William P. Bleha

A new real-time non-coherent to coherent light image converter - The hybrid field effect liquid crystal light valve

A new, high-performance device has been developed for application to real-time coherent optical data processing. The new device embodies a CdS photoconductor, a CdTe light-absorbing layer, a dielectric mirror, and a liquid crystal layer sandwiched between indiumtin-oxide transparent electrodes deposited on optical quality glass flats. The non-coherent image is directed onto the photoconductor; this reduces the impedance of the photoconductor, thereby switching the ac vol-tage that is impressed across the electrodes onto the liquid crystal to activate the device. The liquid crystal is operated in a hybrid field effect mode. It utilizes the twisted nematic effect to create a dark off-state (voltage off the liquid crystal) and the optical birefringence effect to create the bright on-state. The liquid crystal modulates the polarization of the coherent read-out light so an analyzer must be used to create an intensity modulated output beam. Performance figures for the device include: Resolution 100 lines/mm; Input Sensitivity 160 iW/cm2 at 525 nm; Time Response on: 10. msec; off: 15 msec; Contrast >100 : 1 ; Aperture 1 inch by 1 inch.

Photoactivated birefringent liquid-crystal light valve for color symbology display

Aphotoactivated liquid-crystal projection display device that is based upon the field-effect mode of the liquid crystal has been built and tested. The device displays high-resolution dynamic color symbology by means of the optical birefringence of the liquid crystal. The colors that can be displayed simultaneously are white (with gray scale), blue, green, yellow, and magenta on a dark field. The imaging light power required to excite the device varies from 30-900 µW/cm2at 525 nm and the resolution obtained at an MTF of 50 percent is 40 lines/mm. The basic construction of the device is similar to the dynamic scattering photoactivated liquid-crystal light valve.

Recrystallization of thermally evaporated CdS films via an H2S heat−treatment process

It is observed that CdS films, evaporated so that a preferred c−axis columnar crystalline structure exists, undergo a recrystallization of texture in a postevaporation H2S heat−treatment process. This recrystallization process does not involve heavy−metal group−IB impurity doping and takes place at low enough temperatures so that soda−lime glass substrates may be used. These recrystallized films contain crystals of tens of microns in size and possess properties nearer those of CdS crystals than is usually found in CdS films.

Novel charge‐storage‐diode structure for use with light‐activated displays

A theoretical model is described for the photosensor‐substrate structure presently used with the ac liquid‐crystal light valve. This structure consists of a thin film (sputter deposited) of n‐type CdS on which is evaporated a thin film of p‐type CdTe. The model describes the photosensitivity of this structure in terms of the depletion‐width photocapacity of the charge‐storage diode formed by these two coatings. The model developed based on this photocapacity effect allows a consistent explanation of the experimentally observed fact that good device photosensitivity is observed at high frequencies (e.g., 10 kHz), contrary to the prediction that would result from a quantum‐photodiode model. By assuming an electronic‐defect‐center structure that consists of fast and slow electron trap levels and a recombination level, as in the case of a photoconductor, the charge stored in the diode is related to image‐activating light intensity. This provides descriptive information on device response time, gray scale, and...

ac photoresponse of a large‐area imaging CdS/CdTe heterojunction

Experimental data are presented for the photosensor‐substrate structure presently used with the ac liquid‐crystal projection light valve. This structure consists of a thin film (sputter deposited) of n‐type CdS on which is evaporated a thin film of p‐type CdTe. In an earlier publication, a model was presented which describes the photosensitivity of this structure in terms of the depletion‐width photocapacity of the charge‐storage diode formed by these two coatings. The experimental data presented here correlate well with this model. Among the data presented are data on light‐valve display, photosensor gray scale, and response time. The response‐time data are of particular interest because they show that the projection‐light‐valve photosensor is capable of operation at near‐TV rates. The agreement of the experimental data with model and extrapolation from the model imply that large improvements in sensitivity and response time are still quite possible.

A real-time optical data processing device

A novel liquid-crystal electro-optical device useful as a real-time input device in coherent optical data processing is described. The device is a special adaptation of an ac photoactivated liquid-crystal light valve, and utilizes a hybrid field effect (45 deg twisted nematic effect in OFF state and pure optical birefringence of the liquid crystal in ON state). A thin-film sandwich exerts photoelectric control over the optical birefringence of a thin liquid-crystal layer. Liquid-crystal layer thickness is successfully reduced without image degradation. The device offers high resolution (better than 100 lines/mm), contrast (better than 100/1), high speed (10 msec ON, 15 msec OFF), high input sensitivity, low power input, low fabrication cost, and can be operated at below 10 V rms. Preliminary measurements on device performance in level slicing, filtering, contrast reversal, and edge enhancement are under way.

HYBRID LIQUID CRYSTAL LIGHT VALVE -IMAGE TUBE DEVICES FOR OPTICAL DATA PROCESSING

We report here the feasibility of coupling an image intensifier tube directly with the ac liquid crystal light valve, which is briefly described and characterized in the paper, to form a compact, real-time, incoherent-to-coherent image converter. This hybrid device is capable of sensing image intensity levels of less than 10-6 In addition, the same combination can be used as an image amplifier with an intensity gain of up to 10. In the experiments performed we mated an 18 mm aperture micro-channel plate inten-sifier image tube, with a fiber optic faceplate output, with a 50 mm aperture hybrid field effect liquid crystal that has a fiber optic faceplate input. With an incoherent input image intensity of 3 x 10 ft-candle we observed a limiting resolution of 16 1P/mm in the output coherent image. This resolution limit is determined primarily by the image intensifier tube. MTF data for the hybrid device and the components are also presented, along with a photograph of gray scale image capability. The use of other types of image intensifier tubes, such as those having image storage capability, with the light valve is discussed and performance ranges given. Next we summarize the possible uses of the hybrid device to increase the usefulness of real-time optical data processing. Finally, we describe the application of this device to wavelength conversion and image amplification to provide the large screen projection of dynamic ac plasma panel display imagery.

LIQUID CRYSTAL LIGHT VALVE FOR COHERENT OPTICAL DATA PROCESSING

ABSTRACT A new, high performance device has been developed for application to real-time coherent optical data processing. The new device embodies a CdS photoconductor, a CdTe light-absorbent layer, a dielectric mirror and a liquid crystal layer sandwiched between indium-tin-oxide transparent electrodes deposited on optical quality glass flats. The noncoherent image is directed onto the photoconductor; this reduces the impedance of the photoconductor, thereby switching the ac voltage that is impressed across the electrodes onto the liquid crystal to activate the device. The liquid crystal is operated in a hybrid field effect mode. It utilizes the twisted nematic effect to create a dark off-state (voltage off the liquid crystal) and the optical birefringence effect to create the bright on-state. The liquid crystal modulates the polarization of the coherent read-out light so an analyzer must be used to create an intensity modulated output beam. Performance figures for the device include: Resolution >100 lines/mm; Input Sensitivity 160 μW/cm2 at 525 nm; Time Response on: 10 msec; off: 15 msec; Contrast >100:1; Aperture 1 in. by 1 in. The new device is a special adaptation of the ac photoactivated liquid crystal light valve reported elsewhere (1, 2). It represents a development of an earlier dc device that suffered from limited lifetimes (3). Basically, the device consists of a sandwich of thin films that electrically control the optical birefringence of a thin (∼ 2 μm) liquid crystal layer. The device has high resolution (>100 lines/mm, limiting resolution), high contrast (>100:1), high speed (10 msec, on; 15 msec, off) and high input sensitivity (∼ 0.3 ergs/cm2 at threshold). Moreover, it has several practical advantages. It is compact (solid state), low power (several milliwatts), inexpensive to manufacture (thin film technology), and operates from a single, low voltage (5 to 10 V r.m.s.) power supply. In this paper we describe the design, operation, and structure of the device. We discuss in some detail the novel liquid crystal electro-optic mechanism that we use that enables us to reduce the thickness of the liquid crystal layer without sacrificing image quality. Finally, we discuss the performance of the device and show the results of several application experiments. The discussion throughout is primarily qualitative and descriptive in nature. Due to the breadth of the topic we were unable to include the quantitative analysis as well. However, we do plan to publish this material, and where appropriate we have referenced such publications.