JPH0515247B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a light modulation element, an observation device using the same, and a projection type display device, and in particular uses a phase-type diffraction grating and a variable refractive index material such as liquid crystal. The present invention relates to a light modulation element that performs light modulation such as passing light and blocking light, and is suitable for use in displays and light valves in viewfinder systems such as cameras.

(Prior Art) A conventionally well-known light modulation element includes a pair of polarizing plates disposed so that the polarization directions are perpendicular to each other, and a pair of transparent substrates disposed between the pair of polarizing plates facing each other. It consists of an element in which liquid crystal is sealed with the substrate surface subjected to alignment treatment perpendicular to each other, and the alignment state of this liquid crystal is switched between a twisted state and a state perpendicular to the substrate surface to modulate the incident light. The so-called TN
There is a display element using a type of liquid crystal.

This type of display element has a simple structure and is easy to drive, so it is used in a wide variety of applications. However, since it uses two polarizing plates to transmit and block the light beam, the transmittance when transmitting light is low. It could not be said to be a desirable light modulation element from the viewpoint of luminous flux utilization efficiency.

In addition, as a similar type of display element using liquid crystal, there is a so-called guest-host mode liquid crystal element that uses a dye mixed into liquid crystal molecules, but since the dye is present in this display element as well, the transmission during extinction decreases. The rate was at best around 70%.

On the other hand, Japanese Patent Publication No. 53-3928 and US Pat. No. 4,251,137 disclose display elements and variable subtractive color filter elements in which a reflection type or transmission type phase type diffraction grating is combined with a liquid crystal. The elements disclosed in these documents have high luminous flux utilization efficiency and are useful as display elements in camera viewfinders, light valves, and the like.

However, when this type of light modulation element is placed, for example, near the focal plane of a camera's finder system, and a highly bright object (such as a mercury lamp) is observed through the finder system, the light from the bright object becomes When the light is incident on a portion of the diffraction grating section that is displaying, a rainbow-like diffraction image is generated in other regions of the same diffraction grating section, which has the disadvantage of degrading the quality of the display element.

(Problems to be Solved by the Invention) When the present invention uses a diffraction grating to perform display by transmitting or blocking light, light from a high-luminance object is incident on the diffraction grating portion as a display pattern. The object of the present invention is to provide a high-quality light modulation element that prevents a rainbow-like diffraction image from occurring in the display pattern of the diffraction grating section, and an observation device and a projection type display device using the same.

(Means for Solving the Problem) In an optical modulation element having a diffraction grating section, when the thickness of the substrate provided with the diffraction grating section is t and the refractive index is n, the grating lines of the diffraction grating section are The length w in the direction perpendicular to is set to w<4·t·tan {sin ⁻¹ (1/n)}.

(Example) FIG. 1 is an explanatory diagram of the basic configuration of a light modulation element 10 according to the present invention. In the figure, reference numeral 1 denotes a refractive index variable material, which is made of, for example, liquid crystal. 2 is a relief-type diffraction grating made of a material that is transparent to the wavelength used; 3
4 is a transparent electrode, 4 is a transparent substrate made of a transparent optical member, 5 is incident light having arbitrary polarization characteristics, 6a and 6b are mutually orthogonal polarization components of the incident light 5, and 6a is a direction perpendicular to the plane of the paper. The polarized light component 6b indicates a polarized light component in a direction parallel to the plane of the paper orthogonal to the polarized light component 6a.

This light modulation element 10 has transparent electrodes 3 formed on opposing surfaces of a pair of transparent substrates 4, and a relief-type diffraction grating 2 made of a transparent material on one of the transparent electrodes 3 of the pair of transparent substrates 4. is provided, and the grooves (concavities) of the diffraction grating 2 are filled with the refractive index variable material 1. Then, by applying an electric field through the transparent electrode 3, the refractive index of the variable refractive index material 1 is changed.

Next, the principle of operation of the light modulation element 10 shown in FIG. 1 will be explained using an example in which liquid crystal is used as the variable refractive index material 1.

In FIG. 1, it is assumed that in a so-called static state in which no electric field is applied, the liquid crystal 1 is oriented in the direction of the grooves of the diffraction grating 2, that is, in the direction perpendicular to the plane of the paper, and maintains a homogeneous orientation state. Of the polarization components 6a and 6b of the incident light 5 that enters the light modulation element 10 in a static state, the polarization component 6b, which is a component perpendicular to the orientation direction of the liquid crystal 1, has an ordinary refractive index of the liquid crystal 1.
The polarized light component _6a , which is a component parallel to the alignment direction of the liquid crystal 1, senses the extraordinary refractive index n _e of the liquid crystal 1. Here, the refractive index of the material forming the diffraction grating 2 is n _g ,
If the wavelength of the incident light 5 is λ and the thickness of the diffraction grating 2 is T, then when the diffraction grating is rectangular, the diffraction efficiency of the zero-order transmitted diffracted light for each of the polarization components 6a and 6b of the incident light 5 is η _p is roughly expressed by the following equation (1).

η _p 1/2 {1+cos(2π・Δn・T/λ)}……(1
) However, Δn is the refractive index difference between the refractive index n _g of the diffraction grating 2 and the refractive index n _p to ne of the liquid crystal ₁ , and in the state of homogeneous alignment, for the polarization component 6a of the incident light 5, Δn=| n _e −n _g |, and for the polarization component 6b, Δn=|n _p −n _g |.

Therefore, from equation (1), when Δn=0, that is, when n _e =n _g or n _p =n _g , the diffraction efficiency η o of the zero-order transmitted diffracted light becomes η _p =1.

Also, ΔnT=(m+1/2)λ, (m=0, 12,
3,...), the diffraction efficiency η _p becomes η _p =0.

Next, when an electric field is applied to the liquid crystal 1 through the transparent electrode 3, the alignment direction (optical axis direction) of the liquid crystal 1 gradually changes. At this time, the polarization component 6 in the incident light 5
b is the ordinary refractive index of liquid crystal 1 regardless of the application of an electric field.
I feel n _p .

On the other hand, the polarized light component 6a senses a composite refractive index n〓, which is a combination of the extraordinary refractive index n _e and the ordinary refractive index n _p of the liquid crystal 1 at a predetermined ratio, as the amount of applied electric field increases. Here, the composite refractive index n〓 changes as the alignment direction of the liquid crystal 1 changes. When the amount of applied electric field is further increased, the liquid crystal 1 is aligned perpendicularly to the substrate 4 (transparent electrode 3) and becomes in a homeotropic alignment state, and the polarized light components 6a and 6b of the incident light 5 both sense the ordinary refractive index n _p of the liquid crystal 1. saturate. Even in this state, the incident light 5 is (1)
Modulated according to Eq.

FIG. 2 shows that the light modulation element 10 is placed near the focal plane of the finder system, and when a high-luminance object is observed through the finder system, a rainbow-shaped diffraction image is generated in the light modulation region, that is, the diffraction grating. FIG. 2 is an explanatory diagram of factors that reduce the performance of a display element.

In this figure, the same elements as those shown in FIG. Because it is thin, illustration is omitted.

In the same figure, the incident light 5 that has entered the diffraction grating portion 2a of the diffraction grating 2 is optically modulated by the difference in refractive index between the liquid crystal 1 and the diffraction grating 2. generates diffracted light of order.
Among these diffracted lights, the zero-order and low-order diffraction lights 7 with relatively small diffraction angles are emitted as they are from the transparent substrate 4, but some high-order diffraction lights are emitted at the interface between the transparent substrate 4 and air. The light may be totally reflected and enter the diffraction grating section 2a again. The light that has entered the diffraction grating section 2a again is diffracted again, and a part of the diffracted light 7' is
It is emitted to the outside of the transparent substrate 4 with a certain diffraction angle φ,
When it enters the entrance pupil of the observation system, it is recognized as an image. At this time, since the diffraction angle differs depending on the wavelength, the re-incidence position on the diffraction grating section 2a also differs depending on the wavelength, and a deviation in the position is recognized as a rainbow.

Therefore, in this embodiment, the width of the grating line in the normal direction of the diffraction grating part 2a is
It is assumed that the diffracted light is incident on approximately the center of a, and the diffracted light is totally reflected on another surface where the diffraction grating portion 2a is not formed, for example, the surface of the transparent substrate 4 that is in contact with air, and then The length is such that the light does not re-enter the region of , thereby preventing the generation of a rainbow-like diffraction image from the diffraction grating portion 2a.

In FIG. 2, transparent diffraction light was explained as an example, but the same applies to reflected diffraction light.

FIG. 3 is an explanatory diagram of an embodiment in which the light modulation element 10 according to the present invention is applied to a part of a viewfinder system of a camera. In the figure, 10 is a light modulation element, 31 is a flip-up mirror, 32 is a focusing plate, and 3
3 is a condenser lens, 34 is a pentaprism, and 35 is an eyepiece lens. The light transmitted through a photographing lens (not shown) is guided to a finder optical system by a flip-up mirror 31, and is imaged on the focusing surface of a focusing plate 32. The light emitted from the focusing plate 32 is diffused with an intensity according to the F number of the photographic lens and the diffusion characteristics of the lens, and a portion of the light is transmitted to the condenser lens 33, pentaprism 34, and eyepiece lens 35.
reaches the human eye through The light that actually enters the eye is limited by the pupil diameter of the eye, and in the case of a single-lens reflex camera, the light that actually enters the eye is normally set at an aperture angle of about 3° centered on the optical axis on the focusing plate 32. The light is emitted within the range.

The light modulation element 10 in this embodiment is the focusing plate 3
The display/non-display state is selected by a driving device (not shown). In the non-display state, the light modulation element 10 is regarded as a transparent substrate with a uniform refractive index, and the subject image formed on the focal plane is not modulated but is transmitted through the condenser lens 33, pentaprism 34, and eyepiece 35. The image is directly formed on the retina of the eye. In the display state, a portion of the light incident on the light modulation element 10 is diffracted at the display pattern portion made up of the diffraction grating portion. A component of the diffracted light with a large diffraction angle is blown out of the field of view of the eye, so that a portion of the subject light is visually perceived as being attenuated, and the display may overlap with the subject image.

FIG. 4 is a plan view of the light modulation element 10 used in FIG. 3. In the same figure, 4-1, 4-2, 4-
3, 4-4 each have a diffraction grating section, L41, L4
2 is the width in the normal direction of the grating lines of each diffraction grating portion. The direction of the grating portion in each diffraction grating portion is parallel to the longitudinal direction of the region.

The light modulation element 10 shown in the same figure is arranged near the focal plane of the finder system shown in FIG.
If 0 is optically modulated, the diffraction grating parts 2-1, 2-2, 2-3, and 2-4 will be shaded within the field of view of the finder system, forming a rectangular pattern image, which will be observed overlapping the subject image and displayed on the display element. It comes to function as.

FIG. 5 is a cross-sectional view showing the diffraction state of incident light that has entered a part of the diffraction grating section 4-4 of the light modulation element 10 of FIG. In this figure, the same elements as those shown in FIG. 1 are given the same reference numerals. Also transparent substrate 4
The transparent electrode disposed between and the diffraction grating 2 is omitted because it is very thin.

In FIG. 5, t is the thickness of the transparent substrate 4, w is the width in the normal direction of the grating lines of the diffraction grating section 4-4, and φ
is the diffraction angle of the diffracted light in the medium, and 1 is the distance from the incident position of the incident light 5 to the re-diffraction image, that is, the position where the rainbow is generated.

The incident light 5 entering the light modulation element 10 is diffracted due to the difference in refractive index between the liquid crystal 1 and the diffraction grating 2, and diffracted light of a plurality of orders is generated in a plane orthogonal to the grating direction of the diffraction grating. The diffraction angle φ of the diffracted light is the total reflection condition at the interface between the transparent substrate 4 and air (n・sin φ>1;
When n satisfies the refractive index of the transparent substrate 4), total reflection occurs, and when the diffracted light at this time enters the diffraction grating portion again, it is diffracted again, and a rainbow-shaped diffraction image is generated in that area. At this time, the position where the rainbow-shaped diffraction image is generated is determined by the diffraction angle φ and the thickness t of the transparent substrate 4, and using the above-mentioned symbol, 1≈2·t·tanφ.

If there is no diffraction grating section at a distance 1 in the traveling direction of the totally reflected diffracted light, a rainbow-shaped diffraction image will not be generated. In other words, the width in the normal direction of the grating lines of the diffraction grating section, the so-called display pattern width, is made narrower than the distance between the position of the incident light and the position where the rainbow-shaped diffraction image is generated closest to the position of the incident light. Just do it.

By the way, when the light modulation element 10 is applied to a finder system of a camera, it is arranged near the focal plane of the finder optical system, so that the incident light 5 has an incident angle that corresponds to the F number of the photographing lens and the diffusion characteristics of the lens. The light diffracted at 2 also has continuous diffraction angles θ ₁ . Among these diffracted lights, the minimum diffraction angle φ _p of the light totally reflected at the interface between the transparent substrate 4 and air is φ _p = sin ^-1 (1/n), and at this time, the incident light The distance 1' between the position and the position where the diffraction image is generated is 1'=2*t*tan(sin ^-1 (1/n))...(2).

In reality, when high-intensity spot light from a subject is incident on a display pattern, the light incident near the center of the display pattern may produce a rainbow-like diffraction image at the edges of the display pattern. many. Therefore, in such a case, the width of the display pattern should be determined so that rainbow-like diffracted light does not occur. Therefore, if the width of the display pattern, that is, the width w in the normal direction of the grating lines of the diffraction grating section, is set as w<4*t*tan{sin ^-1 (1/n)}...(3) The generation of rainbow-like diffraction images can be substantially prevented.

For example, the light modulation element 10 has a refractive index n=1.53 and a thickness t.
Assuming that a glass substrate of = 0.5 (mm) is used,
The width w of the display pattern at that time may be designed to satisfy w<1.73 (mm).

Moreover, the liquid crystal 1 and the diffraction grating 2 are formed on two transparent substrates 4.
If the two transparent substrates 4 have different thicknesses, the design may be based on the thickness t of the thinner substrate.

FIG. 6 shows the light modulation element 1 of the present invention shown in FIG.
FIG. In the figure, 4 is a transparent substrate, 6-1, 6-2, 6-
Reference numerals 3 and 6-4 each represent a diffraction grating section, and the diagonal lines indicate the direction of the grating lines of the diffraction grating.

In this embodiment, the direction of the grating lines of each diffraction grating section is at a constant angle with respect to the longitudinal direction of each region.

FIG. 7 is an enlarged view of the diffraction grating section 6-4 of the light modulation element 10 of FIG. 6. In the figure, θ is the angle between the grating line and the longitudinal direction of the diffraction grating section 6-4, and La
is the width of the diffraction grating section 6-4, and w is the width of the diffraction grating section 6-4.
is the length of the width in the normal direction of the grid lines.

In this embodiment, the incident light incident on the diffraction grating section 6-4 is diffracted by a refractive index element of the liquid crystal and the diffraction grating (not shown), and a plurality of orders of diffracted light are generated in a plane orthogonal to the grating line direction of the diffraction grating. to occur. Among these diffracted lights, high-order diffracted lights with large diffraction angles are totally reflected at the interface between the transparent substrate 4 and the air, but if the diffraction grating section 6-4 is not present in the direction in which they travel, they will be diffracted again. A rainbow-like diffraction image is not generated.

At this time, the distance L between the incident position of the incident light on the diffraction grating section 6-4 and the position of the diffraction image due to re-diffraction can be calculated using the same symbols as above: L=2*t*tan{sin ^-1 (1 /n)} ...(4).

Therefore, if the width w in the normal direction of the grating lines is set to w<L, it is possible to completely prevent the occurrence of rainbow-like diffraction images, but in reality, it is not necessary to set w<L, and for the reasons mentioned above, If the setting is made to satisfy w<2L=4*t*tan {sin ^-1 (1/n)} (5), the generation of rainbow-shaped diffraction images can be substantially prevented.

For example, as described above, the light modulation element 10 has a refractive index n
= 1.53, thickness t = 0.5 (mm), and angle θ = 45 degrees, then set the width w to satisfy w < 1.22 (mm). good.

If the light modulation element 10 in this embodiment is arranged in the finder system shown in FIG. 3, the diffraction grating portions 6-1, 6-2, 6-3, 6-4 is observed as a rectangular shadow and functions as a display element.

In this embodiment, the diffraction grating section 6-4 may be composed of a plurality of diffraction grating regions 81 having the same or different areas, as shown in FIG.

In the embodiment shown in FIG. 8, w is the width in the normal direction of the grating lines, θ is the angle between the grating lines and the diffraction grating section 8-4, and La is the width of the diffraction grating section 8-4. It is substantially the same as the case shown in FIG. 7 in terms of optical function.

In this example, the conditions for preventing the formation of a rainbow-like diffraction pattern are the same as those shown in FIG.
It suffices to set it so that equation (5) is satisfied.

In each of the above embodiments, two light modulation elements are used.
Although a structure in which a diffraction grating and a liquid crystal are sandwiched between two transparent substrates is shown, it is exactly the same when used as a display element for a phase-type diffraction grating that does not use a liquid crystal. By limiting the width, it is possible to prevent the formation of a rainbow-like diffraction image.

Further, as shown in FIGS. 9 and 10, the light modulation element 10
It may be constructed by laminating a plurality of layers consisting of a diffraction grating and a liquid crystal.

FIG. 9 is an explanatory diagram of the configuration of another embodiment of the optical modulation element 10 of the present invention. In the figure, 1 is a liquid crystal,
91, 92 are diffraction gratings, 93, 94, 95, 96
are transparent electrodes, 97, 98, and 99 are transparent substrates, 5 is incident light, 6a and 6b are polarized components of incident light 5, t ₁ ,
t ₂ and t ₃ are the thicknesses of the transparent substrates 97, 98, and 99, respectively.

The grating direction of the diffraction grating 91 is perpendicular to the plane of the paper (polarization component 6a), and the grating direction of the diffraction grating 92 is parallel to the plane of the paper and horizontal (polarization component 6b). The liquid crystal 1 is oriented in the grating direction of each diffraction grating. Of the incident light 5 that enters the light modulation element 10, a polarized component parallel to the grating direction (polarized component 6b) of the diffraction grating 92 is a component of the polarized light between the diffraction grating 92 (refractive index n _g ) and the liquid crystal 1 (refractive index n _e ). It is diffracted due to the difference in refractive index.
A part of the reflected diffracted light of the diffracted light passes through the transparent electrode 96 and is totally reflected at the interface between the transparent substrate 99 and the air. The size of the area of the diffraction grating serving as the display pattern is first limited by the thickness _t3 of the transparent substrate 99.
Here, the thickness of the transparent electrode 96 is sufficiently smaller than the thickness of the transparent substrate and can be ignored.

On the other hand, a part of the transmitted diffraction light passes through the transparent electrode 95 , the transparent substrate 98 , and the transparent electrode 94 and reaches the diffraction grating 91 . Since the refractive index n _g of the diffraction grating 91 and the refractive index n _p of the liquid crystal 1 are equal to the transmitted diffracted light (polarized light component 6b), the light is transmitted as is and reaches the substrate surface of the transparent substrate 97 via the transparent electrode 93. do. A part of the transmission circuit light is totally reflected at the interface between the transparent substrate 97 and the air, and similarly reaches the diffraction grating 92 via the transparent electrode 93, the diffraction grating 91, the transparent electrode 94, the transparent substrate 98, and the transparent electrode 95. do. The size of the area of the diffraction grating serving as the display pattern is limited by the sum of the thicknesses of the transparent substrates 98 and 97 (t ₁ +t ₂ ).

Here, the thicknesses of the transparent electrodes 93, 94, 95 and the thickness of the diffraction grating 91 are sufficiently small compared to the thicknesses of the transparent substrates 97, 98 and can be ignored.

The size of the area of the diffraction grating for the polarized light component 6b of the incident light 5 is limited by the thinner of the transparent substrate thickness t ₃ and (t ₁ +t ₂ ).

The same applies to the polarized light component 6a of the incident light 5. The incident light 5 (in the direction of the polarization component 6b) that has entered the optical modulation element 10 is diffracted according to the difference between the refractive index n _g of the diffraction grating 91 and the refractive index n _e of the liquid crystal 1. At this time, the size of the area of the diffraction grating for the transmitted diffracted light is limited by the thickness t ₁ of the transparent substrate 97, and for the reflected diffracted light, the size of the area of the diffraction grating is limited by the sum of the thicknesses of the transparent substrates 98 and 99 (t ₂ +
_t3 ). Therefore, the size of the area of the diffraction grating that becomes the display pattern for the polarized light component 6a of the incident light 5 is limited by the thinner of the transparent substrate thickness t ₁ and (t ₂ + t ₃ ). .

Therefore, the size of the area of the diffraction grating that becomes a display pattern for the incident light 5 is limited by the thinner of the thicknesses t ₁ and t ₃ of the transparent substrates 97 and 99 that cause total reflection on the substrate surface.

As described above, the thickness t in equation (3) or (5)
By setting , it is possible to prevent the generation of a rainbow-like diffraction image when used as a display element.

FIG. 10 is an explanatory diagram of the configuration of another embodiment of the optical modulation element of the present invention. In this embodiment, compared to the embodiment shown in FIG. 9, the transparent electrode and transparent substrate disposed between the diffraction gratings 91 and 92 are omitted.

In FIG. 10, the same elements as those shown in FIG. 9 are given the same reference numerals. This embodiment corresponds to the case where the thickness t ₂ of the transparent substrate 98 is t ₂ =0 in the embodiment shown in FIG. 9, and the basic operation is the same as that of the embodiment shown in FIG.

Note that the light modulation element according to the present invention is not limited to being applied to a viewfinder system of a camera as shown in FIG. 3, but may also be applied to a part of a projection system as a display light valve as shown in FIG. 11, for example. You can also do it.

In the figure, 110 is a display light valve;
11 is a light source, 112 is a condenser lens, 11
3 is a transmission type projector, 114 is a projection lens, 115
is a screen.

Illumination light emitted from a light source 111 placed at the focal point of a condenser lens 112 is turned into parallel light by the condenser lens 112, and vertically illuminates a transmission type projector (for example, a photographic film) 113.
An image of the transmission type projector 113 is formed on a screen 115 via a projection lens 114. The display light valve 110 according to the present invention is placed in close proximity to the transmission type projector 113, and a display or non-display state is selected by a drive device (not shown). In the non-display state, the display light valve 110 is regarded as a transparent body with a uniform refractive index, and the light transmitted through the transmission type projector 113 is imaged on the screen 115 via the projection lens 114 without being modulated. In the display state, a portion of the light incident on the display light valve 110 is diffracted at the display pattern portion made up of the diffraction grating. Of the diffracted light, a component with a large diffraction angle is blown out of the pupil of the projection lens, so that a part of the light that has passed through the transmissive projector 113 can be visually recognized and displayed without being attenuated. .

FIG. 12 is a plan view of the display light valve 110. 121 is a diffraction grating section, 4 is a transparent substrate, and the stripes within the diffraction grating section indicate the grating direction. The grating direction of the diffraction grating portion is parallel to the longitudinal direction of the grating region. Of the light incident on the display light valve 110, the light incident on the diffraction grating portion is diffracted at a diffraction angle φ that satisfies φ=sin ⁻¹ (m/n·λ/P).

Here, m is the diffraction order, λ is the wavelength, n is the refractive index of the transparent substrate 4, and P is the diffraction grating pitch.

The low-order diffracted light with a small diffraction angle is transmitted through the projection lens 114.
The image is formed on the screen 115 via the. The diffracted light with a large diffraction angle is blown out of the pupil of the projection lens 114 and therefore does not contribute to image formation. The higher-order diffracted light with a larger diffraction angle is totally reflected at the interface between the transparent substrate 4 and the air, and if a diffraction grating exists in the traveling direction of the totally reflected higher-order diffracted light, the diffraction angle of the diffracted light and the diffraction angle φ A rainbow-like diffraction image similar to that described above is generated at a position determined by the thickness of the transparent substrate 4.

Therefore, by specifying the shape of the diffraction grating portion as described above, it is possible to prevent the formation of a rainbow-like diffraction image.

(Effects of the Invention) As described above, according to the present invention, by specifying the shape and dimensions of the grating lines of the diffraction grating portion, which become the display pattern, as described above, it is possible to prevent the formation of the diffraction grating portion. Using a light modulation element that enables high-quality display, it prevents the diffracted light that has been totally reflected on other surfaces from entering the same diffraction grating section again and creating a rainbow-like diffraction image. It is possible to achieve a viewing device and a projection type display device.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the basic configuration of the light modulation element according to the present invention, FIG. 2 is an explanatory diagram of the occurrence of total reflection of diffracted light in the light modulation element of the present invention, and FIG. 3 is an explanatory diagram of the light modulation element of the present invention. An explanatory diagram when applied to a camera finder system, Fig. 4 is a plan view of the light modulation element used in Fig. 3, Fig. 5 is an explanatory diagram of diffracted light in the light modulation element of Fig. 4, and Fig. 6 , FIGS. 7 and 8 are explanatory diagrams of one embodiment when the light modulation element is used as a display element, and FIGS. 9 and 10 are illustrations of the configurations of other embodiments of the light modulation element of the present invention, respectively. An explanatory diagram, FIG. 11 is an explanatory diagram when the light modulation element of the present invention is applied to a part of a projection system as a display light valve, and FIG. 12 is a plan view of the light modulation element used in FIG. 11. . In the figure, 1 is a refractive index variable material, 2 is a diffraction grating, 3 is a transparent electrode, 4 is a transparent substrate, 5 is incident light, 6a, 6
10 is a light modulation element, 7 is an emitted light, and 4-1, 4-2, 4-3, and 4-4 are diffraction grating sections.

Claims (1)

[Scope of Claims] 1. In an optical modulation element having a diffraction grating section, where t is the thickness of the substrate provided with the diffraction grating section, and n is the refractive index, A light modulation element characterized in that the length w in the direction is w<4·t·tan {sin ⁻¹ (1/n)}. 2. The light modulation element has at least two adjacent substrates, a relief-type diffraction grating section is provided on at least one of the opposing surfaces of the substrates, and the relief-type diffraction grating section and the substrate 2. The light modulation element according to claim 1, further comprising a refractive index variable material filled between the spaces, and a means for controlling the refractive index of the refractive index variable material. 3. The light modulation element according to claim 1, wherein at least a portion of the grating lines are provided obliquely with respect to the outer periphery of the diffraction grating section. 4. The light modulation element according to claim 1, wherein a plurality of the diffraction grating sections are stacked, and grating lines of the plurality of stacked diffraction grating sections are oriented in different directions. 5. The diffraction grating has an irradiation system and a diffraction grating section that is irradiated with a light beam from the irradiation system, and where t is the thickness of the substrate and n is the refractive index of the substrate provided with the diffraction grating section. Observe a light modulation element whose length w in the direction perpendicular to the grid line of the part is w<4・t・tan {sin ^-1 (1/n)} and the light flux modulated by the light modulation element. An observation device having an observation system. 6. In a projection display device having a light source, a light modulation element that modulates the light from the light source, and a projection lens that projects the modulated light, the light modulation element modulates the light from the light source. It has a diffraction grating section, and when the thickness of the substrate including the diffraction grating section is t and the refractive index is n, the length w of the diffraction grating section in the direction perpendicular to the grating lines is w< 4.t.tan {sin ^-1 (1/n)} A projection type display device.