My column this month is a review of the seminar on "Utilization of Infrared Detectors" that was presented on January 17 and 18, 1978, at the Sheraton Universal Hotel in North Hollywood, California. This seminar, organized by Wesley L. (Lum) Eisenman of the Naval Ocean Systems Center, was devoted to materials, testing, astronomy, and devices.
The real-time and parallel processing advantages of coherent optical processors are well known. The ease with which the Fourier transform and correlation operations (that are the hallmark of data processing and pattern recognition) can be performed optically has intrigued researchers for years. This once virgin discipline of optical computing is now maturing and emerging from the laboratory and research arena into deliverable and fielded systems. As this occurs, one of the key technology items that continues to plague program managers and contractors is the real-time and reusable spatial light modulator (SLM) used as the input (and often filter plane) transducer. Without such devices, the full speed advantages of optical processors cannot be realized.
Volume phase holograms are holograms stored throughout a material whose thickness is much greater than the wavelength of light. This paper is a review of the present state of the art in volume holographic storage in ferroelectric crystals. We shall be concerned principally with Fe-doped LiNbO3 since it is the most extensively studied member of this class of materials and to date exhibits the best performance characteristics of any member of this class of materials. Results obtained for other dopants or host materials will be discussed only where important, both because less is known about them and because they are either not as sensitive or do not possess the fixing capability of Fe-doped LiNb0 3. Systems based on this storage technique can be configured either for write/read applications where high recording sensitivity and fast erasure are required or for archival storage where a fixing capability is required.
Spatial light modulation in PLZT ceramics involves variation of the ferroelectric properties in conjunction with the related electrooptic, light scattering, surface deformation, and photosensitive properties of the material. Several combinations of these properties have been used to design a variety of spatial light modulators. Most PLZT light modulators are also capable of non-volatile storage of digital or analog information which can be read out with transmitted or reflected visible light. This paper surveys several types of modulators and compares some of their important characteristics.
The photodichroic optical recording properties of FA, M, and MA type defects and alkali-halide materials are reviewed and new advances are reported. Particular emphasis is placed on the application of crystalline KC1, NaF, and KF to a variety of optical data processing tasks including real-time spectral analysis.
The photoconductor-thermoplastic, or photoplastic, recording medium can serve as an incoherent-to-coherent transducer to insert data into a coherent optical processor in real time. The photoplastic transducer is characterized by good exposure sensitivity, high resolution, and simple construction, and it is well suited to applications requiring image retention before and during readout. In our investigations we measured exposure sensitivities of less than 50 ergs/cm2 , and achieved a write cycle of charge, expose, and develop in less than 10 msec. Because of the band-pass nature of the recording medium, a spatial frequency offset was provided by overlaying the image with a grating. Images were recorded with incoherent light and read out with coherent light with good fidelity, and the transducer was tested as the input to a matched filter processor.
The principle of operation, structure, performance specifications, and numerous examples of the use of the electron-beam addressed potassium dideuterium phosphate (DKDP) light valve in optical processing are reviewed.
This paper presents a review of the theoretical analyses and experimental characterizations of the Itek PROM (Pockels Read-out Optical Modulator). The basic operational principles are discussed, followed by a theoretical description of the dependence of PROM characteristics on fabrication and operational parameters. An extensive section with the results of various characterization programs is provided. Of particular interest are resolution and diffraction efficiency experiments. A discussion of PROM fabrication techniques, materials, and design, both present and future, is included. Finally, examples of some of the many applications for which the PROM may be used are given. Applications include: incoherent image enhancement, image or signal correlation, real-time spatial filter generation, and two-dimensional logical operations.
The structure, principles of operation, specifications, and numerous examples of the use of the optically addressed potassium dideuterium phosphate (photo DKDP) light valve in optical data processing are reviewed.
This paper reviews the hybrid field-effect liquid crystal light valve (LCLV) and its application to real-time coherent optical data processing. The light valve is basically a high resolution optical-to-optical image converter. The device embodies a CdS photoconductor, a CdTe light-absorbing layer, a dielectric mirror, and a biphenyl liquid crystal layer sandwiched between indium-tin-oxide transparent electrodes deposited on optical quality glass flats. The input image is directed onto the photo-conductor to reduce 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 ac operation ensures long operating life for 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 optical birefringence to create the bright on-state. The liquid crystal modulates the phase of the coherent readout light. By an additional analyzer an intensity modulation is created.
Over the past couple of decades, considerable effort has been directed towards identification of methods to quantify the physical characteristics of imaging systems which are essential to effective use of these systems in medical diagnosis. Results of this effort have provided techniques to characterize imaging variables such as resolution, contrast, low contrast perceptability and noise in a reasonably exact manner, and have facilitated the inter-comparison of imaging systems and techniques in attempts to delineate the most effective approach to a particular imaging application. In applying these quantitative analytic techniques to the imaging process, however, one is confronted with the uncertainty of whether the techniques really characterize the variables of importance to the clinician in his use of the imaging process to arrive at a medical diagnosis.
Laboratory comparisons of the resolution, noise, and contrast of five mammography screen-film systems (single screen, single-emulsion film) were made both qualitatively and quantitatively. Subjective single-blind estimations of resolution, noise, and contrast were also made directly from the mammograms of 100 consecutive patients radiographed with each of the screen-film systems. Laboratory and clinical evaluations showed strikingly similar results: improved contrast, minimally reduced resolution, and varying degrees of increased noise for the faster recording systems. Similarities between the results of laboratory and clinical studies should help to establish the clinical relevance of the laboratory analyses, and recognition of the relative precision of the clinical evaluations should allow for their use in place of laboratory studies when sophisticated physics support is not available.
There is a mystique associated with diagnostic radiology that discourages the use of analysis as a means of understanding and improving radiological examinations. While it is true that intuitive manipulation of the generators and detectors that are available can lead to restricted optimizations, it might also be true that the equipment that is available may not have the correct characteristics. Better optimums may be possible with equipment of different, but state-of-the-art, capabilities or characteristics. This paper presents two cases of modelings for mammography that are examples of this situation. It remains to be established whether or not the modeling has diagnostic significance. However, since the conclusions appear to be significant it seems important to pursue such projects. To accomplish this, it is necessary to generate information and data that can be useful to the modelings. The extensive imaging research that has recently occupied many diagnostic radiological physicists is only part of the necessary information. More emphasis needs to be given to research that is useful in helping to describe the details of the x-ray photon image. One of the examples of modeling that is discussed is related to signal-to-noise ratios for the detection of calcifications in mammography. The other is a study of contrast in magnification mammography.
The objective of this work is to relate the performance of radiologists as measured by the probability of detecting a nodular abnormality on a chest film to the properties of the nodule and its surrounding structures measured by a microdensitometer from the film. A parameter called conspicuity has been defined. It consists of measurements of the size, shape, contrast, and the edge gradient of the nodule divided by a measurement of the complexity of the surrounding structures that tend to camouflage the nodule. Conspicuity has been measured for simulated nodules and for a series of real nodules that were originally missed in clinical practice but later detected. Correlation between film reader performance and conspicuity was found.
The question of what spatial frequency response gives optimum image quality in computed tomography (CT) images is studied. We have developed a technique for smoothing these images by using a modified back projection filter with attenuated high frequency response. The results are images with a blurred appearance but dramatically reduced image noise. On processing raw data from the EMI head scanner in this way, grey and white matter distributions are clearly visualized with no increase in radiation dose. These results are analyzed in terms of the Weiner spectrum and correlation properties of CT noise. In particular, a "noise equivalent aperture" is defined in a way analogous to that of Schade. It is shown that the prediction of the simple Rose model of noise sampling cannot explain the improvement produced by smoothing, even when the correlated character of the noise is taken into account.
The results of two types of visual detection experiments are presented and discussed. One set of experiments was done using single bar targets optically superimposed on a variety of backgrounds. The experiments were designed to determine functional relationships between the target contrast at the detection threshold and a number of variables. The variables include bar angular width, bar angular length, background noise level, and viewing distance. The second set of experiments was done using radiographs of aluminum wires. The radiographs were produced using a conventional neuro-angiography system with a 0.38 mm focus. The aluminum wire diameter at the detection threshold was determined as a function of geometric magnification for several screen-film combinations. The aim of the experiments is to determine how to calculate visual signal-to-noise ratios. Some progress has been made toward this goal.
The Air Force Weapons Laboratory (AFWL) has been involved in the study of laser propagation from airborne platforms. A Line Spread Device (LSD) capable of yielding direct measure of a laser beam's Line Spread Function (LSF) was developed and employed in propagation tests conducted in a wind tunnel by AFWL to examine optimal acoustical suppression techniques for laser cavities exposed to simulated aircraft aerodynamic environments. Measurements were made on various aerodynamic fences and cavity air injection techniques that affect the LSF of a propagating laser. Using the quiescent tunnel as a control, the relative effect of each technique on laser beam quality was determined. The optical instrument employed enabled the comparison of relative beam intensity for each fence or mass injection. It was found that fence height had little effect on beam quality but fence porosity had a marked effect, i.e., 58% porosity alleviated cavity resonance and degraded the beam the least. Mass injection had little effect on the beam LSF. The use of a direct LSF measuring device proved to be a viable means of determining aerodynamic "seeing" qualities of flow fields. It could also be applied to static atmospheric "seeing" measurements through various gases and pollutants.
Three studies were conducted to evaluate operator performance with conventional and stereo display systems. The first two studies involved perceptual judgment tasks (a modified Howard-Dolman depth discrimination test and a test of stereopsis, using random dot Julesz patterns), while the third employed a perceptual-motor task requiring end-effector positioning and closure. This third task was designed to approximate the major compo-nents of undersea object recovery missions. In studies One and Two, two methods of displaying stereo information (Fresnel, Field Sequential) were compared. The results of Study One indicate that both of these display systems provide adequate information to enhance performance over that given with a conventional monocular display. Study Two indicates that thresholds of stereo viewing are comparable using the Fresnel system and the Field Sequential system. The data indicate no dif-ference in thresholds over those obtained directly without viewer aids. In Study Three a conventional display was compared with the Field Sequential system used in studies One and Two. The results indicate that the use of stereo cues reduces both response latency and errors significantly. An analysis of performance changes over repeated testing ses-sions indicates significant improvement on both variables. These effects, however, are nondifferential across display systems, and are probably related to the acquisition of manipulator-specific motor-skill learning.
Thin film evaporation and sputtering methods in conjunction with photolithographic fabrication techniques provide a simple means for producing useful electro-optical devices. Multilayer Cd 1-xZnxS (CZS) compound photoconductive film deposited with rf sputtering exhibits a spectral response which can be varied between 360 nm to 530 nm. The CZS films exhibit a rise time of 1 pt sec, have good optical transmission characteristics, exhibit a photoconductive gain of approximately 10, respond to 400 mw laser pulses in a 1 mm spot and provide film breakdown strength of 30 x 104 volt/cm. Multilayer CZS films exhibit less photoconductive gain and higher breakdown strength than single layer films. Sn doped indium oxide (ITO) films are produced with magnetron rf sputtering. The conductivity and transmissivity of the ITO films are critically influenced by the amount of free oxygen present during the sputtering deposition. An 02 partial pressure of 4 x 105 results in greater than 80% transmissivity at 600 nm with a bulk conductivity of 100 mhos/cm. The film transmissivity can be substantially increased in the visible spectrum by post heat treatment in an air oven at 300 C for 20 minutes. A scanning cursor device for graphical data extraction was fabricated using various thin film techniques.
Photography of the CRT image on the scanning electron microscope (SEM) can be done directly on Polaroid film and on Agfa Gevaert (A-G) stabilization paper. The latter requires reversal of image polarity. Image recording may also utilize high-resolution negative materials, subsequently printed on A-G stabilization paper. Comparisons are made, and suggestions for practical SEM photography are presented.
A high resolution fiber optics communications system (FOCS) for analog signals has been designed, built, and tested at the Frank J. Seiler Research Laboratory, USAF Academy, Colorado. The FOCS consists of a small transmitter and receiver pair and an optical waveguide. A combination of voltage-to-voltage (V /F/V) and pulse width modulation (PWM) is used to reconstruct a ±1 0 volt analog output signal from an identical ±10 volt analog input signal. A digital output is easily obtained since the data transmission is pulsed. The FOCS is equally applicable to free-space infrared communications. The advantages of fiber optics have not previously been available for general purpose data acquisition because their application has been limited to dedicated digital systems and to low resolution, high frequency video systems. Data collected on a very general V/F/V system are presented to give the designer information on V/F/V system dynamics at fre-quencies much higher than those considered to be in the V /F /V system's useful range for data acquisition and communication purposes. These data show that the V/F/V process may be useful for servo loop applications at much higher frequencies than it is for data acquisition and communication applications.
Aerospace applications require artificial stars in the laboratory testing of star sensors or trackers. The precise angular testing of these instruments requires that the star simulators or simulacrums provide nearly plane wavefronts and infinitesimal diametral angular subtense of the source. The paper presents performance requirements for the simulator to be used on an advanced space systems program. The paper discusses the parametric and tradeoff studies leading to the pinhole size, radiometric requirements, and collimation tolerance and traces the design of the illumination system. The major lens design aim is to provide a diffraction-limited, long-focal-length objective. Spherical and chromatic aberration and related secondary aberrations such as zonal spherical, secondary color, and spherochromatism need to be corrected to a degree that permits diffraction-limited performance. The paper also outlines procedures followed in the lens design and reviews aspects of mechanical, electrical, and thermal design.