Long before the development of electrical systems and devices, optical systems were being used to perform the col-lection, transmission and storage functions of information processing. In recent years, the mathematical structure common to electrical signal analysis and optics has led to increased interaction between the two disciplines, with tremendous benefit to both. Optics has provided insight into the field of image processing by digital and electrical techniques, including image transmission and coding, image enhancement and restoration, and pattern recognition.1-8 At the same time, the analysis and synthesis of optical systems and advances in microscopy, spectroscopy and interferometry have directly benefited from the insights of signal and communication theory mathematics.9-13
The construction of digital holographic spatial filters using a high precision multiple gray-level plotting device is demonstrated. Examples of the use of such filters in a coherent optical system are presented, and analogous results obtained by computer processing are shown for comparison. The theory of sampled holograms and spatial filters is discussed in some detail.
Computer holography is a subject of interest to a small but increasing number of researchers. This paper briefly describes the concept and history of computer holography, how it works, and its possible applications. Also presented are the recent approaches to computer holography developed at Stanford University, and a procedure for synthesizing this new class of computer holograms. These new on-axis holograms are very efficient both in their use of reconstruction light and in their use of display resolution elements during synthesis. They keep the advantages of the kinoform, a previous approach by IBM, but do not have its limitations. One approach makes use of color reversal film, such as Kodachrome II. Another approach only requires phase-transmittance material, but preserves both amplitude and phase information. Experimental results are presented.
Some recent developments in coherent optical processing are described in this paper. Specifically, the use of one or more elementary gratings to perform useful processing operations and computer-generated spatial filters to obtain generalized transforms are discussed. Applications incorporating nonlinear optical elements and optical feedback into the processing system are also presented.
Finite-size X-ray sources degrade the radiologic image by attenuating the high-frequency components of the image and by phase distorting a significant number of these frequencies. Coherent-optical spatial filtering was employed to correct these distortions in the Fourier transform of the radiologic image. This included the use of amplitude-only and amplitude-and-phase (holographic) filters. The contrast and resolution of X-ray images were improved.
The common hologram is an analog hologram containing photographic grey tones. A binary hologram contains only black or white areas and is made either by computer or by photographic hardclipping of analog holograms. Theoretically and experimentally, the signal/noise superiority of binary over analog holograms is demonstrated in this paper. The diffraction efficiency of binary holograms is typically ten times higher than that of analog holograms, while the noise due to the photographic grains is lower at the same time by a factor one half in the examples discussed. The discussion is presented in the context of holographic data storage with references to other applications where the signal/noise improvement is important.
A hybrid optical/digital image processor is described. The optical portion consists of a real-time two-dimensional electron-beam-addressed KD2PO4 input transducer and a conventional optical data processing system. The Fourier and correlation planes of the optical processor are digitized and analyzed in real-time by a unique digital interface. The system is interfaced to a PDP-1 1 /15 minicomputer and associated storage and display peripherals. Following a brief system description and survey of spatial light modulators, examples of the system's performance and the use of digital computer feedback control of the optical processor in several application areas are presented.
The operation of an electro-optic image modulator device for use in real-time optical processing systems is described. The PROM (Pockel's Read-Out Optical Modulator) is used to provide temporary storage of image or digital data which can then be read out by a laser for coherent optical processing. The stored image may undergo certain operations, such as level slicing, edge enhancement, or contrast inversion by utilizing the electronic characteristics of the device. This feature is useful for incoherent image processing, and also allows zero order suppression in coherent processing applications. The rapid recyclability, high optical quality and high resolution demonstrated by the device are required for practical utilization of the device in the applications described.
The combination of optical preprocessing and interactive digital processing furnishes a powerful technique for image analysis and pattern recognition. We describe in this paper several innovative data-reducing operations easily implemented with an optical system. A particular design for an interactive optical-digital computer now being assembled is discussed and several actual examples of optical preprocessing are given.
A description of a prototype optical-digital system for automatic processing of chest X-rays and preliminary results are presented. The system consists of a digital image scanner used for locating the lung fields and texture measurements, a diffraction pattern sampling unit which provides both annular ring and wedge samples of the Fraunhofer diffraction pattern of lung areas, and a microcomputer for control and computation of decision functions. The film is positioned by a computer-controlled film transport to provide an overall automated system. The system is currently being designed for the Appalachian Laboratory for Occupational Respiratory Diseases and will be used for screening chest X-rays of coal miners for pneumoconiosis.
TOPICS: Interference (communication), Image processing, Signal processing, Data modeling, Digital image processing, Analog electronics, Filtering (signal processing), Optical filters, Digital signal processing, Computer simulations
Image detection noise is a fundamental limitation in picture processing, whether analog or digital. This noise is characteristically signal-dependent and this signal-dependence introduces significant problems in the design of appropriate noise-suppression techniques. This paper outlines some recent results obtained by the authors in the optimum suppression of two types of signal-dependent image noise: film-grain noise and photoelectron shot noise. The work in grain noise suppression involves deriving the minimum-mean-square error Wiener filter for a new form of signal-dependent noise model suggested in earlier work by T. S. Huang. Implementation of these filters by either coherent optical or digital processing techniques is possible. Digital computer simulations of grain noise suppression using two particular cases of this additive, "signal-modulated" noise model were performed. They demonstrate the potential advantages of noise suppression filters which make use of a priori knowledge of the signal-dependent nature of the grain noise. The results of work on linear, unbiased restoration of images recorded in the presence of photoelectron noise are summarized. Additional work in both of these areas is suggested, with a particular need existing for correlating the properties of various models proposed for grain noise with experimental data obtained on emulsions using scanning microdensitome ters.
A synthetic-aperture approach offers one method for obtaining high resolution optical imagery with low-mass, satellite-based optical telescopes. Images recorded sequentially with different aperture configurations are sampled and digitized, and appropriate spatial-frequency-domain processing is performed on a computer to obtain the desired high-resolution imagery. Special steps can be introduced in these post-detection processing operations that allow a substantial reduction in the optical tolerances that must be maintained by the aperture elements in the imaging process.
The article describes a specialized instrument which can measure back focal lengths of lenses to ±.002 inches. It incorporates design features which promote quick, convenient and accurate use by relatively untrained operators.
A simple method of generating pseudo Fizeau fringes on large objects with deformations of the order of a few centimeters is described and demonstrated. A stereo pair of photographs of the object are placed one in each arm of a Mach-Zehnder interferometer and are then imaged onto a common image plane. A knife edge placed in the back focal plane of the imaging lens deforms the straight line fringes of the Mach-Zehnder interferometer to provide relief information of the object. The functional form of the deformed fringes is shown to be equivalent to Fizeau fringes.