JPH11271516A - Diffraction type color filter and liquid crystal display device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the diffraction type color filter which is easily manufactured and nearly free of limitations of material characteristics and has high diffraction efficiency and small wavelength dependency and angle dependency of the diffraction efficiency. SOLUTION: The diffraction type color filter 1 is composed of a diffraction grating and spectrally diffuses white light, made incident at a specific angle to the normal of the diffracting surface of the diffraction grating, by wavelength dispersion. In this case, the diffraction grating 1 is a linear surface relief type and the diffraction vector of the diffraction grating is parallel to the color filter surface and satisfies conditional expressions λ0 /n<Λ<2λ0 /n and 0.50 μm<λ0 <0.60 μm, where Λ is the cycles of the diffraction grating, λ0 is design wavelength, and (n) is the refractive index of a substrate provided with the diffracting surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color filter using a diffraction grating, and more particularly, to a diffraction type color filter which improves the utilization efficiency of illumination light. Further, the present invention relates to a liquid crystal display device using the diffraction type color filter.

[0002]

2. Description of the Related Art A display device using a liquid crystal display element has a configuration (three-panel type) using three display elements corresponding to R (red), G (green), and B (blue), respectively. And a configuration using a single display element (single-plate type). The single-plate type is very advantageous in terms of the size of the apparatus and the cost. 2. Description of the Related Art Conventionally, a generally known single-plate type achieves colorization by providing an absorption filter having R, G, and B patterns on a display element. That is, for example, in a pixel of R, R
Light of other colors is absorbed.

However, when an absorption filter is used, approximately ／ of the light is wasted, and there is a disadvantage that the illumination efficiency is very poor. As a method for solving this, a method using a hologram as a color filter is disclosed. According to this method, the utilization efficiency of the illumination light can be increased.

In a color filter using a hologram, a transmission hologram having a small wavelength dependence of diffraction efficiency is generally used. However, even though the wavelength dependence is small, in a wide wavelength range of R, G, and B,
In the hologram manufactured with the wavelength of G as the center wavelength, B
It is inevitable that the diffraction efficiency at R and R becomes small.
This is because the hologram has a thickness. Therefore, when white light is illuminated, there is a problem that the color balance is deteriorated due to the difference in diffraction efficiency depending on the color. This will be briefly described below.

In calculating the diffraction efficiency of a hologram,
Usually H. Kogelnik, “Coupled Wave Theory for Thick Hol
ogram Gratings ”, Bell. Syst. Tech. J., 48, 2909 (1969). According to this, when the Bragg condition is almost satisfied, diffraction of the first-order light at the design wavelength is performed. The efficiency can be approximated by sin ² (ν), so that ν = π
/ 2, the diffraction efficiency of the primary light is 100% theoretically
Becomes Here, ν depends on various parameters of the hologram. For simplicity, assuming that the lattice vector inside the hologram is parallel to the hologram plane, ν is given by the following equation.

Ν = πn ₁ d / λ cos θ (10) where λ is a design wavelength, n ₁ is a refractive index modulation amount of the hologram layer, d is a thickness of the hologram, and θ is a diffraction angle inside the hologram. It is.

In the case of a color filter using a hologram, n _1, that is, the amount of modulation of the refractive index is 0.02 to 0.02.
It is about 0.04. Therefore, it is understood that sufficient diffraction efficiency cannot be obtained unless d is increased to some extent. Si above
n ² (ν) is a formula for calculating the diffraction efficiency when the light of the design wavelength λ is incident on the hologram at the design angle, and when the wavelength or the incident angle changes, the diffraction efficiency η is given by the following formula: Is calculated.

Η = sin ² ｛(ν ² + ξ ² ) ^1/2 / (1 + ξ ² / ν ² )｝ (11) where ξ is proportional to the deviation of the wavelength or angle from the design value. It is a parameter and is also proportional to the thickness d of the hologram. Therefore, as the thickness of the hologram increases, the diffraction efficiency decreases significantly with respect to a change in wavelength or angle.

In a hologram color filter that needs to separate the R, G, and B colors into desired angle components by a diffraction grating, the diffraction cycle needs to be several times or less the wavelength. In many cases, the thickness d of the hologram is generally designed to be about 10 to 20 times the wavelength in consideration of the diffraction efficiency. For a hologram having such a thickness, see Expression (11). As you can see,
Since the contribution of ξ increases, it is theoretically difficult to construct a device that can obtain a diffraction efficiency that has almost no dependence on changes in wavelength or angle.

In order to reduce the wavelength dependence of the diffraction efficiency, Japanese Patent Application Nos. 7-170922 and 7-29 disclose the invention.
No. 0820 discloses a hologram filter in which two holograms are superimposed or multiplex-recorded. In the hologram filter, the two holograms are such that the spatial wavelength distributions of chromatic dispersion substantially match each other, but the peak wavelengths of diffraction efficiency are different from each other.

Japanese Patent Application No. Hei 7-260007 discloses a hologram filter using three thick holograms optimized for each of three wavelengths.
Since the hologram filter is configured such that each wavelength is filtered by each hologram, the wavelength dependence of diffraction efficiency is reduced.

[0012]

However, in the hologram filter in which the wavelength dependence of the diffraction efficiency is reduced in the prior art, it is necessary to superimpose or multiplex two holograms, which complicates the manufacturing method. . Alternatively, the cost increases because three holograms need to be used. Furthermore, since the thickness of the hologram filter is increased, there is a problem that the change in the diffraction efficiency with respect to the incident angle is further increased.

The characteristics of the hologram depend on the characteristics of the material and are limited by the characteristics of the material. Furthermore, in a projection display device or the like that requires the use of a high-luminance light source, there is a problem such as performance degradation due to aging.

An object of the present invention is to provide a diffraction type color filter which has a simple manufacturing method, is hardly limited by material characteristics, has high diffraction efficiency, and has little wavelength dependence and angle dependence of the diffraction efficiency. And

[0015]

In order to achieve the above object, the invention according to claim 1 comprises a diffraction grating, and a white light incident at a predetermined angle with respect to a normal to a diffraction surface of the diffraction grating. In a diffraction type color filter that disperses light by wavelength dispersion, the diffraction grating is a one-dimensional surface relief type diffraction grating, the diffraction grating vector of the diffraction grating is parallel to the color filter surface, and the period of the diffraction grating is Λ, when the design wavelength is λ ₀ and the refractive index of the substrate on which the diffraction surface is provided is n, Λ, λ ₀ is λ ₀ / n <Λ <2λ ₀ / n, 0.50
The configuration satisfies the condition of μm <λ ₀ <0.60 μm.

The diffraction type color filter having the above-mentioned structure is composed of a diffraction grating having a fine structure having a period on the order of wavelength. Conventional diffraction gratings disclosed for improving the utilization efficiency of illumination light are often volume hologram diffraction gratings using a photopolymer or gelatin dichromate.
Although relief-type diffraction gratings have also been proposed as holograms, there is no specific description of a configuration using a surface-type relief diffraction grating as in the above configuration. The optimum configuration when such a surface relief type diffraction grating is used is different from a conventional diffraction grating.

According to a second aspect of the present invention, there is provided the diffraction type color filter according to the first aspect, wherein the diffraction grating comprises
Is a binary grid of values, where the grid height h is 0.5 <h
/Λ<2.5.

The diffraction type color filter having the above-mentioned structure is composed of a diffraction grating whose grating height is as thin as about a wavelength.

According to a third aspect of the present invention, in the diffractive color filter according to the first or second aspect, the diffraction grating is configured so that substantially parallel white light is emitted by approximately 30 degrees with respect to a normal to a diffraction surface.
The configuration is such that the light is incident with a degree of inclination.

The diffractive color filter having the above configuration can be configured so that the incident angle of white light is approximately 30 degrees and the emission angles of G light are approximately the same. In such a configuration, unnecessary high-order diffracted light hardly occurs.

According to the fourth aspect of the present invention, there are provided the first to third aspects of the present invention.
In any one of the diffractive color filters described above, a cross-sectional shape of the diffraction grating is substantially rectangular.

According to a fifth aspect of the present invention, there is provided a liquid crystal display device for performing a light modulation process on an incident light beam, a white light source for emitting illumination light to be applied to the liquid crystal display device, and a light source having different wavelengths. 5. A liquid crystal display device comprising: a light beam splitting unit for splitting a light beam having a region; and a projection optical system for projecting a light beam subjected to light modulation processing by the liquid crystal display element, wherein the light beam splitting unit is any one of claims 1 to 4. And a microlens array is arranged between the diffractive color filter and the liquid crystal display element.

In the above configuration, for example, the microlens array is arranged so that the diffracted light of G is incident vertically. With this arrangement, light of each color can be focused on the corresponding pixel of the liquid crystal display element by the microlens array.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A diffraction type color filter according to the present invention and a display device using the same will be described below based on embodiments.

<First Embodiment> FIG. 1 is a cross-sectional view of a diffraction type color filter 1 comprising a one-dimensional surface relief type diffraction grating of this embodiment. Color filter 1
Its cross-sectional shape is a rectangular wave shape. h and Λ are the height and period of the grating, respectively. Assuming that λ ₀ is the design wavelength and n is the refractive index of the substrate 3 on which the diffraction surface is provided, h, Λ, and λ ₀ satisfy the following conditional expressions. λ ₀ / n <Λ <2λ ₀ / n (1) 0.50 μm <λ ₀ <0.60 μm (2) 0.5 <h / Λ <2.5... (3)

Conditional expression (1) regulates the size of the grating period. When the value exceeds the upper limit, even at the design wavelength λ ₀ ,
Higher-order light other than the 0th and 1st order is generated, and the diffraction efficiency is reduced. On the other hand, when the value exceeds the lower limit, only the zero-order light is generated and the diffracted light disappears.

Conditional expression (2) regulates the design wavelength. If the lower limit of the condition is exceeded, the reduction in the diffraction efficiency at R becomes large. In addition, when the value exceeds the upper limit, the reduction in diffraction efficiency in B becomes large.

Conditional expression (3) regulates the height of the grating. If the value exceeds the upper limit, the dependence of the diffraction efficiency on the incident angle and the wavelength becomes large. If the lower limit is exceeded, the diffraction efficiency will be very low.

The medium 2 of the two media 2 and 3 constituting the diffraction grating surface of the color filter 1 of the present embodiment is air. By using air, a high refractive index modulation amount can be secured even with a thin diffraction grating, and a high diffraction efficiency can be obtained. The refractive index of the substrate, which is the other medium 3, is n
Then, it is desirable that n satisfies the following conditional expression. n ≦ 1.8 (4) If the upper limit is exceeded, the grating period becomes small, and the production becomes difficult.

The color filter 1 of the present embodiment has a fill
The ing factor (the ratio of the width of the high refractive index portion to the low refractive index portion of the diffraction grating portion) was 0.5, the design wavelength was 0.55 μm (satisfies the conditional expression (2)), and the grating period was 0.55 μm. .

FIG. 2 shows the operation of the diffraction type color filter 1. The diffraction type color filter 1 of the present embodiment is configured to disperse the wavelength of white light incident at an incident angle of 30 ° with respect to the normal to the diffraction surface. Therefore, the diffraction type color filter 1 is arranged such that substantially parallel white light from the light source is incident at an incident angle of about 30 °. The incident white light is diffracted by the diffraction type color filter 1, and the G light is applied to the color filter 1 by about 30 times.
The light is emitted at an angle of ゜, and R and B are emitted at an angle of approximately ± 10 ° with respect to G.

FIG. 3 shows a vector diagram of G light in the diffraction direction. In the figure, ρ represents a propagation vector of incident light, σ represents a diffracted light vector, and K represents a diffraction vector. Note that | ρ
| And | σ | are 2π / λ, respectively, and | K |
、, and in the color filter 1 of the present embodiment, λ = Λ, so that | ρ | = | K |
A triangle having three sides of σ and K is an equilateral triangle.

In the case of such a vector diagram, the diffracted lights of the G and R lights are only the 0th order and the 1st order.
High-order diffracted light such as first-order or second-order light hardly occurs.
For the R light, a slightly reduced first-order diffraction efficiency of less than 100% is generated as the 0-order light, but other high-order light is hardly generated.

As for the B light, the slightly reduced primary diffraction efficiency of less than 100% corresponds to the secondary light and 0%.
Although it is generated as a secondary light, other high-order light is hardly generated. Therefore, high diffraction efficiency can be obtained for light of each wavelength of R, G, and B.

The diffraction grating having the same structure as that of the diffraction type color filter 1 has an S in a wavelength range of 400 nm to 700 nm.
The diffraction efficiency for polarized light was calculated. Note that a rigorous method based on vector analysis theory, RCWA (Rigorous Coupled Wave Analysis), was used for the calculation instead of the approximate expression.

The diffraction efficiency is such that the refractive index of the substrate of the diffraction grating is 1.
52, since the optimal value was obtained when the height of the grating was 0.88 μm, each value of the color filter 1 of the present embodiment is also configured by adopting this optimal value. That is, n = 1.
5, h = 0.88 μm. Note that the substrate refractive index n is 1.
Since it is 52, Λ in this embodiment satisfies conditional expression (1), and the height h of the grid also satisfies conditional expression (3).

FIG. 4 shows the calculated value of the wavelength (λ) dependence of the diffraction efficiency η ₁ obtained by RCWA in the diffraction type color filter 1 of this embodiment. For comparison, FIG. 5 shows a calculated value of the wavelength (λ) dependence of the diffraction efficiency η ₁ in the color filter using the conventional single-layer hologram. FIG.
Has a refractive index n = 1.52, a thickness d = 6 μm, and a wavelength λ =
The values calculated by the Kogelnik equation when optimizing at 0.55 μm are shown. 4 and 5, the color filter 1 of the present embodiment has less wavelength dependence of diffraction efficiency and higher diffraction efficiency than the conventional color filter.

FIG. 6 shows a color filter 1 of this embodiment.
7 shows the calculated value of the dependence of the diffraction efficiency η _{1 on} the incident angle (θ) at. For comparison, FIG. 7 shows a calculated value of the dependence of the diffraction efficiency η _{1 on} the incident angle (θ) in a color filter using a conventional single-layer hologram similar to that in FIG.

As is clear from FIG.
Even with a change of the incident angle of 5 °, the diffraction efficiency is greatly reduced. Therefore, the incident white light is required to have high parallelism. However, the light source has a finite size and it is difficult to make it completely parallel. On the other hand, as can be seen from FIG. 6, in the color filter 1 of the present embodiment, the dependence of the diffraction efficiency on the incident angle is not as high as in the conventional color filter, so that the size of the light source hardly matters.

The diffraction type color filter 1 of the present embodiment
Is a one-dimensional surface relief type diffraction grating, and the grating vector is parallel to the color filter surface. However, if the angle between incident light and diffracted light is approximately 60 °, The configuration is not limited. However, since such a configuration is difficult to manufacture, the configuration of the present embodiment is advantageous.

The substrate material of the diffraction type color filter 1 is as follows.
There is no particular limitation on glass, plastic, optical crystals, and the like as long as the member has a sufficient transmission range in the wavelength region to be used.

The diffraction type color filter 1 can be formed by using a lithography technique (EUV, electron beam, X-ray, etc.) using a state-of-the-art semiconductor manufacturing apparatus. Also, unlike a hologram, it is possible to mold with plastic, glass, or the like by manufacturing a mold from a master, unlike a hologram.

<Second Embodiment> FIG. 8 is a schematic structural view of a liquid crystal display device 4 according to an embodiment of the present invention. The diffraction type color filter 1 used in the liquid crystal display device 4
Has the same configuration as that of the first embodiment, and a description thereof will be omitted.

A part of the white light emitted from the light source 5 is directly reflected from the light source 5, and the rest is reflected by the reflector 6 to be converted into substantially parallel light, and then applied to the diffraction type color filter 1. The diffractive color filter 1 is arranged such that substantially parallel light from the light source 5 and the reflector 6 is incident at an incident angle of about 30 ° with respect to the normal line of the diffraction grating.

The G light diffracted by the diffraction type color filter 1 is bent in the direction of -30 ° with respect to the normal to the diffraction grating, and the R and B lights are separated by a predetermined angle with respect to the G light. Is done. The separated R, G, and B lights are incident on the liquid crystal display unit 10 which is disposed at approximately 30 degrees with respect to the diffraction vector of the color filter 1 and substantially perpendicularly with respect to the G diffracted light.

The liquid crystal display means 10 has a microlens array 7 on the side of the light source 5, and the separated R, G, and B lights are respectively transmitted to the corresponding pixels of the liquid crystal display element 8 by the microlenses of the microlens array 7. 8a.
The light that has been subjected to the light modulation processing in each pixel 8 a is transmitted to the projection optical system 9.
Is projected on the projection surface. By using the diffraction type color filter 1 like the liquid crystal display device 4 of the present embodiment, compared with the case of using the absorption type color filter,
Light utilization efficiency can be greatly improved.

It is desirable that the white light from the light source 5 be arranged such that the polarization directions thereof are aligned by a polarization conversion optical system or the like before entering the color filter 1. This is desirable because light of one polarized light is not wasted. In the case of a configuration in which the polarization directions are aligned, it is desirable that the polarization direction be S-polarization. The diffraction efficiency of the diffraction grating changes depending on the direction of the polarized light. However, in the case of a thin diffraction grating as in the present embodiment, a higher diffraction efficiency is obtained for S-polarized light. Furthermore, color filter 1
Is desirably a configuration in which an antireflection film is formed.

A corresponding absorption type color filter may be provided on the front surface of each pixel 8 of the liquid crystal display means 10. In this way, for example, it is possible to prevent light of an unnecessary color from entering the adjacent pixel 8.

[0049]

According to the present invention, a diffraction grating using a diffraction grating which is a minute diffraction grating having a period on the order of a wavelength and which has a grating height as thin as about a wavelength, while substantially satisfying the Bragg condition. A color filter can be configured. Therefore, it is possible to realize a diffraction type color filter including a diffraction grating having high diffraction efficiency and small wavelength dependence and small angle dependence of the diffraction efficiency.

Further, since the diffraction grating can be formed with a simple structure of a surface relief type without forming a diffraction grating, it can be easily formed. Furthermore, since it is a surface relief mold, it is suitable for mass production because a mold can be manufactured from a master and molded with plastic or glass. Therefore, the manufacturing cost can be reduced.

By appropriately arranging the diffractive color filter, the microlens array, and the liquid crystal display element, a bright display device with excellent color balance can be realized.

[Brief description of the drawings]

FIG. 1 is a sectional view of a diffraction type color filter according to a first embodiment.

FIG. 2 is a view showing the operation of the diffraction type color filter of the first embodiment.

FIG. 3 is a diagram showing a vector diagram of G light in a diffraction direction.

FIG. 4 is a diagram showing a calculated value of the wavelength dependence of the diffraction efficiency of the diffraction type color filter of the first embodiment.

FIG. 5 is a diagram showing a calculated value of wavelength dependence of diffraction efficiency of a color filter using a conventional single-layer hologram.

FIG. 6 is a diagram showing a calculated value of the angle dependence of the diffraction efficiency of the diffraction type color filter of the first embodiment.

FIG. 7 is a diagram showing a calculated value of the angle dependence of the diffraction efficiency of a color filter using a conventional single-layer hologram.

FIG. 8 is a schematic configuration diagram of a liquid crystal display device according to a second embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Diffraction type color filter 3 Substrate 4 Liquid crystal display device 5 Light source 6 Reflector 7 Micro lens array 8 Liquid crystal display element 9 Projection optical system

Claims (5)

[Claims] 1. A diffractive color filter comprising a diffraction grating, which disperses and separates the wavelength of white light incident at a predetermined angle with respect to the normal to the diffraction surface of the diffraction grating, wherein the diffraction grating is one-dimensional. A surface relief type diffraction grating, wherein a diffraction grating vector of the diffraction grating is parallel to a color filter surface, and a period の of the diffraction grating, a design wavelength is λ ₀ , and a refractive index of a substrate on which a diffraction surface is provided is n. , Λ, λ ₀ , λ ₀ / n <Λ <2λ ₀ / n, 0.50 μm <λ ₀ <0.6
A diffraction type color filter satisfying a conditional expression of 0 μm. 2. The diffraction grating according to claim 1, wherein the diffraction grating is a binary binary grating, and a height h of the grating satisfies a conditional expression of 0.5 <h / Λ <2.5. The diffraction type color filter according to the above. 3. The diffraction grating according to claim 1, wherein the diffraction grating is arranged such that substantially parallel white light is incident with an inclination of about 30 degrees with respect to a normal line of the diffraction surface. 3. The diffraction type color filter according to 2. 4. The diffraction type color filter according to claim 1, wherein a cross-sectional shape of the diffraction grating is substantially rectangular. 5. A liquid crystal display device that performs light modulation processing on an incident light beam, a white light source that emits illumination light to be applied to the liquid crystal display device, and divides the illumination light into light beams having different wavelength ranges. 5. A liquid crystal display device comprising a light beam splitting means and a projection optical system for projecting a light beam subjected to light modulation processing by the liquid crystal display element, wherein the light beam splitting means is a diffractive color according to any one of claims 1 to 4. A liquid crystal display device comprising: a filter; and a microlens array disposed between the diffraction type color filter and the liquid crystal display element.