Current Trends In Optical And Hybrid Processing Methods

Brian J. ThompsonCollege of Engineering and Applied Science, University of RochesterRochester, NY 14627IntroductionOptical information processing continues to prosper despite the competition from purely digital computer techniques.This conference attests to this fact but so does the current literature and particularly the recent books, including the twovolumes in the series Topics in Applied Physics; Volume 23, edited by Casasentl (1978) and Volume 48, edited by Lee2 (1981).These two volumes contain a very good summary by many of the leading contributors to the field (Abramson,.Balasubramanian,1 Casasent,l,2 Caulfield, Considine, Gonsalves, Goodman,2 Knight,2 Lee,2 Leith, Rhodes,2 Sawchuk,2Thompson)). The even more recent book edited by Stark3 (1982) has contributions by nineteen authors. Textbooks have notbeen ignored either, with Gaskill's4 book in 1978 on Linear Systems and just a few weeks ago a new and much revised editionof Yu's book on Optical Information Processing5 (1983). The content of these volumes, particularly the first three, givessome overview of current trends in this field; each of the books deals of course with basic concepts (Lee,2 Casasent andCaulfield, Stark3). Specific topics include chapters on biomedical applications (Caulfield)); radar (Leith, King3);crystallography (Thompson1); nondestructive testing (Abramson1); photogrammetry (Balasubramaniani); particle measure-ment (Anderson3); photographic image measurement (Shannon and Cheatham3). Techniques and methodology are also dealtwith in some detail in each of these treatises, including complex spatial filtering (Almeida and Indebetouw3); hybrid systems(Leger and Lee,3 Casasent2); nonlinear processing (Sawchuk and Strand3); interface devices (Knight2); space- variantmethods (Goodman,2 Rhodes3); incoherent techniques (Bartelt et al3; Rhodes and Sawchuk2).All these areas continue to be of importance, with polychromatic and noncoherent methods, devices and parallel andpipeline array processing'being the dominant topics for this meeting.Fourier Plane AnalysisOptical methods using Fourier plane analysis have been quite successful and perhaps the most successful of all opticaltechniques. Figure 1 shows in block diagram form the basic components of a hybrid processor for Fourier plane analysis.The input can have a variety of forms including prerecorded input on film or real objects such as wires or needles, or amultiplicity of objects such as an aerosol. The two -dimensional transform is produced optically and then the transformdetected with a variety of detector arrays. The outputs of these detectors are then digitized and multiplexed for analysis bya piece of hard -wired logic or a microprocessor. On occasion the information can be stored in memory for subsequentanalysis. The basic physics is, of course, straightforward as was essentially understood almost a century ago. Thedifference is the ability to detect and analyze the Fourier transform in essentially real time and hence take advantage ofthe two -dimensional parallel processing available together with its lateral and longitudinal space invariance. Perhaps thebest recent example of optical processing of pre- recorded inputs is the extensive effort in speckle pattern astronomy (seee.g. Roddier6). I would like to choose a different example, that of the analysis of electron micrographs of biologicalsamples. Figure 2a shows an electromicrograph of an elongated viral head structure whose basic assembly unit is a ring ofsix identical asymmetric proteins with the rings lying on a hexagonal net. This net is folded onto a closed cylinder. Whensuch a cylinder is flattened, as happens when the structure is prepared for electron microscopy, two superimposed mirror -symmetric, essentially planar lattices are produced (see e.g. Weyer et al7 1980). These structures are extremely difficult tosee in the electronmicrographic image, but are readily apparent in the diffraction pattern (Fourier transform) as shown inFigure 2b. Furthermore, this transform is easily filtered so that an image of either of the superimposed lattices can beformed. Figure 2e shows the one -sided image reconstruction of the viral structure shown in Figure 2a. The repeat unit is ahexamer of identical proteins lying on a hexagonal net. This method has become quite important and is being used in manylaboratories around the world -- not by optical scientists and engineers but by research biologists and biophysicists (see e.g.Weyer et alb 1981, for other references).There is a growing interest in having inputs that are not images but perhaps contain the image information. Thespeckle patterns mentioned earlier could be considered an example of this kind although they are image plane recordings.Recent work in particle size analysis has combined holographic recording with Fourier plane analysis (Ewan9 1980). In thismethod a far -field hologram of a volume of dynamic particles is recorded; the optical Fourier transform of this hologram isthen formed, thus providing the possibility of automatic readout of the original particle size distribution. This same methodis also being .studied for the determination of velocity distributions as well as size by first recording a doubly exposed holo-gram (Ewan-10 1979, Crane et al11 1982). Figure 3 illustrates the basic concept where (a) is a portion of a doubly exposedfar -field hologram of a two -dimensional field of particles which has been translated between exposures; a magnified portionof this hologram is shown in Figure 3b. Figure 3c shows the Fourier transform of the hologram of Figure 3a. The fringes areproduced because of the double exposure, and hence the fringe spacing is a measure of the movement of the particlesbetween exposures, so that if the time interval between exposures is known, the velocity can be determined. The extensionof this idea has considerable importance.In this class of hybrid systems the most successful in terms of commercialization have been systems using real (asopposed to recorded) inputs. The types of object are those that can be easily transilluminated, such as wire, fibers, needles,solid particulates, liquid droplets, etc. I believe the hypodermic needle measurement system of George (see e.g.Thompson12 1979) is perhaps the best known, while the particulate and droplet measuring systems have the broadest rangeof applicability.