This paper presents a review of the various applications of feedback in optical systems. There are two general areas, optical image processing and bistability. In optical image processing both coherent (optical spatial feedback network internal to an optical resonator) and hybrid (TV) systems are discussed. In the area of bistability there are molecular systems (atoms in an optical resonator), both absorptive and dispersive; hybrid (employing electronic feedback) single-beam and many-element (TV and spatial light modulator-based) systems. All the systems are discussed. A detailed model showing the many facets of molecular bistability is presented along with a discussion of catastrophe theory applied to optical bistability.

The measurement and analysis of thermospheric temperatures and winds with Fabry-Perot spectrometers are reviewed. It is shown how these measurements have been used to establish and define the dynamic behavior of the thermosphere.

A number of readers have asked if Optical Engineering welcomes critical review papers. The answer is: "sometimes." Since that answer is less clear than the question, some explanation is in order.

The use of feedback in optics has many varied facets. Applications range from the solution of partial differential equations to those of binary logic elements. The papers in this issue present a representative coverage of the topic.

"Hybrid" TV optical systems seem to be useful for the translation of electronic feedback concepts into optics. They enable us to achieve active feedback and to simulate temporally varying two-dimensional (x,y)-processes. We report three optical counterparts of electronic feedback applications: the operational amplifier, the analog computer and the flip-flop.

For an array of independent picture elements on a spatial light modulator (SLM), we present a configuration where each element is in one of two stable equilibrium states. Their stability as well as the means for switching between them is exhibited theoretically and experimentally. The distinguishing feature of the array of the independent bistable switching elements is that the implementation of each involves only a single active element in the feedback loop so that optimum use of the active area of the SLM is made.

A study is presented of the variance of the monochromatic absorption coefficient and of the corresponding slant-path molecular transmittance due to errors in the tabulated line-parameter data and in the meteorological profiles used. General mathematical expressions for the variances are derived, which are simple and convenient to use in calculations involving specific atmospheric profiles and spectral line data. The results of calculations at eight representative frequencies for an assumed atmospheric profile and for typical uncertainties in the data indicate that deviations larger than 0.01 should be reasonably expected in transmittance calculations along vertical atmospheric paths.

Intrinsic optical bistability experiments are reviewed and their contributions summarized. Very large nonlinearities in semiconductors give reason to hope that practical devices may evolve in the near future. Characteristics important for practical optical logic devices are discussed: size, temperature range, holding intensity, switching energy, switching times, and wavelength range and stability.

After a brief review of some known steady state properties of an absorptive bistable system, we discuss in some details new aspects of the transient response that follows a sudden perturbation from steady state. The evolution of the perturbed bistable system towards its new steady state consists of two generally distinct phases: one is the local relaxation within each well of the bimodal potential, the second is the tunneling through the local maximum that separates the metastable and stable steady states. We give an analytic description of both phases of the evolution, and give explicit formulas for the local and global relaxation rates.

From the viewpoint of the optical and instrumentation engineer, the instrumentation of environmental optics has two outstanding features: it challenges the instrumentation state of the art, and if successful it is exceedingly useful.

A comprehensive view of some of the considerations in the design of a spaceborne lidar system are presented. The discussion underlines some of the questions of prime scientific interest in the atmosphere which are best addressed by an orbiting lidar.

Fluorescence lidar is an active remote sensing technique for monitoring minor species in the atmosphere. It is based on detection of the fluorescence of atoms and molecules excited with a resonantly tuned probe laser. Rapid quenching of excited states and the associated fluorescence at tropospheric pressures limits the extensive use of fluorescence lidar to the stratosphere and above. Species which can be studied are those which can be excited to fluoresce in the .2µ to 1u, spectral region. Several years of study of Na and K in the mesosphere using ground-based lidar have demonstrated the usefulness of the technique. Detailed calculations of OH lidar proposals for the stratosphere demonstrate its usefulness for measurements of concentrations, temperatures and total pressure. Calculations of NO2 lidar proposals show that its detection is feasible, especially at night with limited range. Some species such as atomic oxygen seem to require novel approaches such as multiphoton excitation to surmount the problems of self-absorption. Experiments currently underway indicate that fluorescence lidar is about to be a major tool in the study of stratospheric composition.

A differential absorption lidar using two simultaneously pulsed CO2 lasers and a direct detection receiver is under development for use in regional scale aircraft measurements of minor species in the troposphere. This paper discusses the applicability of this instrument for regional ozone measurements.

A commercial Fourier transform interferometer system has been installed in a van and used to make long-path absorption and single-ended emission measurements of gaseous pollutant concentrations at a variety of pollutant sources. The system covers the infrared spectral region from 650 to 6000 cm-1 at a maximum resolution of 0.06 cm-1. In the absorption mode, many gaseous pollutants can be detected at parts per billion levels over a one kilo-meter path. In the emission mode, warm gases exiting industrial stacks can be detected at parts per million levels. The temperature of the stack gases can be determined directly from the spectra. This paper describes the interferometer system and results of recent measurements of jet engine, brick kiln, gypsum pond, and industrial stack emissions.

The interferometer-spectrometer is an exceptionally powerful instrument for remotely sensing optical emission spectra associated with the environment. However, when a conventional Michelson interferometer is operated at high spectral resolution, a very narrow viewing field results. For the sensing of emissions which have a significant spatial distribution, which is often the case for environmental species, optical compensation can be employed to open up the field of view. The resulting reduction in observation time for the compensated relative to the uncompensated interferometer is R294/4. For a practical instrument using prism-type compensator elements, the viewing field can be extended to more than ten degrees full angle. Consequently, the observation time required to obtain a spectrum is reduced by more than two orders of magnitude.

The application of Laser Doppler Velocimeters (LDVs) to the measurement of atmospheric processes has produced a wealth of information not previously available to the researcher. This information is providing a better understanding of a variety of both naturally and artificially induced phenomena which in turn is leading to even more varied applications. To date, such applications have included the measurement of wind profiles, dust devils, aircraft wake vortices, and wind shear. Future applications include a measurement program now underway that will permit extensive mapping of severe storm flow fields, a hardware definition study for a satellite-borne system that will allow for wind measurement on a global basis, and a flight program to assess a compact true air speed measurement system. This paper attempts to provide the reader with sufficient information to assess the status of research, development, and applications of atmospheric LDV systems. To accomplish this, an overview of the last thirteen years is presented, with the work grouped into several general areas, and some of the major results summarized. Since the majority of the work in this area has been performed with CO2 systems, a brief description of the principles of operation of typical pulsed and cw systems is provided. An extensive bibliography is provided.

A line of large-format raster-type film recorders, utilizing argon ion laser and rotating-drum techniques, is described. Image formats of up to 40X72 inches are available. The recorders are characterized by high absolute accuracy (down to 20 microns) high incremental accuracy (2.5 microns), continuously variable line-spacing and fairly high speed (up to 1 million points per second). Input is coded digital data from any source. Although developed for graphic arts applications, as part of a series of digital color image processing systems, these recorders can be readily adapted to other applications, including many that now use X-Y film plotters. One present version of the recorder enables exposing most common lithographic films with binary ("line") images as well as with half-tone images of any mesh number and angle. This version is already being applied to decorative, as well as to publications, printing preparation, to computer-aided cartography, and to seismographic and remotely sensed (aerial and satellite) data recording. Another version enables recording of variable density (continuous-tone) films. Future plans include adaptation to the expo-sure of offset plates, further increase in accuracy, and a five-fold increase in speed.

A technique using Fourier transform analysis which is suitable for measuring the specularity of solar glass components in the mrad and sub-mrad is discussed and demonstrated. A brief mathematical background as well as illustrative examples are included. A number of methods for image analysis are discussed with particular emphasis given to electronic integrating detectors. Typical Fourier plane image distributions are given for a few common solar utilization materials, and details of the instrument used to produce the images are considered. The limitations and capabilities of various instruments are outlined along with methods for further enhancing the utility and sensitivity of the technique.

We have found that reliance on theoretical models or incomplete manufacturer's data is not adequate for predicting the result of combining optical and electro-optical components into a system. The complete imaging characteristics of each component must be accurately known if reliable predictions are expected. Inhouse testing then is desirable where possible. Some components have well-established evaluation procedures. Others, such as coherent fiber optics, require various test techniques, some well established; others must be devised if complete characterization is needed. This is especially true where changes in magnification are a feature of the bundle. A description of the techniques we use for testing the modulation transfer function, image rotation, magnification, image size variation, distortion, shear, transmission efficiency, transmission variation, transmission defects, flatness, and acceptance angle are included in this paper.

There are many thousands of luminance photometers in the field, both telephotometers and microphotometers; however, very few of these have scanning microphotometric capability. This paper describes a unique optical system that can be field-interchanged with the standard objective lens, without requiring any modification of the photometer's internal optical system, to convert an existing photometer into a high-resolution scanning microphotometer. One embodiment, called the MicroScanner Spatial Scanner, will be described in detail. Used with any standard Pritchard Photometer, this scanner provides variable-speed scanning of a half-inch (12 mm) of object space with field coverage as fine as 0.0001 in. (0.0025 mm) or as broad as 0.39 in. (10 mm). The system features photometric uniformity within ±0.5% over the entire scan distance, freedom from polarization error, and a half-inch scan distance independent of choice of objective lens.

This paper is concerned with the optical portion of the Voyager spacecraft imaging science subsystem (ISS). After a brief description of the Voyager mission and its photographic aspects, the functional requirements for the television optics are outlined. Environmental considerations that constrained the design are summarized. One section of the paper is devoted to radiation testing and its impact on the design of the wide-angle optics. The main portion of this report, however, is devoted to a description of the Voyager television optics. Finally, Voyager imagery results that are current with the preparation of this paper are presented.

A new technique to reduce the effect of quantization in pulse code modulation image coding is presented. The technique consists of Roberts's pseudonoise technique followed by a noise reduction system. The technique by Roberts effectively transforms the signal dependent quantization noise to a signal independent additive random noise. The noise reduction system that follows reduces the additive random noise. Some examples are given to illustrate the performance of the quantization noise reduction system.

A review is given of recent progress in the development of integrated optical circuits involving the AlGa1-XAs/GaAs double heterostructure system. Various devices utilizing periodic corrugations or gratings are described briefly, whereas alternate attempts to fabricate optical circuits by wet chemical etching are discussed in more detail. The current trend to explore other III-V compounds is considered, with emphasis on the quaternary system GaxIn1-xPYAs1-y. Lattice matching of this quaternary to InP results in long wavelength emission, suitable for use with present optical fibers. A number of reasons are also given for increased investigation of this quaternary lattice-matched to GaAs.

Surface acoustic waves excited in a Si-SiO2-ZnO layered structure can produce a traveling electric field in the silicon substrate. Charges stored in the traveling potential wells can be transferred at high speed and density and with less complexity than conventional charge coupled devices. The monolithic structure under investigation for the surface acoustic wave charge transfer device consists of a silicon substrate, a thin silicon dioxide insulating layer on top of which a ZnO piezoelectric film is deposited by sputtering. The surface acoustic waves are excited by interdigital transducers. The signal charge is injected into traveling potential wells that travel with the velocity of sound. The presence of a thin shorting plate placed on the ZnO film, over the charge transfer region, can enhance the acousto-electric potential at the Si-SiO2 interface, thus resulting in a more efficient device. An 80 MHz, 2p-second surface acoustic wave charge coupled device has successfully been fabricated. An optical application utilizing such a structure is proposed. It can be used in place of a conventional interline transfer design. Surface acoustic waves are launched before the charges are transferred from the sensor region to the transport region.

Optical differential interferometry, initially developed to characterize planar recording media surfaces and subsequently used to detect continuous and pulsed ultrasonic elastic waves, has recently been applied to the measurement of the interface properties of bonded transparent solids. Acousto-optic interaction between dual laser probe beams and periodic particle displacement fields induced by acoustic waves on the boundary permits the qualitative identification of surface conditions at the interface. In this paper, the theory and operation of the real-time interferometric measurement system, an analysis of the acousto-optic interaction, and surface testing results for bonded glass specimens are presented.

Similar field-variables are studied in coherence theory and various laser scattering phenomena. However, a time average replaces the ensemble average in most analyses of coherence. It is demonstrated that when the phase deviation resulting from laser scattering is greater than 27 radians and the average scattered field-intensity is a slowly varying spatial variable, coherence relations can be applied to scattering phenomena by replacing the time average with a spatial average. A gedanken experiment is employed to demonstrate the correspondence of results obtained via coherence theory and scattering analyses. The reduction in observed intensity fluctuations by temporal and spatial averaging is also explicated. The implications of coherence theory results in laser scattering phenomena are discussed as well as the converse.

An x-ray astronomical observatory called the LAMAR, utilizing multiple grazing incidence x-ray telescopes for high sensitivity observations, is being considered by NASA for a Spacelab facility. A LAMAR utilizing Wolter Type I x-ray optics figured by diamond turning is described and its performance compared with a similar facility involving x-ray optics of Kirkpatrick-Baez design. Effective areas, imaging properties, and relative sensitivities of these two LAMAR facilities have been calculated with the aid of computer ray tracing codes. We conclude that the two optical designs provide comparable effective areas. Therefore, the ability to achieve the highest possible angular resolution within cost constraints will be decisive in the choice of x-ray optics for the LAMAR.

Arguments leading to the tentative conclusion that production and measurement techniques developed for low scatter optical surfaces should be directly applicable to low scatter x-ray optics are briefly reviewed, and some of the more useful methods for making and testing high quality optical surfaces are discussed.

The contributions by aerosols to the total extinction as measured by a Barnes transmissometer at San Nicolas Island were determined by subtracting the molecular contributions as calculated using LOWTRAN. These aerosol extinction coefficients are compared to those calculated using (a) aerosol size distributions measured at one end point of the transmissometer and airborne over the optical path, and (b) analytical models (LOWTRAN 3B maritime model and the Munn-Katz model which includes surface wind speed and relative humidity dependencies). The extinction coefficient variation with altitude calculated using the analytical models are also compared with those calculated from measurements at sea of vertical aerosol size distributions.

This paper describes a six-sided single-piece prism which disperses light without deviating it. The prism angles are chosen so that the light undergoes four total internal reflections in the prism. This design eliminates the need for the highly reflective surface coatings commonly used to prevent light loss in direct vision prisms. The prism's performance was demonstrated using light from an argon ion laser. Some applications are suggested.

The throughput of a Hadamard spectrometer is directly proportional to the length of the slits. However, optical aberrations limit the slit length that can be used. The usable length of curved and straight slits are theoretically and experimentally compared for a typical singly encoded Hadamard spectrometer that has a Czerny-Turner plane-grating mounting. For this Hadamard spec-trometer the optical throughput can be increased by a factor of ten by using properly curved slits.

An optical bypass relay can improve the reliability of a repeatered fiber optical computer network. Previous relays suffer from low cross talk isolation, high insertion loss or limitation to single-pole-double-throw operation. We describe an opto-mechanical fiber bypass relay with one moving part. The relay makes use, during assembly, of the fibers themselves both for mutual alignment and as temporary alignment tooling. This eliminates the need for individual alignment of each relay during assembly. Prototype relays exhibit -52 dB cross talk and -1.7 dB loss and ample life expectancy for their intended task.

Stephen Rudin's article [Optical Engineering 19(1), 132 (1980)] on a Rotating Aperture Wheel Device for Improving Radiographic Contrast presents an interesting development that may eventually have wide acceptance and wide application in diagnostic radiography. Unfortunately, there is a most misleading error of nomenclature perpetuated in the summary, in the text, and in Figure 2.