Little in science has created as much interest, excitement, and controversy in the past decade as holography. Holography, or wavefront reconstruction as it was first called, was invented by Dennis Gabor in 1948 in an attempt to improve the resolution of images obtained with an electron microscope. Although Gabor was unable to demonstrate his technique with electron waves, he was able to do so with visible light. The most serious problems Gabor and other early holography experimenters encountered was the pre-sence of a twin image and the absence of a light source suitable for making holograms. Within a decade after its invention, holography appeared destined for obscurity. However, in the early 1960's Leith and Upatnieks at the University of Michigan demonstrated a new approach to holography based on communications theory techniques that made it possible to eliminate the twin image and hence gave higher resolution images than previously available. At about the same time lasers were being developed that provided the coherent light required for making holograms. By 1964 or 1965 holography was causing much interest in the scientific community, and investigators at numerous universities and industries began studying and inventing new holographic techniques. Between 1964 and 1968 more than 500 papers and articles were written on holography. Unfortunately, many of the early predictions on the use of holography failed to materialize, and by 1968 much of the glamor earlier associated with holography began to disappear, contracts were no longer awarded simply because the title contained the word "hologram," and many of the investi-gators began to study other fields.

Holography is ideally suited for the study of dynamic three-dimensional particle fields, and its applications for this pur-pose are almost limitless. The types and selection of configurations are described. A number of examples of current applications are given ranging from the study of micro organisms in biology to ice crystals in meteorology. It is concluded that ultimate image quality arising from a well-designed holography system will normally be slightly lower than the equivalent conventional photographic imaging method. However, such comparison can only be made when the precise object position is known a priori since only then is conventional photography applicable. Holography provides a method for examining, with high resolution, volumes several orders of magnitude larger than can be examined by photography. The most successful systems currently in use are combinations of holography with conventional methods, with the hologram serving primarily as a storage device.

Early holograms were recorded on photographic plates, and today such plates are still the most common holographic recording medium. However, the nature of the holographic process, plus the development of high-power coherent light sources, invited the use of materials other than photographic emulsions. As a result, holograms have been recorded in dichromatic sensitized gelatin, photoresist, electro-optical crystals, photochromic films and glasses, thermoplastics, photopolymers, amorphous semiconductors, and dye, MnBi, vesicular, and diazo films, as well as the more standard silver halide emulsions. In this review, the holographic recording ma-terials currently available are examined along with a few of their applications. Some experimental media are also studied. No effort to rank the materials is made, as the purpose of this review is simply to familiarize the reader with the various options he has in selecting a holographic recording material for his particular application. This approach is felt to be appropriate since most media currently available will form excellent holograms.

Holography has produced many spectacular displays for educational, commercial, scientific and artistic purposes; but it must continue to advance technically and esthetically to establish itself as an impressive and versatile medium. Here we examine the status of holography as an imaging medium, and some prospects for larger, brighter, clearer, deeper and more colorful displays. The properties of available photosensitive materials and light sources, and reasonable costs, impose a structure of compromises on practical imaging.

This article is a survey of the characteristics, technology and applications of Holographic Optical Elements (HOEs), considered as general elements in an optical system. HOEs function by diffraction of light from a generalized grating structure with nonuniform groove spacing. HOEs provide a system of thin film optics. They are capable of unique system functions and configurations, show a rapid variation of optical power and image characteristics with wavelength, have relatively large amounts of astigmatism and coma, and require special consideration of optical efficiency during system design. Comparison of the aberrations of specific, f/3.3 elements shows that the on-axis HOE and the conventional glass lens element have similar aberration levels, while the off•axis HOE has four times as much astigmatism and twice as much coma. We show that these grating aberrations, which appear for conjugate points that are different from the HOE construction points, are proportional to the average surface grating spatial frequency of an off-axis HOE. HOE technology is similar to conventional optics technology, but is less developed. The relative complexity of optical systems with HOEs, and the lack of a suitable aberration theory, produce a relince on computer-based raytracing for system design and development. We give the basic raytracing equations and discuss the special requirements for hologram recording appara-tus and materials. In applications, HOEs will usually provide unique capabilities rather than replace conventional elements, and will usually operate over narrow spectral bandwidths. The use of HOEs in several types of laser optical systems, and in visual displays, appears to be not only advantageous, but also technically and economically feasible.

The theory behind the aberration correction of holographic concave diffraction grating is discussed. From these results, applications in instrumentation design are described. The fields include Spectrophotometry, Spectrofluorimetry, Raman, Polychromators, etc. The new design options are then discussed by comparing the possible available solution for in-strument improvement before and after the advent of the holographic gratings.

Information about arbitrarily shaped wavefronts can be recorded as computer-generated holograms. There are convenient techniques for producing image plane holograms consisting of many narrow fringes similar to interferograms. These diffractive filters are suitable for realization of generalized elements for use in optical systems. These holograms can be applied to accomplish phase transformations, geometrical transformations, and combinations of phase and geometrical transformations. Computer-generation of holograms allows introduction of general carrier configurations. Examples of circular carrier recordings are shown.

There are several methods for optically subtracting one image from another in order to detect differences between scenes or between photographs of scenes. There are many applications for such a technology, including earth resource studies, meteorology, automatic surveillance and/or inspection, pattern recognition, urban growth studies, and bandwidth compression. This paper presents an overview of several techniques for obtaining optical image subtraction, including holographic, interferometric, coding, and positive-negative superposition methods. As part of this review, a table is presented summa-rizing and comparing the characteristics of more than twenty-five approaches. An extensive bibliography is also given.

The ability to record and retrieve three-dimensional infor-mation using holography has prompted many applications of holography to photogrammetry. Among them the significant applications are: 1) holographic recording and mensuration of objects for close-range applications; 2) the synthesis of holographic stereo models from stereo photographs for subsequent mensuration and three-dimensional display; and 3) the development of holographic contouring techniques. In this paper a review of these applications is made. Their relative advantages and disadvantages are also outlined.

Recent work in holographic memories is reviewed. Erasable, digital holographic stores have major remaining problem areas of increasing the memory capacity and developing a good storage material. The modular memory concept and the superposition of holograms in a thick storage media are two approaches to increasing the capacity of a holographic store. There have been several very interesting read-only holographic memories developed for specialized systems applications. The properties of these specialized holographic stores are compared with some bit-oriented optical memories as well as more standard technologies such as semiconductor storage and magnetic disk storage. The advantages and dis-advantages of holographic storage are reviewed. The associa-tive property of holograms should make this form of storage very useful for parallel search type memories and also create the potential for parallel data processing in applications of the future.

Some examples of applications of holographic interferometry to structures are presented in this paper. Some of the reconstruction techniques are novel and have not been reported previously, in particular the moire and blink projection methods to be described. The applications are arranged in increasing order of quantitative or interpretive sophistication. The dis-cussion of the actual holographic technique in each instance is very brief, the details being available in the literature. . . . nothing more can be attempted than to establish the beginning and the direction of an infinitely long road. The pretension of any systematic and definitive completeness would be, at least, a self-illusion. Perfection can here be ob-tained by the individual student only in the subjective sense that he communicates everything he has been able to see." Georg Simmel

The combination of the techniques of holography with those of flow visualization has resulted in one of the most wide-spread uses of holography. The most significant recent contributions in the field comprise solution of practical engineering problems, refinement in analytical and interpretive methods, and an increasing list of current and potential applications. The purpose of this paper is to summarize the meth-1 ods and state-of-the-art of holographic flow visualization. Holographic interferometry, like conventional interferometry, provides a measurement of optical path length between points of space. This can be applied to determine gas density or to observe density gradients and their movement throughout a volume. Moreover, a holographic image of a space is not just limited to interferometric analysis. Such images are amenable to a broad class of optical analysis, including three-dimensional photography and optical filtering methods such as schlieren photography.

Since its discovery in 1969, speckle interferometry has be-come a very active area of research, so much so that there now exists quite a collection of techniques that are applicable to a wide variety of problems. Because of the surprising versatility offered by such a wide range of techniques, the potential user of such technology may become con-fused as to what can and cannot be done, and, further, what process is suitable for what task. This review is provided, therefore, to set the various techniques into context, point out their similarities, and delineate their differences.

In the past, annular coded aperture images have been recon-structed by correlating an appropriately scaled annulus with the coded image. The basic improvement suggested in this paper is the addition of a linear radial frequency weighting in the Fourier plane. Reconstructions of point and disk objects were simulated with a computer program. The results show the advantage of this modification in the processing scheme. When the assumption is made that the detector is an Anger camera, the resolution obtained with the improved processing of the coded image is equal to that obtained with conventional apertures. An actual object consisting of the letter E was imaged with an annular aperture and a scintillation camera. The reconstruction with and without the improved processing is presented. In addition, the annulus is intermediate between the pinhole and the Fresnel zone plate with regard to both collection efficiency and the number of counts required for a given signal-to-noise ratio (SNR). It therefore offers an improvement over pinhole apertures without demanding the increased count rate and resolution required of detectors when Fresnel zone plate coded apertures are used.

Acoustical holography is being used in a broad range of applications which are discussed within the scope of four major areas, namely, industrial testing, medical imaging, seismic holography and undersea imaging.

The principles and methods of scanned transmitter-receiver and scanned object microwave holography and their use in high resolution microwave imaging are discussed. Results of laboratory experiments demonstrating the capabilities of this scanned mode of microwave imaging are presented together with a discussion of potential applications in radar-like imag-ing of remote moving objects and in the visualization of internal structures.

The effects of varying the duration of a stroboscopic flash are considered. If the disk in Fig. 1(a) is rapidly rotated and viewed under continuous illumination which is periodically interrupted for short intervals, then by adjusting the frequency of the inter-ruptions, the stationary negative image in Fig. 1(b) is produced. Standard texts in stroboscopy' and visual percep tion2 do not mention this effect,and we have been unable to find anyone who is familiar with it. There is, however, an early paper by Faraday,3 cited by Flelmholtz,4 which des-cribes related but not identical phenomena. The observed patterns can be predicted using optical arguments. Two time-varying illumination schemes are called complementary if the sum of the intensities of both schemes gives a constant illumination in time. While the disk in Fig. 1(a) is rotating rapidly, it appears a uniform gray under constant illumination, due to time averaging in the visual :system.4 ence, if the spatial pattern of intensity observed on the rotating disk under some illumination scheme is summed with the pattern observed under the complementary scheme, the result must be a uniform gray. Consequently, since illumination with periodic short light flashes gives a positive, image (narrow dark bars on a bright field), illumination by the complement must give a negative image (narrow bright bars on a dark field), Fig. 1(b).

Design and manufacture of infrared sensors is difficult enough for many applications; how ever, high quality radiometric calibration of infrared sensors is generally even more difficult. Specifically for low-background IR space-sensors, the calibration facility required is complex and expensive. In this and in the next column, Drs. Meier and Dauger will discuss a versatile calibration facility and its cali bration uncertainties.