WO1993002375A1

WO1993002375A1 - Membrane light modulating systems

Info

Publication number: WO1993002375A1
Application number: PCT/US1992/005822
Authority: WO
Inventors: Cardinal Warde; Thomas N. Horsky; Craig M. Schiller; George J. Genetti
Original assignee: Optron Systems, Inc.
Priority date: 1991-07-17
Filing date: 1992-07-10
Publication date: 1993-02-04
Also published as: US5287215A; AU2338692A; AU661501B2; JPH06511567A; EP0678197A4; EP0678197A1; US5471341A; CA2113275A1

Abstract

A membrane light modulator (18) comprising a charge transfer plate (30), and having a multiplicity of conductors (35) extending from the rear surface (52) to the front surface (19) of the plate (30). The conductors (35) are supported in an insulating matrix (37). The front side (19) of the transfer plate (30) has a plurality of potential wells (32) defined by insulating walls (15), each potential well (32) constituting a pixel. A plurality of conductors (35) is provided for each pixel. A deformable reflecting membrane (34, 36) comprising a metal layer (36) spans the potential wells (32). An electric potential is provided on the metal layer (36), and a source of electrons (17) is provided for impacting the rear surface (52) of the charge transfer plate (30).

Description

MEMBRANE LIGHT MODULATING SYSTEMS

Background of the Invention Spatial light modulators (SLMs) have numerous potential technical applications such as multispectral infrared target simulation, projection television systems, and optical computer systems. First introduced by Preston in 1968, deformable membrane mirror light modulators (MLMs) , which incorporate a highly reflective membrane as the light modulating element, have generated interest within the applied optics community as good candidates for both adaptive optics and projection display applications. Various means of addressing the two-dimensional deformable membrane mirror have been demonstrated, including electron beam-addressing, optical addressing, and electrical addressing via integrated circuits. These devices have not progressed beyond the development state, hence there are no MLMs on the commercial market. The electron beam- addressed approach is preferred for display applications due to the high resolution and convenience of direct video addressing; however, development of suitable substrates which would decouple the electron beam interaction region from the reflective mirror were lacking. One method of solving this problem is by introducing the charge-transfer plate (CTP) as a means of providing both structural integrity and electrical signal transfer to the mirror elements. This approach has improved the state-of-the-art by yielding a device with a large number of resolution elements, high contrast, and low voltage operation, (see U.S. patent 4,794,296 assigned to the assignee of this application) Such a system is shown in Figure 18 of the above referenced '296 patent and is more particularly described on column 15, lines 21 through 38 thereof. The charge transfer plate creates a two-dimensional electric field which produces a local displacement of the metalized reflective membrane to provide local modulation of the phase output of the two-dimensional light signal reflected from the mirror. These SLMs exhibit very fast response times, can be read out with high optical efficiency, and in principle can incorporate a very large number of resolution elements. Since a deformable mirror SLM is essentially a two-dimensional phase modulating element with a large phase dynamic range, it is well-suited to adaptive optics applications such as wavefront correction, laser beam steering and phase only spatial filtering. With appropriate pixelization of the membrane surface, intensity modulation may be accomplished via the schlieren readout schemes employed by projection display systems such as the Eidophor (G.E.) and the Υ-Ruticon (Xerox). Recent improvements in the MLM and the system for modulating the charge thereon have involved a MLM wherein a membrane is deposited over an array of wells with an addressable electrode at the bottom of each well. Thus, the well and its electrode define an individual pixel. The membrane is coated with a thin light reflecting electrode material held at a static potential. A pixel is activated by establishing a potential difference between the well electrode and the membrane electrode, causing the membrane to deform into the well region in response to electrostatic forces. Hence, the pixel driving voltage induces a local phase modulation on the readout wavefront reflected by the membrane mirror surface. Such a system is described in "Electron Beam Addressed Membrane Light Modulator", Spatial Light Modulators and Applications, 1990 Technical Digest Series, Vol. 14, Optical Society of America, Sept. 1990. Brief Summary of the Invention In one preferred form of the invention a membrane light modulator utilizes a charge transfer plate membrane anode assembly. The charge transfer plate which has a multiplicity of conductors extending from the rear surface to the front surface of the plate. The conductors are supported in an insulating matrix and the front side of the transfer plate has a plurality of recessed wells defined by insulating walls, each recessed well constituting a pixel. A plurality of conductors are preferably provided for each pixel and a metal electrode in the bottom of each recessed well preferably spans a plurality of the conductors. The rear surface of the plate preferably includes a secondary electron-enhancing coating. A deformable reflecting metal surface spans the recessed wells. This metal surface can comprise a thin sheet of unsupported metal, but is preferably formed of a metalized coating on a thin insulating (e.g. plastic) support. In another form of the invention, the anode assembly is such that the support for the mirror membrane is still a plurality of insulating walls defining potential wells with an electrode on the bottom of each potential well. However, the potential wells are not part of a charge transfer plate and the electron beam directly addresses the mirror membrane to modify the charge thereon by either electron depletion or electron accretion. In addition to electron beam addressing of these anode structures with a cathode ray tube, optical addressing by means of a photocathode and microchannel plate assembly, field emitter array or hard-wire addressing are also possible for each of the anode structures cited herein.

Detailed Description of the Invention In order to more fully comprehend the invention reference should be had to the following detailed description of several preferred forms of the invention taken in connection with the attached drawings herein: Figure 1 is a schematic view of an overall system employing an electron-addressed membrane light modulator (e-MLM) in an image projection system. Figure 2 is a schematic sectional view showing a charge transfer plate, metalized membrane and associated electronic and optical components which is an improvement over that prior art CTP illustrated in the cited 1990 technical digest. Figure 3 is a schematic, partial sectional view of one type of charge transfer plate. Figure 4a is a photomicrograph of a metalized membrane overlying a charge transfer plate. Figure 4b is the light distribution showing the sixfold symmetry of the Fourier transform of the membrane mirror of 4a. Figure 5 is a schematic sectional view of a preferred form of the invention. Figure 6 is a block diagram of a comparison between grid stabilized and framed video operation of the e-MLM. Figure 7 is a diagram of an optically-addressed form of the invention, utilizing a photocathode and an electron multiplier assembly a set of microchannel plates to produce the necessary charge image as input to the MLM anode assembly. Figure 8 is a form of the invention, similar to that of Figure 5, except that the electron beam-addressing side of the CTP is patterned so as to realize an integrated grid in registration with the readout side in order to provide improved charging efficiency of the pixel electrodes, and reduce crosstalk between pixels. Figure 9 is a version of Figure 1 modified to accommodate long-wavelength IR projection which requires cryogenic cooling of the system. Figure 10 is a form of the invention in which an insulating substrate (face plate) used in lieu of a CTP. In this form, the electron beam-addressing and the optical readout are achieved on the same face of the device. Figure 11 is a form of the invention similar to that of Figure 10, except that rather than a discontinuous metal mirror coating on the membrane, a uniform dielectric mirror is coated onto the membrane. Figure 12 is a form of the invention similar to Figure 11, except the device is addressed and read out on opposite sides of the substrate. This is achieved by use of a mirror layer embedded within the insulative well structure. Figure 13 depicts the electron gun tube enclosure and readout optics which would be used with the device anode architectures of Figures 10,11, or 12 in those cases where the electron beam and optical readout may occur upon the same surface of the well structure. Figure 14 illustrates a form of the invention which utilizes a field emitter array to produce the necessary charge image on the charge transfer plate. Figure 15 illustrates a form of the invention wherein the charge transfer plate pixels are hard wired to an electronic controller. Figure 16 shows the use of the electron addressed membrane light modulator (e-MLM) to achieve three color projection video. The membrane light modulator of the present invention can be used in a number of applications. These involve various types of high definition display techniques, such as high definition television projection, infrared target simulation, as well as other light modulating systems wherein the spatially modulated light 11 is input to another device, for example, an optical computer. For simplicity, the invention will be initially described in its preferred form where it is used as an ultraviolet, visible or IR image projector. This initial description is not intended to limit the scope of the invention in any wa . A schematic of the projector is shown in Figure 1. Specifically, it consists of a readout light source 10 of the appropriate wavelength which can be a laser, arc lamp or glowbar, for example. It also includes IR collimating optics generally shown at 12, a computer- 14 controlling an electron beam-source 16 addressing a membrane light modulator anode 18. Fourier Transform (FT) spatial filter 20 and an output device, which may be a high gain screen, detector or video camera 24, are provided for viewing the output image. As will be more fully described, the image is impressed onto the mirrored membrane surface 34 of the e-MLM by the video signals of the scanning electron beam 16, and read out by the reflected light as a phase modulation. The phase modulated beam is then converted to a high-contrast, collimated image by the FT spatial filter 20, and displayed in projection. The construction of a prior art membrane light modulator (MLM) anode 18 which has been improved in accordance with one preferred form of the invention is shown in Figure 2. A membrane is deposited over the front surface 19 of a charge transfer plate 30 that is patterned with an array of recessed wells 32 defined by insulating walls 15 and having an addressable electrode 35 at the bottom, each such recessed well 32 constituting a pixel. Thus, the well 32 and its electrode 35 define an individual pixel or resolution element as described in the 1990 technical digest. Additionally, a secondary electron emitter enhancer coating 31 is deposited on the rear surface 52 of the charge transfer plate 30 that receives the electron signal. The purpose of this coating is to enhance the depletion of charge from the surface so images of either positive or negative charge can be written efficiently. The membrane 34 is coated with a thin electrode material 36 held at a static potential. This electrode material is highly reflecting and also acts as a mirror. A pixel is activated by establishing a potential difference between the well electrode 35 and the membrane electrode 36, causing the membrane 34 to deform into the well region 32 in response to the electrostatic force created by the charge on the pin 35. Hence, the pixel driving voltage induces a local phase modulation on the readout wavefront 13 reflected by the membrane mirror surface 36. Since pixel wells 32 are close-packed with high spatial uniformity, the membrane 36 diffracts light efficiently when deformed into the wells 32. By using a schlieren readout scheme 21 (e.g., low pass spatial filter that passes only the zero-order light or a band pass filter that passes only the first-order light) , the phase object at the deformed membrane surface 34 may be converted to an intensity image at the screen 24. The e-MLM consists of an addressing electron beam 16 and an MLM anode assembly generally indicated at 18, as illustrated in Figure 2. The anode consists of a metal mesh grid 44 before the pixelated matrix of wells 32, over which the polymeric, metalized membrane 34 is deposited. The membrane 34 is environmentally protected by an hermetically sealed, IR-transmissive end window 40.^' For use at infrared wavelengths the window material is preferably zinc selenide (ZnSe) or germanium, whereas materials such as glass could be used in the visible and quartz in the ultraviolet. Both sides of the window 40 are anti-reflection (AR) coated to eliminate undesirable light reflected by the window. Referring still to Figure 2, the electron beam 16 writes a charge pattern onto the addressing side of the CTP 30, which is transferred to the bottom of the well 32 by the electrodes 35 as a two-dimensional voltage pattern. The resulting deformed mirror membrane 34,36 phase-modulates the reflected IR readout light 13, which is converted to a projected high-contrast image by the downstream schlieren optics generally shown at 21. (see Figure 1) The heart of the anode assembly 18 is the pixelated structure, called a charge-transfer plate CTP 30. The name derives from its ability to serve as a high-density multi-feedthrough vacuum interface, transferring a two- dimensional charge distribution from vacuum to air. The CTP 30, illustrated in Figure 3, is a wafer of electrically insulating material 37 in which are imbedded a regular matrix of thousands to millions of longitudinally oriented conductive pins 35. The ratio of collective pin cross-sectional area to the CTP area is about 50%. Charge transfer plates 30 with 10 μm diameter pins on 14 μm centers, and 50 urn diameter pins on 70 μm centers are typical. Material may be removed from the pins 35 on one side of the plate 30 so as to form a regular array of recessed wells 32 a few microns deep. The surface is highly polished to an optical flatness of 2λ across the active diameter. Thereafter the enhancer coating 31 is applied to the rear surface 52 of the charge transfer plate 30 by thin film deposition techniques. A polymeric membrane 34 is deposited on the CTP surface that contains the wells 32 such that a reliable bond between the two dielectric surfaces is established due to van der Waals forces. The resultant membrane pixels then assume the symmetry of the wells 32 which may be circular pixels on hexagonal closed packed (HCP) centers or circular pixels on square centers, for example. Upon optical readout of the membrane 34, this symmetry (square or hexagonal) is preserved in the Fourier plane as a diffraction pattern. A high-magnification photograph of the pixelated membrane surface with HCP symmetry and its optical Fourier transform are illustrated in Figures 4(a) and 4(b), respectively. Referring again to Figure 2, we see that one means of addressing of the MLM anode 18 is accomplished by directly addressing the (CTP 30) with a scanning electron beam 16. This approach offers the following advantages: (1) With the appropriate electron gun drive electronics and high-resolution (e.g., vidicon- type) electron gun, each pixel (pin 35) of the CTP anode is individually addressable;

(2) The electron beam current of conventional delivery systems can be large, ranging from tens of nanoamperes to hundreds of microamperes;

(3) A well-established technology standard exists for scanning electron beam imaging systems, as a result of the widescale development of cathode ray tubes and video-based communications (e.g., television) . Electronically, the e-MLM may be viewed as a triode structure, as illustrated in Figure 2. The thermionic cathode 17 at V]<0 emits a primary electron beam 16 which may be intensity modulated by its video grid (not shown) , which strikes a pin 35 (or pins) of the CTP 30. Secondary electrons are emitted from the enhancer coating 31 overlying the pins 35 and collected by the grid 44, resulting in a buildup of positive charge on the pin 35 if the secondary electrons are collected by the grid 44. The local potential of the CTP 30 (which influences the landing energy of the primaries) is determined by the membrane potential at V_m. The landing energy E_p of the primaries is given by: E_p = e(V_k-V_m). The ratio of secondaries to primaries, or 6, is dictated by the particular value of Ep. In the absence of an erase cycle, the throughput (TP) of the e-MLM may be given by: TP (pixels/sec) = i_s/CV, where i_s is the secondary current, C the pixel capacitance, and V the potential difference required to deflect the membrane 34 to full-contrast modulation. The throughput is representative of the signal rate of the e-MLM for information processing. For example, if we desire a 250x250 pixel image framed at 100 Hz, the TP associated with this frame rate is: TP = 250x250x100 = 6.25 x 10⁶ pixels/sec. Hence, the secondary electron current required would, given a pixel capacitance of 0.5 pF and a full-modulation voltage of 70V, be: i_s=TP C V=(6.25xl0⁶pixels/sec) (5xlO-¹³F/pixel) (70V)=220 μA. This level of primary electron current is readily attainable by conventional CRT-type electron guns. In the framed mode, the grid 44 is grounded and a potential difference is established between the CTP pins 35 and the grid 44 by applying a negative DC potential to the membrane metalization layer 36, which drives the CTP pins 35 to that potential by capacitive division. As the electron beam 16 scans across a conductive pin 35 of the CTP 30, secondary electrons are emitted from the enhancer coating 31 covering that pin (35) and are collected by the grounded, planar fine mesh grid 44 held in close proximity to the CTP surface. Since the number of secondary electrons emitted per incoming primary electron, or δ, exceeds unity, a net positive charge accumulates on the pin 35. If an enhancer coating 31 is added to the CTP 30, this increases the charging current i_s according to the increase in δ of the enhancer material: is₂ ^δ ₂-l = isi δ*j_-l Where δ]_ and ₂ are the secondary electron emission coefficients without and with the enhancer coating 31. If the electron beam 16 continues to address that pin 35, charge accumulates until the pin potential stabilizes to the grid potential, (i.e. ground in this case). During framed operation, the beam current 16 can be constrained such that no pixel 32 is allowed to saturate to the grid potential. Thus, by dynamically varying the electron beam current 16 at each pixel location 32, a continuously varying two-dimensional charge image can be written onto the conductors 35. This results in a voltage drop between the membrane 34 and the pins 35 and the associated electrostatic force pulls the membrane 34 into the well 32. Since the pixel capacitances may be on the order of a picofarad, charge storage times can be long (>100s). This necessitates an erase cycle following each write cycle; erasure is easily accomplished by grounding the membrane electrode 36 during electron beam addressing. The device may then be read out by reflecting collimated light 13 off the deformed membrane 34 at a shallow (~10^c) angle of incidence, as indicated in Figure 1. Alternatively, it may be read out at normal incidence with the use of a beam splitter. The phase information encoded upon the reflected wavefront is then processed by Schileren optics, shown generally at 21 in Figure 1. The schlieren system 21 consists of a converging lens 23, field stop 25 a spatial filter 20 and a reimaging lens 22 as illustrated in Figure 1. The converging lens 23 yields the Fourier transform (FT) of the phase object (i.e., the deformed membrane surface) one focal length after the lens. The FT is a diffraction pattern consisting of bright spots of light that possess the symmetry of the CTP 30, as illustrated in Figure 4(b) for a HCP symmetry. For a fully undeformed membrane surface, only the zeroth, or specular, order is present in the FT plane. As pixels 32 are deformed, the diffraction efficiency into the higher orders increases. In the preferred zeroth-order readout scheme an intensity-modulated image of the phase object is obtained by passing only the zeroth order of the FT through the spatial filter 20 before reimaging with the second lens 22. This output image is both collimated and parfocal, and can be displayed onto a screen 24 with variable magnification by a subsequent projector lens. As expected, the zeroth-order readout results in a contrast- reversed image, i.e., black pixels on a white field. A most important aspect of manufacturing the e-MLM is the fabrication of the pixel structures 32 on the readout side of the MLM anode 18. As indicated in Figure 2, one method of forming pixel structures 32 is by etching away portions of the CTP pins 35, leaving the perforated insulating substrate 37 as a support structure for the reflective membrane 34. While this prior art method of producing the "mirror" pixels has proved satisfactory it can be improved. One method has been mentioned above when the secondary emission of the electron impact surface has been increased by use of the enhancer coating 31. Another problem is that the periodicity of the CTP 35 pins may not be perfect. Accordingly, periodicity of the membrane pixels 32 would not be perfectly periodic. This lack of regularity leads to high spatial frequency, static 'noise' in the reconstructed image. Another related problem is that of image contrast. Since image contrast in a schlieren imaging system 21 is attained by interference at the image plane, the apparent pixel fill factor is of extreme importance. For example, approximately 50% pixel fill factor is necessary in order to achieve a perfect intensity null in an anode 18 with HCP pattern of pixels. Another issue is that of dynamic range. In order to modulate long-wavelength light, membrane deflections of several microns may be required. In order to enhance speed, low voltage operation is preferable. To meet these requirements, the ratio of pixel diameter to pixel well depth must be large. For example, it can be shown that the membrane deflection can be expressed by:

where δ is the membrane deflection, ε_Q the permittivity of free space, T the membrane surface tension, V the applied voltage, a the pixel diameter, and D the pixel well depth. Therefore, larger deflections and/or lower operating voltages will be attained by increasing the a/D ratio of the pixel geometry. In addition, the maximum deflection required for device operation should be a fraction (<20%) of the total well depth in order to preserve the parabolic shape of the deflected membrane. This represents about half of the deflection range of the membrane pixel. Since larger deflections require deeper wells, while low voltage operation implies shallower wells, the optimization of pixel well depth (or a/D ratio) can be determined empirically. This provides the ability to continuously vary the geometry of the pixel array and achieves the optimization of device performance. As discussed above, a most important aspect of manufacturing the e-MLM is the fabrication of the pixel structures 32 of the MLM anode 18. Figures 2,5,8,10,11 and 12 show a variety of different anode pixelization schemes. In the Figure 2 , for example, the prior art construction pixelization is provided by the CTP 30 on both the addressing side and readout side of the CTP 30. The pixel well structures 32 are formed by etching away a few microns of the CTP pins 35, leaving the perforated insulating substrate 37 as a support structure for the membrane 34. Therefore, the structure of the membrane pixels 32 is tied to that of the CTP 30, in pixel diameter, pixel pitch, periodicity, packing density, and so on. As mentioned this form of the invention includes the improved enhancer coating 31. Several superior architectures can be employed that decouple the pixel array from the CTP inter-pin pitch. One such improved architecture is illustrated, for example, in Figure 5. In one such case, the membrane pixels 32 are created by patterning an image thereof onto a thin dielectric film 58 coating the face of the CTP 30 via photolithographic techniques. In this process a dielectric film 58 is deposited on the face of the CTP 30 with conventional spin coating or vacuum-deposition techniques. The uniformly thick 2-10 μm coating is overcoated with photoresist which is then exposed by UV ^* light through a mask. After developing the photoresist, the unwanted dielectric material is removed via plasma etching or wet etching to produce a regular array (e.g. rectangular or hexagonal) of say, 100 μm diameter circular pixels on a 138 μm pitch assuming a 70 μm CTP pin pitch. A layer of metal 56, 1000 angstroms thick is then deposited over the remaining surface. When the photoresist is stripped, the remaining metal pattern 56 defines circular pads at the bottom of each pixel 32 by the metal lift-off technique. The dielectric layer 58 has good mechanical properties and provides a smooth surface for membrane attachment. The immediate benefits of this process are twofold: first, the pixel periodicity and lattice geometry are of the highest quality due to established mask-making techniques and alignment procedures. This removes fixed-pattern noise from the schlieren optics image plane. Second, the pixel fill factor, pixel pitch, and well depth are adjustable in a defined fashion, allowing the structure to be optimized for specific optical wavebands and various applications.

EXAMPLE I In one preferred form of the invention of Figure 6, the charge-transfer plate 30 is made according to the methods described in United States Patent No. 4,863,759. The plate is then polished to an optical finish and coated with a polymer (for example polyether-imide) to obtain, say, a 3 μm dielectric layer. The polymer is then overcoated with, say, a 2 μm layer of positive photoresist. The photoresist is soft-baked and exposed to a collimated UV beam passed through a high-contrast mask. After exposure, the photoresist is developed and the CTP 30 is placed in a plasma cha ber. Pixel wells 32 are formed by the removal of polymer material where no photoresist remains. After removal from the plasma-etch chamber, the CTP structure is placed in a vacuum coater where a metal layer 56 is deposited by e-beam or thermal evaporation. Typically 1000 angstroms of aluminum can be deposited. Upon removal from the vacuum coater, the remaining photoresist is stripped, causing the metal pattern 56 to remain only in the bottom of the pixel wells 32. The metalized membrane 34, preferably Parylene, grown by plasma polymerization by Lebow Co. for example, is coated with metal 36 prior to attachment to the support substrate. Silver is usually employed as the reflective metal, though other environmentally-robust metals can also be used. The membrane 34 is attached by slowly bringing it into contact with the substrate under 'tip/tilt' control. The adhesion is due to van der Waals forces which depend on materials parameters of both the substrate and the membrane 34. To optimize the characteristics of the membrane mirror 34, 36 including its zeroth order reflected energy content in the undeflected state, the membrane 34 or underlying substrate may be composed of materials that reduce the tension in the membrane 34 or that reduce the rest-state deflection of the membrane 34 through materials parameters such as Young's modulus of elongation. Alternative membrane materials such as nitrocellulose, polyether- imide, polypropylene, PTFEP, (poly [bis(trifluoroethoxy)-phospazene]) , polyimide, polyimide siloxane, or PET (polyethylene terephthlate) may optimize the actual device performance and are compatible with the process outlined here. Several substrate materials are similarly compatible with this process, including: polyimide, novolac resins, and PTFEP. Alternative processes include direct patterning of photosensitive polymers (UV-curing adhesives, photosensitive polymide, or photoresist) and similar patterning of vacuum-deposited (evaporated or sputtered) dielectric films (such as ZnS, MgF₂, SiO_x, Y₂θ3, or cryolite) . In summary, this improved approach for pixel construction on the MLM anodes has the following advantages:

Improved contrast due to reduced rest-state membrane pull-back.

Enhanced pixel-to-pixel uniformity of response by improving the uniformity of the pixel geometry. Reduced or eliminated fixed-pattern noise from the image by achieving a high degree of periodicity in the pixel array.

Improved image contrast to at least 200:1 by 'tuning' the pixel fill factor.

Phase dynamic range tailored to the specific waveband of interest by controlling the ratio of pixel diameter to pixel well depth. Due to the efficient charge storage characteristics of the CTP 30, considerable image storage times (>10³ sec) have been observed in the e-MLM. Thus, conventional video operation would imply a frame erase after each video field. Since this is wasteful of charge, and also results in non-negligible image flicker, the flickerless mode of operation is preferred. While the conventional video addressing approach described above applies the video signal to the electron gun control grid in order to modulate the delivered beam current, it is also possible to apply the video signal directly to the membrane instead. Thus, the electron beam current will be fixed and simply scanned across the MLM anode 18 at video rates. The potential difference across each pixel well 32 will thus be determined by the instantaneous potential applied to the membrane 34 when the electron beam 16 is impinging on that pixel 32, since the pixel pin electrode 35 stabilizes to the grid potential by the secondary emission process. Such a process is referred to as grid-stabilized operation. A similar scheme has been successfully employed in the operation of the Sodern light valve to eliminate flicker from the display. The differences between framed operation and grid-stabilized operation are illustrated in Figure 6. Apart from the absence of image flicker, the grid- stabilized mode of operation is more charge-efficient than standard video operation by up to a factor of two. In addition to the field of IR target simulation and scene generation, further developments of this class of device present distinct advantages to many areas and systems of significant interest to the military. Important application areas of the device include: high- definition flight simulator displays, infrared scene projection, laser beamsteering and wavefront correction in optical communications and imaging through turbulence, high-intensity large-format displays for C³I and teleconferencing, robotic vision, autonomous vehicle guidance, pattern recognition, parallel processing of large knowledge bases, multispectral image fusion, neural- network-based processing, and industrial inspection. It is the unavailability of low-cost, high resolution, high brightness SLMs that is impeding the development of all areas of optical signal processing and other specialized optical systems such as joint correlators and industrial inspection systems. Concerning the multi-billion-dollar display market, this technology can benefit the high- definition large-format projection displays for television conference rooms, auditoriums and the home. While one embodiment of the invention has been described above wherein an electron beam 16 is scanned across the rear of the transfer plate 30 to create an image modified spatial charge on the light modulating membrane 34, other methods of creating the space charge image can be utilized as shown in Figure 7. In this case, the write signal 63 is an image incident on a photo- electron emitting layer 60 carried on an input window 61. This photo-electron emitting layer 60 emits an image modified stream of electrons through an electron multiplier assembly shown here as a set of microchannel plates (MCP) 62 which transfers the amplified stream of electrons to the rear of the charge transfer plate 30 operating under the influence of grid 44. This anode structure is preferably made in accordance with the present invention. The charge is transmitted to the front thereof; the resultant electrostatic forces deform a membrane 34, 36 whose image-modified deflection is read through an output window 40 by suitable readout beam 13. This general arrangement of elements is similar to Figure 18 of the above mentioned patent 4,794,296 owned by the assignee of this application. Another embodiment of the present invention provides a modified grid arrangement on the rear of the charge transfer plate 30. As shown in Figure 8 an electron beam 16 strikes secondary emitting portions 70 on the rear of the charge transfer plate 30. The impacting electrons cause a large emission of secondary electrons which are then collected by a modified grid 44a which is formed on insulated pedestals 72 created by masking techniques similar to the photolithographic masking techniques discussed above for forming the front surface insulating spacer walls 58 in Figure 5. Referring now to Figure 9 , in those situations where the light to be modulated has a very long wave length (e.g. 8-14 μm) it is necessary to provide a liquid nitrogen cooled jacket 50 to surround the whole system so as to suppress background infrared radiation that could interfere with and mask the IR image being processed by the system. Jacket 50 in turn is surrounded by a thermal shield 71 has a port 85 for charging the jacket with liquid nitrogen and a port 79 for interconnecting the enclosure 86 to a vacuum system (not shown) . A collimated beam from an IR light source 81 is directed through a set of baffles 78 onto the Infrared Membrane Light Modulator (IRMLM) 76. The modulated IR light is reflected onto a combination of two Fourier Transform Mirrors 83 and a magnetically controlled spatial-filter mirror 75 as follows: the modulated IR light is first reflected to one Fourier Transform Mirror 83, then onto the spatial-filter mirror, then back to a second Fourier Transform Mirror from which it is directed at the system under test 74. The IRMLM is directed by a controller 77. In the above discussion of the preferred form of the invention, the electron stream or beam 16 addresses the rear surface 52 of the charge transfer plate 30, and the front of the charge transfer plate 30 carries the metalized membrane mirror 34 which is selectively deflected by the charge transferred through the plate. In a further modification of the invention the electron beam 16 impinges directly on the metalized membrane 34 of an anode structure 18 that does not necessarily employ a charge transfer plate. In this case, each portion of the mirror constituting an isolated pixel 32 is insulated from each other pixel so that an isolated charge can be created at each pixel. This modification is shown in Figure 10. where the electron beam 16 strikes the metalized isolated mirror 36 overlying the pixel potential wells 32. The isolation of the mirror 36 is achieved by gaps 36a in the coating 36. Secondary emission of electrons will create a static charge on the insulated mirror 36 in accordance with the intensity and energy of the electron beam 16. This static charge will then create deformation of the membrane 34 in direct accordance with the charge carried thereby in the same fashion as the deformation of the membrane 34 as discussed in connection with Figures 2 and 5. In this case, the structure could be essentially the same as described for Figure 5 with the exception that the metalized mirror 36 would have gaps 36a electrically isolating each portion of the metalized mirror 36 overlying the individual pixels 32. In Figure 10, the readout light 13 is directly impinged on the mirror surface 36 through the envelope surrounding the source of the electron beams 16. In Figure 11, a similar embodiment of the invention is shown wherein, instead of having individual metal mirror coatings 36, the reflective surface is created by a dielectric mirror 90 such as a multiple alternating layer stack of Si0₂/Ti0₂. In this case, electron beam 16 impingement charges the dielectric mirror 90 negatively or positively and concentrates the charge at the point of impact of the electron beam 16. Accordingly, the pixels 32 will accumulate an amount of charge depending on the beam current, the dwell time of the electron beam 16, and the secondary electron emission coefficient in the case of positive charging. Thus the membrane 34 will be deflected into the potential wells 32 in accordance with the charge carried thereon. A similar system is shown in Figure 12 but in this case the readout light 13 is beamed through the support window 40 to the mirror 84. Note that a charge transfer plate 30 cannot be used as the membrane support since the membrane substrate must be transparent. In Figure 12, a stack of two transparent insulators 80 defining the potential wells 32 is mounted on a transparent electrode 82, such as an indium tin oxide layer, and mirror surfaces 84 are provided between these insulators 80. The isolated mirror pixels 36 are then supported on top of the second insulator stack 80 provided above the mirror surfaces 84. In this case, readout light 13 is modulated in accordance with the degree of deformation of the individual mirror pixels 36 into the potential wells 32. The contrast ratio of the modulation is dependent upon the relative position of the membrane mirror pixels 36 referenced to the mirror surfaces 84 within the insulators 80. In Figure 13 there is shown a schematic assembly of a readout optical system and a modified electron beam addressed membrane spatial light modulator which directs an image modified electron beam directly on the mirror membrane anodes 36 as illustrated in Figure 10, or 11. While several modifications of the invention have been described above where an electron beam cathode ray tube scans the charge transfer plate 30 or the mirrored membrane 36 directly, it is also possible to utilize a source of electrons from a field emitter array 64 which can be electronically addressed. Such a field emitter array 64 is shown in the copending application Serial No. 07/638,317 filed January 4, 1991 and owned by the assignee of the present invention. Similarly, the charge transfer plate 30 can be directly wired by a suitable electronic controller 65 having addressing wires 66 which directly couple to the rear surface 52 of the charge transfer plate 30. Examples of these two systems are shown in Figures 14 and 15. Figure 14 shows a field emitting source 64 of electrons which impact any of the anode structures of Figures 2, 5, 8 or 12 of the present invention. In the direct wired configuration shown in Figure 15 an electronic controller 65 drives each of the pixels 32 by means of discrete metal electrodes 54 provided on the rear side 52 of the charge transfer plate 30. The other (mirror) side of the charge transfer plate 30 is made in accordance with the present invention. In Figure 15 the charge transfer plate anode 56 is preferably made as shown in Figure 5. From the above description, it should be apparent that the novel charge transfer plate 30 of the present invention can be used in a wide range of applications wherever spatial light modulation is desired. The input to the membrane anode structure charge transfer plate may be derived from a vast number of different types of electron sources and scanning can be achieved in numerous ways as described above. An improved high-definition electron-beam-addressed Membrane Projection Display (MPD) System is illustrated in Figure 16 as a three color projection television system. It is comprised of three subsystems as shown: (a) three electron-beam-addressed Membrane Light Modulators (e-MLMs) 41 with their common readout light source 10 and dichroic beam splitters 55, (b) a spatial filtering 20 and projection optical system 43 and (c) an electronic control and computer interface subsystem 45 which performs the necessary image format conversions and implements the control functions that allow the microprocessor to control the light valve. In operation, white light from an arc lamp is fed into the optical system consisting of the three dichroic beamsplitters (BSl, BS2, and BS3) that extract the blue 47, green 48, and red 49 light components, respectively, for readout of the three e-MLMs 41 as shown in Figure 16. Thus e-MLMl 46, for example, which is driven by the blue component 47 of the electronic video signal, phase modulates only the blue component 47 of the image as it reflects from the deformable membrane mirror surface 36 e- MLM1 46. Similarly, the modulators e-MLM2 51 and e-MLM3 53 modulate the green 48 and red 49 components of the readout beam 13. The modulated zero-order components of the blue, green and red light are recombined by the second set of dichroic beamsplitters 55 (see Figure 16) to yield the phase modulated three color signal beam 57. While numerous modifications of the invention have been described, many additional forms thereof will be apparent to one skilled in the art and the invention is not to be limited to the specific forms shown.

Claims

CLAIMS 1. A membrane light modulator characterized by comprising, a charge transfer plate 30, a multiplicity of conductors 35 extending from the rear surface 52 to the front surface 19 of said plate 30, said conductors 35 being supported in an insulating matrix 37, the front side 19 of said transfer plate 30 having a plurality of potential wells 32 defined by insulating walls 15, each said potential well 32 constituting a pixel, a plurality of conductors 35 being provided for each pixel, a deformable reflecting membrane 34,36 comprising a metal layer 36 spanning said potential wells 32, means providing an electric potential on said metal layer 36, means 17 providing a source of electrons 16 for impacting the rear surface 52 of said charge transfer plate 30, and means providing an image defining charge distribution to one of said last two means to provide an image defining deformation to said metal layer 36 at each pixel. 2. The membrane light modulator of claim 1, characterized in that the metal layer 36 is a thin flexible sheet of metal. 3. The membrane light modulator of claim 2, characterized in that the metal layer 36 is a metal coating on a flexible insulating substrate 34, preferably a metalized plastic. 4. The light modulator of claim 1, characterized in that said insulating walls 15 are formed from an insulating layer 72, said walls 15 being created by etching portions of said insulating layer 72 not protected by an overlying photoresist. 5. The membrane light modulator 18 of claim 1, characterized in that said means for providing a source of electrons 16 comprises an electronic controller 65 hard wired 66 to the electrode surface 54, or a photocathode and microchannel plate assembly 62, or a field emitter array 64, or a cathode ray tube 17. 6. A membrane light modulator characterized by comprising, a charge transfer plate 30, a multiplicity of conductors 35 extending from the rear surface 52 to the front surface 19 of said plate, said conductors 35 being supported in an insulating matrix 37, the front side 19 of said transfer plate 30 having a plurality of potential wells 32 defined by insulating walls 15, each said potential well 32 constituting a pixel, a plurality of conductors 35 being provided for each pixel, a metal electrode 56 in the bottom of each potential well 32 spanning a plurality of said conductors 35, a deformable reflecting membrane 34, 36 spanning said potential wells 32, said membrane 34 comprising a metal layer 36, means providing a potential on said metal layer 36, means providing a source of electrons 17 for impacting the rear surface 52 of said charge transfer plate 30, and means providing an image defining charge distribution to said source of electrons 17 to provide an image defining deformation to said metal layer 36 at each pixel. 7. A membrane light modulator characterized by comprising, a charge transfer plate 30, a multiplicity of conductors 35 extending from the rear surface 52 to the front surface 19 of said plate 30, said conductors 35 being supported in an insulating matrix 37, the front side 19 of said transfer plate 30 having a plurality of potential wells 32 defined by insulating walls 15, each said potential well 32 constituting a pixel, a plurality of conductors 35 being provided for each pixel, a metal electrode 56 in the bottom of each potential well 32 spanning a plurality of said conductors 35, a deformable reflecting plastic sheet 34 spanning said potential wells 32, a metal coating 36 on said plastic sheet 34 insulated from said potential wells 32, means providing a modulated stream of electrons 16 to the rear surface 52 of said charge transfer plate 30, means providing a potential on said metal coating 36, means providing an image defining charge distribution to one of said last two means to provide an image defining deformation to said sheet 34 at each pixel, and a light source 10 of predetermined wavelength impinging on said reflective membrane 34, 36 to be modified by the deformation of each pixel on said reflective membrane 34, 36. 8. A device for modulating incident light by impinging said light on a membrane mirror which can be deformed in accordance with an electric charge on individual pixel portions of the membrane mirror, the improvement characterized in that the membrane mirror 34 is supported on a plurality of insulating walls 72 which are formed on an electrode-containing surface 19 to define potential wells 32, each said potential well 32 constituting a pixel so that the bottom of each of said wells 32 has said electrode surface 56 thereon and a charge difference can be created between the electrode 56 and the portion of the membrane 34 overlying each pixel, and means for creating an image defining charge difference across said membrane 34 by addressing the structure with an image modified electron stream 16. 9. The device of claim 8, characterized by a grid 44 adjacent that side of said structure addressed by said electron stream 16. 10. The device of claim 8, characterized in that the mirror membrane 34, 36 comprises discrete mirror segments 36 overlying each pixel, and said electron stream 16 impinges on said mirror 36 and modifies the charge on each pixel by secondary electron emission from said mirror segment 36. 11. The device of claim 8, characterized in that the membrane 34, 36 comprises a dielectric coating 90 and said electron stream 16 impacts said dielectric coating 90 and modifies the charge on each pixel by accumulation or depletion of electrons thereon. 12. The device of claim 8, characterized in that the means for creating the image modified electron stream 16 comprises a matrix addressed field emitter array 64, or a photocathode and microchannel plate assembly 62, or a cathode ray tube 17. 13. The device of claim 6, characterized in that a plurality of light modulators 41 is employed and a different wavelength of light 47, 48, 49 is provided to each modulator 41 and the resultant modulated light beams are combined 57 to give a multi color image. 14. A membrane light modulator characterized by comprising a charge transfer plate 30, a multiplicity of conductors 35 extending from the rear surface 52 to the front surface 19 of said plate 30, said conductors being supported in an insulating matrix 37, the front side 19 of said transfer plate 30 having a plurality of potential wells 32 defined by insulating walls 15, each said potential well 32 constituting a pixel, a deformable reflecting membrane 34,36 spanning said potential wells 32, a metal coating 36 on said membrane 34, means providing a potential on said metal coating 36, means providing a source of electrons 17 for impacting the rear surface 52 of said charge transfer plate 30, means providing an image defining charge distribution to one of said last two means to provide an image defining deformation to said membrane 34 at each pixel, and wherein a plurality of conductors 35 are provided for each pixel and a metal electrode 56 in the bottom of each potential well 32 spans a plurality of said conductors 35. 15. A membrane light modulator characterized by comprising a charge transfer plate 30, a multiplicity of conductors 35 extending from the rear surface 52 to the front surface 19 of said plate 30, said conductors 35 being supported in an insulating matrix 37, the front side 19 of said transfer plate 30 having a plurality of potential wells 32 defined by insulating walls 15, each said potential well 32 constituting a pixel, a deformable reflecting membrane 34,36 spanning said potential wells 32, a metal coating 36 on said membrane 34, means providing a potential on said metal coating 36, means providing a source of electrons 17 for impacting the rear surface 52 of said charge transfer plate 30, means providing an image defining charge distribution to one of said last two means to provide an image defining deformation to said membrane 34 at each pixel, and wherein a secondary electron emitting coating 31 is applied to the rear surface 52 of said charge transfer plate 30 to increase the secondary emission of electrons therefrom and a grid electrode 44a is carried by an insulator structure 72 on the rear surface 52 of said charge transfer plate 30 and having insulating walls 15 corresponding to the insulating walls 15 defining each pixel on the front of the charge transfer plate 30.