Optically addressable spatial light modulator

Claims

What is claimed is:

1. A multi-pixel liquid crystal spatial light modulator having a plurality of picture elements, comprising;

a pair of spaced transparent electrically conductive films,

a photosensitive film carried by one of said conductive films,

a plurality of conductive elements carried by said photosensitive film and arranged to define the position of each individual one of said plurality of modulator picture elements, each individual one of said conductive elements being electrically isolated from all other conductive elements,

an apertured film carried by said photosensitive film substantially coplanar with said conductive elements, said apertured film having apertures formed therein and arranged in the form of the modulator picture elements, so as to define a plurality of areas of uncovered photosensitive film area, each of said uncovered areas of photosensitive film surrounding one of said conductive elements,

liquid crystal activating voltage source means connected to said conductive films,

chiral smectic liquid crystal means confined between said one conductive film and said photosensitive film,

write beam means selectively directable to the side of said photosensitive film opposite from said plurality of conductive elements,

whereby individual picture elements of said modulator are individually addressed by activating the portion of said photosensitive film that is associated with said addressed picture elements,

thereby causing the corresponding conductive-element-defined chiral smectic liquid crystal picture elements to switch states, and read wavefront means directable to a side of said modulator.

2. The spatial light modulator of claim 1 wherein said liquid crystal means is a bistable ferroelectric liquid crystal means, and wherein said photosensitive film is constructed and arranged to define one of the group of photodiode and photoconductor.

3. The spatial light modulator of claim 1 wherein said conductive elements are formed in a geometric shape to minimize the area of diffraction of a light wavefront impinging upon such a shape, and wherein said apertures are of a similar geometric shape.

4. The spatial light modulator of claim 3 wherein said conductive elements are substantially round elements and said apertures are substantially round apertures.

5. The spatial light modulator of claim 1 wherein said apertured film is an opaque film, and including a masking member comprising a plurality of opaque areas corresponding to said plurality of areas of uncovered photosensitive film area, each of said opaque areas overlying one of said plurality of uncovered areas of photosensitive film.

6. The spatial light modulator of claim 5 wherein said masking member is mounted on the opposite side of said liquid crystal means from said photosensitive film.

7. The spatial light modulator of claim 6 wherein said liquid crystal means is a bistable ferroelectric liquid crystal means, and wherein said photosensitive film is constructed and arranged to define one of the group of photodiode and photoconductor.

8. The spatial light modulator of claim 7 wherein said conductive elements are substantially round opaque reflector elements, said apertures are substantially round apertures, and said opaque areas comprises substantially round opaque rings.

9. The spatial light modulator of claim 1 wherein said liquid crystal means is a bistable ferroelectric liquid crystal means, and wherein said apertured electrically film is an electrically conductive film, and including a source of potential adapted to drain charge from unswitched-state ferroelectric liquid crystal means picture element areas connected to said apertured conductive film.

10. The spatial light modulator of claim 9 wherein said photosensitive film is constructed and arranged to define one of the group of photodiode and photoconductor.

11. The spatial light modulator of claim 10 wherein said conductive elements are opaque reflector elements, said apertured film is an opaque film, and including a masking member comprising a plurality of opaque areas corresponding to said plurality of areas of uncovered photosensitive film area, each of said opaque areas overlying one of said plurality of uncovered areas of photosensitive film.

12. The spatial light modulator of claim 11 wherein said masking member is mounted on the opposite side of said ferroelectric liquid crystal means from said photosensitive film.

13. The spatial light modulator of claim 12 wherein said reflector elements are formed in a geometric shape to minimize the area of diffraction of a light wavefront impinging upon such said reflector elements, and wherein said apertures are of a similar geometric shape.

14. The spatial light modulator of claim 13 wherein said reflector elements are substantially round reflectors, said apertures are substantially round apertures, and said opaque areas comprises substantially round opaque rings.

15. The spatial light modulator of claim 1 including plate means for supporting said pair of spaced transparent electrically conductive films, said plate means being selected from the group glass, diamond or diamond-like film and boron nitride film.

16. The spatial light modulator of claim 15 wherein said a photosensitive film is an amorphous silicon photosensitve film.

17. The spatial light modulator of claim 16 wherein said amorphous silicon photosensitive film is configured from the groups p-i-n photodiode film and n-i-n photoconductor film.

18. A method for providing an optically addressable liquid crystal spatial light modulator having a pixel matrix, comprising the steps of;

providing a pair of spaced, light-transparent, conductive films,

providing a light-sensitive film on one of said conductive films in confronting relation to the other of said conductive films,

confining liquid crystal means between said other conductive film and said light-sensitive film,

providing a plurality of isolated electrically conductive elements on said light-sensitive film, said conductive elements being arranged in the pattern of said pixel matrix, and

providing an apertured film on the surface of said light-sensitive film, the apertures in said film being arranged in the pattern of said pixel matrix,

whereby individual pixels of said modulator are addressed by light activation of the portion of said light-sensitive film associated with the conductive elements that are associated with the addressed pixels.

19. The method of claim 18 including the steps of,

providing said an apertured film as an electrically conductive film, and

providing a source of bias potential connected to said apertured film, said source of bias potential being controllable to control the sensitivity threshold of said spatial light modulator pixel matrix.

20. The method of claim 18 including the step of providing ferroelectric liquid crystal means as said liquid crystal means.

21. The method of claim 18 including the step of providing amorphous silicon light-sensitive film as said light-sensitive film.

22. The method of claim 21 wherein said amorphous silicon light-sensitive film is constructed and arranged in accordance with the group of photodiode and photoconductor.

23. The method of claim 18 including the step of providing a masking layer on the opposite side of said liquid crystal means from said light-sensitive film, said masking layer providing an optical mask for the light-sensitive film areas that surround each of said conductive elements.

24. The method of claim 22 including the step of providing a pair of spaced light-transparent plate means for the support of said pair of light-transparent, conductive films.

25. The method of claim 24 wherein said plate means is selected from the group glass, diamond film, diamond-like film and boron nitride film.

26. The method of claim 25 including the step of providing ferroelectric liquid crystal means as said liquid crystal means.

27. The method of claim 26 including the step of providing amorphous silicon light-sensitive film as said light-sensitive film.

28. The method of claim 27 wherein said amorphous silicon light-sensitive film is constructed and arranged in accordance with the group of photodiode and photoconductor.

29. The method of claim 27 wherein said amorphous silicon light-sensitive film is constructed and arranged in accordance with the group p-i-n photodiode and n-i-n photoconductor.

30. The method of claim 18 including the steps of providing said an apertured film as an electrically conductive film,

providing a masking layer on the opposite side of said liquid crystal means from said light-sensitive film, said masking layer providing an optical mask for the light-sensitive film areas that surround each of said conductive elements, and

providing a source of bias potential connected to said apertured film, said source of bias potential operating to to drain charge from adjacent portions of said liquid crystal means.

31. The method of claim 30 including the step of adjusting the sensitivity threshold of said spatial light modulator by adjusting the magnitude of said source of bias potential.

32. The method of claim 31 including the step of providing a pair of spaced light-transparent plate means for the support of said pair of light-transparent and conductive films.

33. The method of claim 32 wherein said plate means is selected from the group glass, diamond film, diamond-like film and boron nitride film.

34. The method of claim 33 including the step of providing ferroelectric liquid crystal means as said liquid crystal means.

35. The method of claim 34 including the step of providing amorphous silicon light-sensitive film as said light-sensitive film.

36. The method of claim 35 wherein said amorphous silicon light-sensitive film is constructed and arranged in accordance with the group of photodiode and photoconductor.

37. The method of claim 35 wherein said amorphous silicon light-sensitive film is constructed and arranged in accordance with the group p-i-n photodiode and n-i-n photoconductor.

38. A reflection mode optically addressable spatial light modulator in the form of an array of pixel cells, comprising;

ferroelectric liquid crystal film means,

photosensitive film means,

a plurality N of pixel cells, each cell of which is defined by one of a plurality N of electrically conductive reflectors, each reflector having one side thereof in physical contact with one side of said liquid crystal film means, and having the other side thereof in physical contact with one side of said photosensitive film means,

opaque apertured film means having one side thereof in physical contact with said one side of said liquid crystal film means, and having the other side thereof in physical contact with said one side of said photosensitive film means, said apertured film means having a plurality N of small apertures positioned to coincide with said N reflectors, each reflector being spaced from its adjacent aperture so as to define an exposed annular area of said photosensitive film means surrounding each of said reflectors,

a source of liquid crystal switching voltage connected between the other side of said liquid crystal film means and the other side of said photosensitive film means,

write illumination means operable to selectively illuminate said other side of said photosensitive film means,

individual pixel cells of said modulator being addressed by activating selected portions of said photosensitive film means using said write illumination means, the selected portions defining a data input to be stored in the modulator,

the photosensitive film means when so activated operating to connect said source of switching voltage in a series circuit extending through said selected portions of said photosensitive film means and through the associated reflectors to the associated portions of said liquid crystal film means, and

read illumination means for direction modulator reading illumination toward said other side of said liquid crystal film means.

39. The light modulator of claim 38 wherein

said photosensitive film means comprises photodiode film means, and

said source of liquid crystal switching voltage operates

to apply a first voltage polarity between said other side of said liquid crystal film means and said other side of said photodiode film means, to thereby erase all portions of said liquid crystal films means, and

to subsequently apply a voltage of a second polarity between said other side of said liquid crystal film means and said other side of said photodiode film means,

said write illumination means being operable during the period of application of said second polarity to store data in said modulator, and

said read illumination means being operable thereafter to read data stored in said modulator.

40. The light modulator of claim 39 wherein said source of liquid crystal switching voltage includes a period of substantially zero potential, and wherein said read illumination means is operable to read data stored in said modulator during said period of substantially zero potential.

41. The light modulator of claim 39 wherein said apertured film means is electrically conductive, and wherein said other side of said liquid crystal film means and said apertured film means are both connected to the same electrical potential level.

42. The light modulator of claim 41 wherein said photodiode film means comprise amorphous silicon photodiode film means.

43. The light modulator of claim 42 including a plurality N of annular opaque members positioned to coincide with said N reflectors, said opaque elements being located adjacent said other side of said liquid crystal film means, and each of said opaque members overlying one of said exposed annular areas of said amorphous silicon photodiode film means surrounding each of said reflectors.

44. The light modulator of claim 38

wherein said photosensitive film means comprises a film photoconductor portion for each pixel cell of said modulator,

wherein each pixel cell includes film resistor means having one end portion electrically connected to the reflector of its pixel cell, said film resistor means having a second end portion,

wherein said source of liquid crystal switching voltage comprising a direct current source connected to apply a first voltage polarity to said other side of said photoconductor film means, and to apply a voltage of an opposite polarity to said second end portion of said resistor means, and

means connecting said other side of said liquid crystal film means to a polarity intermediate said first and second polarity.

45. The light modulator of claim 44 wherein said source of liquid crystal switching voltage is of a variable magnitude, and means operable to vary the magnitude of said source in order to control the write illumination sensitivity threshold of said modulator.

46. The light modulator of claim 45 wherein said photoconductor portions comprise amorphous silicon photoconductor portion.

47. The light modulator of claim 46 including a plurality N of annular opaque members positioned to coincide with said N reflectors, said opaque element being located adjacent said other side of said liquid crystal film means, and each of said opaque members overlying one of said exposed annular areas of said photoconductor portion surrounding each of said reflectors.

48. A transmission mode optically addressable spatial light modulator in the form of an array of pixel cells, comprising;

ferroelectric liquid crystal film means,

photosensitive film means,

a plurality N of pixel cells, each cell of which is defined by one of a plurality N of electrically conductive transparent elements, each transparent element having one side thereof in physical contact with one side of said liquid crystal film means, and having the other side thereof in physical contact with one side of said photosensitive film means,

apertured film means having one side thereof in physical contact with said one side of said liquid crystal film means, and having the other side thereof in physical contact with said one side of said photosensitive film means, said apertured film means having a plurality N of small apertures positioned to coincide with said N transparent elements, each transparent element being spaced from its adjacent aperture so as to define an exposed annular area of said photosensitive film means surrounding each of said transparent elements,

a source of liquid crystal switching voltage connected between the other side of said liquid crystal film means and the other side of said photosensitive film means,

write illumination means operable to selectively illuminate said other side of said photosensitive film means,

individual pixel cells of said modulator being addressed by activating selected portions of said photosensitive film means using said write illumination means, the selected portions defining a data input to be stored in the modulator,

the photosensitive film means when so activated operating to connect said source of switching voltage in a series circuit extending through said selected portions of said photosensitive film means and through the associated conductive elements to the associated portions of said liquid crystal film means, and

read illumination means for direction modulator reading illumination toward said liquid crystal film means.

49. The light modulator of claim 48 wherein

said photosensitive film means comprises photoconductor film means, and

said source of liquid crystal switching voltage operates

to apply a first voltage polarity between said other side of said liquid crystal film means and said other side of said photoconductor film means, to thereby erase all portions of said liquid crystal film means, and

to subsequently apply a voltage of a second polarity between said other side of said liquid crystal film means and said other side of said photoconductor film means,

said write illumination means being operable during the period of application of said second polarity to store data in said modulator, and

said read illumination means being concurrently operable during the period of application of said second polarity to read data stored in said modulator.

50. The light modulator of claim 49 wherein said apertured film means is electrically conductive, and wherein said apertured film means is connected to a voltage source of variable magnitude, and means operable to vary the magnitude of said source in order to control the write illumination sensitivity threshold of said modulator.

51. The light modulator of claim 50 including a plurality N of annular opaque members positioned to coincide with said N conductive elements, said opaque elements being located adjacent said other side of said liquid crystal film means, and each of said opaque members overlying one of said exposed annular areas of said photoconductor film means surrounding each of said conductive elements.

FIELD OF THE INVENTION

This invention relates to the field of optical systems and elements, and more specifically to optically addressable spatial light modulators or light valves incorporating liquid crystal means.

BACKGROUND OF THE INVENTION

Spatial light modulators (SLM), as the term is used herein, are optical masks having a plurality of small pixel areas that are individually and selectively switchable. SLMs are used to modulate a reading optical wavefront, causing the wavefront to be either transmitted through the SLM (i.e. a transmission mode device), or to be reflected from the SLM (i.e. a reflection mode device), with a polarization that is determined by the modulator. These optical masks are usually two-dimensional, and comprise a plurality of small picture elements (i.e. pixels or PELS) that may be arranged in a two or three dimensional matrix of rows and columns.

In an optically addressable SLM, a write beam(s), for example a visible laser beam(s), programs or activates the individual pixels of the SLM to subsequently rotate the polarization of a read beam(s), for example an infrared laser beam(s). The write beam(s) programs the SLM by activating individual photosensitive pixel areas of the SLM. That is, those modulator areas that are to be programmed to rotate the polarization of a read beam(s) are activated by a write beam(s). U.S. Pat. No. 4,538,884 is an example. In the device of this patent, a pair of glass plates 1a and 2a support a pair of transparent electrodes 2a and 2b. A photoconductive layer 9, which can be amorphous silicon, is supported on electrode layer 2b. A plurality of aluminum reflectors 8 are incorporated into a transparent insulating layer 7 and are supported on the surface of the photoconductive layer, with the reflectors directly adjacent the photoconductive layer. An apertured shading layer 5 of carbon or metal is carried on the transparent insulating layer, each apertures 6 facing one of the reflectors. The space intermediate transparent insulating layer 7 and transparent electrode 2a is occupied by a liquid crystal 3. Other light activated spatial light modulators are shown in U.S. Pat. Nos. 4,619,501 and 4,655,554 wherein activating light produces a surface charge pattern that modulates the index of refraction of an electro-optic crystal; U.S. Pat. No. 4,693,561 wherein the voltage applied to aluminum strips that are in contact with a liquid crystal operate to destroy the twist of the liquid crystal in the grating space between the aluminum strips, and wherein activating light operates on a photoconductive layer to reduce the potential in the grating spaces, whereby light modulation occurs; and U.S. Pat. No. 4,679,910 which discloses two liquid crystal cells in series, a photoconductive substrate being disposed between the cells, and a mirror being disposed after the second cell. Other patents that may be of interest are U.S. Pat. No. 3,855,579 which shows an optic relay having a multi-layer dielectric mirror in direct contact with an electro-optical material; U.S. Pat. No. 3,824,002 which describes a liquid crystal light valve wherein all electrically conductive elements are isolated from the liquid crystal, and as a result, photoactivation must be accomplished by using the principle of impedance matching; U.S. Pat. No. 4,368,386 which discloses a liquid crystal image converter that can be configured to operate in either the transmissive or the reflective mode; and U.S. Pat. No. 4,277,145 which discloses a reflective mode liquid crystal display device wherein a writing beam in the presence of an applied voltage causes a transition in the liquid crystal to the isotropic phase, whereupon an image may be readout by light that passes through the liquid crystal and is reflected from a reflecting electrode.

The use of amorphous silicon photoconductor means in a liquid crystal device is suggested in the article "Amorphous silicon photoconductor in a liquid crystal spatial light modulator", by Paul R. Ashley and Jack H. Davis, APPLIED OPTICS, Jan. 15, 1987, Vol. 26, No. 2, at pages 241-246.

The use of amorphous silicon photoconductor means and ferroelectric liquid crystal means in a liquid crystal device is suggested in the article "High-speed light valve using an amorphous silicon photosensor and ferroelectric liquid crystals", by N. Takahashi, H. Asada, M. Miyahara and S. Kurita, APPLIED PHYSICS LETTERS, Vol. 51, No. 16, Oct. 19, 1987.

The use of a ferroelectric liquid crystal in an SLM having reflective mode photodiode or photoconductive portions is suggested in THE PROCEEDINGS OF SPIE--THE INTERNATIONAL SOCIETY FOR OPTICAL ENGINEERING, Vol. 754, Jan. 13-15, 1987, at pages 207-212, incorporated herein by reference.

SUMMARY OF THE INVENTION

The present invention relates to optically addressable spatial light modulators utilizing a chiral smectic liquid crystal element having plural states, for example plural state electroclinic (smectic A.sup.*), or two state ferroelectric (smectic C.sup.*). Data is stored in the modulator by causing selected pixel areas of the liquid crystal to switch from one stable state to the other stable state under the influence of a writing beam(s) of light and an applied electric field. The written modulator is non-volatile in that, in the absence of an erasing means, the liquid crystal molecules remain in their last-set state. Liquid crystals of this type are birefringent, with the orientation of their optical axis being dependent on the molecular orientation of the liquid crystal.

Exemplary embodiments of a modulator constructed and arranged in accordance with the invention comprise a two-dimensional array of pixel cells, for example an X-Y square or rectangular array of cells. Since the modulator of the invention may take a variety of physical forms, the invention is not to be limited to either a two dimensional array or to an X-Y array.

The plurality of modulator pixels are defined by a like plurality of small, individual, isolated electrically conductive elements. In a transmission mode modulator constructed in accordance with the invention, these conductive elements are transparent to a reading wave front. In a reflection mode modulator constructed in accordance with the invention, these conductive elements operate to reflect the reading wave front, i.e. they are reflectors. Each conductive element is in physical contact with a continuous film of a liquid crystal means, the film being located on one side of the conductive elements. The other side of each conductive element is in physical contact with a continuous photosensitive layer or film, this film being located on the other side of the conductive element from the liquid crystal layer or film. As is conventional, thin alignment layers may be provided immediately adjacent both sides of the liquid crystal layer, to orient the liquid crystal in a desired direction.

The modulator is written or programmed by activating selected pixel areas or portions of the photosensitive layer (i.e. the areas or portions of the photosensitive layer that are defined by the individual conductive elements) using a write light beam(s). These selected pixel portions define input data of one form or another that is to be stored in the modulator.

When the photosensitive layer is so activated, a switching electrical field or voltage is applied to the associated pixel areas of the liquid crystal, by way of a circuit that extends through the photosensitive layer's activated pixel portions and through the associated conductive elements. When an electric field is applied across a pixel area of the liquid crystal, the molecules therein rotate into one of two stable states. As a result, selected pixel areas of the liquid crystal switch, and the input data is stored therein in the form of switched pixel areas.

Subsequently, the data stored in the modulator can be read out by way of a reading light beam(s) that enters the modulator from either side of the modulator in the case of a transmission mode modulator, or that enters the modulator from the opposite side from the side of entry of the write beam(s) in the case of a reflection mode modulator. As is desired, the write and read light beams can be differentiated from one another by their wavelength, intensity, and/or timing.

The above mentioned photosensitive layer functions to convert absorbed writing light energy into localized electric field areas within the adjacent liquid crystal area. These localized electric field areas determine the localized state of the adjacent pixel area(s) of the liquid crystal.

Preferably, but without limitation thereto, the photosensitive portion of the modulator comprises a hydrogenated amorphous silicon (a-Si:H) based multilayer film.

The individual photosensitive layer pixel areas or cells of the modulator can be constructed and arranged to operate in a photodiode mode or in a photoconductor mode.

A bipolar source of switching voltage may be applied to the pixel cells to facilitate the various modes of operation of the modulator, such as, for example, erasing the ferroelectric photodiode mode modulator, or erasing the ferroelectric continuous-layer photoconductor mode modulator. A bipolar source of switching voltage and full matrix illumination may be applied to the pixel cells to facilitate erasing the ferroelectric discontinuous-layer photoconductor mode modulator.

In a liquid crystal spatial light modulator in accordance with the invention, the individual pixel conductive elements may be in direct physical contact with the liquid crystal. More specifically, in the present invention a liquid crystal is confined between a conductive film and a photosensitive film. The individual modulator pixels are defined by a like plurality of small conductive elements, which may be reflectors, that are carried on the photosensitive film.

The liquid crystal material of the present invention is preferably, but without limitation thereto, a ferroelectric liquid crystal material of the type described in U.S. Pat. No. 4,367,924, incorporated herein by reference.

An apertured film is also carried by the photosensitive film, substantially coplanar with the conductive elements or reflectors. The apertures in this film surround each of the conductive elements, leaving an exposed ring of photosensitive film surrounding each of the conductive elements. This apertured film may be either opaque to or transmissive of the reading radiation.

A plurality of light-blocking rings are carried in overlying relation to these exposed rings of photosensitive film, to thereby block reading radiation from striking this portion of the photosensitive film when the reading radiation is directed to the modulator from that side of the modulator.

An object of the invention is to provide a multi-pixel liquid crystal spatial light modulator having an array of picture elements, the modulator having a pair of substantially parallel, closely spaced, and transparent plates, a pair of transparent and electrically conductive films, one film being being carried by each of the plates in facing relationship, a photosensitive film carried by one of the conductive films, and the modulator's picture elements being defined by a conductive-element pattern that is carried by the photosensitive film, the conductive-element pattern having a plurality of picture elements arranged in the shape of said array, each individual picture element of the pattern being isolated from all other elements of the pattern, and each element of the pattern defining an individual picture element of the modulator, activating voltage means connected to the conductive films, and liquid crystal means confined between the one conductive film and the photosensitive film, whereby individual picture elements of the modulator are addressed by activation of the portion of the photosensitive film that is associated with individual conductive elements.

Another object of the invention is to provide a method for constructing a liquid crystal spatial light modulator comprising a two dimensional pixel matrix, comprising the steps of; providing a pair of spaced light-transparent and conductive films, providing a light-responsive film on one of the conductive films in confronting relation to the other of the conductive films, confining a liquid crystal between the other conductive film and the light-responsive film, providing a plurality of isolated conductive elements on the light-responsive film, the conductive elements being arranged in the two dimensional form of the pixel matrix, providing an apertured layer substantially coplanar with the conductive elements, the apertures in the layer being arranged in the two dimensional form of the pixel matrix so as to coincide with the locations of the conductive elements, whereby individual pixels of the modulator are addressed by light activating the portion of the light-responsive film that is associated with individual conductive elements.

As a feature of the invention, a layer of opaque rings is spaced to overlie the ring areas of the light-responsive film that lie between each conductive element and its adjacent portion of the apertured layer.

As a feature of the invention, the apertured layer may be connected to ground potential, or to a source of bias potential.

As a feature of the invention, the liquid crystal comprises a ferroelectric liquid crystal, and as a further feature, a bipolar source of potential may be applied to the conductive films to facilitate the various modes of operation of the modulator, such as writing, reading and erasing.

As a feature of the invention, the individual pixel portions of the photosensitive or light-responsive film may be configured as photodiodes or photoconductors. These and other objects and advantages of the invention will be apparent to those of skill in the art upon reference to the following detailed description of the invention, which description makes reference to the drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an enlarged plan view of an optically addressable spatial light modulator 10 in accordance with the invention wherein the individual pixel cells 11 of the modulator are arranged in an 8 by 8 X--Y matrix,

FIG. 2 is an enlarged view of a 3 by 3 portion of the modulator of FIG. 1, showing individual pixel reflectors 12 that are associated with each modulator pixel 11,

FIG. 3 is a view similar to FIG. 2 wherein an apertured layer 13 is shown coplanar with reflectors 12,

FIG. 4 is a view similar to FIG. 3 wherein an overlying layer of opaque rings 15 is shown, the rings 15 operating to block reading light from passing to the exposed, ring-shaped portions 16 of the FIG. 3 photosensitive layer,

FIG. 5 is a side view of one