Optical modulating element and apparatuses using it

Abstract: A color variable filter for obtaining tranmission light or irradiation light of desired wavelength band electrically in a short time by using liquid crystal, and a novel electronic appliance using such color variable filter. The light entering a color variable filter 1 from one side passes through a liquid crystal element Ci composed of polarizer PLi, liquid crystal panel Pi and polarizer PL(i+1), and the distribution of the transmission light intensity versus the wavelength is nearly a normal distribution centered on a specific wavelength determined on the basis of retardation Δnd of the liquid crystal panel Pi in the liquid crystal element Ci. On the basis of the half-value width W necessary in this distribution, the retardation Δnd of the liquid crystal panel Pi and the number N of the liquid crystal elements Ci are so determined to obtain the band width Rout in the wavelength corresponding to the type of the color of the light desired. The retardation Δnd of the liquid crystal panel Pi is changed, by a voltage regulating circuit 5, so that the transmission light intensity distribution of the liquid crystal element Ci may be maximum at the desired wavelength λO. Therefore, the wavelength of the light leaving the color variable filter 1 may be selected.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to an optical modulating element which is capable of changing, continuously or in stages, the wavelength of a transmitted light or a reflected light easily and at a high speed by controlling physical quantities such as voltage, temperature, etc. and electronic apparatuses such as direct viewing display apparatus, transmission display apparatus, image pickup apparatus, copying machine, printing & platemaking apparatus, facsimile communication apparatus, light communication apparatus or image processing apparatus, etc. constituted by using such optical modulating element. 2. Description of the related art Liquid crystal display is utilized in a wide variety of fields such as watch, desk calculator, computer terminal, word processor, TV receiver, etc. As a typical display mode used for those purposes, there is the so-called TN (Twisted Nematic) mode in which the liquid crystal molecules in the liquid crystal cell are twisted by approximately 90° as initial orientation. TN mode is designed to make a monochromic indication by disposing a liquid crystal cell between a pair of polarizers and by utilizing the optical characteristics of this cell i.e. the optically activating characteristics in the absence of application of display voltage and the racemizing characteristics in the presence of application of display voltage. As for color display, the indication is made in multiple colors (indicated in prescribed plural number of colors) or in full colors (indicated in stepless colors) by providing a micro color filter of extremely small dimensions in red, blue and green for example and mixing colors by utilizing the light switching characteristics of TN mode. This principle is currently adopted for the display apparatus of a small liquid crystal TV receiver realized by application of active matrix drive or simple matrix drive. As a display system widely used in the display apparatus for word processor, we may cite STN (Super Twisted Nematic) mode having a cell structure similar to that of TN mode and in which the twisting angle of the liquid crystal is set at 180° - 270°. This mode is characterized in that the twisting angle of the liquid crystal is increased to no less than 90° and the set angle in the direction of polarization of the polarizer is optimized to reflect the sudden deformation in the molecular orientation due to increase of applied voltage in the change of birefringence of the liquid crystal so as to realize electro-optical characteristics having a sharp threshold value. It is therefore suitable to simple matrix drive. On the other hand, this mode has the drawback of presenting a yellow-green or dark blue tint as the background color for indication because of birefringence of the liquid crystal. As a method to improve this, there is a technique which consists in correcting the color by placing an optical compensation panel or a phase difference plate formed by high polymer molecules such as polycarbonate, etc. on the STN panel for indication to enable monochromic indication, and a liquid crystal display apparatus (LCD) of such structure is currently on the market under the name of "Paper White LCD". For color indication of this apparatus, an indication in multiple colors and in full colors is possible based on an operating principle similar to that of the TN mode. For services requiring a wide visual angle, the so-called GH (Guest Host) mode is used in which a pigment with different absorbencies in the direction of major axis and the direction of minor axis (dichromatic pigment) is added to the liquid crystal. This system can be classified into Heilmeyer type which uses a polarizer, White/Tiller type (phase transfer type) which does not use any polarizer and 2-layer type, etc. In any case, the operating principle of the system consists in controlling the orientation of pigments through the orientation of liquid crystal molecules by voltage to use the difference of absorbency in the direction of pigment molecules for the indication. Color indication becomes possible by either using pigments which absorb a part of the visible light or combining a colored filter with a GH cell using a pigment which turns into black. Another method of color indication consists in installing an element capable of controlling the wavelength of the transmitting light on the front face or the rear face of a monochromic display and switching the wavelength of that transmitting light sequentially to obtain a color indication. As an example of application of this method to a projection display, there is a technique of sealing a high polymer molecule distributed liquid crystal in a CdSe-TFT panel (TFT = thin film transistor), installing a disc provided with red, green and blue filters in front of a light source and making an indication on the TFT-LCD in synchronization with the change in color of the illuminated light by the rotation of the disc to obtain a full-color indication. On the other hand, a color indication is also progressing in electronic apparatus other than display apparatus such as image pickup apparatus (camera), color sensor, copying machine, etc. Image pickup apparatus is currently forming a large market as the performances of CCD (= charge coupled device) improve with the progress of semi-conductor manufacturing techniques. Especially, loading of CCD in video cameras is greatly contributing to the expansion of the home use market. The demand is increasing rapidly also for color sensors as they came to be loaded on video camera for adjustment of white balance although they were used only for industrial purposes in the past. Those electronic apparatuses are provided with a color filter consisting of a synthetic resin material, etc. on the front face of the photoelectric transfer face and constructed in a way to calculate the characteristics of the entire light of incidence by detecting the characteristics of specific wavelength components of the light incident on the photoelectric transfer element. What is important about the structure of an electronic apparatus having such photoelectric transfer element is the structure of the color filter in which filters corresponding to the wavelength components of light or red (R), green (G) and blue (B) are placed in parallel on the photoelectric transfer face in the same way as in the display apparatus. Namely, the three filters of red, green and blue constitute one picture element on the display. In addition to the method of thus decomposing the light of incidence into 3 primary color components, there is also a proposal of a construction in which filters of auxiliary colors only (cyan, magenta and yellow) or 3 primary colors mixed with those auxiliary colors are placed in parallel. In a copying machine, a full color indication is reproduced by irradiating a white color on the document to be copied, writing optical images of 3 primary colors on a photosensitive drum by switching the red, green and blue filters of reflected light sequentially by using a mechanical construction, and then transferring those optical images on the recording paper a plural number of times through the intermediary of a color developer. In this principle, full color copying is also possible by irradiating in advance the source lights of 3 primary colors on the document to be reproduced by using color filters of red, green and blue, decomposing the copy image into 3 primary colors and then treating them on the photosensitive drum in the same way as above. The sequential switching control of color filters in this case is performed mechanically in normal cases. In a digital copying machine, the image of the copy to be reproduced is submitted to photoelectric transfer by means of a solid state imaging element (CCD) and then to treatment of digital image signal to reproduce the original picture in color. The solid state imaging element used in this case is the same as the CCD and its operating principle also follows the explanation given earlier. As mentioned above, color indication constitutes one of the development trends of today in various display apparatuses or image input/output apparatuses such as imaging element, color sensor, copying machine, etc. However, the current technology of color indication still leaves a lot of problems to be solved. Namely, regarding display apparatuses, we know a structure in which micro color filters of red, green and blue, for example, are provided for each picture element to be displayed on the front face of a liquid crystal display apparatus making a monochromic indication for the purpose of realizing a multicolor or full-color indication. In such a panel structure, a drop of resolution is produced because micro color filters of a plural number of lights are combined into one picture element to be displayed. In the case where you reduce the size of picture element to be displayed of the liquid crystal display apparatus making the indication to prevent that problem, an increase of resistance is produced because of the reduction in size of the electrode through which the indication signal flows and this leads to the production of unevenness in the density of image displayed between the position where the signal is fed and a position apart from the position of signal feed in the liquid crystal display apparatus. Moreover, it is also necessary to set the manufacturing environments at an extremely high degree of purity and this produces problems such as drop of yield ratio in manufacture under the influence of dust, etc. On the other hand, in a display apparatus using the GH (guest host) mode, we know a technology of using pigments which absorb a part of the visible light to realize a multicolor or full-color indication. In such a case, it is necessary to place guest host liquid crystals supplemented with different kinds of pigment, but this technology has a problem that the picture elements to be displayed of each layer do not agree with one another because of parallax error when they are seen from a diagonal direction. A solution to this problem may be given by a technology of reducing the thickness of the glass substrate of each block stacked panel. In this case, however, the glass substrate is extremely liable to be broken in the manufacturing process of the liquid crystal display apparatus, presenting a problem of difficulty of handling. Moreover. in a guest host liquid crystal using pigments capable of displaying the black color, it is necessary to use micro color filters as mentioned before and this leads to the basic problem of drop in resolution. On the other hand, in the technology of applying the red, green and blue lights to display apparatus by switching them sequentially, the resolution improves compared with the case of use of micro color filters because one picture element of the display apparatus directly turns into the picture element to be displayed. In the past, however, there was a problem with the miniaturization or the durability of the apparatus because the switching of colors is realized with a mechanical construction by providing color filters of 3 colors in one same direction on a disc for example and driving the disc for turning, etc. It also has a problem of noise. On the other hands, in image pickup apparatus or the color sensors, there is a problem of drop in resolution as in display apparatuses because of the structure of providing micro color filters. Another problem is that the structure of the apparatus gets complicated because of use of micro color filters of fine structure. In the color copying machine, the switching operation of filters for decomposing the light irradiated on the document or the reflected light from the document into 3 colors is performed either mechanically or by providing separate light sources for the respective colors as mentioned before, and this leads to problems such as increase in size, drop in resolution or noise as mentioned before. When providing separate light sources for the respective colors, there are such problems as increase in the size of structure, service life of light source, purity of color and (increase of) power consumption. "Electronic Letters", 18 September 1975, Vol. 11, No. 19 pp471-472, on which the preambles of claims 1 and 12 are based, discloses an optical filter comprising a vertical stack of full-wave plates. The optic axis of a full-wave plate is at an angle a the optic axis of the preceding full-wave plate. The stack of full-wave plates is placed between input and output polarisers, which have their transmission axes arranged parallel to one another. Each full-wave plate consists of a liquid crystal panel. "Proceedings of the SID", 32 (1991) No 3, pp183-186 discloses a Lyot filter having at least three birefringent plates. The thickness of a plate is twice the thickness of the preceding plate. Each full-wave plate consists of a liquid crystal panel. WO 84/04402 discloses an birefringent filter which consists of a liquid crystal panel. The optical properties of the filter can be tuned by applying a suitable voltage across the liquid crystal. EP-A-0 138 456 discloses a colour filter comprising two twisted nematic liquid crystal panels an three colour selective polarisers. The filter can transmit light of one of three different colours or white light. A first aspect of the present invention provides an optical modulating element for modulating light of wavelength λ 0 , the modulating element comprising: input and output polarisers; a plurality of liquid crystal panels (PI, where 1 ≤ I ≤ N) disposed between the polarisers, a nematic liquid crystal having birefringence and a twisting angle of 0° being sealed in each panel; and means for changing the retardation Δnd of each liquid crystal panel, where Δn is the refractive index anisotropy of the nematic liquid crystal and d is the thickness of the liquid crystal layer contained in the liquid crystal panel; whereby, in use, light is incident upon the input polariser, passes through the liquid crystal panels in sequence, and is output through the output polariser; wherein the alignment direction of the first liquid crystal panel is at a pre-determined angle with respect to the transmission axis of the input polariser; and wherein the alignment direction of the second and successive liquid crystal panels crosses the alignment direction of the immediately preceding liquid crystal panel at twice the pre-determined angle α; characterised in that the transmission axis of the output polariser is at the pre-determined angle to the alignment direction of the last liquid crystal panel whereby the transmission axis of the output polariser is at an angle 2Nα to the transmission axis of the input polariser; and in that, in use, each liquid crystal panel is adjusted to be a half-wave plate in which the retardation Δnd satisfies the equation Δnd/λ 0 = (2m-1)/2 where m is an integer, whereby when linearly polarised light of wavelength λ 0 is incident on one of liquid crystal panels the plane of polarisation of the light is rotated by twice the predetermined angle by the liquid crystal panel. In a preferred embodiment the liquid crystal panel is constituted by a pair of transparent electrodes between which a liquid crystal is sealed, and the means for changing the retardation Δnd of each liquid crystal panel are voltage application means for applying either an AC voltage or a DC voltage to the liquid crystal. In a preferred embodiment the retardation changing means is temperature adjustment means for adjusting the temperature of the respective liquid crystal panels. In a preferred embodiment, when transmitting the light of a wavelength of λ 0 through the modulating element, the retardation Δnd of each liquid crystal panel and the number of the liquid crystal panels are selected in such a way that the width at half maximum of the intensity distribution of the transmitted light centring on the wavelength λ 0 of the transmitted light and the intensity of light of wavelengths other than λ 0 may be predetermined values. A second aspect of the present invention provides a display apparatus for displaying a coloured image, the apparatus having a light source for generating light containing wavelengths corresponding to a plurality of colours, said apparatus including an optical modulating element, said apparatus displaying successive images corresponding to different colours within a time period corresponding to the time period for an afterimage effect, the optical modulating element switching the colour of the light that passes after a time period for display of the image for each colour; wherein the optical modulating element is an optical modulating element as specified above. A third aspect of the present invention provides a light detecting apparatus having an optical modulating element disposed on the input side of a light detecting means for outputting a detecting signal corresponding to the intensity of the incident light; wherein the optical modulating element is an optical modulating element as defined above. A fourth aspect of the present invention provides a colour copying machine having a light source for generating light containing a plurality of wavelengths; an optical modulating element for selecting a desired wavelength of light from the light source; a document or photoelectric transfer means for forming an optical image of the document and for converting optical images into electric signals; and imaging means for forming colour images on a recording medium by using the developers for the plurality of colours based on the electric signals obtained by the photoelectric transfer means, wherein the optical modulating element is an optical modulating element as defined above. A fifth aspect of the present invention provides a printing and platemaking apparatus having a light source for generating light containing a plurality of wavelengths, an optical modulating element for selecting a desired wavelength of light from the light source; a document or photoelectric transfer means for forming an optical image of the document and for converting optical images into electric signals, and plate materials processing means for forming the image of a document for each colour on the plate materials for a plural number of different colours, based on the electric signals obtained by the photoelectric transfer means, wherein the optical modulating element is an optical modulating element as defined above. A sixth aspect of the present invention provides an image input/output apparatus for photographing object materials for a plurality of colours, having: photographing means for photographing through an optical modulating element; input means for storing image data from the photographing means in an image memory for each colour and forming colour images; and either indicating means for displaying images or output means for outputting colour images by switching the colour of transmitted light with the optical modulating element, based on the image data of a plural number of colours stored in the image memory; wherein the optical modulating element is an optical modulating element as defined above. A seventh aspect of the present invention provides an image operating apparatus comprising a light source which produces a light of a plural number of wavelengths, an optical modulating element which passes only specific wavelength components of the light from the light source, and a plural number of transmission or reflection type displaying elements for either transmitting or reflecting the light of the remaining wavelength components, wherein the optical modulating element is an optical modulating element as defined above. An eighth aspect of the present invention provides a lighting apparatus having: a light source which produces a light of a plurality of wavelengths; and an optical modulating element passing only specific wavelength components of light from the light source; wherein the optical modulating element is an optical modulating element as defined above. A ninth aspect of the present invention provides a method of modulating light of wavelength λ 0 , using a modulating element (101) comprising: input and output polarisers (2,3); a plurality of liquid crystal panels (PI, where 1 ≤ I ≤ N) disposed between the polarisers, a nematic liquid crystal having birefringence and a twisting angle of 0° being sealed in each panel; and means for changing the retardation Δnd of each liquid crystal panel, where Δn is the refractive index anisotropy of the nematic liquid crystal and d is the thickness of the liquid crystal layer contained in the liquid crystal panel; whereby, in use, light is incident upon the input polariser (2), passes through the liquid crystal panels in sequence, and is output through the output polariser (3); the method comprising the steps of: disposing the first liquid crystal panel (P1) so that its alignment direction (A1) is at a pre-determined angle (α) with respect to the transmission axis (B1) of the input polariser (2); and setting the alignment direction (A1) of the second and successive liquid crystal panels (PI) at twice the pre-determined angle (2α) to the alignment direction (A{I-1}) of the immediately preceding liquid crystal panel (P{I-1}); characterised in that it further comprises the step of disposing the output polariser so that the transmission axis (B2) of the output polariser (3) is at the pre-determined angle (α) to the alignment direction (AN) of the last liquid crystal panel (PN) whereby the transmission axis (B2) of the output polariser (3) is at an angle 2Nα to the transmission axis (B1) of the input polariser (2); and in that it further comprises the step of adjusting the retardation Δnd of each liquid crystal panel such that the liquid crystal panels are half-wave plates having a retardation given by the equation Δnd/λ 0 = (2m-1)/2 where m is an integer, whereby when linearly polarised light of wavelength λ 0 is incident on each of the liquid crystal panels the plane of polarisation of the light is rotated by twice the predetermined angle (2α) by the liquid crystal panel. According to the invention, in the case where a light incident on the optical modulating element from one side transmits through a liquid crystal element consisting of first polarizer, a liquid crystal panel and the second polarizer, the distribution of wavelengths of the intensity of transmitted light becomes about close to the normal distribution centering on the specific wavelength determined based on the retardation of the liquid crystal Panel in the liquid crystal element. The retardation of the liquid crystal is determined based on the width at half maximum necessary in this distribution and the number of the liquid crystal elements is determined in such a way that the band width of required wavelengths corresponding to the kinds of color of the light which you wish to obtain with the optical modulating element may be obtained. The retardation of the liquid crystal is changed by means for changing retardation in such a way that the intensity of transmitted light of the desired wavelength in the distribution of intensity of transmitted lights relating to the wavelengths of the transmitted light of the liquid crystal element may be largest. Therefore, you can select the desired wavelength of light irradiated from the optical modulating element. This makes it possible to obtain the light of the desired color with the optical modulating element. In case of using such optical modulating element as color filter, the construction of the filter can be much miniaturized and simplified compared with a micro color filter in which fine color filters of red, green and blue colors, for example, are integrated or an apparatus which switches the colors of light obtained by loading the 3-color filter in the circumferential direction of a disc which is driven for turning. Moreover, when such optical modulating element is combined with means for displaying of transmitted type or reflection type, the optical modulating element selectively passes the light of a plural number of wavelength and displays the images corresponding to the Plural number of colors within the period of afterimage effect of human beings by the displaying means respectively. This enables construction of a color display apparatus. In addition, it is also possible to realize a compact and lightweight electronic apparatus with much improved indication quality by using the optical modulating element for a component producing the light of a plural number of color in not only a display apparatus but also any electronic apparatus which uses the light of colors of a plural number of wavelengths. As mentioned above, in the case where a light incident on the optical modulating element from one side transmits through a liquid crystal element consisting of first polarizer, a liquid crystal panel and the second polarizer, the distribution of wavelenghts of the intensity of transmitted light becomes about close to the normal distribution centering on the specific wavelength determined based on the retardation of the liquid crystal panel in the liquid crystal element. The retardation of the liquid crystal is determined based on the width at half maximum necessary in this distribution and the number of the liquid crystal elements is determined in such a way that the band width of required wavelenghts corresponding to the kinds of color of the light which you wish to obtain with the optical modulating element may be obtained. The retardation of the liquid crystal is changed by means for changing retardation in such a way that the intensity of transmitted light of the desired wavelength in the distribution of intensity of transmitted lights relating to the wavelengths of the transmitted light of the liquid crystal element may be largest. Therefore, we can select the desired wavelength of light irradiated from the optical modulating element. This makes it possible to obtain the light of the desired color with the optical modulating element. When such optical modulating element is combined with means for displaying of transmission type or reflection type, the optical modulating element selectively passes the light of a plural number of wavelength and displays the images corresponding to the plural number of colors within the period of afterimage effect of human beings by the displaying means respectively. This enables construction of a color display apparatus. In addition, it is also possible to realize a compact and lightweight electronic apparatus with much improved indication quality by using the optical modulating element for a component producing the light of a plural number of color in not only a display apparatus but also any electronic apparatus which uses the light of colors of a plural number of wavelengths. An optical modulating element is constituted by disposing a plural number of liquid crystal panels, in which a liquid crystal having birefringence is sealed, between at least one pair of polarizers, the axis of polarization of the polarizer on the outlet side is selected to be parallel to the direction of polarization of the straight polarizing component or to the direction of major axis of the elliptical polarizing component of the output light of the liquid crystal panel in the last stage, and provided with means for changing the retardation of each liquid crystal panel. Moreover, each liquid crystal panel is constituted by sealing liquid crystal in at least one pair of transparent electrodes, and the retardation changing means are voltage application means for applying either an AC voltage or a DC voltage to the liquid crystal after adjustment. The retardation changing means are temperature adjustment means for adjusting the temperature of the respective liquid crystal panels. When passing the light of a wavelength of λ0 only, the retardation Δnd of each liquid crystal panel is selected in a way to satisfy [formula 3] Δnd λ 0 = (2m-1) 2 m≧1 for the anisotropy of refraction index of liquid crystal Δn, layer thickness of liquid crystal d and integer m. When passing the light of a wavelength of λ0 only, the retardation and the number of layers of the respective liquid crystal panels is selected in such a way that the width at half maximum in the distribution of intensity of transmitted light centering on the wavelength λ0 of the transmitted light and the intensity of the lights other than that of a wavelength λ0 may become equal to the predetermined values. Moreover, the display apparatus makes an indication by irradiating the light of one light source containing the wavelength of a plural number of colors on transmission or reflection type means for displaying and an optical modulating element, it displays the images corresponding to the plural number of colors on displaying means within the period of afterimage effect, the optical modulating element switching the color of the light that passes within the period of display of the image for each color, the optical modulating element being constituted by disposing a plural number of liquid crystal panels, in which a liquid crystal having birefringence is sealed, between at least one pair of polarizers, the axis of polarization of the polarizer on the outlet side is selected to be parallel to the direction of polarization of the straight polarizing component or to the direction of major axis of the elliptical polarizing component of the output light of the liquid crystal panel in the last stage, and the optical modulating element provided with means for changing the retardation of each liquid crystal panel. Furthermore, an optical modulating element is disposed on the incidence side of the light detecting means which outputs detecting signal corresponding to the intensity of the light of incidence, the optical modulating element being constituted by disposing a plural number of liquid crystal panels, in which a liquid crystal having birefringence is sealed, between at least one pair of polarizers, the axis of polarization of the plarizer on the outlet side is selected to be parallel to the direction of polarization of the straight polarizing component or to the direction of major axis of the elliptical polarizing component of the output light of the liquid crystal panel in the last stage, and the optical modulating element provided with means for changing the retardation of each liquid crystal panel. In a color copying machine, an optical modulating element is disposed in either one light source producing lights containing the wavelength of a plural number of colors, a document or photoelectric transfer means on which the optical image of the documet is formed and which convert optical images into electric signals, provided with means for forming color images on a recording medium by using the developers for the plural number of colors based on the electric signals obtained by the photoelectric transfer means, the optical modulating element being constituted by disposing a plural number of liquid crystal panels, in which a liquid crystal having birefringence is sealed, between at least one pair of polarizers, the axis of polarization of the polarizer on the outlet side is selected to be parallel to the direction of polarization of the straight polarizing component or to the direction of major axis of the elliptical polarizing component of the output light of the liquid crystal panel in the last stage, and the optical modulating element provided with means for changing the retardation of each liquid crystal panel. In a printing & platemaking apparatus, an optical modulating element is disposed in either one light source producing lights containing the wavelength of a plural number of colors, a document or photoelectric transfer means on which the optical image of the document is formed and which converts optical images into electric signals, provided with processing plate materials means for forming the image of document for each color on the plate materials for a plural number of different colors which become printing plates, based on the electric signals obtained by the photoelectric transfer means, the optical modulating element being constituted by disposing a plural number of liquid crystal panels, in which a liquid crystal having birefringence is sealed, between at least one pair of polarizers, the axis of polarization of the polarizer on the outlet side is selected to be parallel to the direction of polarization of the straight polarizing component or to the direction of major axis of the elliptical polarizing component of the output light of the liquid crystal panel in the last stage, and the optical modulating element provided with means for changing the retardation of each liquid crystal panel. An image input/output apparatus photographs object materials for a plural number of colors with means for photographing through an optical modulating element, it inputs color images by storing image data from photographing means in an image memory for each color, it either indicates images with means for displayng or outputs color images by switching the color of transmitted light with the optical modulating element, based on the image data of a plural number of colors stored in the image memory, the optical modulating element being constituted by disposing a plural number of liquid crystal panels, in which a liquid crystal having birefringence is sealed, between at least one pair of polarizers, the axis of polarization of the polarizer on the outlet side is selected to be parallel to the direction of Polarization of the straight polarizing component or to the direction of major axis of the elliptical polarizing component of the output light of the liquid crystal panel in the last stage, and the optical modulating element provided with means for changing the retardation of each liquid crystal panel. An image operating apparatus is provided with a light source which produces a light of a plural number of wavelengths, an optical modulating element which Passes only specific wavelength components of the light from the light source, and a plural number of transmission or reflection type displaying elements which either transmit or reflect the light of the remaining wavelength components, the optical modulating element being constituted by disposing a plural number of liquid crystal panels. in which a liquid crystal having birefringence is sealed, between at least one pair of polarizers, the axis of polarization of the plarizer on the outlet side is selected to be Parallel to the direction of polarization of the straight polarizing component or to the direction of major axis of the elliptical polarizing component of the output light of the liquid crystal panel in the last stage, and the optical modulating element provided with means for changing the retardation of each liquid crystal panel. Moreover, a lighting apparatus irradiates the light from a light source which produces a light of a plural number of wavelength through an optical modulating element as some of the lights of the plural number of colors, the optical modulating element being constituted by disposing a plural number of liquid crystal panels, in which a liquid crystal having birefringence is sealed, between at least one pair of polarizers, the axis of polarization of the plarizer on the outlet side is selected to be parallel to the direction of polarization of the straight polarizing component or to the direction of major axis of the elliptical polarizing component of the output light of the liquid crystal panel in the last stage, and the optical modulating element provided with means for changing the retardation of each liquid crystal panel. According to the invention, the light incident on the optical modulating element from the side of one polarizer and passing through the polarizer is converted into a linearly polarized light. When this linearly polarized light passes through a plural number of liquid crystal panels, the light of the linearly polarized light turns successively at an angle dependent on the wavelength and the degree of birefringence because of the birefringence of each liquid crystal panel. Because the birefringence of the liquid crystal panel is dependent on wavelength, the angle at which the light of the linearly polarized light which passed the polarizer turns each time when it passes through the liquid crystal panel varies with each wavelength. On the other hand, a polarizer is disposed on the exit side of the liquid crystal panel of the last stage. In the case where we change the retardation of the liquid crystal in each liquid crystal panel by means for changing retardation, we can select as required the wavelength of the light either the direction of polarization of linearly polarized component or the direction of major axis of the elliptically polarized component of which is parallel to the axis of polarization of the other polarizers, from among the lights coming out of the liquid crystal panel of the last stage in correspondence to that change. This makes it possible to pass the light of the desired color by means of an optical modulating element. When such optical modulating element is combined with means for displaying of transmission type or reflection type, the optical modulating element selectively passes the light of a plural number of wavelength and displays the images corresponding to the plural number of colors within the period of afterimage effect of human beings by the displaying means respectively. This enables construction of a color display apparatus. In addition, it is also possible to realize a compact and lightweight electronic apparatus with much improved indication quality by using the optical modulating element for a component producing the light of a plural number of color in not only a display apparatus but also any electronic apparatus which uses the light of colors of a plural number of wavelengths. As described above, according to the invention, the light incident on the optical modulating element from the side of one polarizer and passing through the polarizer is converted into a linearly polarized light. When this linearly polarized light passes through a plural number of liquid crystal panels. the light of the linearly polarized light turns successively at an angle dependent on the wavelength and the degree of birefringence because of the birefringence of each liquid crystal panel. Because the birefringence of the liquid crystal panel is dependent on wavelength, the angle at which the light of the linearly polarized light which passed the polarizer turns each time when it passes through the liquid crystal panel varies with each wavelength. On the other hand, another polarizer is disposed on the exit side of the liquid crystal panel of the last stage. In the case where we change the retardation of the liquid crystal in each liquid crystal panel by means for changing retardation, we can select as required the wavelength of the light either the direction of polarization of linearly polarized component or the direction of major axis of the elliptically polarized component of which is parallel to the axis of polarization of the other polarizers, from among the lights coming out of the liquid crystal panel of the last stage in correspondence to that change. This makes it possible to pass the light of the desired color by means of an optical modulating element. When such optical modulating element is combined with means for displaying of transmission type or reflection type, the optical modulating element selectively passes the light of a plural number of wavelength and displays the images corresponding to the plural number of colors within the period of afterimage effect of human beings by the displaying means respectively. This enables construction of a color display apparatus. In addition, it is also possible to realize a compact and lightweight electronic apparatus with much improved indication quality by using the optical modulating element for a component producing the light of a plural number of color in not only a display apparatus but also any electronic apparatus which uses the light of colors of a plural number of wavelengths. An optical modulating element is realized by disposing alternately a plural number of liquid crystal panels constituted by sealing liquid crystal between a pair of transparent substrates and a plural number of color polarizers, and is provided with a plural number of liquid crystal elements constituted by liquid crystal panel and color polarizers disposed on both sides of the liquid crystal panel and means for switching the drive voltage applied to each liquid crystal panel to a plural number of different states so that the respective liquid crystal elements may transmit the lights of different wavelengths or transmit the light of desired wavelength(s). Moreover, an optical modulating element is realized by disposing alternately a plural number of liquid crystal panels constituted by sealing liquid crystal between a pair of transparent substrates and a plural number of polarizers including color polarizer, and is provided with a plural number of liquid crystal elements constituted by liquid panel and color polarizers disposed on both sides of the liquid crystal panel and means for switching the drive voltage applied to each liquid crystal panel to a plural number of different states so that the respective liquid crystal elements may transmit the lights of different wavelengths or transmit the light of desired wavelength(s). Moreover, in an optical modulating element, when passing the light of a wavelength of λ only, each liquid crystal panel is, for the anisotropy of refraction index of liquid crystal Δn, natural spiral pitch p, [formula 4] p · Δn >λ p; Natural spiral pitch Δn; Anisotropy of refraction index of liquid crystal λ; Wavelength of light of incidence, and in the main wavelength of the light, it is selected in a construction to satisfy [formula 5] µ = (2m) 2 -1 m; Integer [formula 6] µ = 2Δnd/λ d; Thickness of liquid crystal layer When passing the light of a wavelength of λ only, the retardation Δnd of each liquid crystal panel is selected in a way to satisfy [formula 7] Δnd λ = (2m'-1) 2 m'; Integer for the anisotropy of refraction index of liquid crystal Δn, layer thickness of liquid crystal d and integer m'. Moreover, a display apparatus makes an indication by irradiating the light of one light source containing the wavelength of a plural number of colors on transmission or reflection type means for displaying and an optical modulating element, it displays the images corresponding to the plural number of colors on the displaying means within the Period of afterimage effect, the optical modulating element switching the color of the light that passes within the period of display of the image for each color, the optical modulating element being an optical modulating element constituted by disposing alternately a plural number of liquid crystal panels constituted by sealing liquid crystal between a pair of transparent substrates and a plural number of polarizers including color polarizer, and the optical modulating element constituted with a plural number of liquid crystal elements constituted by liquid panel and color polarizers disposed on both sides of the liquid crystal panel and means for switching the drive voltage applied to each liquid crystal panel to a plural number of different states so that the respective liquid crystal elements may transmit the lights of different wavelengths or transmit the light of desired wavelength(s). Moreover, a light detecting apparatus has an optical modulating element which outputs a detecting signal corresponding to the intensity of the light of incidence on the incidence side of the light detecting means, the optical modulating element being an optical modulating element constituted by disposing alternately a plural number of liquid crystal panels constituted by sealing liquid crystal between a pair of transparent substrates and a plural number of polarizers including color polarizer, and the optical modulating element constituted with a plural number of liquid crystal elements constituted by liquid panel and color polarizers disposed on both sides of the liquid crystal panel and means for switching the drive voltage applied to each liquid crystal panel to a plural number of different states so that the respective liquid crystal elements may transmit the lights of different wavelengths or transmit the light of desired wavelength(s). Moreover, in a color copying machine an optical modulating element is disposed in either one light source containing the wavelength of a plural number of colors, a document or photoelectric transfer means on which optical images of the document is formed and which convert optical images into electric signals, provided with means for forming color images on a recording medium by using the developers for the plural number of colors based on the electric signals obtained by the photoelectric transfer means, the optical modulating element being an optical modulating element constituted by disposing alternately a plural number of liquid crystal panels constituted by sealing liquid crystal between a pair of transparent substrates and a plural number of polarizers including color polarizer, and the optical modulating element constituted with a plural number of liquid crystal elements constituted by liquid panel and color polarizers disposed on both sides of the liquid crystal panel and means for switching the drive voltage applied to each liquid crystal panel to a plural number of different states so that the respective liquid crystal elements may transmit the lights of different wavelength or transmit the light of desired wavelength(s). Moreover, in a printing & platemaking apparatus an optical modulating element is disposed in either one light source containing the wavelength of a plural number of colors, a document or photoelectric transfer means on which optical image of the document is formed and which convert optical images into electric signals, provided with processing plate materials means for forming the image of document for each color on the plate materials for a plural number of different colors which become printing plates, based on the electric signals obtained by the means of photoelectric transfer, the optical modulating element being an optical modulating element constituted by disposing alternately a plural number of liquid crystal panels constituted by sealing liquid crystal between a pair of transparent substrates and a plural number of polarizers including color polarizer, and the optical modulating element constituted with a plural number of liquid crystal elements constituted by liquid panel and color polarizers disposed on both sides of the liquid crystal panel and means for switching the drive voltage applied to each liquid crystal panel to a plural number of different states so that the respective liquid crystal elements may transmit the lights of different wavelengths or transmit the light of desired wavelength(s). Moreover, an image input/output apparatus photographs object materials for a Plural number of colors with means for photographing through an optical modulating element, it either inputs color images by storing image data from means for photographing in an image memory for each color or indicates images with means for displaying and outputs color images by switching the color of transmitted light with the optical modulating element. based on the image data of a plural number of colors stored in the image memory, the optical modulating element being an optical modulating element constituted by disposing alternately a plural number of liquid crystal panels constituted by sealing liquid crystal between a pair of transparent substrates and a plural number of polarizers including color polarizer, and the optical modulating element constituted with a plural number of liquid crystal elements constituted by liquid panel and color polarizers disposed on both sides of the liquid crystal panel and means for switching the drive voltage applied to each liquid crystal Panel to a plural number of different states so that the respective liquid crystal elements may transmit the lights of different wavelengths or transmit the light of desired wavelength(s). Moreover, an image operating apparatus is provided with a light source which Produces a light of a plural number of wavelengths, an optical modulating element which passes only specific wavelength components of the light from the light source, and a plural number of transmission or reflection type displaying elements which either transmit or reflect the light of the remaining wavelength components, the optical modulating element being constituted by disposing alternately a plural number of liquid crystal panels constituted by sealing liquid crystal between a pair of transparent substrates and a plural number of polarizers including color polarizer, and the optical modulating element constituted with a plural number of liquid crystal element constituted by liquid panel and color polarizers disposed on both sides of the liquid crystal panel and means for switching the drive voltage applied to each liquid crystal panel to a plural number of different states so that the respective liquid crystal elements may transmit the lights of different wavelength or transmit the light of desired wavelength(s). Moreover, a lighting apparatus irradiates the light from a light source which produces a light of a plural number of wavelengths through an optical modulating element as some of the lights of the plural number of colors, the optical modulating element being constituted by disposing alternately a plural number of liquid crystal panels constituted by sealing liquid crystal between a pair of transparent substrates and a plural number of polarizers including color polarizer, and the optical modulating element constituted with a plural number of liquid crystal elements constituted by liquid panel and color polarizers disposed on both sides of the liquid crystal panel and means for switching the drive voltage applied to each liquid crystal panel to a plural number of different states so that the respective liquid crystal elements may transmit the lights of different wavelengths or transmit the light of desired wavelength(s). According to the invention, of the light incident on the optical modulating element from one side, the light of specific wavelengths is polarized into a linearly polarized light when passing through the color polarizer while the light of wavelengths other than the specific wavelengths concerned is not converted into a linearly polarized light but is delivered retaining the state of the light of incidence about as it is. In the case where the drive voltage applied to the liquid crystal panel is mutually switched among a plural number of different states by using means for switching, it is possible for the liquid crystal elements consisting of the liquid crystal panel and the color polarizers on both sides to transmit the lights of wavelengths different from one another or transmit the light of desired wavelength. Thus it becomes possible to switch for passing or not the light of a wavelength corresponding to the color of the color polarizer on the upstream side in the direction of passage of light of the respective liquid crystal elements. By performing this switching operation for each of the plural number of liquid crystal elements, we can obtain colors and non color produced with the mixing of the color of each color polarized plate and those colors. Such action can also be achieved when each of the plural number of liquid crystal elements is constructed in a way to have a liquid crystal panel and a plural number of polarizers including a plural number of color polarizers. Such a simplified and compact construction can achieve an indicating action with improved indication quality. When such optical modulating element is combined with means for displaying type or reflection type, the optical modulating element selectively passes the light of a plural number of wavelengths and displays the images corresponding to the plural number of colors within the period of afterimage effect of human beings by displaying means respectively. This enables construction of a color display apparatus. In addition, it is also possible to realize a compact and lightweight electronic apparatus with much improved indication quality by using the optical modulating element for a component producing the light of a plural number of color in not only a display apparatus but also any electronic apparatus which uses the light of colors of a plural number of wavelengths. As mentioned above, according to the invention, of the light incident on the optical modulating element from one side, the light of specific wavelengths are polarized into a linearly polarized light when passing through the color polarizer while the light of wavelengths other than the specific wavelengths concerned is converted into a linearly polarized light of a direction different from that of the linearly polarized light of specific wavelengths and is delivered in the wavelength band of the light of incidence. In the case where the drive voltage applied to the liquid crystal panel is mutually switches among a plural number of different states by using means for switching, it is possible for the liquid crystal elements consisting of the liquid crystal panel and the color polarizers on both sides to transmit the lights of wavelength different from one another or transmit the light of desired wavelength. Thus it becomes possible to switch for passing or not the light of a wavelength corresponding to the color of the color polarizer on the upstream side in the direction of passage of light of the respective liquid crystal elements. By performing this switching operation for each of the plural number of liquid crystal elements, we can obtain colors and non color produced with the mixing of the color of each color polarized plate and those colors. Such action can also be achieved when each of the plural number of liquid crystal elements is constructed in a way to have a liquid crystal panel and a plural number of polarizers including a plural number of color polarizers. Such a simplified and compact construction can achieve an indicating action with improved indication quality. When such optical modulating element is combined with means for displaying of transmission type or reflection type, the optical modulating element selectively passes the light of a plural number of wavelengths and displays the images corresponding to the plural number of colors within the period of afterimage effect of human beings by displaying means respectively. This enables construction of a color display apparatus. In addition, it is also possible to realize a compact and lightweight electronic apparatus with much improved indication quality by using the optical modulating element in not only a display apparatus but also any electronic apparatus which uses the light of colors of a plural number of wavelengths. BRIEF DESCRIPTION OF THE DRAWINGS Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description taken with reference to the drawings wherein: Fig. 1 is a systematic diagram of the optical modulating element 1 of a comparative example, Fig. 2 shows the construction of the liquid crystal panel Pi, Fig. 3 is a diagram illustrating the optical construction of the optical modulating element 1, Fig. 4 is a diagram showing the distribution of intensity of transmitted light in each liquid crystal panel Pi, Fig. 5 is a graph explaining the voltage dependence of the anisotropy of refraction index Δn, Fig. 6 is a graph explaining the optical characteristics of the optical modulating element 1, Fig. 7 is a systematic diagram of another comparative example, Fig. 8 is a systematic diagram of another comparative example, Fig. 9 is a graph explaining the optical characteristics of the optical modulating element 1a, Fig. 10 is a block diagram of a light modulating element 1b of another comparative example, Fig. 11 is a decomposed perspective view of a liquid crystal display apparatus 26 of an example of the invention, Fig. 12 is a block diagram of the liquid crystal display apparatus 26, Fig. 13 is a systematic diagram of a projection type display apparatus 35, Fig. 14 is a block diagram of a color camera apparatus 41, Fig. 15 is a block diagram of a color sensor 47, Fig. 16 is a systematic diagram of a color copying machine 57, Fig. 17 is a systematic diagram of a color printing & platemaking apparatus 63, Fig. 18 is a block diagram of a color facsimile apparatus 79, Fig. 19 is a systematic diagram of a reading element 81a, Fig. 20 is a systematic diagram of a reading element 81b, Fig. 21 is a systematic diagram of the optical modulating element 101 of an example of the invention, Fig. 22 is a diagram showing the optical construction of the optical modulating element 101, Fig. 23 is a diagram explaining the behavior of the linearly polarized light in the liquid crystal panel P1, Fig. 24 is a graph explaining the wavelength dependence of the anisotropy of refraction index Δn, Fig. 25 is a graph explaining the wavelength dependence of the intensity of the transmitted light, Fig. 26 is a diagram explaining the action of the optical modulating element 101, Fig. 27 is a graph showing the relation between the response time τ and the cell thickness d in the optical modulating element 101, Fig. 28 is a block diagram indicating an example of construction of the light modulating element 101a of another example, Fig. 29 is a systematic diagram of an image operating apparatus, and Fig. 30 is a diagram explaining the action of the image operating apparatuses 90a - 90c. Now referring to the drawing, preferred embodiments of the invention are described below. [Embodiment 1] (Comparative Example) Fig. 1 is an exploded perspective view showing an example of a photomodulation element 1, Fig. 2 is a diagram showing the structure of the photomodulation element 1, and Fig. 3 is a diagram showing an optical composition of the photomodulation element 1. The photomodulation element 1 comprising a polarizer PL0, a liquid crystal panel P1 having a structure as described below, a polarizer PL1, a liquid crystal panel P2, a polarizer PL2, ---, a liquid crystal panel PN, and a polarizer PLN, which are sequentially arranged on a same optical axis. Herein, the polarizer PLO liquid crystal panel P1, and polarizer PL1 compose a liquid crystal element C1 which is a constitutional unit, while the polarizer PL1, liquid crystal panel P2, and polarizer PL2 compose a liquid crystal element C2. Thereafter, similarly, the photomodulation element 1 possesses liquid crystal elements C1 to CN. In each liquid crystal panel P1, the applied voltage is individually controlled by plural voltage regulating circuits 5 connected commonly to a power source 4. As a modified example, a common applied voltage may be applied to each liquid crystal panel P1 from a common voltage regulating circuit 5. The liquid crystal panels P1 possess an identical composition, and on a pair of glass substrates 6, 7, for example, transparent electrodes 8, 9 composed of ITO (indium tin oxide) are formed. Covering the transparent electrodes 8, 9, for example, orientation films 10, 11 made of polyvinyl alcohol or the like are formed, and a liquid crystal 12 possessing double refractive characteristic is placed between them as being sealed in a layer thickness d, and the peripheral edge is sealed vith a sealing material 13. The orientation films 10, 11 on each liquid crystal panel Pi are oriented by rubbing treatment or the like. In each liquid crystal panel Pi, as shown in Fig. 2 (2), the orientation directions of the light incident side and exit side are parallel to each other, as indicated by arrow Ai (i = 1 to N). Refer to Fig. 1 and Fig. 3. In the embodiment, the polarizers PL0 to PLN shown in Fig. 1 all possess polarization planes BL to BN in a same direction, while the orientation direction Ai of the liquid crystal panels P1 to PN is selected in a state of crossing at an angle α (for example, 45 degrees). In other embodiments of the invention, as described later, the polarization direction Bi' (1≦i≦N) of at least one polarizer PLI' at the light exit side may be selected, as shown in Fig. 3, in a cross Nicol's state orthogonal to the polarization direction Bi of the remaining polarizer PLi. In the structural example in Fig. 1, the transmissivity T of the light passing through the liquid crystal element C1 disposed between parallel Nicol's lenses is expressed as follows. [formula 8] T = cos 2 δ 2 (α = 4 5 ° ) where [formula 9] δ = 2Π λ · Δ nd Supposing the retardation (Δnd)i of the i-th liquid crystal element ci to be [formula 10] (Δnd) i=k i - 1 (Δnd) where i = 1 to N, (Δnd)1 = Δnd is retardation of liquid crystal element C1, k : 1 < k < 3, preferably k = 2, then the transmission light intensity T of transmitting through the liquid crystal element CN of the final stage is [formula 11] T = ( sin 2 N (δ/2) 2 N sin (δ/2) ) 2 Fig. 4 explains the action of each liquid crystal element Ci. In formula (8), the transmissivity T becomes the maximum when [formula 12] δ 2 = 2 m Π m: integer From formulas (9) and (12), the wavelength λ(m)1 when the transmission light intensity reaches the maximum in the liquid crystal element C1 is [formula 13] λ (m) 1 = Δnd/2 m The state of distribution of such transmission light intensity is shown in Fig. 4 (1). In Fig. 4 (1), λ(1), λ (2), --- are expressed as λ1, λ2, ---. On the other hand, at the i-th liquid crystal element Ci, the wavelength λ(m)i where the transmission light intensity is the maximum is, in the case of k = 2, [formula 14] λmi = 2 i-1 Δnd/2m = 2 i-1 λ(m)1 i : 1, 2, ---, N That is, the light of the wavelength of 2 i-1 times the wavelength λ(m)1 given in formula (13) at the liquid crystal element C1 passes at the maximum transmissivity. Fig. 4 (2) and Fig. 4 (3) show the distribution of transmission light intensity of liquid crystal elements C2, C2. In this way, when the incident light sequentially passes the liquid crystal elements C1, C2 ---, in the distribution curves shown in Figs. 4 (1) through 4 (3), only the wavelength corresponding to the peak portion pass, while the light of the remaining wavelengths is cut off, or the transmission light intensity is suppressed. Thus, from the photomodulation element 1 in the composition shown in Fig. 1, for example, the light of the wavelengths λ1, λ6 shown in Fig. 4 (4) is obtained. Conforming to such principle, if desired to obtain the light of desired wavelength λ0 from the photomodulation element 1 shown in Fig. 1. the integer m in formula (12) is obtained from the condition where the transmission light intensity of each liquid crystal element Ci may reach the maximum at this wavelength λ0, that is, the condition of Equation 13 obtained from Equations 12 and 9 where the transmissivity T of Equation 8 is the maximum, and the condition of determining the desired degree of the width at half maximum W shown in Fig. 4 (4) of the transmission light obtained finally from the photomodulation element 1, and the retardation Δnd of the liquid crystal element C1 is determined from Equation 13. Here, what defines the half-value width W of the transmission light obtained finally is the widths at half maximum W1, W2, W3, --- shown in Fig. 4 (1) to Fig. 4 (3) of each liquid crystal element Ci, and these half-value widths wi (i = 1 to N) are determined by the pitch of the transmission light intensity distribution curve, and this pitch is determined by the integer m as shown in Equation 13. Besides, the number of layers N of the liquid crystal element Ci is determined so that the bandwidth Rout of the transmission light of the photomodulation element 1 shown in Fig. 4 (4) may be a specified width. That is, the number of layers N is selected so that the bandwidth Rout may be determined in order that the wavelength λ6 may be outside of the visible light band R. On the other hand, if the wavelength λ6 is within the visible light band R, the polarization direction of at least one polarizer at the exit side of the light should be set in a state of orthogonally crossing the remaining polarizers, and as shown in Fig. 7, for example, the polarization plane BN of the final polarizer PLN (in this example, N = 3) is select in a cross Nicol's state with the remaining polarizers PL0, PL1, PL2, so as to use the photomodulation element la for cutting off the light of the wavelength λ6 with the polarizer PLN. As shown in Fig. 5, the refractive index anisotropy Δn of the liquid crystal 12 of each liquid crystal panel Pi is known to vary with the applied voltage V, and when the refractive index anisotropy Δn varies, the wavelength λ(m)i of the transmission light as the maximum transmissivity is changed from formula (13). In other words, when the voltage applied to each liquid crystal panel Pi is properly adjusted by using the voltage adjusting circuit 5 shown in Fig. 1, the lights of wavelengths of red, green and blue are obtained from the photomodulation element 1 as shown in Fig. 6, and therefore the light of any desired wavelength may be obtained. In this way, the photomodulation element 1 may be used as a color filter for full color display. Here, to satisfy formula (10), the refractive index anisotropy Δn is varied in each liquid crystal panel Ci, or the layer thickness d of the liquid crystal 12 is varied. In this example, the layer thickness d of each liquid crystal panel Pi is changed. That is, the layer thickness di of the i-th liquid crystal panel Pi is determined as [formula 15] di = 2 i-1 d where i = 1 to N. If d1 = d, it is the layer thickness of the liquid crystal 12 of the liquid crystal panel P1. In its application, such as the use of the photomodulation element 1 in combination with active matrix liquid crystal display device for monochromatic display for multicolor or full-color display, a high speed is required in the color change. To operate the photomodulation element 1 at high speed, it is necessary that the dislocation of the liquid crystal molecules in the array direction corresponding to specified change of applied voltage be done at high speed. Accordingly, the layer thickness d of each liquid crystal panel Pi should be as small as possible, but in this embodiment since the cell thickness d of each liquid crystal panel Pi is selected to be different individually as mentioned above, as shown in Fig. 8, a modified example is possible, in which each liquid crystal panel Pi is formed in multiple layers, the cell thickness d is suppressed in individual liquid crystal panels Pi, and the response of each liquid crystal panel Pi is improved. That is, the liquid crystal element C2 comprises polarizer PL1. liquid crystal panels P21, P22, and polarizer PL2, and the liquid crystal element C3 comprises polarizer PL2, liquid crystal panels P31, P32, P33, P34, and polarizer PL3. The sum of the layer thicknesses of the liquid crystals of the liquid crystal panels P21, P22 is controlled to be the value specified in Equation 15. The intensity distribution of the transmission light in this modified example was measured, and the distribution as shown in Fig. 9 was obtained. On the other hand, to obtain a high response speed, it is necessary to consider also the viscosity of the liquid crystal material. The viscosity of the liquid crystal is desired to be as low as possible, and generally at 35 centipoise (cp) or less, the effect for realizing the invention is shown, but morepreferably it is desired to be 25 centipoise or less, as clarified by the experience of the present inventors. The liquid crystal materials showing such characteristics include biphenyl compound, phenylester compound, cyclohexane compound, phenylpyrimidine compound, dioxane compound, tolan compound, alkenyl compound, and fluorine compound, or their mixtures. Using the liquid crystal composition made of such materials, a photomodulation element as shown in Fig. 1 was composed, and the applied voltages were changed over to obtain three wavelengths λ1, λ2, λ3 as shown in Fig. 6, and the response speed was measured, and a high speed response of several milliseconds was obtained. Incidentally, the one field period of 1/60 sec of the television video signal of the NTSC system is about 17 msec. Hence, for example, it is possible to realize a composition in which a red image, a green image and a blue image are displayed for 5 msec each in one field period, and the photomodulation element 1 changes over in synchronism to transmit the lights of red, green and blue color. It is thus possible to display in multiple colors or full colors by using the photomodulation element 1. That is, a color filter of small size, light weight, and simple structure is realized. Fig. 10 is a block diagram showing the constitution of a photomodulation element 1b as a modified example. In this example, the driving voltage applied to the plural liquid crystal panels P1 to Pn is selected properly in a changeover circuit 25 for changing over the outputs of a high frequency power source 23 for generating a relatively high frequency fH (e.g. 10 kHz), and a low frequency power source 24 for generating a relatively low frequency fL (e.g. 100 Hz), and the selected output is adjusted in a voltage adjusting circuit 5 as in the preceding embodiment, and is supplied to each liquid crystal panel Pi. That is, the liquid crystal used in each liquid crystal panel Pi is a two-frequency driving liquid crystal, for example, TX2A (manufactured by BDH) in which the dielectric constant anisotropy Δε is positive at relatively low frequency voltage, and the dielectric constant anisotropy Δε is negative at high frequency voltage, and it is used, for example, at a layer thickness of d = 5 µm by using a 5 µm spacer. That is, when the driving voltage from the low frequency power source 24 is adjusted in the voltage regulating circuit 5, and is supplied to the liquid crystal panel Pi, the liquid crystal of each liquid crystal panel Pi changes from the homogeneous array gradually to a homeotropic array as the applied voltage increases, and corresponding to this change, the refractive index anisotropy Δn decreases. To return to the initial homogeneous array from this state, the driving voltage from the high frequency power source 23 is used, and is adjusted to a low voltage in the voltage regulating circuit 5. Hence, a high speed restoration is realized, and the operation of the photomodulation element 1b may be done at high speed, for example, 5 to 10 ms. The two-frequency driving liquid crystal is, in order to widely define the effective operating temperature range of the liquid crystal and to realize low viscosity, composed of a mixed liquid crystal by using a material of relatively weak polarity as the base material, such as alkoxyphenyl cyclohexycarboxylate, alkylphenyl cycloxycarboxylate, and alkoxyphenyl cyclohexane, and further adding a material of low dielectric dispersion frequency, for example, Moreover, using a material having a large negative value of dielectric constant anisotropy Δε such as 2,3-dicy ano-1,4-hydroquinone derivatives given below, the overall dielectric constant anisotropy and cutoff frequency (that is, the frequency for changing over positive and negative polarity of the dielectric constant anisotropy Δε) are adjusted. In the preceding example, the number N of the liquid crystal panels Pi for composing the photomodulation elements 1, 1a, 1b is not particularly limited numerically, but are properly selected in consideration of the color purity of the light of specified wavelengths obtained by using the photomodulation elements 1, 1a, 1b. Besides, the types of the polarizers 2, 3 and liquid crystal material, the conditions of the layer thickness d of the liquid crystal 12, and the installation conditions of the liquid crystal panels Pi and polarizers 2, 3 are properly selected. In these examples, all polarizers PL0 to PLN are explained as parallel Nicol's arrangement, but in order to obtain the desired bandwidth or transmission wavelength, setting of vertical Nicol is possible at least in one liquid crystal element Ci. According to the experiment by the present inventors, at the number of layers of N = 3, the composition of vertical Nicol for the liquid crystal element C3 at the exit side has been confirm to present a favorable filter action as a color filter of 400 to 700 nm. Similar effects may be achieved by using other liquid crystal materials, such as the liquid crystal material with negative dielectric constant anisotropy Δε including nemtatic liquid crystal, the liquid crystal panel composed of surface treatment agent in the vertical orientation system, and smectic liquid crystal. In the foregoing examples, a glass substrate is used as the substrate for the photomodulation elements 1, 1a, 1b, but as a modified example, by replacing with a plastic substrate such as synthetic resin material, the weight may be reduced. Furthermore, for the purpose of correcting the wavelength characteristic of the transmission light, a phase different plate composed of high molecular film such as polycarbonate and polyvinyl alcohol is inserted between the polarizers 2, 3, or a specified dichroic pigment may be added to the liquid crystal material. Or, instead of the voltage regulating circuit 5 used in the photomodulation element 1, it may be composed to control the temperature of the liquid crystal 12 of each liquid crystal panel Pi. Explained below is an application example of the photomodulation elements 1, 1a, 1b into electronic appliances. Fig. 11 is an exploded perspective view of a liquid crystal display device 26 as an embodiment of an electronic appliance, and Fig. 12 is a block diagram showing an electric composition of the liquid crystal display device 26. In the drawings, for the sake of simplicity, the diffusion plate, polarizer and other peripheral materials are omitted. The liquid crystal display device 26 comprises a back light source 27 for generating white light as surface light source, a liquid crystal display element 28 of active matrix type applying a nonlinear switching element for high , speed monochromatic display (thin film transistor, metal-insulation film-metal structure switching element, etc.), and a liquid crystal color filter 29. This display device is within the scope of this invention only if the colour filter 29 is within claim 1. The composite video signal supplied in the liquid crystal display device 26 is fed into a synchronous separation circuit 30, and a synchronous signal SY is separated, and the remaining color signal SC is fed into a color demodulation circuit 31, which produces image signals SR, SG, SB corresponding to the red, green and blue color. The video signals of each color SR, SG, SB are sequentially selected in a changeover circuit 32, and are fed Into a display control unit 33. The display control unit 33 scans the liquid crystal display element 28, and monochromatically displays the color image signals SR to SB. On the other hand, the synchronous signal SY is fed into a control circuit 34, and the control circuit 34 operates the changeover circuit 32 is synchronism with the synchronous signal SY, and also controls the voltage regulating circuit 5 connected to the liquid crystal panels P1 to PN, and synchronously changes over the liquid crystal color filter 29 as to sequentially transmit the lights of red, green and blue, corresponding to the monochromatic display of each color displayed by the liquid crystal display element 28. Here, one field period of the composite video signal of NTSC system is 1/60 sec, that is, about 17 msec, and when monochromatic images corresponding to red, green and blue images are sequentially displayed within this one field period, the liquid crystal color filter 29 changes over the colors at high speed in synchronism as mentioned above. Thus, using the liquid crystal color filter 29 of the invention, the liquid crystal display device 26 for multicolor display or full color display is realized. Therefore, in the liquid crystal display device 26 of the invention, as compared with the constitution employing the conventional microcolor filter, the resolution is three times or more higher substantially, and the display quality is outstandingly improved, while the manufacturing process is simplified, the yield is improved, and the constitution is simplified because the microcolor filter of microstructure is not formed on the liquid crystal display element 28. Fig. 13 is a schematic diagram of a projection type display device 35 as other embodiment of the electronic appliance of the invention. In the projection type display device 35, for example, a light source 36 for generating a white light, an ultraviolet ray cut-off filter 37, the liquid crystal color filter 29, and a lens 38 are arranged on a same optical axis, and the liquid crystal display element 28 and lens 38 are disposed on the same optical axis, and the optical image is projected and displayed on a screen 40. The display device 35 is within the scope of the invention only if the colour filter 29 is within the scope of claim 1. In such projection type display device 35, hitherto, for example, a disc mechanically rotated and driven is disposed between the ultraviolet ray cut-off filter 37 and lens 38, and this disc is a mechanical color filter forming color filters of red, green and blue in the circumferential direction. As compared with such constitution, in this embodiment, mechanical driving parts are not needed, and the durability is enhanced, noise is reduced, the life is extended, and the structure is downsized. Fig. 14 is a block diagram of a color image pickup device 41 as another embodiment of electronic appliance of the invention. The color image pickup device 41 of the embodiment comprises a solid-state image pickup element 42 composed of CCD element or the like, and the liquid crystal color filter 29 is installed at the light incident side to the solid-state image pickup element 42. The image pickup device 41 is within the scope of the invention only if the colour filter 29 is within the scope of claim 1. A control circuit 43 controls a scanning circuit 44 to scan the rasters of the solid-state image pickup element 42. By every scanning of one screen of the solid-state image pickup device 42, the voltage regulating circuit 5 is controlled so that the liquid crystal color filter 29 may sequentially transmit the lights of, for example, red, green and blue color. Therefore, the image signals corresponding to the red image, green image and blue image of the object of taking are sequentially read out from the solid-state image pickup element 42, and each image signal is converted into a digital signal in an analog/digital converter 45, and is read into the control circuit 43. The control circuit 43 is combined with a synchronous signal generating circuit 46 for generating a synchronous signal for synchronously controlling the scanning circuit 44 and voltage regulating circuit 5. In this way, the color image pickup device 41 of the embodiment is free of the necessity of using microcolor filter at the light incident side of the solid-state image pickup element 42 as explained in the prior art, and the resolution is substantially improved by three times or more, and the image quality is enhanced. Besides, as compared with the case of mounting microcolor filter, the manufacturing procedure and construction are simplified, and the yield is improved. Fig. 15 is a block diagram of a color sensor 47 as a different embodiment of the electronic appliance of the invention. The color sensor 47 is used as the means of reading the reference white copy when adjusting the white balance in, for example, facsimile communication equipment. The color sensor 47 comprises a photoelectric converting part 51 composed of p-layer 48, i-layer 49 and n-layer SO of amorphous silicon, a back electrode 52 formed on the n-layer 50, a transparent electrode 53 composed of ITO (indium tin oxide) or the like formed on the p-layer 48, and a sensor main body 55 including a glass substrate 54. At the light incident side of the sensor main body 55, the liquid crystal color filter 29 is installed. The colour sensor 47 is within the scope of the invention only if the colour filter 29 is within the scope of claim 1. The control circuit 43 controls the voltage regulating circuit 5 so that the liquid crystal color filter 29 may sequentially transmit the lights of, for example, red, green and blue. Upon every changeover of colors, the voltage detecting circuit 56 detects the voltage between the back electrode 52 and transparent electrode 53. That is, the transmission light intensity of every color can be detected. In such color sensor 47, conventionally, three color filters of red, green and blue are mounted on the sensor main body 55, and the back electrode 52 and transparent electrode 53 are separated into individual color filters. As compared with such prior art, the structure is simplified and downsized in this embodiment. Fig. 16 is a block diagram of a color copier 57 as a further different embodiment of the electronic appliance of the invention. In the color copier 57, the light source 36 is a white light source which is used when reading the document image by emitting light to the document 58, and the liquid crystal color filter 29 is placed between the light source 36 and the document 58. The colour copier 57 is within the scope of claim 1 only if the colour filter 29 is within the scope of claim 1. The reflected light from the document 58 enters an image sensor 61 such as CCD element, through plural reflectors 59 and an optical device 60. That is, the document 58 is scanned every time the liquid crystal color filter 29 is changed over, for example, to red, green and blue, and the image sensor 61 reads the document image corresponding to each color. The output of the image sensor 61 is fed into a printing device 62, and the printing device 62 produces a color copy by printing plural times on a single recording paper, by using color developers of plural colors corresponding to the three colors. The printing device 62 is composed of, for example, laser generating device, photosensitive drum on which laser beam is emitted, and developing device containing the color developers. In such color copier 57, therefore, the mechanical structure for changing over the colors of the light source light emitted to the document 58 is not needed, and the structure is simplified and the durability is enhanced. As other modified example of the embodiment, instead of the liquid crystal color filter 29 installed near the light source 36, it may be disposed on the optical path before the image sensor 61 as indicated by double dot chain line in Fig. 12. In such constitution, too, the same effects as mentioned above may be brought about. Fig. 17 is a schematic diagram of a color print platemaking device 63. The platemaking device 63 has a halogen lamp 64 as the light source, and the light source light is led into a document cylinder 65 composed of a right cylindrical glass or the like, and is focused on a color document film 68 installed in the document cylinder 65 through reflector 66 and converging lens 67. The light transmitting through the color document film 68 is led into a half-mirror 71 through pickup lens 69 and reflector 70. The light reflected by the half-mirror 71 enters a photo tube 73 through green or red filter 72. On the other hand, the light transmitting through the half-mirror 71 is reflected by a reflector 74, and enters a photo tube 75 by passing through the liquid crystal color filter 29. The platemaking device 63 is within the scope of the invention only if the colour filter 29 is within the scope of claim 1. The current corresponding to the quantity of incident light from the photo tubes 73, 75 is converted into a voltage in a current/voltage converter 76, and is given to a control circuit 77, and the control circuit 77 controls a platemaking mechanism 78, and makes a plate material for lithography, letterpress, intaglio or screen print. In such color print platemaking device 63, too, it is free from the need of installing filters of red, green and blue, and plural photo tubes corresponding to them, and the structure may be downsized and simplified. Fig. 18 is a block diagram of a color facsimile apparatus 79 for transmitting color images. The color document 58 to be read is read by a reading element 81 laminating the liquid crystal color filter 29 and line image sensor 80, every time the liquid crystal color filter 29 is change over in three colors. The facsimile apparatus is within the scope of the invention only if the colour filter 29 is within the scope of claim 1. That is, the reading processing circuit 82 controls the voltage regulating circuit 5, changes over the color of the light passing through the liquid crystal color filter 29 sequentially in the three colors, while the line image sensor 80 is scanned in synchronism. The obtained image data of each color is read by the reading processing circuit 82, and the data is compacted or extracted in the image processing circuit 83, or stored in the image memory 84. The image data obtained by reading the color document 58 is sent out in the telephone circuit network through a network control unit 86 by making a call to the destination of communication through a communication control unit 85. On the other hand, the image data sent out from the telephone circuit network is extracted or processed in the image processing circuit 83, through the network control unit 86 and communication control unit 85, and a color image is recorded on a recording paper by color recording means 88 composed of, for example, thermal transfer ribbons of three colors or ink jet nozzles of three colors, through a recording processing circuit 87. The structure of such color facsimile apparatus 79 is the so-called on-chip type having the liquid crystal color filter 29 mounted directly on the line image sensor 80. In such color facsimile apparatus 79, hitherto, for example, microcolor filters of three colors are installed on the line image sensor 80. Therefor, in this embodiment, as compared with such prior art, the structure is smaller and simpler. Fig. 19 is a schematic diagram showing a reading element 81a in another structural example of the color facsimile apparatus 79. In this embodiment, for example, the light is emitted to the color document 58 from the white light source 36, and its reflected light enters the line image sensor 80 through rod lens 89 and liquid crystal color filter 29. In such embodiment, conventionally, instead of the liquid crystal color filter 29, the reflected light from the color document 58 was separated into three colors by moving three color filters parallel, for example, by mechanical system. In this embodiment, as compared with such prior art, the structure is simplified and reduced in size. Fig. 20 is a schematic diagram showing a reading element 81b in a different structural example of the color facsimile apparatus 79. In this embodiment, the light from the white light source 36 is separated into three colors sequentially in time series through the liquid crystal color filter 29, and the reflected light from the color document 58 of the light source light enters the line image sensor 80 through the rod lens 89. The reading element 81b is within the scope of the invention only if the colour filter 29 is within the scope of claim 1. In this embodiment, too, the same effects as mentioned in the preceding embodiment are brought about. Besides, moreover, the photomodulation element of the invention may be used as the photo selector for picking up a light signal of desired frequency out of the light signals of various frequencies coexisting in the optical communication apparatus. Furthermore, it is known that the retardation Δnd of the liquid crystal 12 varies with the temperature, and therefore instead of the voltage regulating circuit 5 in the foregoing embodiments, it is also possible to modify so as to employ a constitution adjusting the temperature of each liquid crystal panel Pi. [Embodiment 2] Fig. 21 is an exploded perspective view showing a structural example of a photomodulation element 101 in an embodiment of the invention, and Fig. 22 is a diagram showing an optical composition of the photomodulation element 101. The photomodulation element 101 is laminated of plural, or N in this embodiment, liquid crystal panels Pi (i = 1 to N) having the constitution as described below, and polarizers 2, 3 are arranged at both sides of the direction of array. Each liquid crystal panel Pi is controlled of the applied voltage individually by plural voltage regulating circuits 5 connected commonly to a power source 4. As a modified example, a common applied voltage may be given to each liquid crystal panel Pi from a common voltage regulating circuit 5. The individual liquid crystal panels Pi are composed alike as shown in Fig. 2, and transparent electrodes 8, 9 made of, for example, ITO (indium tin oxide) are formed on a pair of glass substrates 6, 7. Covering the transparent electrodes 8, 9, for example, orientation films 10, 11 composed of polyvinyl alcohol are formed, and a liquid crystal 12 having double refractive characteristic such as ZL1-1565 (manufactured by Merck) or the like is sealed in between them in a cell thickness d = 2.5 µm, and the peripheral edge is sealed with a sealing material 13. The orientation films 10, 11 of the liquid crystal panels Pi are oriented by rubbing or other treatment. In each liquid crystal panel Pi, the orientation directions of the light incident side and exit side are parallel to each other as shown in Fig. 2 (2), as indicated by arrow Ai (i = 1 to N). Refer to Fig. 21 and Fig. 22. The orientation direction A1 of the liquid crystal panel P1 closet to the polarizer 2 in the polarization direction B1 of the polarizer 2 is selected in a state of crossing at an angle a (for example, 15 degrees). The orientation direction A2 of the liquid crystal panel P2 adjacent to the liquid crystal panel P1 is selected in a direction of inclining by an angle 3α to the polarization direction B1. Thereafter, similarly, the orientation direction AN of the N-th liquid crystal panel PN is selected in a direction of inclining by angle (2N-1)α to the polarization direction B1. The polarization direction of the polarizer 3 at the exit side of the final liquid crystal panel PN is selected in a direction of inclining at an angle 2Nα to the polarization direction B1 of the polarizer 2. That is, the polarization direction B1 of the polarizer 2 and the orientation direction A1 of the liquid crystal panel P1 form an angle of α, and the orientation directions Ai, Ai+1 of the adjacent liquid crystal panels Pi, Pi+1 (i = 1 to N-1) are determined so as to incline sequentially by angle 2α each in the same direction. The polarization direction B2 of the polarizer 3 is selected in a direction of inclining angle α to the same direction as the orientation direction AN of the final liquid crystal panel PN. Fig. 23 is a diagram for explaining the behavior when the light becoming a linear polarized light after passing through the polarizer 2 passes through the liquid crystal panel P1. As shown in Fig. 23 (1), the light entering the polarizer 2 is a linear polarized light parallel to the polarization direction B1 regardless of the wavelength of the light, and enters the liquid crystal panel P1. The orientation direction A1 of the liquid crystal panel P1 crosses with the polarization direction B1 by angle α. Here, when the retardation Δnd of the liquid crystal panel P1 (Δn: refractive index anisotropy of liquid crystal, d: layer thickness of liquid crystal) and the wavelength λ0 of the incident light are in the relation of [formula 3] Δnd λ 0 = ( 2m - 1 ) 2 m≧1 the light of the wavelength λ0 passing through the liquid crystal panel P1 becomes a linear polarized light turning from the polarization direction B1 by angle 2α due to the double refractive characteristic. The optical axis direction of the exit side of the light of the wavelength λ0 satisfying formula (3) in the liquid crystal panel 1 is shown by arrow C1 in Fig. 22. Hereinafter, of the light leaving the liquid crystal panel Pi, the optical axis direction of the light leaving as linear polarized light satisfying Equation 3 is indicated by arrow Ci. This phenomenon may be explained as follows. As shown in Fig. 23 (1), the light of the linear polarization entering the liquid crystal panel P1 is a linear polarized light near the incident side end of the liquid crystal panel as shown in Fig. 23 (2), but as passing through the liquid crystal panel P1, it is sequentially converted into elliptical polarization, circular polarization, and elliptical polarization, and near the exit side end it becomes linear polarization again. The optical axis direction of the linear polarization at the exit side is deviated from the polarization direction B1 of the polarizer 2 by angle 2α. Thus, when the light of the wavelength λ0 leaving the liquid crystal panel P1 as linear polarization enters the liquid crystal panel P2, if formula (3) is satisfied about the liquid crystal panel P2, the light of the wavelength λ0 is emitted again from the liquid crystal panel P2 as linear polarization. Its optical axis direction C2 shown in Fig. 22 is turned by angle 2α from C1 which is the optical axis direction of the incident light to the liquid crystal panel P2. That is, it is turned from the polarization direction B1 of the polarizer 2 by 4α. Thereafter, similarly, by selecting so that all liquid crystal panels Pi up to the N-th final liquid crystal panel PN may satisfy formula (3), the light of the wavelength λ0 leaving the liquid crystal panel PL as linear polarization is turned from the polarization direction B1 of the polarizer 2 by the angle 2Nα. Therefore, by selecting the polarization direction B2 of the polarizer 3 in the same direction as the optical axis direction of the light of the wavelength λ0 at this time, the light of the wavelength λ0 will pass through the polarizer 3. On the other hand, from the polarizer 2, the light of arbitrary wavelength is emitted as linear polarization, but the refractive index anisotropy Δn of the liquid crystal is dependent on wavelength as shown in line 14 in Fig. 24, and formula (3) is not satisfied. That is, the light of other wavelength than λ0 becomes circular polarization or elliptical polarization when leaving the liquid crystal panel P1, and when passing through the polarizer 3, the rate of being absorbed or dispersed by the polarizer 3 is notably increased as compared with the light of the wavelength λ0. Therefore, the exit light from the polarizer 3 shows the distribution state having the maximum value at wavelength λ as indicated by line 15 in Fig. 25 which is a graph showing the relation between the wavelength λ of the exit light and the transmission light intensity 1. On the other hand, lines 15, 16, 17 in Fig. 25 are graphs when the number of liquid crystal panels Pi laminated cn the photomodulation element 101 in Fig. 21 is N1, N2 and N3 (N1 > N2 > N3). That is, according to the experiment by the present inventors, the noise light intensities 11, 12, 13 which are intensities of the light of other wavelength than the wavelength λ0 in the exit light from the polarizer 3, and the half-value widths W1, W2, W3 of the lines 15 to 17 were decreased as the number of laminates N of the liquid crystal panel Pi increased, and the color purity of the light leaving the polarizer 3 was confirmed to be enhanced. It is also known that the apparent refractive index anisotropy Δn of the liquid crystal 12 varies depending on the voltage V applied between the transparent electrodes 8, 9 as indicated by line 18 in Fig. 5 above. Therefore, in Fig. 24, when attempted to have light of wavelength λ0, the refractive index anisotropy required from line 14 is Δn0, and to obtain the refractive index anisotropy Δn0, it is known enough to apply voltage V0 from line 18 in Fig. 5. Thus, by properly adjusting the voltage applied to the liquid crystal panels Pi through the voltage regulating circuit 5, the light of arbitrary wavelength is obtained from the photomodulation element 101. Actually, when desired to obtain a desired wavelength, or a desired color purity for specific light, that is, when desired to properly determine the value of wavelength λ for giving the maximum value of the distribution curve of the transmission light intensity in Fig. 25, or half-value width W and noise light intensity I, it is possible to determine by comprehensively considering the retardation of the liquid crystal panel Pi, number of laminates, the refractive index of the liquid crystal 12, or wavelength characteristics of the polarizers 2, 3. The present inventor investigated the photomodulation element 101 having the above constitution, and measured the distribution of transmission light intensity by varying the voltage applied to the liquid crystal panels Pi by using the voltage regulating circuit 5. The result is shown by graph in Fig. 26. Lines 119, 120, 121 in Fig. 26 indicate that the applied voltages V1, V2, V3 are in the relation of [formula 16] V1 < V2 < V3 That is, in lines 119 to 121, it is shown that the transmission lights of red, green and blue are obtained at a relatively favorable color purity. As a modified example of the invention, it is possible to realize by installing a polarizer between liquid crystal panels (Pi and Pi+1, 1≦i≦N-1), and setting the polarization direction of the polarizer parallel to the linear polarized light leaving each liquid crystal panel P1 or the wavelength λ0 of notice or the major axis direction (Ci) of the elliptical polarized light close to the linear polarized light. At this time, the light of the wavelength λ0 of notice passes through the photomodulation element 101 according to the principle of operation described above, but the light of other wavelengths is a circular polarized light or elliptical polarized light of which major axis is not parallel to the polarization direction of intermediate polarizer, and hence its passing is blocked. It is hence possible to compose the photomodulation element 101 capable of picking up the desired light efficiently. Besides, in addition to the double refractive characteristic in the foregoing embodiment about the liquid crystal 12 of the liquid crystal panels Pi, by twisting the orientation of the liquid crystal molecules to increase the optical characteristic, it is also possible to compose the photomodulation element capable of picking up the light of arbitrary light in the same principle. That is, by twisting the orientation of the liquid crystal molecules, the light entering the liquid crystal panels Pi in liner polarization state leave as elliptical polarization having a certain eccentricity, but by properly selecting the retardation or twist angle of the liquid crystal as mentioned above, it is possible to form an elliptical polarization close to the linear polarization as far as possible. In this regard, the modified example may be realized. When such photomodulation element 101 is combined with, for example, active matrix liquid crystal display device of monochromatic display in an attempt to realize multicolor or full-color display, a high speed is required in color changes in its application example. To operate the photomodulation element 101 at high speed, the dislocation of the liquid crystal molecules with respect to specified change of applied voltage must be done at high speed. Accordingly, the layer thickness d of the liquid crystal 12 must be as small as possible. Hence, by modifying by supposing m = 1 in formula (3), we obtain [formula 17] Δnd = λ0/2 Accordingly, when λ0 = 650 nm is determined, from formula (17), it follows that Δnd = 0.32 µm. Assuming the refractive index anisotropy Δn = 0.13 of ordinary nematic liquid crystal, the layer thickness is d = 2.5 µm. Seeing that the layer thickness of the liquid crystal in ordinary TN type liquid crystal display device is 5 to 12µm, in this case, the layer thickness of the liquid crystal 12 is about 1/5 to 1/2 smaller as compared with the existing liquid crystal display device. The response speed to the change of the applied voltage of the liquid crystal is known to be inversely proportional to d 2 , concerning the layer thickness d of the liquid crystal, generally. That is, the relation between the layer thickness d and response time τ is indicated by line 22 in Fig. 27. More specifically, as the cell thickness becomes smaller, the response speed becomes faster. In the actual numerical condition given above, the response speed is about 4 to 25 times faster than the response speed of the conventional liquid crystal display device of TN type or the like. On the other hand, the minimum value of the layer thickness of the liquid crystal 12 among the presently available liquid crystal display devices is about 5 µm, and to design the layer thickness smaller than this, it is known that the refractive index anisotropy of the liquid crystal material must be Δn = 0.07 or more in Equation 13. This numerical value is the value satisfied by the refractive index anisotropy of the ordinary TN type liquid crystal, and hence the photomodulation element 101 of the invention may be composed by using the ordinary TN liquid crystal. On the other hand, to obtain a high response speed, it is also necessary to consider the viscosity of the liquid crystal material. The viscosity of the liquid crystal should be as low as possible, and generally the effect of the invention is exhibited at 35 centipoise (cp) or less, but preferably it is desired to be 25 centipoise or less, as clarified by the experience of the present inventors. Liquid crystal materials showing such characteristics include biphenyl compound, phenylester compound, cyclohexane compound, phenyl pyrimidine compound, dioxane compound, tolan compound, alkenyl compound, and fluorine compound, or their mixtures. Using a liquid crystal composition made of such materials, a photomodulation element as shown In Fig. 21 was composed, and the applied voltage was changed over to obtain three wavelengths λ1, λ2, λ3 as shown in Fig. 26, and the response speed was measured. As a result, a high speed response of several milliseconds was obtained. Incidentally, the one field period of television video signal of NTSC system is 1/60 second or about 17 msec. For example, it is hence possible to compose a constitution in which the red image, green image, and blue image are displayed for 5 msec each in one field period, and the photomodulation element 101 changes over in synchronism to pass the red, green and blue light. It is thus possible to realize a multicolor display or full-color display by using the photomodulation element 101. That is, a color filter of small size, light weight, and simple structure is realized. Fig. 28 is a block diagram showing the constitution of a photomodulation element 101a as a modified example of the invention. In this embodiment, the driving voltage applied to plural liquid crystal panels P1 to PN is properly selected between a high frequency power source for generating a relatively high frequency fH and a low frequency power source 24 for generating a relatively low frequency fL by a changeover circuit 25 which changes over their outputs, and the selected output is adjusted in a voltage regulating circuit 5 as in the preceding embodiments, and is supplied to each liquid crystal panel Pi. In other words, the liquid crystal used in each liquid crystal panel Pi is a two-frequency driving liquid crystal of which dielectric constant anisotropy Δε is positive at a relatively low frequency voltage, and negative at a high frequency voltage. That is, when the driving voltage from the low frequency power source 24 is adjusted in the voltage regulating circuit 5 and supplied to each liquid crystal panel Pi, as the applied voltage increases, the liquid crystal of each liquid crystal panel Pi gradually changes from the homogeneous array to the homeotropic array, and the refractive index anisotropy Δn varies corresponding to this change. To return to the initial homogeneous array from this state, the driving voltage from the high frequency power source 23 is used and is adjusted to a proper voltage In the voltage regulating circuit 5. As a result, the state is returned at high speed, and the operation of the photomodulation element 101a is also high in speed. The two-frequency driving liquid crystal is, in order to define widely the effective operating temperature range of the liquid crystal and to realize low viscosity, composed of a mixed liquid crystal as a base material of a relatively weak polarity, such as alkocyphenyl cyclohexyl carboxylate, alkylphenyl cycloxycarboxylate, and alkoxyphenyl cyclohexane, which is blended with a material of low dielectric dispersion frequency, such as chemical formula (1)-(4). Furthermore, using a material having a large negative value of dielectric constant anisotropy Δε , such as 2,3-dicyano-1,4-hydroquinone derivaties as shown chemical formula (5)-(7), the overall dielectric constant anisotropy and cut-off frequency (the frequency for changing over the dielectric constant anisotropy Δε between positive and negative polarity) are adjusted. In the foregoing embodiments, the number N of liquid crystal panels Pi for composing the photomodulation elements 101, 101a is not particularly defined, but is properly selected in consideration of the color purity of the light of the desired wavelength obtained by using the photomodulation elements 101, 101a and the like. Besides, the type and number of polarizers (2, 3), the type of liquid crystal material, the condition of layer thickness d of liquid crystal 12, and installation conditions of liquid crystal panels Pi and polarizers 2, 3 are properly selected. Besides, similar effects are obtained by using other liquid crystal materials, such as the liquid crystal panel composed of liquid crystal material with negative dielectric constant anisotropy Δε as represented by nematic liquid crystal and a surface treatment agent of vertical orientation type, and ferrodielectric liquid crystal. In the embodiments, the glass substrate was used as the substrate for photomodulation elements 101, 101a, but it may be replaced by a synthetic resin plastic substrate in a modified example, so that the weight may be reduced. Furthermore, for the purpose of correcting the wavelength characteristic of the transmission light, a phase difference plate made of high molecular film such as polycarbonate and polyvinyl alcohol may be placed between the polarizers 2, 3, or a material adding a specific dichoic pigment may be used as the liquid crystal material. Hereinafter are explained application examples of photomodulation elements 101, 101a into electronic appliances. Meanwhile, the photomodulation elements 101, 101a in these embodiments may be used as liquid crystal color filter 29 in various electronic appliances shown in Figs. 11 to 20. In such cases, too, same effects are obtained. As a example to show that the invention may be applied also in an image processing device, the image processing system disclosed in the Journal of Electronic Information Communication Society C-II, J73, pp. 703-712 (1990), is cited and described below. Fig. 29 is a schematic diagram showing the constitution of an image processing device 90 as a different embodiment of the invention. The image processing device 90 comprises a white light source 36 and a liquid crystal color filter 29 composed of the photomodulation elements 101, 101a, and the light from the liquid crystal color filter 29 enters a color polarizer 91, for example, a composition composed of liquid crystal display element 90 of active matrix type for monochromatic display, color polarizer 93, similar liquid crystal display element 94 and color polarizer 95, and the light leaving the color polarizer 94 is the processed image. Here, the color polarizers 91, 93, 95 convert the light of predetermined specific wavelength into linear polarized light, but do not convert the light of other wavelengths. Fig. 30 is a diagram to explain the operation of the image processing device 90. Fig. 30 (a) shows the image processing device 90a for calculating the logic products (G1, G2) of display images G1, G2 of the liquid crystal display elements 92, 94. In this example, the color polarizers 91, 93, 95 convert the light of wavelength λ1 into linear polarized light all in one direction. Therefore, the light of linear polarization passing through the color polarizer 91 is turned only by a specific angle in the non-passing region indicated by shaded area of the liquid crystal display element 92, for example, and the linear polarized light directly passes through in the transmission region without shaded area. Therefore, the light in the passing region directly passes through the color polarizer 93, and then the portion corresponding to the non-passing region indicated by shaded area of the liquid crystal display element 94 is turned, and the light directly passes through the portion corresponding to the passing region without shaded area. When this transmitted light passes through the color polarizer 95, the light turned by the liquid crystal element 94 is cut off, and the light not turned is passed. The light turned by the liquid crystal display element 92 is cut off by the color polarizer 93. Therefore, a semicircular display portion is obtained as processed image 96. Fig. 30 (2) shows the structure of the image processing device 90b for realizing the logic sum (G1 + G2) of the display images G1, G2 similar to Fig. 30 (1). In this case, the lights of two wavelengths λ2, λ3 are used. The color polarizer 91 converts only the light of wavelength λ2 into linear polarized light, and the color polarizer 93 converts the lights of both wavelengths λ2, λ3 into linear polarized light, and the color polarizer 95 converts only the light of wavelength λ3 into linear polarized light. Therefore, the light of the wavelength λ2 is converted into linear polarized light through the color polarizer 91, and the light of wavelength λ3 is directly passed through the color polarizer 91 without being converted into linear polarized light. The light of the wavelength λ2 passes through the liquid crystal display element 92 and color polarizer 93, and then passes only through the disc region without shaded area in the liquid crystal display element 92. When such light of wavelength λ 2 gets into the liquid crystal display element 94, it passes without being turned in the portion without shading, and is turned and passes through the shaded area, but when the color polarizer 95 passes all the light of wavelength λ2, the transmission light in the shape of the disk region is obtained. On the other hand, the light of wavelength λ3 is first converted into linear polarized light when passing through the color polarizer 93, and therefore by passing through the liquid crystal display element 94 and color polarizer 95, the light of the rectangular range without shading passes through the liquid crystal display element 94. Therefore, as the processed image 96, a superposed image of the circular image G1 and rectangular image G2, that is, the image of the logic sum is obtained. Fig. 30 (3) shows the structure of the image processing device 90 for obtaining an inverted image of inverted image (G1EXORG2) of exclusive logic sum of the images G1, G2 similar to the above example. At this time, using monochromatic light of wavelength λ4, the color polarizers 91, 95 convert the light of this wavelength λ4 into linear polarized light, but the color polarizer 93 directly passes the light of the wavelength λ4. That is, the color polarizer 93 may not be necessary, or a transparent glass may be used. The light of the wavelength λ4 passes through the color polarizer 91 to be converted into linear polarized light, and this light directly passes through the circular region without shading in the liquid crystal display element 92, and is turned by the specified angle in the shaded region. When the light of such state enters the liquid crystal display element 94, the light entering the shaded area of the liquid crystal display element 94 is turned by a specific angle. That is, when passing through the liquid crystal display element 92, the revolved light is further revolved to be a linear polarized light parallel to the direction of polarization of the color polarizer 95. The range not revolved in the liquid crystal display element 92 is revolved in the shaded area in the liquid crystal display element 94, and is cut off in the color polarizer. A similar phenomenon occurs in the transmissing part without shading in the liquid crystal display element 94, and therefore, the inverted image of the exclusive OR image of the images G1, G2 as shown in Fig. 30 (3) is obtained as the processed image. Such image processing devices 90, 90a to 90c easily generate and select arbitrary wavelength necessary for operation by using the liquid crystal color filter 29 as shown in Fig. 29, and may be preferably executed in image processing for overlapping plural images or excluding a portion from a certain image. Besides, in the optical communication appliance, the photomodulation element of the invention may be used as the optical selector for picking up an optical signal of desired frequency, from various mixed various frequency optical signals. Furthermore, the retardation Δnd of the liquid crystal 12 is known to vary depending on temperature, and instead of the voltage regulating circuit 5 in the preceding embodiment, a modified example is possible to adjust the temperature of the liquid crystal panels Pi. The invention may be embodied in other specific forms. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description.

Claims: An optical modulating element (101,101a) for modulating light of wavelength λ 0 , the modulating element comprising: input and output polarisers (2,3); a plurality of liquid crystal panels (PI, where 1 ≤ I ≤ N) disposed between the polarisers, a nematic liquid crystal having birefringence and a twisting angle of 0° being sealed in each panel; and means for changing the retardation Δnd of each liquid crystal panel, where Δn is the refractive index anisotropy of the nematic liquid crystal and d is the thickness of the liquid crystal layer contained in the liquid crystal panel; whereby, in use, light is incident upon the input polariser (2), passes through the liquid crystal panels in sequence, and is output through the output polariser (3); wherein the alignment direction (A1) of the first liquid crystal panel (P1) is at a pre-determined angle (α) with respect to the transmission axis (B1) of the input polariser (2); and wherein the alignment direction (AI) of the second and successive liquid crystal panels (PI) crosses the alignment direction (A{I-1}) of the immediately preceding liquid crystal panel (P{I-1}) at twice the pre-determined angle; characterised in that the transmission axis (B2) of the output polariser (3) is at the pre-determined angle (α) to the alignment direction (AN) of the last liquid crystal panel (PN) whereby the transmission axis (B2) of the output polariser (3) is at an angle 2Nα to the transmission axis (B1) of the input polariser (2); and in that, in use, each liquid crystal panel is adjusted to be a half-wave plate in which the retardation Δnd satisfies the equation Δnd/λ 0 = (2m-1)/2 where m is an integer, whereby when linearly polarised light of wavelength λ 0 is incident on one of liquid crystal panels the plane of polarisation of the light is rotated by twice the predetermined angle by the liquid crystal panel. An optical modulating element as claimed in claim 1, wherein the liquid crystal panel is constituted by a pair of transparent electrodes (8,9) between which a liquid crystal (12) is sealed, and the means for changing the retardation Δnd of each liquid crystal panel are voltage application means (4,5) for applying either an AC voltage or a DC voltage to the liquid crystal. An optical modulating element as claimed in claim I wherein the retardation changing means is a temperature adjustment means for adjusting the temperature of the respective liquid crystal panels. An optical modulating element as claimed in claim 1, wherein when transmitting the light of a wavelength of λ 0 through the modulating element, the retardation Δnd of each liquid crystal panel (101,101a) and the number of the liquid crystal panels are selected in such a way that the width at half maximum of the intensity distribution of the transmitted light centring on the wavelength λ 0 of the transmitted light and the intensity of light of wavelengths other than λ 0 may be predetermined values. A display apparatus (26,35) for displaying a coloured image, the apparatus having a light source (27,36) for generating light containing wavelengths corresponding to a plurality of colours, said apparatus including an optical modulating element, said apparatus displaying successive images corresponding to different colours within a time period corresponding to the time period for an afterimage effect, the optical modulating element switching the colour of the light that passes after a time period for display of the image for each colour; wherein the optical modulating element is an optical modulating element as defined in any of claims 1 to 4. A light detecting apparatus (41,47) having an optical modulating element disposed on the input side of a light detecting means (42,51) for outputting a detecting signal corresponding to the intensity of the incident light; wherein the optical modulating element is an optical modulating element as defined in any of claims 1 to 4. A colour copying machine (57) having a light source (36) for generating light containing a plurality of wavelengths; an optical modulating element for selecting a desired wavelength of light from the light source; a document or photoelectric transfer means (61) for forming an optical image of the document and for converting optical images into electric signals; and imaging means (62) for forming colour images on a recording medium by using the developers for the plurality of colours based on the electric signals obtained by the photoelectric transfer means, wherein the optical modulating element is an optical modulating element as defined in any of claims 1 to 4. A printing and platemaking apparatus (63) having a light source (64) for generating light containing a plurality of wavelengths, an optical modulating element for selecting a desired wavelength of light from the light source; a document or photoelectric transfer means (66,67,73,75,76) for forming an optical image of the document and for converting optical images into electric signals, and plate materials processing means (77,78) for forming the image of a document for each colour on the plate materials for a plural number of different colours, based on the electric signals obtained by the photoelectric transfer means, wherein the optical modulating element is an optical modulating element as defined in any of claims 1 to 4. An image input/output apparatus (41,79) for photographing object materials (58) for a plurality of colours, having: photographing means (42,80) for photographing through an optical modulating element; input means (82,83) for storing image data from the photographing means in an image memory (84) for each colour and forming colour images; and either indicating means (88) for displaying images or output means (85,86) for outputting colour images by switching the colour of transmitted light with the optical modulating element, based on the image data of a plural number of colours stored in the image memory; wherein the optical modulating element is an optical modulating element as defined in any of claims 1 to 4. An image operating apparatus comprising a light source which produces a light of a plural number of wavelengths, an optical modulating element which passes only specific wavelength components of the light from the light source, and a plural number of transmission or reflection type displaying elements for either transmitting or reflecting the light of the remaining wavelength components, wherein the optical modulating element is an optical modulating element as defined in any of claims 1 to 4. A lighting apparatus having a light source which produces a light of a plurality of wavelengths; and an optical modulating element passing only specific wavelength components of light from the light source; wherein the optical modulating element is an optical modulating element as defined in any of claims 1 to 4. A method of modulating light of wavelength λ 0 , using a modulating element (101) comprising: input and output polarisers (2,3); a plurality of liquid crystal panels (PI, where 1 ≤ I ≤ N) disposed between the polarisers, a nematic liquid crystal having birefringence and a twisting angle of 0° being sealed in each panel; and means for changing the retardation Δnd of each liquid crystal panel, where Δn is the refractive index anisotropy of the nematic liquid crystal and d is the thickness of the liquid crystal layer contained in the liquid crystal panel; whereby, in use, light is incident upon the input polariser (2), passes through the liquid crystal panels in sequence, and is output through the output polariser (3); the method comprising the steps of: disposing the first liquid crystal panel (P1) so that its alignment direction (A1) is at a pre-determined angle (α) with respect to the transmission axis (B1) of the input polariser (2); and setting the alignment direction (AI) of the second and successive liquid crystal panels (PI) at twice the pre-determined angle (2α) to the alignment direction (A{I-1}) of the immediately preceding liquid crystal panel (P{I-1}); characterised in that it further comprises the step of disposing the output polariser so that the transmission axis (B2) of the output polariser (3) is at the pre-determined angle (α) to the alignment direction (AN) of the last liquid crystal panel (PN) whereby the transmission axis (B2) of the output polariser (3) is at an angle 2Nα to the transmission axis (B1) of the input polariser (2); and in that it further comprises the step of adjusting the retardation Δnd of each liquid crystal panel such that the liquid crystal panels are half-wave plates having a retardation given by the equation Δnd/λ 0 = (2m-1)/2 where m is an integer, whereby when linearly polarised light of wavelength λ 0 is incident on each of the liquid crystal panels the plane of polarisation of the light is rotated by twice the predetermined angle (2α) by the liquid crystal panel.

Inventors: ISHII YUTAKA, YAMAMOTO YOSHITAKA, ISHII,YUTAKA, YAMAMOTO,YOSHITAKA
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: G02F1
IPC codes: G02F1