As most readers of Optical Engineering already know, a majority of issues of this journal are "special issues" - issues in which many of the papers are devoted to a "special" topic related to the field of optical engineering. In both 1986 and 1987, nine of the twelve issues of the journal were special issues. During 1988, lit appears that we will have only seven special issues, although I hope to increase that number for 1989.

Tunable Lasers-Reviewed by Charles E. Byvik, NASA; Light-Reviewed by Freeman F. Hall, Jr., Harrier Consultants

Optical technology has emerged as a viable solution to the growing demand to increase the throughput of high speed processors and computers. Although higher speed and denser integrated circuits are being developed, it appears that faster switching speeds in digital circuits will not provide an adequate solution to the bottleneck problem of computing systems for such tasks as real-time distortion-invariant pattern recognition and associative memory. Even supercomputers using new computing architectures and subnanosecond gate delays do not have sufficient speed for such real-time operations. Optical systems offer the advantages of inherent parallelism and high speed with superior interconnection capability, which allow for the processing of millions of simultaneous operations. The lack of electromagnetic interference in optics is ideally suited for neural network based proces-sors, which require a high degree of interconnectivity and global communications properties. Analog optical computers are particularly attractive for the processing of large stochastic data, while the more precise digital computers break down when confronted with such random problems. The immunity to electromagnetic interference can also be used advantageously in VLSI interconnections technology and board-to-board communications to reduce the pinout problem and to improve flexibility and performance. For these reasons, optical technology has become a major research and development effort at many industrial, government, and university laboratories both nationally and internationally.

Coherent optical processors for product inspection are discussed, and data from a case study are presented. An optical processor that produces both Fourier and Hough transform data is presented. The Fourier transform data are wedge/ring sampled to produce in-plane distortion-invariant features to determine the product's attribute (good or defective) and its orientation. Hough transform data are used for new quantitative mensuration information on good products. In one case, optical Hough transforms are quantitatively shown to yield more consistent results than do digital realizations.

We have previously designed correlation filters that detect multiple views of an object while remaining specific to the object. We now present a method of modifying the filters to reject clutter, which is defined as additive structured noise. The clutter is rejected by adding to the filters components that are designed to force the contribution of the clutter in the output correlation to be zero. The additional clutter-rejection components are orthogonal to the object harmonics and do not affect the detection process. In the specific example of rotation-invariant detection, the resulting filter has two salient features: it recognizes the object regardless of its rotation or intensity and rejects the clutter regardless of its rotation or intensity.

Shape-changing, or morphological, transformations (e.g., erosion, dilation, median filtering, and other ranked-order filtering) on binary imagery can be obtained by optically convolving the input image with a disk or other binary spread function and thresholding the output. Gray-scale images can be processed if the input is decomposed into a sequence of binary "slices" by a variable thresholding operation (threshold decomposition). The slices undergo shape-changing transformations and are then added together to produce the output gray-scale image. Median filtering to remove "salt-and-pepper" noise from a gray-scale image is demonstrated. The use of gray-scale (as opposed to binary) convolution kernels allows extension of the method to a more general class of nonlinear filtering operations that includes stack filters.

After a brief survey of several optical methods for computing the geometric moments of an image, we introduce an algorithm that calculates these geometric moments from the intensity of the Hartley transform of the image. The performance of this algorithm is analyzed with the help of simulations as several important parameters are varied. These include the sampling interval in the Hartley transform plane, the input signal-to-noise ratio, the signal-to-noise ratio in the detector plane, positional misalignment of the detectors, the variation in the widths of the detectors, the number of bits in the detector analog-to-digital converters, and the length of the differentiating filters used. We also discuss how to obtain all of the geometric moments even though the sign information in the Hartley transform is ignored and how to extend the algorithm to two dimensions.

The performance of inexact processors can be improved at the cost of throughput by using parallel redundant computations to correct errors at the processor's output. Error correcting codes applicable to binary data strings have previously been suggested for application to optical processors. We demonstrate that multilevel block codes can likewise be applied. Specific attention is given to error correction multilevel optical matrix-vector multipliers. The per-formance of the multilevel block code is compared to that of multilevel error correction codes formulated for VLSI processors and bar code readers. In residue-coded form (in which the matrix-vector multiplication is performed conventionally), the multilevel block code is shown overall to require fewer resolvable levels at the output.

We investigate the performance of a recently introduced bipolar joint transform image correlator when multiple reference objects and single and multiple targets are present at the input plane. The bipolar joint transform correlator uses nonlinearity at the Fourier plane to threshold the Fourier trans-forms' interference intensity to only two values, 1 and -1. The performance of the bipolar correlator is compared to the classical joint transform image correlator when multiple objects are present at the input plane. We show that the performance of the bipolar correlator is substantially superior to that of the classical joint transform image correlator in the areas of autocorrelation peak intensity, autocorrelation peak to sidelobe ratio, autocorrelation bandwidth, and discrimination sensitivity. It is shown that when multiple objects are present at the input plane, the classical joint transform image correlator produces large cross-correlation signals and poorly defined autocorrelation signals with large bandwidth. The large cross-correlation signals are comparable in intensity with the autocorrelation peak and can falsely indicate the presence of many targets. On the other hand, the bipolar joint transform image correlator produces well-defined autocorrelation signals with very low cross-correlation level. Computer simulations are used to test both types of correlators, and 3-D plots of the correlation functions are provided and discussed.

A new photopolymer from Polaroid has been investigated for producing holographic lens arrays. Arrays such as this may be important elements in optical computing architectures requiring large numbers of parallel interconnections.

An experimental coherent optical correlator implementing real-time binary phase-only filters (BPOFs) and a matching computer model that simulates and predicts correlator performance are used to investigate target recognition and location and character recognition for images of interest in industrial applications. Good quantitative agreement between the computer model and the experiment is exhibited over hundreds of correlation pairs. Rotation and scale sensitivities are measured, and three types of composite smart filter are applied to reduce these sensitivities while maintaining good clutter object rejection. The effect of edge enhancing the subject images is investigated. Simple and composite BPOFs also demonstrate promising character recognition performance. The performance of magneto-optic spatial light modulators implementing BPOFs is validated, as is the use of computer simulations to predict performance. The potential of hybrid adaptive correlators based on this approach for industrial machine vision applications is demonstrated.

The influence of ionic conductivity in photographic film emulsions is explored as a possible mechanism for infrared presensitization.

The results of an analysis of the precision pointing and tracking accuracy possible between satellite-borne optical linking systems using measured platform disturbance data are discussed. Measured vibration data from the Landsat satellite were used to generate the power spectral density parameter needed to model the baseframe noise environments for the two-satellite system. Three control subsystem configurations were evaluated: gyro-stabilized, mass-stabilized, and complementary filter. The sources of error included in the analysis were sensor noise from the optical detector, host satellite baseframe vibrational noise, and frictional and bearing noise. The results of the study indicate that a 1 µrad rms pointing and tracking accuracy may be achieved with either the gyro-stabilized or the complementary filter approach.

As part of the Landsat Thematic Mapper (TM) engineering evaluation effort, there have been several studies to measure a modulation transfer function (MTF). In contrast to work that measured or modeled only the optical and electronic components of the MTF, our goal was to measure the complete imaging system MTF from actual imagery. Measurements of the TM MTF using targets of opportunity have been reported previously. This paper describes new results using a specially designed target. To arrive at an estimate of the system MTF, the TM point spread function (PSF) was measured using a two-dimensional array of black squares constructed at the White Sands Missile Range in New Mexico. The design of the target creates 1/4-pixel shifts of "point sources" throughout the 30-m instantaneous field of view (IFOV) of the TM. This shifting allows exploitation of sample-scene phasing to effectively resample the PSF at a 7.5-m sample interval and thereby avoid aliasing in the MTF. A method for sorting the image data and "assembling" a PSF and a technique for correcting the rotation of the target relative to the sensor scan direction are discussed. A technique for partial elimination of variations in the scene background also is presented. The resulting estimates of the TM imaging system MTF compare favorably with the MTF predicted from a system model.

An AIGaAs laser transmitter has been developed that dichroically combines six single-mode 830 nm laser diodes in a 100 A total band-pass. Single-mode operation of each diode is necessitated by the 20 A diode-to-diode channel spacing. An external etalon is utilized for each laser diode to maintain single-mode operation (greater than 25 dB side mode suppression) in the presence of wideband modulation [1000 mega-pulses per second (Mpps)]. A universal header accommodates diodes from various manufacturers and includes an impedance-matching net-work as well as a thermistor and heater for diode closed-loop temperature control. Diffraction-limited optics are used to maximize the combined beam far-field irradiance for free-space applications. A 218 urad output beam with less than 1.05:1 ellipticity was measured for the combined output.