Driving method for spatial light modulator and projection display system

Abstract: A driving method for a spatial light modulator can provide bright image, images of high contrast and resolution with no persistence and instability, and can be used in a projection display system. The spatial light modulator is prepared by sandwiching a ferroelectric liquid crystal layer 105 between a first substrate and a second substrate. The first substrate is prepared by sequentially laminating a transparent conductive electrode 102 and a photoconductive layer with rectifying properties 103 on a glass substrate 101. On the photoconductive layer 103, a reflective layer 104 and an alignment layer 106 for aligning a liquid crystal layer 105 are then laminated. The second substrate is prepared by laminating a transparent conductive electrode 107 and an alignment layer 108 on a glass substrate 109. Alternating current voltage having a waveform of inconsistent cycles is applied to a section between the transparent conductive electrodes102, 107.

Description: This invention relates to a driving method for a spatial light modulator applied to optical processors, projection display systems, and the like, and further relates to a projection display system applying the driving method. Optically addressed spatial light modulators applying a liquid crystal layer basically include a photoconductive layer, a liquid crystal layer which has varying light transmittivity by the application of an electric field, and two transparent conductive electrodes sandwiching the photoconductive layer and the liquid crystal layer. (Spatial light modulators mentioned below indicate the optically addressed spatial light modulators.) The spatial light modulators are driven by the application of voltage from an outside source to a section between the transparent conductive electrodes. When writing light is irradiated to the photoconductive layer, the electrical resistance of the photoconductive layer changes. Then, voltage applied to the liquid crystal layer varies, thus changing the orientation of liquid crystal molecules. As a result, functions such as the thresholding operation of light, wavelength conversion, incoherent-to-coherent conversion and image storage can be achieved, so that the spatial light modulators are a key device for information processing. When readout light with high intensity is irradiated from the direction opposite the direction of writing light and written information is read by reflection, light amplifying properties are added to the spatial light modulators. Therefore, the modulators can be used as a projection display system, and are expected to be used as general-purpose devices. Besides the projection display system applying the above-mentioned optically addressed spatial light modulator, the practical projection display systems include the system of projecting with three cathode ray tubes (CRT) having high brightness, and the system of projecting an active matrix liquid crystal light valve with a light source of high brightness. In the system of projecting with CRT, a color image is obtained by displaying images on R (red), G (green) and B (blue) CRT having high brightness and 5-7 inches in the diagonal direction and by projecting and converging the images on a screen through three projection lenses. However, since CRT has to display with high brightness so as to provide a bright picture, the resolution and contrast are poor. There is also a problem in that the projection apparatus is heavy. In the system of projecting an active matrix liquid crystal light valve with a light source of high brightness, images are displayed on three (R, G and B) liquid crystal panels or on one liquid crystal panel which includes R, G and B color filters in one body. The images are then read by a highly bright light source for backlight such as a metal halide lamp and a halogen lamp, thus projecting the images onto a screen. Compared with the system of projecting with CRT, a projection apparatus can be small and light in this system. However, in order to provide images of high resolution, the picture element size of a liquid crystal panel has to be small. As a result, the ratio between the size of a picture element and a shading area (a transistor section for driving a liquid crystal layer) becomes large, thus lowering the aperture ratio of the picture element and darkening images. As described above, there is a trade-off between resolution and brightness. In the projection display systems applying the CRT or the active matrix liquid crystal light valve, both resolution and brightness cannot be accomplished. In the system of applying the optically addressed spatial light modulator, images are input to a photoconductive layer by CRT, and the images are read by reflection while a light source of high brightness is irradiated from the side of a liquid crystal layer. The images are then projected onto a screen through projection lenses. In this system, the projection apparatus can be kept small and light. Bright images of high resolution are also obtained, thus solving the above-mentioned problems of resolution and brightness. A hydrogenated amorphous silicon (a-Si:H) thin film having high sensitivity with respect to visible light is generally applied as a photoconductive layer constituting a spatial light modulator. As a liquid crystal layer, a ferroelectric liquid crystal which is capable of rapid response is applied in general. The waveform shown in Fig. 14 is proposed as the waveform of an alternating current voltage driving the spatial light modulator (Y. Tanaka et al., Japanese Journal of Applied Physics, 33 (6A), 1994, pp. 3,469-3,477). In period T w when negative voltage V w is applied, input images are provided to the a-Si:H (photoconductive) layer, and the images are written in the ferroelectric liquid crystal layer. In period T e when positive voltage V e is applied, the written images are erased. In the conventional driving method of a spatial light modulator mentioned above, half-tone display becomes possible even in the spatial light modulator, applying a bistable ferroelectric liquid crystal, by setting erasing voltage V e larger than writing voltage V w . Bright output images can also be provided by setting erasing period T e (off-state (dark state) in the spatial light modulator) shorter than T w (on-state (bright state) in the modulator). However, as in the conventional driving method, the liquid crystal layer gradually switches to the on-state by setting T w long, even if writing light is not irradiated. Thus, the contrast of output images in the spatial light modulator radically declines. In addition, since T e is short, the images written in the writing period (T w ) remain even after T e (persistence phenomenon). The sticking phenomenon, which is the persistence phenomenon lasting for more than one minute, can also be found. The persistence phenomenon or the sticking phenomenon is solved by lengthening cycle so as to make the actual erasing period (T e ) longer, by setting the erasing period longer than the writing period under a constant cycle, or by setting the applied voltage (V e ) larger in the erasing period (T e ). However, if the erasing period is lengthened, the time aperture ratio of the spatial light modulator declines, so that the output images become dark. When the applied voltage (V e ) in the erasing period is set large, a large portion of the erasing voltage (V e ) remains in the liquid crystal layer even in the writing period (T w ) after the erasing period. As a result, light of large intensity is required to write in images, thus lowering writing sensitivity, the resolution and contrast of written images, and the resolution and contrast of output images of the spatial light modulator. As in the above-mentioned conventional driving method of a spatial light modulator, the transmittivity of a liquid crystal layer becomes large with a longer writing period (T w ) even when writing light is not irradiated. Thus, the contrast of output images declines. This problem is caused by the electrostatic capacity of the liquid crystal layer being equal or smaller than the capacity of the photoconductive layer. In order to solve the problem, the electrostatic capacity of the photoconductive layer can be set much smaller than the capacity of the liquid crystal layer, so that the photoconductive layer has to be five times as thick as the liquid crystal layer. However, when the photoconductive layer is thickened, the thickness of the liquid crystal layer becomes uneven due to the warp or deformation of a substrate by the increase in stress of the photoconductive layer. As a result, the uniformity of quality of output images radically worsens, and the manufacturing cost of spatial light modulators increases since the time required for forming a photoconductive layer increases. Cycles can be shortened so as to set the actual writing period shorter or the writing period under constant cycles can be set shorter than the erasing priod, thus solving the problems mentioned above. However, when writing light with large intensity is used it becomes necessary to switch a liquid crystal layer in a short period, thus lowering the writing sensitivity of the spatial light modulators, the resolution and contrast of written images and the resolution and contrast of output images. When an image, display device providing a two dimensional image, by scanning from one point to another (such as CRT) is applied as a means of writing images in a projection display system using a spatial light modulator, the frame frequency of CRT and the frequency of the driving waveform of the spatial light modulator resonate. As a result, a "beat", which is the distribution of brightness having a certain spatial cycle, is found on the output images of the spatial light modulator. If the beat is clearly found, the picture quality of images declines considerably due to the generation of a contrast band on the images. The contrast band shifts as time passes. When the speed of the shifting is high, the band is perceived as flickering, so that looking at the images becomes difficult. The beat becomes especially more severe with a spatial light modulator using a photoconductor with a rectifing property and a ferroelectric liquid crystal as a liquid crystal which switches according to a polarity of applied voltage because the output image repeats on and off forcibiy in response to the frequency of driving AC voltage, the driving frequency of the spatial light modulator and the frame frequency of CRT become easy to resonate with each other. At any frequency of driving waveform, the beat is generated even though there is a difference in the level of the beat. The frequency of driving waveform can be set higher than 1KHz, so that the frequency becomes too high for human eyes to sense the frequency of beat. However, output images become darker since the time aperture ratio of the spatial light modulator is reduced. EP-A-0 617 312 discloses a spatial light modulator and a method for driving the same. Said method includes the steps of keeping the intensity threshold values of a spatial light modulator constant, and changes at least one of the minimum value of the driving voltage in the writing period (V w ), the maximum value of the driving voltage in the erasure period (V e ), and the width of the writing period (T w ), to enable a half-tone display that is not varied with the lapse of time. Specifically, these parameters V w , V e and T w are changed while measuring the brightness on the screen, and a feedback is performed. Hence, if there is no change of the brightness on the screen, these parameters are constant. It is an object of the invention to provide for an output image of high contrast and resolution in which beat is inhibited and persistence and sticking are not found. This object is achieved with the features of the claims. When alternating current voltage with inconsistent cycles is applied as a driving waveform, long and short writing periods which are influenced by the length of cycles are provided. In the long writing period, the liquid crystal layer is likely to switch even in a state with no irradiation of writing light, but the intensity of writing light can be reduced. In the short writing period, on the other hand, the switching of the liquid crystal layer in the state with no irradiation of writing light can be prevented. However the intensity of writing light becomes high. Therefore, due to the existence of long and short writing periods and the nonlinear properties of the liquid crystal layer, the merits both of long and short writing periods can be obtained, and the weak points of each period can become unnoticed. As a result, the switching of the liquid crystal with no irradiation of writing light is prevented, and the intensity of writing light can also be weakened, so that output images of high contrast and resolution are provided. In addition, since the cycles are short and long, there are also short and long erasing periods. When the erasing period is short, written images cannot be erased completely, thus generating the persistence or sticking. However, the persistence or the sticking is removed immediately in the long erasing period, and human eyes cannot detect the persistence or sticking in the output images. If the first or the second voltage in each cycle is not constant, the following properties are found by applying the first voltage as erasing voltage and the second voltage as writing voltage. When the erasing voltage is large, the persistence and the sticking are prevented. With small erasing voltage, residual erasing voltage left in the liquid crystal layer during the writing period is reduced. The intensity of writing light is reduced when the writing voltage is large. With small writing voltage, the liquid crystal no longer switches naturally by irradiating no writing light in the writing period. As a result, the contrast and resolution of the output image improves. From these advantages, images of high contrast and resolution whose persistence or sticking is unnoticed are provided. The properties mentioned below are found by using the first voltage as erasing voltage and the second voltage as writing voltage, when the second voltage in one cycle of alternating current voltage is not constant. In other words, the erasing voltage in each cycle is shifted from high to low as time passes. When the erasing voltage is high, written images are completely deleted, thus preventing the persistence and sticking. Just before the writing period, the erasing voltage becomes low, and voltage applied to the liquid crystal layer at the early stages of the writing period becomes small, thus weakening the intensity of writing light. Therefore, images of high contrast and resolution whose persistence or sticking is unnoticed are provided. On the other hand, when the writing voltage in each cycle is changed from high to low as time passes, the intensity of writing light can be reduced at the early stage with high voltage. The problem of switching the liquid crystal layer with no irradiation of writing light is solved, by applying small voltage of the later stage and images of high resolution and contrast are provided. If the ratio between the period of the first voltage and the period of the second voltage is not constant, the following properties are found by applying the period of the first voltage as the erasing period and the period of the second voltage as the writing period. When the ratio between the erasing period and the writing period is large, the brightness of output images decline. However, the generation of persistence or sticking can be prevented. In addition, the liquid crystal layer no longer switches naturally with no irradiation of writing light. If the ratio is small, the persistence or the sticking is likely to be generated. There is also a problem in that the liquid crystal layer naturally switches with no irradiation of writing light. However, output images can be lightened. In other words, due to the existence of large and small ratios between the erasing period and the writing period, the merits of both a large ratio and small ratio are found and the weak points of these ratios become unnoticed. Therefore, output images of high contrast and brightness are provided. If the first voltage is larger than the second voltage, a half-tone display becomes possible even with a spatial light modulator using bistable ferroelectric liquid crystals, by applying the first voltage as the erasing voltage and the second voltage as the writing voltage. Bright output images are provided by applying the period of the first voltage as the erasing period (off-state (dark state) in the spatial light modulator) and the period of the second voltage as the writing period (on-state (light state) in the modulator) when the period of the first voltage is shorter than the period of the second voltage. Output images of stable brightness are also provided if the cycle of alternating current voltage ranges from T o /10 to 10T o where T o is the median cycle. When the second voltage in one cycle of alternating current voltage has at least one maximum or minimum value, sensitivity to the writing light of the spatial light modulator varies with respect to time, so that the brightness distribution of output images generated from the brightness distributions of a writing and reading optical system and a writing optical system become small. At least one voltage selected from the group consisting of the first voltage and the second voltage ranges from V o /10 to 10V o where V o is a time average value equal to {the sum of (voltage multiplied by application time per cycle) for at least ten voltage cycles} divided by {the sum of (application time per cycle) for at least ten voltage cycles}, so that output images of stable brightness are provided. When the range of the ratio between the period of the first voltage and the period of the second voltage is from 0.1 to 10, output images of stable brightness are provided by applying the period of the first voltage as the erasing period and the period of the second voltage as the writing period. Photocarriers are efficiently generated by the irradiation of writing light when the photoconductive layer has rectifying properties, so that the photocarriers are efficiently transported to the liquid crystal layer. If the liquid crystal layer consists of at least one material selected from the group consisting of ferroelectric liquid crystals and antiferroelectric liquid crystals, the liquid crystal layer can be thinned. Thus, the photoconductive layer can also be thin. The ferroelectric liquid crystals and the antiferroelectric liquid crystals are capable of quick response and are useful since they have memory properties. When the ferroelectric liquid crystals, the antiferroelectric liquid crystals, or a mixture of the ferroelectric and antiferroelectric liquid crystals are used for the liquid crystal layer, images written in the layer can be erased by the application of forward bias. Fig. 1 is a cross-sectional view of a spatial light modulator applied to one embodiment of the driving method of the invention. Fig. 2A is a cross-sectional view of another spatial light modulator applied to one embodiment of the driving method of the invention. Fig. 2B is a cross-sectional view of the spatial light modulator of the invention. Fig. 3 is a schematic view of a projection display system of the invention. Fig. 4 shows an alternating current voltage waveform applied to one embodiment of the driving method of the invention. Figs. 5 to 13 show further examples of alternating current voltage waveforms. Fig. 14 shows the driving voltage waveform of a conventional spatial light modulator. This invention will be described by referring to the following illustrative examples and attached figures. Fig. 1 is a cross-sectional view of the spatial light modulator of one embodiment of the invention. As shown in Fig. 1, a transparent conductive electrode 102 (for example, ITO (indium-tin oxide), conductive oxide such as ZnO and SnO 2 , or a semi-transparent metal thin film such as Cr, Au, Pt and Pd) and a photoconductive layer 103 made of an amorphous semiconductor are sequentially formed on a transparent insulating substrate 101 (for instance, a heat resistant glass substrate, fused silica substrate or sapphire substrate). On photoconductive layer 103, a reflector 104 and an alignment film 106 for aligning liquid crystal layer 105 are laminated, thus preparing a first substrate. A transparent conductive electrode 107 (e.g., ITO (indium-tin oxide), conductive oxide such as ZnO and SnO 2 , or a semi-transparent metal thin film such as Cr, Au, Pt and Pd) and an alignment film 108 for aligning a liquid crystal layer 105 are sequentially formed on a transparent insulating substrate 109 (for example, a heat resistant glass substrate, fused silica substrate, or sapphire substrate), thereby preparing a second substrate. Liquid crystal layer 105 is sandwiched between the first and second substrates. The spatial light modulator is driven by applying alternating current voltage from an AC power supply 114 which is connected to a section between transparent conductive electrodes 102 and 107. As the alternating current voltage, voltage having a waveform shown in Fig. 4, for example, is applied. In the figure, the period of applying negative voltage (V w ) is a writing period (T w ) for writing images in the spatial light modulator; the period of applying positive voltage (V e ) is an erasing period (T e ) for erasing written images. When writing light 110 is irradiated from the side of transparent insulating substrate 101 to photoconductive layer 103 during the application of negative voltage (V w ) to the spatial light modulator, the electric resistance of photoconductive layer 103 at a section where writing light 110 is irradiated changes. Thus, the voltage across corresponding of liquid crystal layer 105 increases, changing the orientation of liquid crystal molecules. The orientation of the liquid crystal molecules is observed as reflecting light from reflector 104 by an optical system of a polarizer 111 and an analyzer 112 while readout light 113 is irradiated from the side opposite to the direction of writing light 110 (side of transparent conductive electrode 109). Instead of the optical system of polarizer 111 and analizer 112, one polarizing beam splitter can also be applied. By referring to Figs. 4-13, specific examples of alternating current voltage waveform applied to the spatial light modulator from the AC power supply are explained below. Fig. 4 shows an alternating current voltage waveform in which the frequency 1/T (cycle T=T e +T w ) is changed at each cycle. (Erasing voltage V e , writing voltage V w , and the ratio (T e /T w : duration ratio) between erasing period T e and writing period T w are set constant.) Flickering is not detected by human eyes at the upper limit of the fluctuation range of cycle, T; the liquid crystal layer can respond at the lower limit of the range. The lower limit depends on the material of liquid crystals and the thickness of the liquid crystal layer. However, the specific range of cycle T (frequency 1/T) is preferably from 1µ sec to 1 sec (from 1Hz to 1MHz). It is more preferable that the range is from 10µ sec to 0.1 sec (from 10Hz to 100kHz), and is further preferable that the range is from 100µ sec to 0.33 sec (from 30Hz to 10kHz). By applying an alternating current voltage waveform having inconsistent cycles T, long and short writing periods T w are generated due to the length of cycles T. When writing period T w is long, the liquid crystal layer is likely to switch even with no irradiation of writing light. However, on the other hand, the intensity of writing light is weakened. With a short writing period T w , the intensity of writing light becomes large, but the switching of the liquid crystal layer with no irradiation of writing light can be prevented. Thus, with the existence of long and short writing periods T w and the nonlinear properties of the liquid crystal layer, the merits of both long and short writing periods T w are found, and negative aspects of each period become unnoticed. As a result, the switching of the liquid crystal layer with no irradiation of writing light is prevented, and the intensity of writing light can be kept small, thus providing output images of high contrast and resolution. Because of the long and short cycles T, there are short and long erasing periods T e . When erasing period Te is short, the deletion of written images is unsatisfactory. Thus, the persistence or sticking is likely to occur. However, in long erasing period T e , the persistence or sticking is removed, so that human eyes cannot detect those phenomena. Due to the existence of short and long erasing periods Te and the nonlinear of liquid crystal layer, the merits of each short and long erasing period are obtained, and the negative aspects of the periods become unnoticed. As a result, output images of high contrast and resolution with no preceived persistence and sticking are obtained. If the frequencies are changed in a wide range at each cycle and the spatial light modulator is used as a display, inconsistency is found in the brightness of images. When cycle T is changed from T o /10 to 10T o with respect to center cycle T o , images of stable brightness are provided. The specific range of T o is from 200 µ sec to 20m sec. Writing period Tw is longer than erasing period Te to obtain a bright image when the module is applied as a display. In other words, the duration ratio (T e /T w ) is preferably less than 1. However, when the module is applied as an optical processor, a hologram system and the like, the duration ratio is preferably from 0.01 to 2, or more preferably from 0.05 to 1. Figs. 5A to 5D show an alternating current voltage waveform in which only writing voltage V w is changed at each cycle with the passage of time, and cycles T, duration ratio (T e /T w ) and erasing voltage V e are kept constant. Writing voltage V w shifts from the initial value (V w1 ) to maximum value (V w2 ), and then to V w3 . In the figures, four patterns shift from the initial value to the maximum value and then to V w3 . The patterns of the change in writing voltage V w are not limited to these examples. As long as the time of reaching the maximum value (V w2 ) is within writing period T w , the pattern is not particulary limited. The change in writing voltage V w fluctuates the sensitivity of the spatial light modulator with respect to writing light 110 as time passes. In other words, the spatial light modulator has the highest sensitivity at maximum value V w2 , and the output images become the brightest with respect to writing light having a certain intensity. Therefore, the brightness distribution of output images generated by the brightness distributions of a writing optical system and a reading optical system is minimized when this alternating voltage current waveform is applied as a driving waveform. Similarly, as shown in Figs. 6A to 6D, an alternating current voltage waveform of shifting writing voltage V w from initial voltage (V w1 ) to minimum value (V w2 ) and then to V w3 along with the brightness distribution of output images can be applied as a driving waveform. Fig. 7A shows an alternating current voltage waveform with changing erasing voltage V e at each cycle while cycles T, duration ratio (T e /T w ) and writing voltage (V w ) are set constant. However, in Fig. 7A; erasing voltage V e varies regularly. When this alternating current voltage waveform is applied as a driving waveform, the properties as described below are found. With a large erasing voltage V e , the persistence or sticking is prevented. When erasing voltage V e is small, the erasing voltage remaining in the liquid crystal layer in writing period T w is reduced. Thus, the writing sensitity does not decline, and bright images with no persistence and sticking are obtained. Fig. 7B shows an alternating current voltage waveform with changing writing voltage V w at each cycle while cycles T, duration ratio (T e /T w ) and erasing voltage (V e ) are set constant. However, in Fig. 7B, the writing voltage varies regularly. When this alternating current voltage waveform is applied as a driving waveform, the following properties are found. When the writing voltage is high, the intensity of writing light is lessened. With low writing voltage V w , the natural switching of the liquid crystal layer with no irradiation of writing light is prevented, so that bright images of high resolution and contrast are obtained. Figs. 8A to 8C show an alternating current voltage waveform in which erasing voltage V e changes regularly while cycles T, duration ratio (T e /T w ) and writing voltage (V w ) are kept constant. In Fig. 8A, cycles having low erasing voltage (V e2 ) are repeated (1) times after one cycle having high erasing voltage (V e1 ). Furthermore, after one cycle of high erasing voltage (V e1 ), cycles having low erasing voltage (V e2 ) are repeated (m) times. In Fig. 8B, cycles having high erasing voltage (V e1 ) are repeated (n) times after one cycle of low erasing voltage (V e2 ); cycles of high erasing voltage (V e1 ) are repeated (u) times after one cycle having low erasing voltage (V e2 ). In Fig.8C, cycles having high erasing voltage (V e1 ) are repeated (n) times after cycles having low erasing voltage (V e2 ) are repeated (1) times ; cycles having high erasing voltage (V e1 ) are repeated (u) times after cycles having low erasing voltage (V e2 ) are repeated (m) times. When (1), (m), (n) and (u)≧1, (1) can be either equal or unequal to (m), and (n) can be equal or unequal to (u). Therefore, when erasing voltage (V e ) is large, the persistence and sticking are prevented. With small erasing voltage (V e ), residual erasing voltage in the liquid crystal layer during the writing period is reduced. As a result, output images of high contrast and resolution with no persistence and sticking are obtained. Figs. 9A to 9C show an alternating current voltage waveform with regularly changing writing voltage V w while cycles T, duration ratio (T e /T w ) and erasing voltage V e are kept constant. In Fig. 9A, cycles of high writing voltage V w1 are repeated (q) times after one cycle having low writing voltage V w2 ; cycles of high writing voltage V w1 are repeated (r) times after one cycle having low writing V w2 . In Fig. 9B, cycles of low writing voltage V w2 are repeated (s) times after one cycle having high erasing voltage V w1 ; cycles of low writing voltage V w2 are repeated (t) times after one cycle having high erasing voltage V w1 . In Fig. 9C, cycles of low writing voltage V w2 are repeated (s) times after cycles having high writing voltage V w1 are repeated (q) times, cycles having high writing voltage are repeated (r) times, and cycles of low writing voltage V w2 are repeated (t) times. When (q), (r), (s) and (t) are one or larger than one, (q) is equal or unequal to (r). In addition, (s) is equal or unequal to (t). Therefore, when the writing voltage is large, the intensity of writing light can be reduced. The natural switching of the liquid crystal layer with no irradiation of writing light is prevented when the writing voltage is small. As a result, bright images of high resolution and contrast are provided. In Figs. 8A to 8C, and in Figs. 9A to 9C the erasing voltage or the writing voltage has two types of values. However, the erasing voltage or the writing voltage may have three or more types of values. In Figs. 8A to 8C low erasing voltage (V e2 ) and high erasing voltage (V e1 ) have two types of cycle numbers. (The cycle numbers of the low erasing voltage are (1) times and (m) times. The cycle numbers of the high erasing voltage are (n) times and (u) times.) However, the low erasing voltage and the high erasing voltage can have three or more types of cycle numbers. In Figs. 9A to 9C, high writing voltage V w1 and low writing voltage V w2 have two types of cycle numbers. (The cycle numbers of the high writing voltage are (q) times and (r) times. The cycle numbers of the low writing voltage are (s) times and (t) times.) However, the high writing voltage and the low writing voltage can have three or more types of cycle numbers. If erasing voltage V e or writing voltage V w in the alternating current voltage waveforms shown in Figs. 6 to 9 is changed in a wide range, the brightness of images become inconsistent. In order to obtain the images of stable brightness, the erasing voltage or the writing voltage is preferably changed from V o /10 to 10V o where V o is a time average value equal to {the sum of (voltage multiplied by application time per cycle) for at least ten voltage cycles} divided by {the sum of (application time per cycle) for at least ten voltage cycles}. Fig. 10A shows an alternating current voltage waveform in which only the duration ratio (T e /T w ) changes at each cycle while cycles T, erasing voltage V e and writing voltage V w are kept constant. Fig. 10B shows an alternating current voltage waveform changing only duration ratio (T e /T w ) at each cycle while writing period T w , erasing voltage V e and writing voltage V w are kept constant. Fig. 10C, shows an alternating current voltage waveform varying only writing period T w at each cycle so as to change the duration ratio (T e /T w ) while erasing period T w'-e erasing voltage V e and writing voltage V w are kept constant. When the duration ratio is large, the brightness of output images decline. However, the generation of persistence or sticking, and the natural switching of liquid crystal layer with no irradiation of writing light are prevented. When the duration ratio is small, the persistence or sticking is unlikely to occur. Even though the natural switching of the liquid crystal layer with no irradiation of writing light is likely to occur, output images can be brightened. Due to the existence of large and small duration ratios and the nonlinear properties of the liquid crystal layer, the merits of the large and small duration ratios are found, and the negative aspects of the duration ratios become unnoticed. As a result, the bright output images of high contrast and resolution with no persistence and sticking are obtained. If the duration ratios at each cycle of the alternating current voltage waveform of Figs. 10A to 10C are changed in a wide range and the spatial light modulator is applied as a display, the brightness of images becomes inconsistent. In order to provide images of stable brightness from the spatial light modulator applied as a display, the duration ratios (T e /T w ) are preferably in the range from 0.1 to 10. Fig. 11A shows an alternating current voltage waveform in which frequency 1/T and erasing voltage V e change at each cycle while duration ratios (T e /T w ) and writing voltage (V w ) are set constant. Fig. 118 shows an alternating current voltage waveform in which frequency 1/T and writing voltage V w change at each cycle while duration ratios (T e /T w ) and erasing voltage (V e ) are set constant. The properties provided from the application of the alternating current voltage waveform of changing frequency 1/T and erasing voltage V e at each cycle as a driving waveform are as follows. In other words, with a short erasing period T e , the deletion of written images is not sufficient, and the persistence or sticking is likely to occur. However, the persistence or sticking is removed in the long erasing period, so that human eyes cannot detect those phenomena. Due to the existence of short and long erasing periods and the nonlinear properties of the liquid crystal layer, the merits of the short and long erasing periods are found, and the negative aspects of the periods are unnoticed. As a result, output images of high contrast and resolution with no persistence and sticking are provided. The effects mentioned below are found when the alternating voltage waveform with changing frequency 1/T and writing voltage V w at each cycle is applied as a driving waveform. With long writing period T w , the liquid crystal layer is likely to switch with no irradiation of writing light, but the intensity of writing light can be reduced. When writing period T w is short, the intensity of writing light becomes large. However, the switch of the liquid crystal layer with no irradiation of writing light is prevented. Due to the existence of long and short writing periods and the nonlinear properties of the liquid crystal layer, the benefits of the long and short writing periods are found, and the negative aspects of the periods are unnoticed. As a result, the switching of the liquid crystal layer with no irradiation of writing light is prevented, and output images of high contrast and resolution are provided. In Fig. 12, erasing voltage V e and writing voltage W w in each cycle vary at each cycle as time passes. The figure shows an alternating current voltage waveform with changing frequency 1/T and duration ratios T e /T w at each cycle. Since this alternating current voltage waveform has the properties of the alternating current voltage waveforms shown in Figs. 4 and 10 bright images of high resolution and contrast with no persistance and sticking are obtained. In Fig. 12, there are two types of change in erasing voltage V e (from V e1 to V e2 and from V e2 to V e3 ). The change in the erasing voltage is not limited to two types, and can be one type or three or more types. The types of the change in erasing voltage V e may be the same as or different from the types of change in writing voltage V w . When liquid crystals having a memory function such as ferroelectric liquid crystals are used, voltage may not be applied continuously in the erasing period or the writing period as in the alternating current voltage waveforms shown in Figs. 4 to 12, but can be applied only in a short period as in Figs. 13A to 13C. Fig. 13A shows an alternating current voltage waveform with frequency 1/T varying at each cycle while erasing voltage V e , writing voltage V w and duration ratios (T e /T w and T e1 /T w1 ) are set constant. shows an alternating current voltage having two values of erasing voltage V e and writing voltage V w while cycles T and duration ratios (T e /T w and T e1 /T w1 ) are kept constant. Each of the two values of the erasing voltage and the writing voltage appears every other cycle. Fig. 13C' shows an alternating current voltage waveform with periods T e1 and T w1 for the application of erasing voltage V e1 and writing voltage V w1 varied at each cycle while cycles T, erasing voltage V e and writing voltage V w are set constant. Nematic liquid crystals, super-twist nematic liquid crystals, ferroelectric liquid crystals, antiferroelectric liquid crystals, polymer-dispersed liquid crystals or the like are applied for liquid crystal layer 105. When the ferroelectric liquid crystals or the antiferroelectric liquid crystals are applied, the thickness of liquid crystal layer 105 is kept small, so that photoconductive layer 103 is kept thin. The ferroelectric and antiferroelectric liquid crystals are useful since they are capable of quick response and have a memory function. These properties are obtained even when the mixed material of ferroelectric liquid crystals and antiferroelectric liquid crystals is applied. The transmittivity of ferroelectric liquid crystals has a steep threshold characteristic with respect to voltage, so that the liquid crystals are a suitable material for carrying out a threshold treatment in response to input light. When the polymer-dispersed liquid crystals are used, alignment films 106 and 108 become unnecessary. Polarizer 111 and analyzer 112 also are not required. As a result, output light becomes bright and an element structure and an optical system become simple. Liquid crystal layer 105 is sealed with resin, and spacers (not shown in Fig. 1) are mixed in liquid crystal layer 105 so as to arrange the thickness. Beads made of alumina, glass or quartz, glass fiber powder, or the like are used as the spacers. The spacers are also mixed in the resin sealing liquid crystal layer 105. Alignment films 106 and 108 for aligning the liquid crystals are SiO x oblique evaporated layers or organic polymer thin films, made of polyimide, polyvinyl alcohol or the like and treated with a rubbing treatment. A material that can be formed as a film in a wide area at a relatively low temperature (less than 400°C), can generate photocarriers efficiently in response to the irradiation of writing light 110 and can efficiently transport the photocarriers to the side of liquid crystal layer 105 is preferable for photoconductive layer 103. More specifically, a single layer of hydrogenated amorphous semiconductor such as a-Si:H, hydrogenated amorphous germanium (a-Ge:H), hydrogenated amorphous silicon carbide (a-Si 1-x C x :H where 0<x<1), hydrogenated amorphous silicon germanium (a-Si 1-x Ge x :H), hydrogenated amorphous germanium carbide (a-Ge 1-x C x :H), and hydrogenated amorphous germanium nitride (a-Ge 1-x N x :H), or a laminated layer including of at least two layers of the above-mentioned hydrogenated amorphous semiconductor is applied. Halogen atoms such as F and Cl, and hydrogen may be added to the hydrogenated amorphous semiconductor mentioned above, thus efficiently reducing a dangling bond which works as a carrier trap. Moreover, a small amount (for instance, 0.1-10% by atom) of oxygen (O) atoms or nitrogen atoms may be added to the semiconductor. If photoconductive layer 103 has rectifying properties, photocarriers are efficiently generated with respect to the incidence of writing light 110. Then, the photo carriers are transported efficiently to the side of liquid crystal layer 105. The rectifying properties are added to photoconductive layer 103 when p/i, i/n and p/i/n structures are formed inside the photoconductive layer (i layer is an undoped layer). In order to form a p-type layer, a p-type impurity such as B, Al and Ga can be added at 1×10 -4 -10 atom %. The thickness of the p-type layer is preferably 1-10 3 nm, more preferably 2-3×10 2 nm, and most preferably 5-30 nm. An n-type layer can be formed by adding an n-type impurity such as P, As and Sb at 1×10 -4 -10 atom %. The n-type layer is preferably 1-3×10 3 nm thick, more preferably 10-2×10 3 nm, and most preferably 50-1×10 3 nm. When liquid crystals which switch due to the polarity of voltage (e.g., ferroelectric liquid crystals, antiferroelectric liquid crystals, etc.) are used for liquid crystal layer 105, images written in liquid crystal layer 105 can be erased by the application of forward bias. The thickness of photoconductive layer 103 is determined by the correlation with liquid crystal layer 105, but is generally 0.5-10µm. As reflector 104, a multi-layered dielectric mirror, in which a thin film of a large dielectric constant material such as TaO 2 and Si and the thin film of a small dielectric constant material such as MgF and SiO 2 are alternately laminated, is used. Figs. 2A and 2B show other examples of the spatial light modulator of the invention. In the spatial light modulators shown in the figures, metallic thin films made of a material with a large reflectance such as Al, Ag, Mo, Ni, Cr, Mg and Ti are discontinuously formed as the reflector, so that an insular reflector 201 arranged in a two-dimensional matrix or mosaic state is applied. If the reflector is formed continuously, no potential difference is generated and the formation of images becomes impossible. Each section of insular reflector 201 corresponds to one picture element. Photoconductive layer 103 between areas of insular reflector 201 is removed by etching, thus preventing the horizontal diffusion of photocarriers and providing high resolution corresponding to the arrangement of insular reflector 201. When images are read out by irradiating light with large intensity, readout light 113 enters photoconductive layer 103, which generates photocarriers, through gaps between the sections of insular reflector 201. As a result, the undesirable switching of liquid crystal layer 105 occurs. It is preferable to remove photoconductive layer 103 between the sections of insular reflector 201 entirely as shown in Fig. 2B. However, photoconductive layer 103 can be left as shown in Fig. 2A as long as it is at a thickness so that visible rays are hardly absorbed and can transmit (less than 1.5µm thick, or more preferably less than 0.5µm). Moreover, a light absorbing layer 202 for absorbing visible rays (for instance, organic polymer in which carbon particles are dispersed, organic polymer mixed with black pigment or black dye, or an inorganic thin film such as a-C:H, a-Ge:H and a-Ge 1-x N x ) may be formed in the gaps between the sections of the insular reflector 202, so that readout light 113 leaked from the reflector can be efficiently absorbed. In order to completely shield out readout light 113, a metal light blocking film 203 made of Al, Ag, Mo, Ni, Cr or Mg can be formed on the bottom of the gaps. If an insulating film 204 is formed on the gaps, electric insulation between the sections of insular reflector 201 becomes complete. The insulating film 204 is made of an inorganic insulating material such as SiO x , SiN x , SiC x , GeO x , GeN x , GeC x , AlO x , AlN x , BC x , and BN x , or an organic insulating material such as polyimide, polyvinyl alcohol, polycarbonate, poly-p-xylene, polyethylene terephthalate, polypropylene, poly(vinyl chloride), poly(vinylidene chloride), polystyrene, poly(ethylene tetrafluoride), poly(ethylene chloride trifluoride), polyvinylidene fluoride, propylene hexafluoride-ethylene tetrafluoride copolymer, ethylene trifluoride-vinylidene copolymer fluoride, polybutene, polyvinyl butyral, and polyurethane. Example 1 As shown in Fig. 1, a 0.05-0.2µm thick ITO film was formed on a glass substrate 101 by a sputtering method, and a transparent conductive electrode 102 was then formed. The substrate was then placed in a plasma CVD apparatus, and the substrate was heated by a heater at 280°C after the vacuum chamber was exhausted to less than 1×10 -5 Torr. To the vacuum chamber, 400sccm B 2 H 6 having 10ppm (1ppm=1×10 -6 ) and diluted with He, 1sccm SiH 4 , and 0.2sccm C 2 H 2 were introduced. The pressure of the chamber was maintained at 0.5-0.8 Torr. Plasma was generated by applying 20-30W radio frequency electric power of 13.56MHz frequency to the electrode, so that a 5-50nm thick p-type a-Si 1-x C x :H layer was formed on transparent conductive electrode 102. After exhausting the vacuum chamber to a high vaccum level, 100sccm H 2 and 40sccm SiH 4 were introduced to the chamber. The pressure in the chamber was set to 0.5-0.8 Torr. Then, a 2-5µm thick i-type a-Si:H layer was formed on the p-type a-Si 1-x C x :H layer by generating plasma with the application of 15-30W radio frequency electric power of 13.56MHz to the electrode. The vacuum chamber was again exhausted to a high vacuum level, and 160sccm N 2 and 1sccm GeH 4 were then introduced to the chamber. The pressure in the chamber was maintained at 0.5 Torr. Plasma was generated by applying 20W radio frequency electric power of 13.56MHz frequency to the electrode, so that a 0.3-1µm thick i-type a-Ge 1-x N x :H layer (0.1≦x≦0.4) was formed on the i-type a-Si:H layer. As a result, a photoconductive layer 103 having rectifying properties was formed on transparent conductive electrode 102. Then, 1.5×10 2 nm thick Si and SiO 2 layers were alternately laminated for three to ten layers each on photoconductive layer 103 by a sputtering deposition method, thus forming a multi-layered dielectric reflective layer 104. A polyimide alignment layer 106 treated with a rubbing treatment was then laminated on multi-layered dielectric reflective layer 104. A spatial light modulator (1) was manufactured by sandwiching a 0.8-1.3µm thick ferroelectric liquid crystal layer 105 between glass substrate 101 and a glass substrate 109 which was already laminated with a transparent conductive electrode 107 (ITO) and a polyimide alignment film 108. Instead of the i-type a-Ge 1-x N x :H layer of the photoconductive layer 103, an n-type a-Si:H layer was formed by applying PH 3 :50-100sccm, having 100ppm density and diluted with H 2 , and SiH 4 :5-20sccm, thus manufacturing a spatial light modulator (2). An alternating current voltage having a waveform shown in Fig. 4 (erasing voltage V e =15V, writing voltage V w =-3V, duration ratio (T e /T w )=1/10, change in cycle T=1-16msec) was applied to a section between transparent conductive electrodes 102 and 107 of spatial light modulators (1) and (2). White light was used as writing light 110, and a He-Ne laser (633nm) was applied as readout light 113. The voltage was applied so as to set transparent conductive electrode 102 positive. The operation of the spatial light modulator is now explained below. Writing light 110 was irradiated while negative voltage V w was applied for reverse-biasing photoconductive layer 103. Thus, voltage applied to liquid crystal layer 105 increased, switching the liquid crystals from the off-state to on-state. The on-state of the liquid crystals were observed as reflecting light from reflector 104 by irradiating readout light 113 from the side opposite to the side of writing light 110. Positive voltage V e for biasing photoconductive layer 103 forward was applied, so that liquid crystal layer 105 was changed to the off-state with or without the irradiation of writing light 110. Under these operational conditions, spatial light modulator (1) had 150-280µW/cm 2 photosensitivity, 30-50µsec rise time, and 25-50 lp (line pairs)/mm (MTF=10%) resolution. On the other hand, spatial light modulator (2) had 90-120µ W/cm 2 photosensitivity, 30-50µsec rise time, and 20-401p/mm (MTF=10%) resolution. Spatial light modulators (1) and (2) were inserted in the projection display apparatus shown in Fig. 3. As shown in Fig. 3, the projection display apparatus includes of a spatial light modulator 304, an AC power supply 311, a cathode ray tube (CRT) 303, an image formation lens (image formation means) 307, a light source for projection 302, and a lens for projection 305. The AC power supply is connected to the transparent conductive electrodes of spatial light modulator 304, and is used for driving the modulator. The cathode ray tube (CRT) is applied as a writing light source (image, input means) providing images to spatial light modulator 304. The image formation lens is for focusing images output from CRT 303 on the photoconductive layer of spatial light modulator 304. The light source for projection reads out the output images from spatial light modulator 304. The lens for projection enlarges the output images from spatial light modulator 304 by 40 times onto a screen 301 having a white color diffusing surface. In Fig. 3, 306 indicates a polarizing beam splitter, 308 is a relay lens system, 309 is a prepolarizer, and 310 is a supplementary lens. A metal halide lamp including a reflector is used as a light source for projection 302. The output waveform from AC power supply 311 has the same properties mentioned above. While negative voltage V w for reverce-biasing photoconductive layer 103 was applied, images displayed on CRT 303 were written in spatial light modulator 304. The written images were then projected onto screen 301. When positive voltage V e was applied, photoconductive layer 103 was biased forward, thus erasing the written images. Illuminance on spatial light modulator 304 was 2,000,000 lx when metal halide lamp 302 was on. The black and white contrast on screen 301 was 200:1 for both spatial light modulators (1) and (2). The resolution was evaluated by a resolution chart, and was 900 TV lines. The images projected onto screen 301 had no fluctuation of brightness and "beat". The brightness distribution around the center of screen 301 was within ± 2%. As a comparison, an alternating current voltage having a conventional waveform as shown in Fig. 14 (erasing voltage V e =15V, writing voltage V w =-3V, duration ratio (T e /T w )=1/10, cycle T=6m sec) was applied to spatial light modulators (1) and (2), and projected images were tested. According to the results, beat (flickering due to bands with different brightness) was found on the images, and it was difficult to view the images. The brightness distribution around the center of screen 301 was about ±20% because of the beat. In the projection display apparatus shown in Fig. 3, written images are provided by CRT 303. However, instead of the CRT, another display such as a liquid crystal display, a plasma display, an electro-luminescent device, a light emitting diode array, a laser diode with a two-dimensional scanning system using a polygon mirror or an acousto-optical device may be used. Example 2 As shown in Fig. 2 (a), a 0.05-0.2µm thick ITO film was formed on a glass substrate 101 by a sputtering method, thus forming a transparent conductive electrode 102. As in Example 1, a 5-50nm thick p-type a-Si 1-x C x layer, 1.4-4.0µm thick i-type a-Si:H layer, and 0.1-1.0µm n-type a-Si:H layer were sequentially laminated on transparent conductive electrode 102, thus forming a photoconductive layer 103. On the surface of photoconductive layer 103, Cr was laminated at 2×10 2 -5× 10 2 nm thickness by a vacuum evaporation method, and was then patterned by photolithography, thus forming an insular reflector 201. The shape of insular reflector 201 was 24µm× 24µm square, and the reflector was arranged in a 1000×2000 matrix condition with 2µm gap in-between. Besides the photolithography, a lift-off method can also be applied to form the insular reflector. The a-Si:H layer of photoconductive layer 103 between insular reflector 201 was removed by etching, thus forming grooves. By a vacuum evaporation method, 50-100nm thick Al was deposited on insular reflector 201 and the grooves. Insular reflector 201 had the two-layered structure of Al film and Cr film. The Al film formed on the grooves shields out readout light 113, and was a metal light blocking film 203. An insulating film 204 made of polyimide was also formed on the grooves at 1×10 2 -3×10 2 nm thickness. Resist including carbon particles was coated and filled in the grooves, thereby forming a light absorbing layer 202. The polyimide film and the resist film on insular reflector 201 were removed by a dry etching. On insular reflector 201 and light absorbing layer 202, a 10-30nm thick polyimide film was then formed, and was treated with a rubbing treatment, thus forming a polyimide alignment film 106. As a result, a first substrate was prepared. Similarly, a second substrate was prepared by laminating a transparent conductive electrode 107 (ITO) and a polyimide alignment film 108 on a glass substrate 109. A 0.8-2µm thick ferroelectric liquid crystal layer 105 was sandwiched between the first and the second substrate, so that a spatial light modulator (3) shown in Fig. 2 (a) was prepared. A spatial light modulator (4) shown in Fig. 2 (b) was also prepared by removing the entire photoconductive layer 103 between insular reflector areas 201 by etching. As in Example 1, spatial light modulators (3) and (4) were evaluated. According to the results, both had 80µW/cm 2 photo sensitivity and 30µ sec rise time. As in Example 1, spatial light modulators (3) and (4) were inserted in the projection display apparatus shown in Fig. 3, and output images on a screen 301 were tested. The alternating current voltage waveform shown in Fig. 4 was applied as the output waveform from an AC power supply 311. More specifically, the output waveform had 15V erasing voltage V e , -1.5V writing voltage V w , and 1/10 duration ratio (T e /T w ). The cycle had 0.4-30m sec fluctuation width with respect to 3m sec central cycle. As a comparison, alternating current voltage having a conventional waveform (erasing voltage V e =15V, writing voltage V w =-1.5V, duration ratio (T e /T w )=1/10) shown in Fig. 14 was applied, and the output images were tested. With the conventional alternating current voltage waveform, the brightness distribution around the center of screen 301 was within ±35%, and it was difficult to view the images since a clear beat was found. However, when the alternating current voltage waveform shown in Fig. 4 (waveform of the invention) was applied, the brightness distribution around the center of screen 301 was within ±2.5%, and beautiful images with no beat were observed. When the fluctuation width of the cycle was 0.01-100m sec with respect to 3m sec central cycle, undesirable light and shade of images were observed. Example 3 Spatial light modulators (3) and (4) were applied to the projection display systems shown in Fig. 3. Alternating current voltage having a waveform shown in Fig. 4 was applied from an AC power supply 311, and output images on a screen 301 were tested. More specifically, the alternating current voltage waveform had 15V erasing voltage V e , -2.5V writing voltage V w , and 1/5 duration ratio (T e /T w ). The cycle had ±1.4m sec fluctuation width with respect to 16.7m sec central cycle. The brightness distribution around the center of screen 301 was within ±2.5%, and beautiful images with no beat were obtained. Example 4 Spatial light modulators (3) and (4) were used in the projection display system shown in Fig. 3. Alternating current voltage having a waveform shown in Figs. 5A to 5D was applied from an AC power supply 311, and output images on a screen 301 were tested. More specifically, the alternating current voltage waveform had 15V erasing voltage V e ; -1V initial writing voltage V w , -4V maximum V w2 and -2V V w3 , 1/10 duration ratio (T e /T w ), and 16.7m sec cycle T. Picture images of high contrast (200:1) and uniform brightness were obtained. (There was only a 10% reduction in brightness relative to the brightness at the center when the angle of view was 0.9.) No persistence and sticking were observed. However. the disbribution of brightness was increased by 30% with 0.9 angle of view when the conventional alternating current voltage waveform shown in Fig. 14 was applied. Example 5 Spatial light modulators (3) and (4) were used in the projection display system shown in Fig. 3. Alternating current voltage having a waveform shown in Fig. 7A was applied from an AC power supply 311, and output images on a screen 301 were tested. More specifically, the alternating current voltage waveform had -1.5V writing voltage V W , 1/10 duration ratio (T e /T W ), and 1m sec cycle T. The range of erasing voltage Ve was from 0.5V to 50V with respect to 5V average voltage at 10 cycles. Picture images of high contrast (180:1) and high resolution (950TV) were obtained. No persistence and sticking were observed. When the range of erasing voltage Ve was from 0.1V to 100V with respect to 5V average voltage at 10 cycles, the brightness of images declined by 20%. Thus, it was not preferable. Example 6 Spatial light modulators (3) and (4) were used in the projection display system shown in Fig. 3. Alternating current voltage having a waveform shown in Fig. 7B was applied from an AC power supply 311, and output images on a screen 301 were tested. More specifically, the alternating current voltage waveform had -1.5V erasing voltage V e , 1/10 duration ratio (T e /T w ), and 1m sec cycle T. The range of writing voltage V w was from -15V to -0.15V with respect to - 1.5V average voltage at 10 cycles. Picture images of high contrast (180:1) and high resolution (1000TV) were obtained. When the range of writing voltage V w was from -50V to -0.05V with respect to -1.5V average voltage at 10 cycles, the contrast declined to 20:1 and was not preferable. Example 7 Spatial light modulators (3) and (4) were used in the projection display system shown in Fig. 3. Alternating current voltage having a waveform shown in Fig. 8C was applied from an AC power supply 311, and output images on a screen 301 were tested. More specifically, the alternating current voltage waveform had -2.5V writing voltage V w , 1/10 duration ratio (T e /T w ), and 1.25m sec cycle T. High erasing voltage V e1 was 20V while low erasing voltage V e2 was 15V, and (l), (m), (n) and (u) were set from 1 to 50. As a result, images of high contrast (150:1) and high resolution (950TV) were obtained. No persistance and sticking were observed. However, when (l) and (m) were set 50 times or more higher than (n) and (u), residual images of about 150m sec were found and were not preferable. With (n) and (u) 50 times higher than (1) and (m), the contrast declined to 80:1, and the resolution also decreased to 700TV. Furthermore, the brightness of images declined fully by 20%. Example 8 Spatial light modulators (3) and (4) were used in the projection display system shown in Fig. 3. Alternating current voltage having a waveform shown in Fig. 9C was applied from an AC power supply 311, and output images on a screen 301 were tested. More specifically, the alternating current voltage waveform had 15V erasing voltage V e , 1/10 duration ratio (T e /T w ), and 1.25m sec cycle T. High writing voltage V w1 was -1V while low writing voltage V w2 was -5V, and (q), (w), (r) and (t) were set from 1 to 50. As a result, images of high contrast (180:1) and high resolution (1000TV) were obtained. However, when (q) and (r) were set 50 times or more higher than (s) and (t), the contrast declined to less than 50:1 and was not preferable. With (s) and (t) 50 times greater than (q) and (r), the brightness of images declined fully by 50%. Example 9 Spatial light modulators (3) and (4) were used in the projection display system shown in Fig. 3. Alternating current voltage having a waveform shown in Fig. 10A was applied from an AC power supply 311, and output images on a screen 301 were tested. More specifically, the alternating current voltage waveform had 15V erasing voltage V e , and -1.5V writing voltage V w , at 330Hz frequency. The range of erasing period Te was from 0.01m to 10m sec with respect to 0.1ms average value at 10 cycles. As a result, images of high contrast (150:1) and high resolution (950TV) were obtained. However, when the range of erasing period Te was set from 0.001m sec to 30m sec with respect to 0.1m sec average value at 10 cycles, undesirable flickering was found in the image, images. Example 10 Spatial light modulators (3) and (4) were used in the projection display system shown in Fig. 3. Alternating current voltage having a waveform shown in Fig. 10B was applied from an AC power supply 311, and output images on a screen 301 were tested. More specifically, the alternating current voltage waveform had 15V erasing voltage V e , -1.5V writing voltage V w , and 16m sec writing period T w . The fluctuation width of erasing period Te was from 0.07m sec to 7m sec with respect to 0.7m sec average value at 10 cycles. As a result, images of high contrast (150:1) and high resolution (950TV) were obtained, and no persistence and sticking were found. However, when the range of erasing period T e was set from 0.007m sec to 16m sec with respect to 0.7m sec average value at 10 cycles, undesirable flickering was found in the images. Example 11 Spatial light modulators (3) and (4) were used in the projection display system shown in Fig. 3. Alternating current voltage having a waveform shown in Fig. 10C was applied from an AC power supply 311, and output images on a screen 301 were tested. More specifically, the alternating current voltage waveform had 15V erasing voltage V e , -1.5V writing voltage V w , and 0.03m sec erasing period T e . The range of writing period T w was from 0.16m sec to 16m sec with respect to 1.6m sec average value at 10 cycles. As a result, images of high contrast (180:1) and high resolution (1000TV) were obtained. However, when the range of writing period T w was set from 0.016m sec to 160m sec with respect to 1.6m sec average value at 10 cycles, undesirable flickering was found in the images. Example 12 Spatial light modulators (3) and (4) were used in the projection display system shown in Fig. 3. Alternating current voltage having a waveform shown in Fig. 11A was applied from an AC power supply 311, and output images on a screen 301 were tested. More specifically, the alternating current voltage waveform had -1.5V writing voltage V w , 1/10 duration ratio (T e /V w ), and 10-20V range of erasing voltage V e . The cycle had 1-10m sec range with respect to 3.3m sec central cycle. As a result, images of high contrast (150:1) and high resolution (950TV) were obtained, and no persistence and sticking were found. Example 13 Spatial light modulators (3) and (4) were used in the projection display system shown in Fig. 3. Alternating current voltage having a waveform shown in Fig. 11B was applied from an AC power supply 311, and output images on a screen 301 were tested. More specifically, the alternating current voltage waveform had +15V erasing voltage V e , 1/10 duration ratio (T e /V w ), and -0.5 to - 5V range of writing voltage V w . The cycle had 1-10m sec range with respect to 3.3m sec central cycle. As a result, images of high contrast (180:1) and high resolution (1000TV) were obtained. Example 14 Spatial light modulators (3) and (4) were used in the projection display system shown in Fig. 3. Alternating current voltage having a waveform shown in Fig. 12 was applied from an AC power supply 311, and output images on a screen 301 were tested. More specifically, the alternating current voltage waveform had 25V erasing voltage V e1 , 15V V e2 , and 10V V e3 ; and -5V writing voltage V w1 , -2V V w2 , and -0.5V V w3 . The average duration ratio (T e /V w ) at 10 cycles was 1/10, and the range was 1/100-1. The average value of cycle T at 10 cycles was 3.3m sec, and the range was 1-10m sec. As a result, images of high contrast (180:1) and high resolution (1000TV) were obtained, and no persistence and sticking were found. Example 15 Spatial light modulators (3) and (4) were used in the projection display system shown in Fig. 3. Alternating current voltage having a waveform shown in Fig. 13A was applied from an AC power supply 311, and output images on a screen 301 were tested. More specifically, the alternating current voltage waveform had 15V erasing voltage V e , -5V writing voltage V w , and 1 duration ratios (T e /T w and T e1 /T w1 ). The average value of cycle T at 10 cycles was 3m sec, and the range was 0.3-30m sec. As a result, images of high contrast (120:1) and high resolution (800TV) were obtained, and no persistence and sticking were found. The spatial light modulators mentioned above can also be applied as an element for displaying a dynamic hologram. In the projection display apparatus shown in Fig. 3, a color image, can be output onto a screen when three CRTs, each for providing image of R (red), G (green) and B (blue) are combined with three spatial light modulators and a color separation optical system and, (if necessary, a color composition optical system) inserted into a readout optical system.

Claims: A method of driving a spatial light modulator comprising (a) two transparent insulating substrates (101,109) having transparent electrodes (102,107) a photoconductive layer (103), a liquid crystal layer (105), and a reflector (104); (b) the photoconductive layer (103) the liquid crystal layer (105), and the reflector (104) being sandwiched between the transparent insulating substrates (101,109); the reflector (104) being sandwiched between the photoconductive layer (103) and the liquid crystal layer (105), (c) wherein a cycle of a voltage waveform includes a period T e of first voltage V e with predetermined polarity and a period T w second voltage V w , with a polarity opposite to the predetermined polarity of the first voltage V e , (d) wherein the voltage waveform is applied between the transparent electrodes (102,107); and (e) wherein a period T of the cycle of the voltage waveform, T=T e + T w , is changed at each cycle and a ratio T e /T w between the period T e of the first voltage and the period T w of the second voltage, is constant. The method of claim 1, wherein the period T of the cycle of the voltage wave form is changed at each cycle within a range from T o /10 to 10T o where T o is a center period. The method of claim 2,wherein the center period T o ranges from 200 µ sec to 20msec. The method of claim 1, 2 or 3, wherein the first voltage V e and/or the second voltage V w is changed according to a predetermined rule within a plurality of cycles of the voltage wareform, or the second voltage V w has at least one maximum value or one minimum value within one cycle. The method of claim 1, wherein the first voltage is capable of taking a large value V e1 , and a small value V e2 , and cycles having the first voltage of V e1 are repeated n, n≧1, times after cycles having the first voltage of V e2 are repeated 1,1≧1, times; cycles having the first voltage of V e1 are repeated u, u≧1, times after cycles having the first voltage of V e2 are repeated m, m≧ 1, times. The method of claim 1, wherein the second voltage is capable of taking a large value V w1 and a small value V w2 ; and cycles having the second voltage of V w2 are repeated s, s≧1, times after cycles having the second voltage of V w1 are repeated q, q≧1, times; cycles having the second voltage of V w2 are repeated t, t≧1, times after cycles having the second voltage of V w1 are repeated r, r≧1, times. The method of any of claims 4 to 6, wherein at least one voltage selected from the group consisting of the first voltage and the second voltage ranges from V o /10 to 10V o where V o is a time average value equal to the sum of voltage multiplied by application time per cycle for a number of at least ten voltage cycles divided by the sum of application time per period for said number of voltage periods. The method of claim 1, wherein the first voltage is changed at each cycle. The method of claim 1, wherein the second voltage is changed at each cycle. The method of claim 1, wherein the first and second voltages change with time within one cycle. The method of claim 1, wherein the period of first voltage T e , has a sub-period of zero voltage, and the period of second voltage T w has a sub-period of zero voltage. The method of any of claims 1 to 11, wherein the photoconductive layer has rectifying properties. The method of any of claims 1 to 11, wherein the liquid crystal layer comprises at least one material selected from the group consisting of ferroelectric liquid crystals and antiferroelectric liquid crystals. A projection display system comprising a spatial light modulator, an AC power supply; an image input means; an image formation means; a light source; and projection lenses; the spatial light modulator comprising two transparent insulating substrates (101,109) having transparent electrodes (102,107); a photoconductive layer (103); a liquid crystal layer (105); and an dielectric reflector (104); the photoconductive layer (103), the liquid crystal layer (105), and the reflector (104) being sandwiched between the transparent insulating substrates (101,109), the dielectric reflector (104) being deposited on one plane between the photoconductive layer (103) and the liquid crystal layer (105); wherein the AC power supply drives the spatial light modulator connected to a section between the transparent electrodes (102, 107); wherein the image input means provides image to the spatial light modulator; wherein the image formation means forms image output from the image input means on the photoconductive layer; wherein the light source reads out image output from the spatial light modulator; wherein an alternating current output from the AC power supply has the voltage waveform used in the method of any of claims 1 to 11. The system of claim 14 wherein the image input means comprises a cathode ray tube.

Inventors: AKIYAMA KOJI, OGAWA HISAHITO, OGIWARA AKIFUMI, TANAKA YUKIO, TSUTSUI HIROSHI, AKIYAMA, KOJI, OGAWA, HISAHITO, OGIWARA, AKIFUMI, TANAKA, YUKIO, TSUTSUI, HIROSHI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD
Application year: 1995
Main classification: G09G3
IPC codes: G09G3