JPH08201776A - Method for driving spatial optical modulation element and projection type display - Google Patents

Abstract

PURPOSE: To provide a method for driving a spatial optical modulation element capable of outputting an image whose contrast ratio is high by using DC voltage which is not constant as the driving voltage wavelength of the spatial optical modulation element. CONSTITUTION: A transparent conductive electrode 102 and a photoconductive layer 103 having rectifying property are successively laminated on a glass base plate 101, and a reflection layer 104 and an oriented film 106 where a liquid crystal layer 105 is oriented are arranged in order on the photoconductive layer 103, and a ferroelectric liquid crystal layer 105 whose thickness is 0.8-1.5μm is interposed between the oriented film 106 and a transparent insulating base plate 109 where a transparent conductive electrode 107 and an oriented film 108 are laminated. The DC voltage (114) where a high voltage period and a low voltage period appear alternately is impressed between the electrodes 102 and 107.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for driving a spatial light modulator used in an optical arithmetic unit, a projection type display, etc.
Further, it relates to a projection type display using the driving method.

[0002]

2. Description of the Related Art The basic structure of a conventional photo-address type spatial light modulator using liquid crystal (hereinafter, a spatial light modulator is a photo-address type device unless otherwise specified) is based on a photoconductive material. A liquid crystal layer whose light transmittance changes by application of a layer and an electric field is sandwiched between two transparent conductive electrodes facing each other. This element is driven by externally applying a voltage between both transparent conductive electrodes. When writing light is applied to the photoconductive layer in this state, the electric resistance of the photoconductive layer changes and the voltage is applied to the liquid crystal layer. The applied voltage changes, and the read light passing through the liquid crystal layer is modulated according to the magnitude of the voltage. Since this operation can be used to realize functions such as optical thresholding, wavelength conversion, incoherent / coherent conversion, and image memory, the spatial light modulator is positioned as a key device for optical information processing. ing. Further, if the read light having a high light intensity is incident from the direction opposite to the write light and the written content is read out by the reflection type, the optical amplification function can be realized. Therefore, it can be applied also as a projection type display and is expected as an optical functional element having excellent versatility.

At present, as a projection type display which has been put into practical use, in addition to the one using this photo-address type spatial light modulator, three high brightness cathode ray tubes (hereinafter referred to as C
(Hereinafter abbreviated as RT), and projection of an active matrix liquid crystal light valve with a high brightness light source.

According to the CRT method, an image is displayed on three R, G, B high-intensity CRTs each having a diagonal size of 5 to 9 inches, and the image is projected on a screen by three projection lenses to be combined to produce a color image. To get an image. However, this method has problems that the CRT must be displayed with high brightness in order to obtain a bright screen, the resolution and contrast are low, and the weight of the projection apparatus is heavy.

The method of the active matrix liquid crystal light valve is as follows: R, G, B three liquid crystal panels or R, G, B
An image is displayed on a single liquid crystal panel integrated with a B color filter, read out by a high-intensity backlight light source such as a metal halide lamp or a halogen lamp, and projected on a screen. There is an advantage that the device can be made small and lightweight. However, in order to obtain a high-resolution image, the pixel size of the liquid crystal panel must be reduced, and as a result, the ratio of the light-shielding area (transistor portion for active matrix driving) to the pixel size increases, and the pixel size increases. However, there is a problem that the aperture ratio is decreased and the image becomes dark. As described above, the resolution and the brightness are in a trade-off relationship, and the projection display using the CRT and the active matrix liquid crystal light valve cannot satisfy both of them.

On the other hand, when the photo-addressable spatial light modulator is used, an image is input to the photoconductive layer by a CRT and the image is read out from the liquid crystal layer side by a high-intensity light source in a reflective type.
It is a method of projecting on a screen by a projection lens. This method is an epoch-making method that can provide a bright screen with a high resolution while being small and lightweight, and can solve the problem of the trade-off between brightness and resolution that is present in conventional displays.

As a photoconductive material forming the spatial light modulator, a hydrogenated amorphous silicon (hereinafter abbreviated as a-Si: H) thin film having a high photosensitivity to visible light is used as a liquid crystal layer and has a high-speed response. Ferroelectric liquid crystal (hereinafter abbreviated as FLC) is generally used. As a waveform for driving such a spatial light modulator, a waveform as shown in FIG. 8 has been proposed (Gee-model et al., US Pat. No. 5,17).
8,445). In this voltage waveform, the input image is _aS during the period (T _w ) in which the negative voltage is applied.
The image is written to the FLC layer by being applied to the i: H layer, and the written image is erased during the period (T _e ) in which the positive voltage is applied.

[0008]

However, this conventional spatial light modulator has a problem that the contrast ratio of an output image is small. This is due to the element structure in which the capacitance of the liquid crystal layer is smaller than the capacitance of the photoconductive layer. That is, most of the writing voltage (V _w ) is applied to the liquid crystal layer, and the liquid crystal layer gradually starts to switch even when there is no irradiation of the writing light, and the black portion of the output image becomes white. In order to prevent this, it is necessary to make the capacitance of the photoconductive layer smaller than that of the liquid crystal layer. For that purpose, the film thickness of the photoconductive layer must be at least four times as large as that of the liquid crystal layer. However, if the thickness of the photoconductive layer is increased in this way, problems such as deterioration in the uniformity of the thickness of the photoconductive layer and warpage or deformation of the substrate occur. Such a phenomenon leads to unevenness in the thickness of the liquid crystal layer, appears as uneven brightness in the output image, and significantly deteriorates the quality of the spatial light modulator. In addition, the time for forming the photoconductive layer becomes long, which increases the cost of the device. Therefore, the method of increasing the film thickness of the photoconductive layer cannot provide a high quality spatial light modulator at low cost.

As described above, it is difficult to output a high-contrast image in the state where the drive waveform of the conventional spatial light modulator is used, which is a problem in the projection type display using the spatial light modulator. It was a big obstacle to practical use.

In order to solve the above-mentioned problems of the prior art, it is an object of the present invention to provide a method for driving a spatial light modulator capable of outputting a high-contrast image and a projection type display which is low in cost and high in mass productivity. It is a thing.

[0011]

In order to achieve the above-mentioned object, a method of driving a spatial light modulator according to the present invention is a method of photoconductive in a region sandwiched by two transparent insulating substrates having transparent conductive electrodes. In a method of driving a spatial light modulator including at least a layer and a liquid crystal layer, a direct current waveform having a non-constant voltage is applied between the transparent conductive electrodes.

In the above structure, the photoconductive layer is
It is preferable to have a rectifying property. Further, it is preferable that the voltage of the DC waveform having the above-mentioned structure changes in a cycle of 1 × 10 ⁻⁶ to 1 second, and the polarity of the voltage of the DC waveform is forward bias with respect to the photoconductive layer. Preferably, the DC waveform is such that a high voltage period and a low voltage period are alternately repeated, and it is preferable that the low voltage period is shorter than the high voltage period, and
It is preferable that the high voltage of the DC waveform has twice or more the magnitude of the low voltage, and the low voltage of the DC waveform is
It is preferably 0.

Further, in the above structure, the liquid crystal layer preferably has one or more stable states, and at least a reflecting mirror is preferably provided between the photoconductive layer and the liquid crystal layer.

The projection display according to the present invention comprises a spatial light modulator having at least a photoconductive layer and a liquid crystal layer in a region sandwiched between two transparent insulating substrates having transparent conductive electrodes, and the transparent conductive substrate. Direct current power source for driving the spatial light modulation element connected to the conductive electrode, image input means for displaying an image for inputting to the spatial light modulation element, and an image displayed on the image input means for displaying the image. An image forming unit for forming an image on the photoconductive layer, a light source for reading out an image output from the spatial light modulator and a projection lens are at least components, and the voltage of the DC waveform output from the DC power supply is not constant. It is characterized by

Further, in the above structure, it is preferable that at least a reflecting mirror is formed between the photoconductive layer and the liquid crystal layer, and that the image input means is a cathode ray tube. However, it is preferable that the high voltage period and the low voltage period are alternately repeated, and that the high voltage period is at least ⅕ or less of the low voltage period.

[0016]

According to the method of driving a spatial light modulator of the present invention, a DC voltage waveform having a non-constant voltage, that is, a DC waveform having at least a high voltage period and a low voltage period is used. At this time, if an image is written in the spatial light modulator during the period when the voltage is low and the written image is erased during the period when the voltage is high, the applied voltage at the time of writing has the same polarity as that at the time of erasing, so The liquid crystal layer does not switch when a voltage is applied. Therefore, the black portion of the output image does not appear white. Further, when the photoconductive layer is irradiated with light during image writing, the internal voltage formed in the photoconductive layer is generated with a polarity opposite to the applied voltage and switches the liquid crystal layer. Images can be written even if the polarity is used. However, in order to perform the switching of the liquid crystal layer completely, it is preferable that the writing voltage in the low voltage period is smaller than the internal voltage of the photoconductive layer.

As described above, it is possible to eliminate the inconvenient state in which the liquid crystal layer naturally switches even without the writing light, and the liquid crystal layer can be sufficiently switched by the irradiation of the writing light. The output image of the modulator has high contrast.

[0018]

EXAMPLES The present invention will be described in more detail below with reference to examples.

FIG. 1 is a sectional view showing an embodiment of the spatial light modulator according to the present invention. As shown in FIG. 1, on a transparent insulating substrate 101 (for example, a glass substrate), a transparent conductive electrode (for example, ITO (indium-tin oxide),
A conductive oxide 102 such as ZnO or SnO ₂ ) and a photoconductive layer 103 made of an amorphous semiconductor are sequentially stacked. Further, on the photoconductive layer 103, a reflecting mirror 104 and an alignment film 106 for aligning the liquid crystal layer 105 are sequentially arranged. An alignment film 108 is also uniformly formed on the transparent conductive electrode 107 on the opposite side. In FIG. 1, 10
Reference numeral 9 is a transparent insulating substrate (for example, a glass substrate).

The spatial light modulator is driven by applying a DC voltage (for example, the voltage waveform shown in FIG. 4) from a DC power supply 114 connected between the transparent conductive electrodes 102 and 107. However, in FIG. 4, when the voltage is high, it is the erasing period, and when it is low, the writing period. When the photoconductive layer 103 is irradiated with the writing light 110 while the writing voltage is applied to the spatial light modulation element, the voltage applied to the liquid crystal layer 105 in the portion where the light is irradiated increases, The alignment state of the liquid crystal molecules changes according to the magnitude of this voltage. This change in the alignment state is caused by the polarizer 111
By using the optical system in which the reading light 113 is incident from the direction opposite to the writing light 110 by using an optical system in which the reading light 113 and the analyzer 112 are combined, it can be observed as reflected light from the reflecting mirror 104. However, the polarizer 111 and the analyzer 1
12 may be replaced with one polarization beam splitter.

Specific examples of applied voltage waveforms applied to the spatial light modulator by the DC power source 114 are shown in FIGS. The waveform shown in FIG. 4 shows the magnitude of the applied voltage during erasing (hereinafter referred to as V _e ), the magnitude of the applied voltage during writing (hereinafter referred to as V _w ), and the erasing period (T _e ).
And the writing period (T _w ) ratio (T _e / T _w : hereinafter referred to as duty ratio) and the period (T = T _e + T _w ) are constant. In the range of the repetition frequency of V _e and V _w , the lower limit is such that flicker cannot be visually sensed, and the upper limit is such that the liquid crystal layer can respond. The latter depends on the material of the liquid crystal used and the thickness of the liquid crystal layer, but a specific frequency (cycle) range is preferably 1 Hz or more and 1 MHz or less (1 μs or more and 1 s or less), and preferably 10 Hz or more and 50 kHz or less ( 20 μs or more 0.1
s or less), optimally 30 Hz or more and 10 kHz or less (10
0 μs or more and 0.33 s or less). Further, the magnitude of V _e with respect to V _w must be at least twice or more in order to increase the contrast ratio of the liquid crystal layer. Also, if T _w is made longer than T _e , the period during which the output image is output becomes longer.
It is better to make _w as long as possible.

FIG. 5 shows an example in which the repetition frequency and the duty ratio are constant and only the voltage during writing is changed with the passage of time in each cycle. In this example, the writing voltage V _w1 is smaller and the writing voltage V _{w2 is} larger.
Although the case of changing to is shown, there is no problem in the form of change other than those listed here. By changing the write voltage from V _w1 to V _{w2 in} this manner, malfunction of the liquid crystal layer 105 (switching to the ON state without irradiation of writing light) is prevented even if T _w is long.

FIG. 6 shows an example in which the repetition frequency and the duty ratio are kept constant and only the voltage during erasing is changed with the passage of time in each cycle. The difference in the manner in which the erasing voltage changes from the lower V _e1 to the higher V _e2 is shown, but there is no problem in the form of the change other than those mentioned here. By thus changing the erasing voltage from V _e1 to V _e2 , there is an effect that the written image is completely erased even if T _e is short. FIG. 7 shows the erase voltage (V _e ) and the write voltage (V _w ) of FIGS. 5 and 6.
This is an example of the case in which the changes of (1) and (2) are made at the same time, and the effects of FIGS.

Materials used for the liquid crystal layer 105 include nematic liquid crystals, super twisted nematic liquid crystals, ferroelectric liquid crystals, antiferroelectric liquid crystals, and dispersion type liquid crystals in which liquid crystals are dispersed in a polymer. When the ferroelectric liquid crystal or the anti-ferroelectric liquid crystal is used, the thickness of the liquid crystal layer 105 can be made small, so that the electric impedance can be made small, and the film thickness of the photoconductive layer 103 can be set thin.
It is useful because it can respond at high speed and has a memory function. This feature is also obtained using a mixture of ferroelectric and antiferroelectric liquid crystals. In addition, since the transmission characteristic of the ferroelectric liquid crystal has a threshold value characteristic that is steep with respect to the voltage, it is an optimum material for performing threshold processing on the input light. At least 1 like ferroelectric or antiferroelectric liquid crystals
When a liquid crystal having three or more stable states is used, it is effective to set one of the stable states to an OFF state in which an output image is black so that a high contrast ratio can be obtained.

On the other hand, when the dispersion type liquid crystal is used, the alignment film 1
Since 06 and 108 are not necessary and the polarizer 111 and the analyzer 112 are not necessarily required, there are advantages that the output light becomes bright and the element structure and the optical system become simple. In addition, the liquid crystal layer 105 is sealed with resin, and a spacer (see FIG. 1) is used to adjust the layer thickness.
(Not shown in the figure) is mixed in the liquid crystal layer 105. As the spacer, beads of alumina or quartz or powder of glass fiber is usually used. Further, these spacers are also mixed in the resin that seals the liquid crystal layer 105. Alignment films 106 and 108 for aligning liquid crystal
Is an SiO _x oblique deposition film or a rubbing-treated organic polymer thin film such as polyimide or polyvinyl alcohol.

As the material forming the photoconductive layer 103,
A film that can be formed on a large area at a relatively low temperature (400 ° C. or less) and has a function of efficiently generating photocarriers when the writing light 110 is incident and efficiently transporting the photocarriers to the liquid crystal layer 105 side. preferable. Specific examples of materials and structures used include a-Si: H and a-G.
e: H (hydrogenated amorphous germanium), a-Si _1-x C
_x : H (hydrogenated amorphous silicon carbide, where 0 <
x <1), a-Si _1-x Ge _x : H (hydrogenated amorphous silicon germanium), a-Ge _1-x C _x : H (hydrogenated amorphous germanium carbide), a-Ge _{1- x} N _x : H is a single layer of a hydrogenated amorphous semiconductor such as hydrogenated amorphous germanium nitride, or a stacked layer formed by combining two or more of these materials.

It should be noted that halogen atoms such as F and Cl may be added to the hydrogenated amorphous semiconductor in the same manner as hydrogen in order to effectively reduce dangling bonds which act as carrier traps. , And / or a trace amount (for example, 0.1 to 10 atom%) of oxygen (O) atom or nitrogen (N) atom may be contained. In addition, the photoconductive layer 103
In order to increase the internal voltage (diffusion potential) formed in the above, it is preferable to have a rectifying property. For this purpose, p / i, i / n, p / n, and p / i / n structures may be formed inside (however, the i layer means an undoped layer).

In order to form a p-type layer, an element such as B, Al, or Ga as a p-type impurity may be added to the above material in the range of 1 × 10 ⁻⁴ atom% to 10 atom%. The thickness of the p-type layer is 10 to 10,000 angstroms (Å), preferably 20 to 3,000 angstroms (Å), and most preferably 50 to 300 angstroms (Å).
It is said. In order to form an n-type layer, an element such as P, As, Sb, or Se as an n-type impurity is added to the above material at 1 × 10 ^−4.
It is advisable to add it in the range of 10 to 10 atomic%.

The film thickness of the n-type layer is 10 to 10,000.
0 angstrom (Å), preferably 100 to 5,0
00 angstrom (Å), optimally 500 to 3,0
It is said to be 00 angstrom (Å). Also, Au, P
A junction with a metal such as d, Pt, Ag, Al, or Cr may be formed to form a Schottky barrier so that the photoconductive layer 103 has a rectifying property. In this way, the rectifying property is provided by the photoconductive layer 10.
3 makes it possible to efficiently generate and transport photocarriers with respect to the incidence of the writing light 110, and to apply a large forward bias (V _e ) to the image written in the liquid crystal layer 105 to increase the speed. It has the advantage that it can be erased. The film thickness of the photoconductive layer 103 is determined in relation to the film thickness of the liquid crystal layer 105, but is usually 0.5 to 30 μm,
It is preferably 1 to 10 μm, and most preferably 1 to 5 μm.

As the reflecting mirror 104, a multi-layer dielectric reflecting mirror is used in which thin films having a large dielectric constant such as TaO ₂ and Si and thin films having a small dielectric constant such as MgF and SiO ₂ are alternately laminated. As another method, as shown in FIG. 2, the reflecting mirror 201 may be formed of Al, Ag, M having a large reflectance.
A metal thin film made of o, Ni, Cr, Mg, Ti or the like is formed in an island shape and arranged in a two-dimensional matrix shape or a mosaic shape. However, FIG. 2 is a sectional view showing one embodiment of the spatial light modulator according to the present invention. Island reflector 2
01 must be separated as shown in FIG. 2, but the shape does not matter. The reason why the island-shaped reflecting mirror 201 is separated in this way is that if they are continuous, the same potential is generated and no potential difference occurs, so that image formation becomes impossible.

The individual reflecting mirrors 201 are respectively
It corresponds to one pixel in the output image. Further, the photoconductive layer 103 between the island-shaped reflecting mirrors 201 needs to be at least removed by etching for the purpose of preventing lateral diffusion of carriers and increasing the resolution of the device. By doing so, the lateral flow of carriers between the island-shaped reflecting mirrors 201 is suppressed, and the resolution of the device can be increased. However, if it is read as it is with high intensity light, the read light 113 enters from the gap between the island-shaped reflecting mirrors 201 to the photoconductive layer 103 that controls the generation of photocarriers, causing the liquid crystal layer 105 to malfunction. . To prevent this, the island reflector 2
It is desirable to completely remove the photoconductive layer 103 between 01 and 01, but as shown in FIG. 2, a film thickness that allows visible light to be transmitted with little absorption, preferably 1 μm or less, and most preferably 0.5 μm or less. The photoconductive layer 103 may be set to remain. Further, in the groove portion, a light absorption layer 202 made of a material that absorbs visible light (for example, an organic polymer polymer in which carbon particles are dispersed, an organic polymer polymer in which a black pigment or a black dye is mixed, or a -C: H, a-
Ge: H, a-Ge 1 _{over x} N _x: composed of an inorganic thin film such as H) is by forming, it is possible to absorb the reading light 113 coming leaking from the gap islands reflector 201 effectively.

Further, in order to complete the blocking of the reading light 113, Al, Ag, Mo, Ni, Cr,
A metal light shielding film 203 such as Mg may be formed on the bottom of the hole. In order to complete the electrical insulation between the island-shaped reflecting mirrors 201, SiO _x , SiN _x , SiC _x , GeO _x , GeN _x ,
Inorganic insulators such as GeC _x , AlO _x , AlN _x , BC _x , BN _x or polyimide, polyvinyl alcohol, polycarbonate, polyparaxylene, polyethylene terephthalate, polypropylene, polyvinyl chloride, polyvinylidene chloride, polystyrene, polytetrafluoride Ethylene, polytrifluoroethylene chloride, polyvinylidene fluoride, hexafluoropropylene-tetrafluoroethylene copolymer, ethylene trifluoride-
An insulating film 204 made of an organic insulating material such as vinylidene fluoride copolymer, polybutene, polyvinyl butyral or polyurethane may be formed in the groove portion.

However, as in the case of the spatial light modulator for optical calculation, when the light with high intensity is not used as the reading light 113, it is not necessary to provide the reflecting mirror 104, and the photoconductive layer 103 and the alignment film are not necessary. The interface with 106 can be substituted.

The present invention will be described in more detail with reference to specific examples. (Example 1) As shown in the sectional view of FIG.
01 on the ITO film by a sputtering method at 0.05-0.
A film having a thickness of 2 μm was formed to form a transparent conductive electrode 102. Next, this was placed in a plasma CVD apparatus, the inside of the vacuum vessel was evacuated to 1 × 10 ⁻⁶ Torr or less, and then heated to 280 ° C. by a heater. Concentration 10p diluted with He
pm 400Scc the B ₂ H ₆ in (1 ppm = 1x10 ^-6)
m and SiH ₄ : 1 sccm, C ₂ H ₂ : 0.2 scc
After introducing m into the vacuum container and adjusting the pressure to 0.8 Torr, a high frequency power of 30 W with a frequency of 13.56 MHz was applied to the electrodes to generate discharge, and p-type a- with a thickness of 50 to 180 angstroms (Å). A Si _1-x C _x : H layer was formed.

Subsequently, the inside of the vacuum vessel was once evacuated to a high vacuum, and then H ₂ : 100 sccm and SiH ₄ : 40 sc.
cm was introduced into a vacuum container, an i-type undoped a-Si: H layer of 1 to 1.8 μm was similarly formed at a pressure of 0.8 Torr and a high frequency power of 15 W, and the inside of the vacuum container was evacuated again. Then, PH diluted with H ₂ at a concentration of 5000 ppm
_3: 100 sccm and SiH _4: 40 sccm was introduced into the vacuum container, after adjusting the pressure to 0.8 Torr, the 0.3~1μm under conditions of high-frequency power 15W n-type a-S
An i: H layer was formed to produce a photoconductive layer 103.

Then, Si and Si are formed by the sputter deposition method.
Polyimide alignment film 1 which is obtained by laminating O ₂ alternately in multiple layers to form a multilayer dielectric reflection layer 104, and then performing a rubbing treatment.
06 was laminated. Then, a ferroelectric liquid crystal layer 105 having a thickness of 0.8 to 1.3 μm is sandwiched between this and a glass substrate 109 on which a transparent conductive electrode 107 made of ITO and a polyimide alignment film 108 are laminated, and a spatial light modulator ( 1) was produced.

Between the transparent conductive electrodes 102 and 107 of this spatial light modulator (1), the DC voltage waveform (V
_{e = 15V, V w = 0.05V} , T e / T w = 1/10, T
Was applied for 16.7 ms), white light was used as the writing light 110, and a He—Ne laser (633 nm) was used as the reading light 113 to examine the operation. However, the applied voltage was set so that the transparent conductive electrode 102 became +.

The operation of the spatial light modulator will be briefly described below. The writing light 110 is applied to the photoconductive layer 103 when a low voltage (V _w ) is applied, whereby the voltage applied to the liquid crystal layer 105 approaches the internal voltage of the photoconductive layer 103, and the liquid crystal switches from the off state to the on state. The output light of the read light 113 is output. After that, when a high voltage (V _e ) is applied which forward biases the photoconductive layer 103, the liquid crystal layer 105 is switched to the off state regardless of the irradiation of the writing light 110.

Under these operating conditions, the spatial light modulator (1) has a photosensitivity of 50 to 80 μW / cm ² , a rise time of 30 to 50 μs, and a resolution of 50 to 70 lp (li).
nepairs) / mm (MTF = 50%) and the contrast ratio was 200: 1.

An embodiment in which this spatial light modulator is incorporated in a projection type display device as shown in FIG. 3 will be described. In FIG. 3, 301 is a screen having a white diffusion surface, 302 is a projection light source, a metal halide lamp with a reflector, 303 is a CRT as a writing light source, and 304 is a writing light source.
Is a spatial light modulator, and 305 is a projection lens system that magnifies the output image of the spatial light modulator 304 on the screen 301 by 40 times. 306 is a polarization beam splitter,
Reference numeral 307 denotes an imaging lens system for forming an image displayed on the CRT 303 on the photoconductor layer 103 of the spatial light modulator 304, 308 a relay lens system, 309 a pre-polarizer,
Reference numeral 310 is an auxiliary lens, and 311 is a DC power supply for driving the spatial light modulator 304. The voltage waveform of the DC power supply 311 is the same as that used in the above characteristic evaluation.

Here, the formation, erasure of the projected image and the result projected on the screen in this embodiment will be briefly described. While the voltage of V _w is being applied, the CRT 303
The image displayed on the screen is written in the spatial light modulator 304, and the written image is projected on the screen 301. The written image is erased during the period when the voltage V _e is applied. The illuminance on the spatial light modulator 304 when the metal halide lamp 302 is turned on is 2 million lx.
However, the contrast ratio of the obtained image on the screen 301 was 150: 1.

Next, for comparison, a conventional voltage waveform (V _e = 15 V, V _w = −5 V, T _e / T _w = 1 as shown in FIG. 8 is used.
/ 10, T = 16.7 ms) was applied to the element (1) to examine the projected image. As a result, it was found that the contrast ratio was reduced to 50: 1.

In FIG. 3, the writing image is given by the CRT 303, but another display is used instead of the CRT.
For example, a liquid crystal display, a plasma display, an electroluminescent element, a light emitting diode array, a semiconductor laser having a two-dimensional scanning device, or the like may be used.

(Embodiment 2) As shown in the sectional view of FIG.
ITO having a thickness of 0.05 to 0.2 μm on the glass substrate 101
The transparent electrode 102 is formed by a sputtering method, and p of 50 to 500 angstrom (Å) is formed in the same manner as in the first embodiment.
Type a-Si _1-x C _x : H layer / 1.4 to 1.8 μm thick i-type undoped a-Si: H layer / 0.1 to 0.3 μm type a
A photodiode made of —Si: H was formed, and a photoconductive layer 103 was formed. Then, Cr: 5000 angstrom (Å) thickness was deposited on the surface of the photoconductive layer 103 by a vacuum deposition method, and patterning was further performed by a photoetching method to form an island-shaped reflecting mirror 201. At this time, the shape of the island-shaped reflecting mirror 201 is a square of 24 μm square and 2 μm.
The 1000 × 2000 reflecting mirrors are arranged in a matrix form while maintaining the intervals (however, the island-shaped reflecting mirrors 201
Can be formed not only by photolithography but also by lift-off method). Next, using the island-shaped reflecting mirror 201 as a mask, the a-Si: H layer 103 between the island-shaped reflecting mirrors 201 was removed by etching by a dry etching method.

Next, about 1000 is applied from above by a vacuum deposition method.
Angstrom (Å) thick Al was vapor-deposited. This time,
This Al film is to be formed on the island-shaped reflecting mirror 201 and in the groove, and the island-shaped reflecting mirror 201 has a double structure of Al / Cr, and the portion formed in the groove blocks the reading light 113. To be the metal light-shielding film 203. Next, the insulating film 204 made of polyimide was formed. Further, a resist containing carbon particles was applied and filled in the groove to form a light absorption layer 202, and then a rubbing-treated polyimide alignment film 106 was laminated. Between this and the transparent conductive electrode 107 of ITO and the glass substrate 109 on which the polyimide alignment film 108 manufactured similarly to the alignment film 106 is laminated.
The ferroelectric liquid crystal layer 105 having a thickness of 8 to 1.5 μm was sandwiched to complete the spatial light modulator (2) shown in FIG.

When this spatial light modulator (2) was evaluated in the same manner as in Example 1, the photosensitivity was 80 μW / c.
m ² , rise time 30 μs, contrast ratio 2
It was 50: 1.

Next, this element was mounted on the projection display apparatus shown in FIG. 3 in the same manner as in Example 1 and the output image thereof was examined. The output waveform from the DC power supply 311 at this time is V _e = 15 V, V _w = 0 V, T _{e in} the waveform of FIG.
/ T _w = 1/10 and a repetition period T = 16.7 ms (repetition frequency 60 Hz) were used. For comparison, a conventional voltage waveform (V _e = 1 as shown in FIG. 8 is used.
_{5V, V w = -5V, T} e / T w = 1/10, frequency =
It was also investigated when 60 Hz) was used. As a result, when the waveform of the conventional example was used, the contrast ratio was low and the image was unclear, but when the voltage waveform of the DC voltage of this example was not constant, a high contrast ratio was obtained and a beautiful image was obtained. I was able to output.

(Embodiment 3) In a projection type display device using the spatial light modulator (2) constructed in Embodiment 2,
As the output waveform from the DC power supply 311, in the waveform of FIG. 5, V _e = 15 V, V _w1 = 0 V, V _w2 = 0.2 V, T _e
/ T _w = 1/20, T = 16.7 ms,
The image on screen 301 was examined. As a result, 20
It was confirmed that a beautiful image with a high contrast ratio of 0: 1 or more was obtained.

(Embodiment 4) In a projection type display device using the spatial light modulator (2) constructed in Embodiment 2,
As the output waveform from the DC power supply 311, in the waveform of FIG. 6, V _e1 = 15 V, V _e2 = 20 V, V _w = 0.05 V,
The image on the screen 301 was examined by using T _e / T _w = 1/10 and T = 1.6 ms. As a result, no afterimage or image sticking was sensed in the image, and the contrast ratio was high (200: 1).

(Embodiment 5) In a projection type display device using the spatial light modulator (2) constructed in Embodiment 2,
In the waveform of FIG. 7 as the output waveform from the DC power supply 311, V _e1 = 15 V, V _e2 = 20 V, V _w1 = 0 V, V _w2 =
0.2V, using those _{T e / T w = 1/} 20, T = 8ms, examined the image on the screen 301. As a result, no afterimage or image sticking was detected in the image, and a clear image could be confirmed with a high contrast ratio (250: 1).

The element produced as described above also functions as a spatial light modulator for displaying a dynamic hologram.

The liquid crystal layer of the present invention is not limited to those described in the above embodiments, the photoconductive layer is not limited to those described in the above embodiments, and the insulating film, the light absorption layer, and the applied voltage waveform are not limited thereto. It goes without saying that the present invention is not limited to the examples given above. Further, in the projection system of FIG. 3, by combining three CRTs for displaying R, G, and B three colors and three spatial light modulators, and incorporating a color separation optical system and a color synthesis optical system into the optical system, It goes without saying that a color image can be output on the screen.

[0053]

As described above, according to the method for driving a spatial light modulator according to the present invention, by using a DC waveform having a non-constant voltage, it is possible to reduce the floating of the black portion of the output image. Images with high contrast ratio can be output.

According to the projection type display of the present invention, since the black portion of the output image does not rise,
Images with high contrast ratio can be output.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an embodiment of a spatial light modulator according to the present invention.

FIG. 2 is a cross-sectional view showing another embodiment of the spatial light modulator according to the present invention.

FIG. 3 is a configuration diagram of a projection type display device used in this embodiment.

FIG. 4 is a waveform diagram showing a first example of a DC voltage waveform used in this example.

FIG. 5 is a waveform diagram showing a second example of the DC voltage waveform used in this example.

FIG. 6 is a waveform diagram showing a third example of the DC voltage waveform used in this example.

FIG. 7 is a waveform diagram showing a fourth example of the DC voltage waveform used in this example.

FIG. 8 is a drive voltage waveform diagram of a conventional spatial light modulator.

[Explanation of symbols]

 101,109 Transparent Insulating Substrate 102,107 Transparent Conductive Electrode 103 Photoconductive Layer 104 Reflective Layer 105 Liquid Crystal Layer 106,108 Alignment Film 110 Writing Light 111 Polarizer 112 Analyzer 113 Readout Light 114 DC Power Supply 201 Island Reflector 301 Screen 302 Metal halide lamp 303 CRT 304 Spatial light modulator 305 Projection lens system 306 Polarization beam splitter 307 Imaging lens system 308 Relay lens system 309 Pre-polarizer 310 Auxiliary lens 311 DC power supply

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kunihito Ogawa 1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric Industrial Co., Ltd.

Claims (13)

[Claims] 1. A method of driving a spatial light modulator comprising at least a photoconductive layer and a liquid crystal layer in a region sandwiched by two transparent insulating substrates having transparent conductive electrodes, wherein the transparent conductive electrodes are provided. A method for driving a spatial light modulator, characterized in that a direct current waveform having a non-constant voltage is applied between them. 2. The method for driving a spatial light modulator according to claim 1, wherein the photoconductive layer has a rectifying property. 3. The method of driving a spatial light modulator according to claim 1, wherein the voltage of the DC waveform changes at a cycle in the range of 1 × 10 ⁻⁶ to 1 second. 4. The method for driving a spatial light modulator according to claim 2, wherein the polarity of the voltage of the DC waveform is forward bias with respect to the photoconductive layer. 5. The direct current waveform is such that a high voltage period and a low voltage period are alternately repeated, and the low voltage period is shorter than the high voltage period. Driving method for spatial light modulator. 6. The method of driving a spatial light modulator according to claim 1, wherein the liquid crystal layer has one or more stable states. 7. The method of driving a spatial light modulator according to claim 5, wherein the high voltage of the DC waveform has a magnitude that is at least twice as high as the low voltage. 8. The method of driving a spatial light modulator according to claim 5, wherein the low voltage of the DC waveform is 0. 9. The method for driving a spatial light modulator according to claim 1, further comprising at least a reflecting mirror between the photoconductive layer and the liquid crystal layer. 10. A spatial light modulator comprising at least a photoconductive layer and a liquid crystal layer in a region sandwiched between two transparent insulating substrates having transparent conductive electrodes, and the spatial light modulator connected to the transparent conductive electrodes. DC power supply for driving the spatial light modulator, image input means for displaying an image to be input to the spatial light modulator, and an image displayed on the image input means formed on the photoconductive layer And a light source and a projection lens for reading an image output from the spatial light modulator, which are at least components, and the voltage of the DC waveform output from the DC power supply is not constant. Type display. 11. The projection type display according to claim 10, wherein at least a reflecting mirror is formed between the photoconductive layer and the liquid crystal layer. 12. The projection display according to claim 10, wherein the image input means comprises a cathode ray tube. 13. The direct current waveform is such that a high voltage period and a low voltage period are alternately repeated, and the high voltage period is at least ⅕ or less of the low voltage period. Item 10. The projection display according to item 10.