C plate formed by discotic liquid crystals, a negative A plate formed by discotic liquid crystals, and combinations of these retarders, such as a combination of a positive C plate formed by vertically oriented rod-shaped liquid crystals and a positive A plate formed by horizontally oriented rod-shaped liquid crystals. Furthermore, a B plate, which is a biaxial refractive index plate, can also be used as the retardation plate. The B plate preferably has an Nz factor of 0.1 to 0.9.

[0091] In the filter used in the optical system of the present invention, 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 first liquid crystal layer and the second liquid crystal layer, the liquid crystal wavelength dispersion in the liquid crystal layers 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 first liquid crystal layer and the second liquid crystal layer to make the liquid crystal wavelength dispersion in the liquid crystal layers a strong forward dispersion, a bandpass filter with a narrower transmission wavelength range can be obtained.

[0092] 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. Among these, dichroic dyes are preferred. Infrared light refers to, for example, light with a wavelength of 700 to 900 m. 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.

[0093] There is no limitation on the amount of the infrared absorbing dye added to the first and second liquid crystal layers, and it may be set appropriately depending on the width of the transmission wavelength range required for the bandpass filter.

[0094] In the filter used in the optical system of the present invention, the first and second liquid crystal layers may contain a liquid crystal elastomer. The first and second liquid crystal layers containing a liquid crystal elastomer may be formed using a liquid crystal elastomer, or may be formed of a normal liquid crystal compound other than an elastomer and then contain a liquid crystal elastomer.

[0095] In this way, by including a liquid crystal elastomer in the first and second liquid crystal layers, the first and second liquid crystal layers can be made elastic, and the thickness of the liquid crystal layer can be changed by stretching or shrinking the filter in the plane direction. By changing the thickness of the liquid crystal layer, the Δnd of the liquid crystal layer can be changed. As a result, in the bandpass filter, it is possible to change the wavelength range of light transmitted through the filter. In other words, by including a liquid crystal elastomer in the first and second liquid crystal layers, the wavelength range can be changed by stretching and shrinking the liquid crystal layer, i.e., the filter, and active wavelength control is possible in the bandpass filter.

[0096] 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 chiral agent, a crosslinking agent, 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.

[0097] When the first and second liquid crystal layers 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.

[0098] A filter using such a liquid crystal polarization interference element can be used at any wavelength, i.e., the optical system of the present invention using such a filter can be used for any electromagnetic wave, such as ultraviolet light, visible light, infrared light, terahertz waves, and millimeter waves.

[0099] As described above, the optical system of the present invention includes a light source unit, a filter using such a liquid crystal polarization interference element, and a light receiving unit. In such an optical system of the present invention, for example, by using the above-mentioned filter as a bandpass filter, a wavelength separation element, or the like, an optical system with little loss of received light can be realized.

[0100] 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.

[0101] Various optical elements can be incorporated into the optical system of the present invention. Figure 6 conceptually shows an example of combining a condenser lens with the optical system of the present invention. The optical system shown in Figure 6 has a light source unit 90, a condenser lens 92, the above-mentioned filter 30, and a light receiving unit 96. Note that various known condenser lenses can be used.

[0102] In the optical system of the present invention shown in Figure 6, the filter using the liquid crystal polarization interference element is the filter 30 shown in Figure 4. However, the optical system of the present invention is not limited to this. In other words, the optical system of the present invention can use various filters that use the liquid crystal polarization interference element described above, such as the filter 10 shown in Figure 1 and a filter composed only of a liquid crystal polarization interference element. In this respect, the same applies to the optical systems of the present invention described below.

[0103] In the optical system shown in FIG. 6 , divergent light emitted by a light source 90 is collected by a collecting lens 92 and received by a light receiving unit 96 for photometry. In such an optical system, the above-described filter 30 (bandpass filter) is disposed at the light divergence or collection unit. In the illustrated example, the above-described filter 30 is disposed at the light collection unit. As described above, the filter used in the optical system of the present invention exhibits little wavelength shift when light is incident from an oblique direction and exhibits the same wavelength bandpass performance for light incident from both the front and oblique directions. Therefore, this filter can collect light of desired wavelengths over a wide angular range at the light receiving unit. This allows the optical system of the present invention to achieve high light receiving efficiency and little light reception loss. The optical system of the present invention using a collecting lens in this manner can be used, for example, in an imaging system. This imaging system can achieve high light receiving efficiency when light emitted from a light source unit 90, which is the object to be imaged, is collected by a collecting lens and highly collected on the light receiving unit 96, which is an imaging element. Furthermore, because the filter used in the present invention has a wide bandpass performance, using a focusing lens with a high numerical aperture makes it possible to realize a thin, compact optical system. 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 constructing a system that achieves high efficiency similar to that described above. Specifically, the light from the optical fiber output terminal and / or the light focused by a lens contains a mixture of light from various angles. Therefore, the optical system 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.

[0104] As described above, this optical system can also suitably use a filter 30 in which the second polarizer 14 is a reflective linear polarizer. In this case, as conceptually shown in Figure 11, the filter 30 is disposed at an angle to the optical axis of the condenser lens 92, and a light receiving unit 96a for reflected light is provided at a position where the light reflected by the filter 30 is collected. In this way, both the transmitted light and the reflected light, which have been wavelength-separated by the filter 30, can be received by the light receiving unit and measured.

[0105] As shown in Figures 6 and 11 , 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 16 in the filter used in the optical system of the present invention exhibits very little wavelength shift due to oblique incidence of light. That is, in the optical system of the present invention, the filter 30 has little angle dependency, and therefore functions as a 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. That is, in the optical system of the present invention, the filter 30 functions as a bandpass filter and wavelength separation element that acts appropriately on specific wavelengths for light incident at various angles through the lens, as well as for diverging and condensing light. This angle dependency of the filter 30 also applies to various embodiments described below that do not include a lens.

[0106] FIG. 7 conceptually illustrates an example of an optical system incorporating a beam splitter according to the present invention. The optical system shown in FIG. 7 includes a light source 90, a beam splitter 98, the filter 30 described above, 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 then received by the corresponding light-receiving units 96 for photometry. In this optical system, the filter 30 (bandpass filter) described above is disposed in the region after the split light. The filter 30 exhibits bandpass performance for the same wavelength for split light traveling in different angular directions. As a result, high light-receiving efficiency is achieved in each of the multiple light-receiving units 96 that receive the split light. The optical system of the present invention, which uses the beam splitter 98 in this manner, 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.

[0107] As described above, this optical system can also suitably use a filter 30 in which the second polarizer 14 is a reflective linear polarizer. In this case, as conceptually shown in Fig. 12, a light receiving unit 96a for reflected light is provided in the optical path of light reflected by the filter 30. This allows both transmitted light and reflected light, which have been wavelength-separated by the filter 30, to be received by the light receiving unit and measured.

[0108] FIG. 8 conceptually illustrates an example of combining a light-guiding element (light guide plate) with the optical system of the present invention. The optical system illustrated in FIG. 8 includes a light source unit 90, a light-guiding element 100, the filter 30 described above, 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, the filter 30 (bandpass filter) described above is disposed at the exit position of the propagating light from the light-guiding element 100. The filter 30 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.

[0109] In the optical system of the present invention, the above-described filter 30 and the light receiving unit 96 may be provided adjacent to each other, as conceptually shown in FIG. 9 . In the optical system shown in FIG. 9 , divergent light emitted from the light source unit 90 reaches the above-described filter 30 (bandpass filter) from various angular directions. The filter 30's wide-angle bandpass performance allows light of the 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 the optical system of the present invention using the above-described filter 30 allows the filter 30 to exhibit the desired bandpass performance over a wide angle, which is effective in reducing the thickness of the system.

[0110] Furthermore, in the optical system of the present invention in which a filter and a light receiving unit are adjacent to each other, multiple filters with different center wavelengths of transmitted light may be arranged. FIG. 10 shows an example. The optical system shown in FIG. 10 is similar to the example shown in FIG. 9 in that the filter (bandpass filter) used provides 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 filters 30a and 30b with different center wavelengths of transmitted light, and multiple light receiving units 96 corresponding to each filter. This allows the optical system shown in FIG. 10 to simultaneously receive light of different wavelengths. Note that filters 30a and 30b basically have the same configuration and effects as filter 30. Specifically, the optical system of the present invention shown in FIG. 10 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 Figure 10 uses two filters with different center wavelengths of transmitted light, the optical system of the present invention is not limited to this. 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 filters with different center wavelengths of transmitted light, i.e., the number of wavelengths to be measured. Furthermore, the filter used in the optical system of the present invention may be a single filter capable of handling multiple wavelengths, formed by patterning multiple liquid crystal polarization interference elements with different transmission center wavelengths in-plane. In this case, a pattern with different retardation values ​​is formed, which can be achieved by varying either the film thickness or the birefringence of the liquid crystal layer in-plane. The patterning may be either discrete or continuous.

[0111] It should be noted that each of the above-described optical systems can also be used with a configuration in which the first polarizer 12 and the second polarizer 14 in the filter are arranged in parallel Nicols. In other words, when each of the above-described optical systems uses a filter with the first polarizer 12 and the second polarizer 14 arranged in parallel Nicols, the transmission and absorption / reflection actions of the filter are reversed between the wavelength at which the liquid crystal polarization diffraction element acts as a λ / 2 retardation plate and other wavelengths, as described above.

[0112] The optical system of the present invention has been described in detail above, but the present invention is not limited to the above-described examples, and various improvements and modifications may be made without departing from the spirit of the present invention.

[0113] 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.

[0114] <Fabrication of Bandpass Filter 1 (Corresponding to Examples)> [Preparation of Compositions] The following compositions C-1, C-2, D-1, and D-2 were prepared as liquid crystal compositions for forming a liquid crystal layer in which the liquid crystal compound is twisted and aligned in the thickness direction. In each composition, "C" indicates that the liquid crystal compound is primarily a rod-shaped liquid crystal compound, and "D" indicates that the liquid crystal compound is primarily a discotic liquid crystal compound. In each composition, "1" indicates that the chiral dopant induces a right-handed twist in the liquid crystal compound, and "2" indicates that the chiral dopant induces a left-handed twist in the liquid crystal compound. Therefore, composition C-1 is a liquid crystal composition containing a rod-shaped liquid crystal compound as the main component and forming a liquid crystal layer in which the twist direction of the liquid crystal compound in the thickness direction is right-handed; composition C-2 is a liquid crystal composition containing a rod-shaped liquid crystal compound as the main component and forming a liquid crystal layer in which the twist direction of the liquid crystal compound in the thickness direction is left-handed; composition D-1 is a liquid crystal composition containing a discotic liquid crystal compound as the main component and forming a liquid crystal layer in which the twist direction of the liquid crystal compound in the thickness direction is right-handed; and composition D-2 is a liquid crystal composition containing a discotic liquid crystal compound as the main component and forming a liquid crystal layer in which the twist direction of the liquid crystal compound in the thickness direction is left-handed.

[0115] Composition C-1 ----------------------------------- Rod-shaped liquid crystal compound L-1 100.00 parts by mass Chiral agent Ch-A 0.058 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 Co., Ltd.) 1.00 parts by mass Leveling agent T-1 0.08 parts by mass Methyl ethyl ketone 2000.00 parts by mass

[0116] Composition C-2 ----------------------------------- Rod-shaped liquid crystal compound L-1 100.00 parts by mass Chiral agent Ch-B 0.099 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 Co., Ltd.) 1.00 part by mass Leveling agent T-1 0.08 parts by mass Methyl ethyl ketone 2000.00 parts by mass

[0117] Composition D-1 --------------------------------------------------- Discotic liquid crystal compound L-2 80.00 parts by mass Discotic liquid crystal compound L-3 20.00 parts by mass Polymerization initiator (Irgacure (registered trademark) 907, manufactured by BASF) 5.00 parts by mass Megafac F444 (manufactured by DIC Corporation) 0.50 parts by mass Chiral agent Ch-2 0.033 parts by mass Methyl ethyl ketone 300.00 parts by mass

[0118] Composition D-2 --------------------------------------------------- Discotic liquid crystal compound L-2 80.00 parts by mass Discotic liquid crystal compound L-3 20.00 parts by mass Polymerization initiator (Irgacure (registered trademark) 907, manufactured by BASF) 5.00 parts by mass Megafac F444 (manufactured by DIC Corporation) 0.50 parts by mass Chiral agent Ch-3 0.033 parts by mass Methyl ethyl ketone 300.00 parts by mass

[0119] Rod-shaped liquid crystal compound L-1

[0120] Discotic liquid crystal compound L-2 Discotic liquid crystal compound L-3

[0121] Leveling agent T-1

[0122] Chiral agent Ch-A Chiral agent Ch-B Chiral agent Ch-2 Chiral agent Ch-3

[0123] (Formation of Alignment Film) A glass substrate was prepared as a support. The following coating liquid for forming an alignment film was applied onto the support by spin coating. The support on which the coating film of the coating liquid for forming an alignment film had been formed was dried on a hot plate at 60°C for 60 seconds to form an alignment film P-1.

[0124] 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 ---------------------------------------------------

[0125] Material for photo alignment

[0126] (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 transmission 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 support, and the clockwise direction is taken as positive.

[0127] A first liquid crystal layer was formed by applying composition C-1 to this alignment film P-1. That is, composition C-1 was first applied to the alignment film P-1, heated, and then cured with ultraviolet light to prepare a liquid crystal fixing layer. More specifically, the liquid crystal fixing layer was prepared by applying composition C-1 to the alignment film P-1 to obtain a coating film, heating this coating film to 80°C on a hot plate, and then irradiating the coating film with ultraviolet light of 365 nm at 300 mJ / cm using a high-pressure mercury lamp in a nitrogen atmosphere at 80°C. 2 The coating film was irradiated with a radiation dose of 1000 rad / s at 1000 rad / s to fix the alignment of the liquid crystal compound, thereby forming a first liquid crystal layer. The thickness of the first liquid crystal layer was 0.86 μm. A 0.86 μm thick region of a discotic liquid crystal compound was formed thereon using Composition D-1 to form a first liquid crystal layer. A 0.86 μm thick region of a discotic liquid crystal compound was formed on the first liquid crystal layer using Composition D-2, and a 0.86 μm thick region of a rod-shaped liquid crystal compound was formed on the first liquid crystal layer to form a second liquid crystal layer set. This liquid crystal layer set formation was repeated four times to form a liquid crystal polarization interference element having eight liquid crystal layers (four liquid crystal layer sets) as shown in Table 1 below (see FIG. 4 ).

[0128]

[0129] The Δn and the twist angle φ of the liquid crystal compound were measured using an AxoScan (manufactured by Axometrics). The twist angle φ of the liquid crystal compound is the angle with respect to the direction of the transmission axis of the wire grid polarizer used to expose the alignment film, and the clockwise direction is defined as positive. The same applies to the following examples.

[0130] The alignment film was peeled off from the liquid crystal polarization interference element thus produced, and a linear polarizer was placed on one side of the stacking direction of the liquid crystal polarization interference element so that its transmission axis coincided with the direction of the linearly polarized light irradiated onto the alignment film, and another linear polarizer was placed on the other side of the stacking direction so as to be in a crossed Nicol configuration, thereby producing a bandpass filter 1 as shown in Fig. 4. The linear polarizer used was a polarizer configured by laminating transparent protective films on both the front and back surfaces of a polyvinyl alcohol film that had been oriented by adsorption of iodine.

[0131] The maximum transmittance, central wavelength (central wavelength of transmitted light) showing the maximum transmittance, and half-width of the transmitted light of the fabricated bandpass filter 1 were measured using a spectroradiometer SR-3 manufactured by Topcon Technohouse Co., Ltd. The results were that the maximum transmittance was 99%, the central wavelength of the transmitted light was 550 nm, and the half-width of the transmitted light was 120 nm.

[0132] Furthermore, for the fabricated bandpass filter 1, a spectroradiometer SR-3 manufactured by Topcon Technohouse Corporation was used to measure the wavelength shift (absolute value) when light was incident at a polar angle of 60° relative to when light was incident at a polar angle of 90°. Incident light from a polar angle of 60° was measured from two directions, at azimuth angles of 0° and 90°, and the average value was taken as the measured value. As a result, the wavelength shift of the center wavelength of the transmitted light was less than 5 nm.

[0133] <Preparation of Bandpass Filter 2 (Corresponding to Comparative Example)> (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. Composition B-1 ----------------------------------- Rod-shaped liquid crystal compound L-1 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 below: 0.08 parts by mass Methyl ethyl ketone 2000.00 parts by mass

[0134] Rod-shaped liquid crystal compound L-1

[0135] Leveling agent T-1

[0136] 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.

[0137] 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.

[0138] 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.

[0139] The fabricated liquid crystal polarization interference element was placed between polarizers arranged in a crossed Nicol configuration to produce bandpass filter 2. Bandpass filter 2 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 fabricated bandpass filter 2 was measured for maximum transmittance, center wavelength and half-width of transmitted light, and wavelength shift of the center wavelength in the same manner as bandpass filter 1. As a result, the fabricated bandpass filter 2 had a maximum transmittance of 99%, a center wavelength of transmitted light of 550 nm, and a half-width of 120 nm. The wavelength shift of the center wavelength was also 90 nm.

[0140] The results of measuring the characteristics of this bandpass filter indicate that, compared to a case in which the liquid crystal compound in the first and second liquid crystal layers does not have a twisted orientation in the thickness direction, as in bandpass filter 2 (corresponding to a comparative example), the liquid crystal compound in the first and second liquid crystal layers has a twisted orientation in the thickness direction, as in bandpass filter 1 (corresponding to an example), thereby making it possible to reduce wavelength shift when light is obliquely incident. These features of bandpass filter 1 have the effect of increasing light-receiving efficiency in various optical systems shown in the following examples.

[0141] [Preparation of Absorptive Linear Polarizer] An absorptive linear polarizer was prepared by the following procedure. (Preparation of Cellulose Acylate Film 1) <Preparation of Core Layer Cellulose Acylate Dope> The following composition was charged into a mixing tank and stirred to dissolve each component, thereby preparing a cellulose acetate solution to be used as the core layer cellulose acylate dope. ------------------------------------------------ Core Layer Cellulose Acylate Dope ---------------------------------------------------------------- Cellulose acetate having an acetyl substitution degree of 2.88: 100 parts by mass Polyester compound B described in the examples of JP 2015-227955 A: 12 parts by mass Compound F below: 430 parts by mass Methylene chloride (first solvent): 64 parts by mass Methanol (second solvent): ------------------------------------------------

[0142] Compound F

[0143] <Preparation of Outer Layer Cellulose Acylate Dope> 10 parts by mass of the following matting agent solution was added to 90 parts by mass of the prepared core layer cellulose acylate dope to prepare a cellulose acetate solution to be used as the outer layer cellulose acylate dope.

[0144] Matting agent solution - Silica particles with an average particle size of 20 nm (AEROSIL R972, manufactured by Nippon Aerosil Co., Ltd.) 2 parts by mass - Methylene chloride (first solvent) 76 parts by mass - Methanol (second solvent) 11 parts by mass - The above-mentioned cellulose acylate dope for the core layer 1 part by mass -

[0145] <Preparation of Cellulose Acylate Film 1> The prepared core layer cellulose acylate dope and outer layer cellulose acylate dope were filtered through a filter paper with an average pore size of 34 μm and a sintered metal filter with an average pore size of 10 μm. Then, the core layer cellulose acylate dope and the outer layer cellulose acylate dope on both sides were simultaneously cast onto a drum at 20°C from a casting nozzle (band caster). The film was then peeled off while the solvent content was approximately 20% by mass, and both ends of the film in the width direction were fixed with tenter clips. The film was then stretched in the transverse direction at a stretch ratio of 1.1 times while being dried. The film was then transported between the rolls of a heat treatment device and further dried to prepare an optical film with a thickness of 40 μm, designated as Cellulose Acylate Film 1. The Re of the obtained Cellulose Acylate Film 1 was 0 nm.

[0146] (Formation of Photo-Alignment Layer PA1) The coating liquid S-PA-1 for forming an alignment layer, which will be described later, was continuously applied onto the cellulose acylate film 1 using a wire bar. The support on which the coating film was formed was dried with hot air at 140°C for 120 seconds. Subsequently, the coating film was irradiated with polarized ultraviolet light (10 mJ / cm 2 A photo-alignment layer PA1 was formed by irradiating the substrate with light (using an ultra-high pressure mercury lamp). The film thickness was 0.3 μm.

[0147] ------------------------------------------------------------------ (Coating liquid for forming alignment layer S-PA-1) ------------------------------------------------------------------ Polymer M-PA-1 (shown below) 100.00 parts by mass Acid generator PAG-1 (shown below) 5.00 parts by mass Acid generator CPI-110TF (shown below) 0.005 parts by mass Xylene 1220.00 parts by mass Methyl isobutyl ketone 122.00 parts by mass

[0148] Polymer M-PA-1

[0149] Acid generator PAG-1

[0150] Acid generator CPI-110TF

[0151] (Formation of Optically Absorbent Anisotropic Layer P1) On the obtained photo-alignment layer PA1, the following coating solution S-P-1 for forming an optically absorbent anisotropic layer was continuously applied using a wire bar to form a coating layer. The formed coating layer was then heated at 140°C for 30 seconds and cooled to room temperature (23°C). Next, it was heated at 90°C for 60 seconds and cooled again to room temperature. Thereafter, an LED lamp (center wavelength 365 nm) was used to apply the coating solution S-P-1 to form an optically absorbent anisotropic layer. The coating layer was then heated at 140°C for 30 seconds and cooled to room temperature again. The coating layer was then heated at 90°C for 60 seconds and cooled again to room temperature. After that, an LED lamp (center wavelength 365 nm) was used to apply the coating solution S-P-1 to form an optically absorbent anisotropic layer. The coating layer was then heated at 90°C for 60 seconds and cooled again to room temperature. The coating layer was then heated at an illuminance of 200 mW / cm. ２ The optically absorptive anisotropic layer P1 was formed on the photo-alignment layer PA1 by irradiating the layer for 2 seconds under the irradiation conditions of 1.6 μm.

[0152] 0.25 parts by mass of dichroic substance D-1 shown below 0.36 parts by mass of dichroic substance D-2 shown below 0.59 parts by mass of dichroic substance D-3 shown below 2.21 parts by mass of polymer liquid crystal compound M-P-1 shown below 1.36 parts by mass of low molecular weight liquid crystal compound M-1 shown below 0.200 parts by mass of polymerization initiator IRGACURE OXE-02 (manufactured by BASF) 0.026 parts by mass of surfactant F-3 shown below Cyclopentanone 46.00 parts by mass Tetrahydrofuran 46.00 parts by mass Benzyl alcohol 3.00 parts by mass

[0153] Dichroic substance D-1 Dichroic substance D-2 Dichroic substance D-3

[0154] Polymer liquid crystal compound M-P-1

[0155] Low molecular liquid crystal compound M-1

[0156] Surfactant F-3

[0157] In this manner, an absorptive linear polarizer was produced.

[0158] (Preparation of Reflective Linear Polarizer) A reflective linear polarizer was prepared as follows.

[0159] [Preparation of Coating Liquid for Reflective Layer] [Coating Liquid R-1 for Reflective Layer] The composition shown below was stirred and dissolved in a container kept at 70° C. to prepare Coating Liquid R-1 for Reflective Layer. Unlike Composition (C) of the bandpass filter, in the coating liquid for the reflective layer, R represents a coating liquid using a rod-like liquid crystal compound.

[0160] -------------------------------------------------- Coating liquid R-1 for reflective layer -------------------------------------------------- Methyl ethyl ketone 120.9 parts by mass Cyclohexanone 21.3 parts by mass Mixture X of the following rod-shaped liquid crystal compounds 100.0 parts by mass Photopolymerization initiator B 1.00 part by mass Chiral agent A 4.18 parts by mass Surfactant F1 0.1 part by mass -------------------------------------------------- Mixture X of rod-shaped liquid crystal compounds

[0161] In the above mixture X, the numerical values ​​are in mass %. R is a group bonded via an oxygen atom. Furthermore, the average molar absorption coefficient of the above rod-shaped liquid crystal compound in the wavelength range of 300 to 400 nm was 140 / mol cm.

[0162] Chiral agent A

[0163] Surfactant F1

[0164] Photopolymerization initiator B

[0165] The chiral agent A is a chiral agent whose helical twisting power (HTP) is reduced by light.

[0166] [Reflective Layer Coating Solution R-2] The reflective layer coating solution R-2 was prepared in the same manner as the reflective layer coating solution R-1, except that the amount of chiral agent A added was changed as shown in Table 2 below.

[0167]

[0168] [Coating Solution D-1 for Reflective Layer] The composition shown below was stirred and dissolved in a container kept at 50° C. to prepare Coating Solution D-1 for Reflective Layer. Here, as with the composition of the bandpass filter, D represents a coating solution using a discotic liquid crystal compound.

[0169] -------------------------------------------------- Coating liquid D-1 for reflective layer -------------------------------------------------- 80 parts by mass of discotic liquid crystal compound (A) below 20 parts by mass of discotic liquid crystal compound (B) below 10 parts by mass of polymerizable monomer E1 below 0.3 parts by mass of surfactant F2 below 3 parts by mass of photopolymerization initiator (Irgacure 907, manufactured by BASF) 5.45 parts by mass of the chiral agent A above Methyl ethyl ketone 290 parts by mass Cyclohexanone 50 parts by mass --------------------------------------------------

[0170] Discotic Liquid Crystal Compound (A) Discotic Liquid Crystal Compound (B)

[0171] Polymerizable Monomer E1

[0172] Surfactant F2

[0173] [Reflective Layer Coating Solutions D-2 and D-3] These were prepared in the same manner as Reflective Layer Coating Solution D-1, except that the amount of chiral agent A added was changed as shown in Table 3 below.

[0174]

[0175] [Preparation of Reflective Circular Polarizer 1 (Selective Reflection Layer)] A 100 μm thick PET film (A4265, manufactured by Toyobo Co., Ltd.) was prepared as a temporary support, and the PET surface on which the easy-adhesive layer was not formed was subjected to a rubbing treatment. The prepared reflective layer coating solution R-1 was applied using a wire bar coater, and then dried at 110°C for 72 seconds. Thereafter, the reflective circular polarizer was subjected to a rubbing treatment at 100°C under a low-oxygen atmosphere (100 ppm or less) with an illuminance of 80 mW / cm. 2 , irradiation amount 500mJ / cm 2 The coating was then cured by irradiating it with light from a metal halide lamp at a discharge rate of 150 W min / m to form a first blue light reflective layer (first cholesteric liquid crystal layer) made of a cholesteric liquid crystal layer. The light was irradiated from the cholesteric liquid crystal layer side. The coating thickness was adjusted so that the film thickness of the first blue light reflective layer after curing would be 2.6 μm. Next, the surface of the first blue light reflective layer was irradiated with light from a metal halide lamp at a discharge rate of 150 W min / m. 2 After the corona treatment, the reflective layer coating liquid D-1 was applied onto the corona treated surface using a wire bar coater.

[0176] Subsequently, the coating film was dried at 70°C for 2 minutes, and after the solvent was evaporated, it was heat-aged at 115°C for 3 minutes to obtain a uniformly oriented state. Thereafter, the coating film was kept at 45°C and irradiated with ultraviolet light (300 mJ / cm) using a metal halide lamp in a nitrogen atmosphere. 2 ) and cured to form a second blue light reflective layer (second cholesteric liquid crystal layer) on the first blue light reflective layer. Light was irradiated from the cholesteric liquid crystal layer side. At this time, the coating thickness was adjusted so that the film thickness of the second blue light reflective layer after curing was 2.0 μm.

[0177] Next, the reflective layer coating solution D-2 was applied onto the second blue light reflective layer using a wire bar coater. Subsequently, the coating film was dried at 70°C for 2 minutes, and after the solvent was evaporated, it was heat-aged at 115°C for 3 minutes to obtain a uniformly oriented state. Thereafter, this coating film was kept at 45°C and irradiated with ultraviolet light (300 mJ / cm) using a metal halide lamp under a nitrogen atmosphere. 2 ) and cured to form a green light reflective layer (third cholesteric liquid crystal layer) on the second blue light reflective layer. Light was irradiated from the cholesteric liquid crystal layer side. At this time, the coating thickness was adjusted so that the film thickness of the green light reflective layer after curing was 2.7 μm.

[0178] Next, the reflective layer coating solution R-2 was applied onto the green light reflective layer using a wire bar coater, and then dried at 110°C for 72 seconds. Thereafter, the coating solution was dried in a low-oxygen atmosphere (100 ppm or less) at 100°C with an illuminance of 80 mW and an irradiation dose of 500 mJ / cm. 2 The coating was cured by irradiating it with light from a metal halide lamp (1000 W / m²), thereby forming a red light reflective layer (fourth cholesteric liquid crystal layer) on the green light reflective layer. The light was irradiated from the cholesteric liquid crystal layer side. The coating thickness was adjusted so that the red light reflective layer after curing had a thickness of 3.4 μm.

[0179] Next, the red light reflecting layer surface was subjected to a discharge of 150 W·min / m 2 After corona treatment at 70°C, the reflective layer coating solution D-3 was applied to the corona-treated surface using a wire bar coater. Subsequently, the coating film was dried at 70°C for 2 minutes, and after the solvent was evaporated, it was heat-aged at 115°C for 3 minutes to obtain a uniform alignment state. Thereafter, this coating film was kept at 45°C and irradiated with ultraviolet light (300 mJ / cm) using a metal halide lamp under a nitrogen atmosphere. 2 ) and cured to form a yellow light reflective layer (fifth cholesteric liquid crystal layer) on the red light reflective layer. Light was irradiated from the cholesteric liquid crystal layer side. At this time, the coating thickness was adjusted so that the film thickness of the yellow light reflective layer after curing was 3.4 μm.

[0180] By the above procedure, a reflective circular polarizer 1 (selective reflection layer) having the first to fifth cholesteric liquid crystal layers in this order was obtained.

[0181] Table 4 shows the central reflection wavelength and film thickness of each cholesteric liquid crystal layer of the produced reflective circular polarizer 1. Here, the central reflection wavelength shown in Table 4 corresponds to the central wavelength of the reflected light of the above-mentioned cholesteric liquid crystal layer. The central reflection wavelength (central wavelength of the reflected light) was confirmed by creating a film in which each cholesteric liquid crystal layer was coated in a single layer. The film thickness was confirmed using an SEM.

[0182]

[0183] In this manner, a reflective circular polarizer 1 (selective reflection layer) was produced.

[0184] [Preparation of λ / 4 Retardation Plate] A λ / 4 retardation plate was prepared by the following procedure.

[0185] [Preparation of Retardation Plate 1] Reverse wavelength dispersion retardation plate 1 was prepared with reference to the method described in paragraphs 0151 to 0163 of JP 2020-084070 A. Retardation plate 1 had Re = 146 nm and Rth = 73 nm.

[0186] [Preparation of Positive C Plate 1] Positive C Plate 1 was prepared by adjusting the film thickness with reference to the method described in paragraphs 0132 to 0134 of JP 2016-053709 A. However, the support was changed from a polyethylene terephthalate film (PET film) to a triacetyl cellulose film (TAC film). Positive C Plate 1 had Re = 0.1 nm and Rth = -80 nm.

[0187] The prepared retardation plate 1 and a positive C plate 1 were laminated together to prepare a λ / 4 retardation plate.

[0188] [Preparation of Reflective Linear Polarizer] The prepared λ / 4 retardation plate and reflective circular polarizer 1 (selective reflection layer) were laminated in this order to prepare a reflective linear polarizer. The transmission axis of the reflective linear polarizer was set at an angle shifted by 45° from the slow axis of retardation plate 1.

[0189] [Preparation of Bandpass Filter 3 (Wavelength Separation Element)] Bandpass filter 3 was prepared by arranging an absorptive linear polarizer, a liquid crystal polarization interference element, and a reflective linear polarizer in this order. The same liquid crystal polarization interference element as bandpass filter 1 (corresponding to the example) was used. The surface of the reflective linear polarizer facing the liquid crystal polarization interference element was the surface of retarder 1. The transmission axes of the absorptive linear polarizer and the reflective linear polarizer were perpendicular to each other. Bandpass filter 3 was prepared 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 of the liquid crystal polarization interference element were stacked. The transmittance of the prepared bandpass filter 3 was measured. Incident light was incident from the absorptive linear polarizer. The wavelength (center wavelength) and half-width showing maximum transmittance, as well as the wavelength shift and side lobe, were measured using a spectroradiometer "SR-3" manufactured by Topcon Technohouse Corporation. In addition, reflectance was also measured, similar to transmittance. Similarly to bandpass filter 1, the wavelength shift (absolute value) was measured when light was incident at a polar angle of 60° relative to when it was incident at a polar angle of 90°. Incident light from a polar angle of 60° was measured from two directions: azimuth angles of 0° and 90°, and the average was taken as the measured value. Transmittance measurements showed that the fabricated bandpass filter 3 transmitted light with a center wavelength of 550 nm, a half-width of 120 nm, a wavelength shift of less than 5 nm, and a side lobe of 10%. The size of the side lobe is the ratio of the side lobe transmittance to the transmittance of the center wavelength. Reflectance measurements also showed opposite characteristics to transmittance, with non-transmitted light reflected by the reflective linear polarizer on the exit side and exiting from the entrance side. Non-transmitted light refers to the component whose polarization is not changed by the liquid crystal layer. This demonstrated that wavelength shift in a bandpass filter (wavelength separation element) when light is incident obliquely can be significantly suppressed.

[0190] [Fabrication of Bandpass Filter 4 (Wavelength Separation Element)] Bandpass Filter 4 was fabricated in the same manner as Bandpass Filter 3, except that the reflective linear polarizer in Bandpass Filter 3 was replaced with a broadband dielectric multilayer film (manufactured by 3M, product name: APF). The fabricated Bandpass Filter 4 was subjected to the same measurements as Bandpass Filter 3. The results showed that the center wavelength of the transmitted light of the fabricated bandpass filter was 550 nm, the half-width was 120 nm, the wavelength shift was less than 5 nm, and the side lobes were 10%. Furthermore, the reflectance was measured and found to have the opposite characteristics to the transmittance, with non-transmitted light being reflected by the reflective polarizer on the exit side and exiting from the entrance side. The non-transmitted light is the component whose polarization is not changed by the liquid crystal layer. This demonstrated that the wavelength shift of the bandpass filter (wavelength separation element) when light is incident obliquely can be significantly suppressed.

[0191] [Fabrication of Bandpass Filter 5 (Wavelength Separation Element)] Bandpass filter 5 was fabricated in the same manner as bandpass filter 3, except that the reflective linear polarizer in bandpass filter 3 was replaced with a wire grid polarizer manufactured by THORLABS. The fabricated bandpass filter 5 was subjected to the same measurements as bandpass filter 3. As a result, the center wavelength of the transmitted light of the fabricated bandpass filter was 550 nm, the half-width was 120 nm, the wavelength shift was less than 5 nm, and the side lobes were 10%. Furthermore, the reflectance was measured, and the results showed opposite characteristics to the transmittance, with non-transmitted light being reflected by the reflective polarizer on the exit side and exiting from the entrance side. The non-transmitted light is the component whose polarization is not changed by the liquid crystal layer. This demonstrated that the wavelength shift of the bandpass filter (wavelength separation element) when light is incident obliquely can be significantly suppressed.

[0192] For bandpass filters 1, 3 to 5, bandpass filters were prepared in which the linear polarizer was arranged in parallel Nicols rather than in crossed Nicols, and the characteristics were measured in the same manner. The transmission axis of the linear polarizer was aligned with the transmission axis of the linear polarizer on the light incident side. As a result, in the embodiment corresponding to bandpass filter 1, light except for that near 550 nm was transmitted, and in the embodiments corresponding to bandpass filters 3 to 5, the center wavelength of the reflected light was 550 nm, and the wavelengths of the transmitted light and reflected light were reversed. Furthermore, in all bandpass filters, for light of each wavelength, characteristics similar to those of bandpass filters in which the polarizers were arranged in crossed Nicols were obtained.

[0193] EXAMPLES The optical performance of the optical system of the present invention was evaluated by optical simulation (Camerium Lighting Simulator CAD) as follows.

[0194] [Examples 1 and 1-2] Using the fabricated bandpass filter 1, an optical system including a light source unit, a condenser lens, a bandpass filter, and a light-receiving unit as shown in FIG. 6 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. Furthermore, in Example 1-2, an optical system as shown in FIG. 11 was constructed using a bandpass filter 3 that uses a reflective linear polarizer, and evaluation was performed. [Comparative Example 1] An optical system similar to that of Example 1 was constructed using a bandpass filter 2, and performance evaluation was similarly performed.

[0195] [Example 2, Example 2-2] Using the fabricated bandpass filter 1, an optical system having a light source unit, a beam splitter, a bandpass filter, and two light receiving units as shown in FIG. 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, and the distance between the two light receiving units was 30 mm. Furthermore, in Example 2-2, an optical system as shown in FIG. 12 was constructed using a bandpass filter 3 that uses a reflective linear polarizer, and evaluation was performed. [Comparative Example 2] An optical system similar to that of Example 2 was constructed using bandpass filter 2, and performance evaluation was similarly performed.

[0196] [Example 3] Using the fabricated bandpass filter 1, an optical system having a light source unit, a light guide element, a bandpass filter, and a light receiving unit, 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. [Comparative Example 3] Using bandpass filter 2, an optical system similar to that of Example 3 was constructed, and performance evaluation was similarly performed.

[0197] [Example 4] Using the fabricated bandpass filter 1, an optical system was constructed as shown in Figure 9, which had a light source unit, a bandpass filter, and a light receiving unit, with the bandpass filter and the light receiving unit adjacent to each other. 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 4] Using bandpass filter 2, an optical system similar to that of Example 4 was constructed, and performance evaluation was similarly performed.

[0198] Example 5: Bandpass filter 3 was fabricated by changing the central wavelength of transmitted light to 1550 nm by thickening the first and second liquid crystal layers of the liquid crystal polarization interference element in bandpass filter 1. As described above, the central wavelength of transmitted light through bandpass filter 1 is 550 nm. Using the fabricated bandpass filters 1 and 3, an optical system was constructed as shown in FIG. 10 , including a light source unit, two bandpass filters (first and second), and two light-receiving units (first and second), with the bandpass filters and light-receiving units adjacent to each other. Furthermore, these adjacent configurations were arranged in a direction that did not overlap with the path of light from the light source unit. The performance of the intensity of light passing through this optical system was evaluated. The light source unit used a light source that continuously emitted light with wavelengths from 550 to 1550 nm. Furthermore, the intensity of light at a wavelength of 550 nm was evaluated for the first light-receiving unit, and the intensity of light at a wavelength of 1550 nm was evaluated for the second light-receiving unit. Comparative Example 5 Bandpass filter 4 was produced in bandpass filter 2, with the center wavelength of transmitted light set to 1550 nm, in the same manner as bandpass filter 1. Bandpass filter 2 and bandpass filter 4 were used to configure an optical system similar to that in Example 5, and performance evaluation was similarly performed.

[0199] As a result of the evaluation of the optical systems described above, in all of Examples 1 to 5, the intensity of light passing through the optical systems was 20 times or more higher than the results for the corresponding Comparative Examples 1 to 5. This demonstrates the effect of light entering the light receiving section over a wide angular range in the Examples, which are optical systems according to the present invention, because the bandpass performance of the bandpass filter, i.e., the filter of the present invention, remains unchanged even when the angle of the light ray incident on the light receiving section is large. Furthermore, in Examples 1-2 and 1-3, it was also possible to receive reflected light from the bandpass filter 3. This demonstrates that the present invention provides an optical system with minimal light reception loss. The above results clearly demonstrate the effectiveness of the present invention.

[0200] As an optical system having the function of a bandpass filter, it can be suitably used in various optical devices.

[0201] 10, 30, 94a, 94b Filter 12 First polarizer 14 Second polarizer 16, 46 Liquid crystal polarization interference element 18 Liquid crystal compound (rod-shaped liquid crystal compound) 20, 32 First liquid crystal layer 24, 34 Second liquid crystal layer 26, 36 Liquid crystal layer set 40 Discotic liquid crystal compound 90 Light source unit 93 Condenser lens 96, 96a Light receiving unit 98 Beam splitter 100 Light guide element

Claims

1. An optical system having a light source unit, a filter, and a light receiving unit, wherein the filter includes a liquid crystal layer assembly including three or more sets in the thickness direction, the liquid crystal layer assembly including a first liquid crystal layer formed by fixing a liquid crystal compound twisted and oriented in the thickness direction, and a second liquid crystal layer formed by fixing a liquid crystal compound twisted and oriented in the thickness direction, wherein a twisting direction of the liquid crystal compound in the second liquid crystal layer is opposite to a twisting direction of the liquid crystal compound in the first liquid crystal layer, and in the liquid crystal layer assembly, an alignment direction of the liquid crystal compound on a surface of the first liquid crystal layer on the second liquid crystal layer side is parallel to an alignment direction of the liquid crystal compound on a surface of the second liquid crystal layer on the first liquid crystal layer side, and a twisting angle of the liquid crystal compound in the first liquid crystal layer is equal to a twisting angle of the liquid crystal compound in the second liquid crystal layer, and having a liquid crystal polarization interference element.

2. The optical system according to claim 1, wherein the filter arranges a first polarizer, the liquid crystal polarization interference element, and a second polarizer in this order.

3. The optical system according to claim 2, wherein a transmission axis of the first polarizer and a transmission axis of the second polarizer are orthogonal to each other.

4. The optical system according to claim 2, wherein a transmission axis of the first polarizer and a transmission axis of the second polarizer are parallel to each other.

5. The optical system according to claim 3 or 4, wherein the first polarizer and the second polarizer are absorption type linear polarizers.

6. The optical system according to claim 3 or 4, wherein the first polarizer is an absorption type linear polarizer and the second polarizer is a reflection type linear polarizer.

7. The optical system according to claim 6, wherein the reflection type linear polarizer has a selective reflection layer including at least one cholesteric liquid crystal layer and a λ / 4 retardation plate.

8. The optical system according to claim 7, wherein the selective reflection layer has a cholesteric liquid crystal layer R formed using a rod-shaped liquid crystal compound and a cholesteric liquid crystal layer D formed using a disc-shaped liquid crystal compound.

9. The optical system according to claim 7, wherein the λ / 4 retardation plate is made of an inverse dispersion type liquid crystal compound.

10. The optical system according to claim 7, wherein the λ / 4 retardation plate includes a C plate.

11. The optical system according to claim 6, wherein the reflection type linear polarizer is made of a dielectric multilayer film.

12. The optical system according to claim 6, wherein the reflection type linear polarizer is a wire grid polarizer.

13. When the total number of stacked layers of the first liquid crystal layer and the second liquid crystal layer is N, and the twist angle of the liquid crystal compound in the first liquid crystal layer and the second liquid crystal layer is ±φ [°], the following formula 0.9×(129.05×N -0.961 ) ≤ |φ| ≤ 1.1×(129.05×N -0.961 ) is satisfied. The optical system according to claim 1 or 2.

14. The optical system according to claim 1 or 2, wherein the filter has a retardation plate between one or both of the polarizers and the liquid crystal polarization interference element, and a slow axis in the plane of the retardation plate is parallel to an absorption axis of any one of the polarizers.

15. The optical system according to claim 1 or 2, which has a condenser lens.

16. The optical system according to claim 1 or 2, which has a beam splitter.

17. The optical system according to claim 1 or 2, which has a light guide element.

18. The optical system according to claim 1 or 2, wherein the filter and the light receiving portion are adjacent to each other.

19. The optical system according to claim 18, which has a plurality of the filters having different center wavelengths of transmitted light.

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