Due largely to the visual appeal of its three-dimensional imagery, holography has been one of the more fascinating branches of optics for Iboth academics and system designers since the production of the first high-quality laser holographic image in the early 1960s. Much of the effort in the early years was devoted to understanding holography theoretically and to making better-quality holograms. Those interested in practical application of the technique were concerned mainly with holography's imaging properties, but researchers, system designers, and industrial users were quick to realize that holography had important applications lin measurement, microscopy, nondestructive testing, and other fields. Many of these other applications were discussed in Optical Engineering's first. Applications of Holography Special Issue, published in September/October 1985. The present Special Issue, Applications of Holography II, continues that theme with a collection of papers concentrating on the practical features of holography and its diverse applications.

Five related holographic methods for imaging through inhomogeneous media are described and compared. The methods are one- and two-way phase conjugation, superresolution with source broadening and with spectral broadening, and combined superresolution and one-way phase conjugation. The advantages and limitations of each are described.

This paper gives a comparative analysis of different techniques of continuous readout of volume holograms in photorefractive crystals. The techniques rely on self-diffraction of recording beams and can be used as a basis for developing adaptive holographic interferometers that ensure a high degree of reliability of interferogram formation. The use of adaptive interferometers for the analysis of vibrations and displacements of real objects is illustrated using the photorefractive Bi12Ti020 crystal. This crystal exhibits the highest holographic sensitivity among presently known photorefractive crystals for recording by cw lasers in the red region of the spectrum. The applicability of photorefractive crystals for conversion of phase variations of recording light beams into intensity variations is considered. This enhances the sensitivity of interferometers to vibrations to be measured. The experimental measurements of vibration amplitudes as low as 1 A using Bi12Ti020 and a 0.63 µm laser are reported.

Electron micrographs of volume phase holograms recorded in DMP-128 reveal microstructure that is responsible for holographic activity. Solid and porous layers alternate with a spacing commensurate with the recorded fringe pattern. The difference in material density between the solid and porous regions accounts for the refractive index modulation and therefore the holographic activity of DMP-128 holograms. The pores of the holograms are interconnected and can be filled with many low and moderate viscosity liquids. Diffraction efficiency, bandwidth, and wavelength of maximum efficiency are profoundly and predictably affected by filling the hologram pores.

Broadband imaging systems that contain holographic lenses can be lightweight and have large apertures. We report a Fresnel diffraction analysis of an imaging system that consists of three lenses of arbitrary dispersion. A general solution is found for the wavelength dependence of the lenses to simultaneously correct the imaging system for both longitudinal and lateral paraxial chromatic aberration. As a special case, we describe an optical system that uses holographic lenses to produce a well-corrected image in broadband light. Experimental results that demonstrate the system performance in both laser and broadband illumination are reported.

Reflective computer-generated holographic elements are used in a quasi-optical fashion to modify both the phase and polarization of a high power, coherent microwave beam. Theory and design for both one- and two-component systems are discussed along with some experimental results.

Diffractive optical elements have the potential to improve the performance of infrared optical systems. An approach to constructing high quality diffractive elements has been developed using standard integrated circuit techniques. It is possible to implement arbitrary phase profiles since the elements are computer generated.

Holographic filters for protection against visible-wavelength lasers offer potentially high visual transmittance owing to a narrow spectral notch, but the angular dependence of the spectral notch position dictates a trade-off between eye protection and visual transmittance. The fundamental physical properties and design parameters of holographic filters are discussed. The relative merits of various exposure and substrate configurations for laser-protective eyewear are compared. Emphasis is placed on single-beam exposure, surface-conformal fringe structures in which the local Bragg angle is determined by the fringe spacing as opposed to the fringe tilt. This type of hologram is readily made free from flare or multiple images in transmission. Performance is evaluated in terms of visual transmittance versus eye protection, including retinal area and eye rotation. The relationship between angular and spectral response of holographic laser filters determines the exposure source for optimum performance to be roughly coincident with the center of eye rotation, regardless of the substrate geometry. Performance may be improved by locating the filters a greater distance from the eye. A more dramatic improvement in performance may be achieved by increasing the curvature of the substrate so that it is concentric with the eye.

The curved holographic combiner of an aircraft wide field of view head-up display is described. The complete design procedure is presented, from initial layout to the completed construction optics design. A description of the display optics in addition to the hologram construction optics is included.

An algorithm is presented for the computation of large size fast Fourier transforms (FFTs) for computer-generated holograms. The algorithm is useful when the recording device has a very large space-bandwidth product and the computation is under memory restrictions. The number of complex operations required using this algorithm is slightly larger than the number of operations necessary using the FFT algorithm.

Two iterative algorithms for synthesis of binary computer-generated holograms (CGHs) for image reconstruction are described: alternating projections onto constraint sets (POCS) and direct binary search (DBS). Comparisons with conventional methods for CGH synthesis show that POCS and DBS yield substantially lower mean-squared reconstruction error and higher diffraction efficiency. The best method is DBS, but it is also the most computationally intensive. To ameliorate this disadvantage, an acceleration technique is presented for DBS. With this technique, the design of binary holograms with moderate space-bandwidth product becomes feasible. For example, the computation required to design a hologram with 256K elements is reduced by a factor of 121.

Holographic content-addressable memory is used in an optical symbolic-substitution-based system to realize an efficient and high speed optical multiplier. Such a system is also capable of performing image processing operations. Consequently, it is used to obtain a fast gray-level histogram and histogram equalization of an input image.

The three-dimensional imagery available from holography can provide additional information to a business machine operator and visual interest to encourage use of the display. Both of these factors can enhance the functionality of the user interface. This paper describes several display concepts that use holographic images for man-machine interfaces and the implementation of some of these ideas. A hybrid display that incorporates a holographic image and a liquid crystal display has been developed.

Considerations in using holographic optical elements and a laser/detector hybrid device for optical disk systems are discussed. The process for fabricating the holographic optical elements is briefly reviewed.

A novel method for obtaining a tapered profile in photoresist is proposed. On the mask near the main features we add some slits smaller than the resolution limit of the projection optics at a distance also smaller than the resolution limit. In such a way, a controlled sloped profile can be obtained. We have applied this method to open tapered contacts and vias (contacts between two levels of metal) without using the thermal reflow technique of borophos-phosilicate glass, optimizing the configuration of the subresolved slits by means of a simplified mathematical model. There is experimental evidence that contacts can still be opened up to the diffraction limit of the projection optics.

The effect of signal and background shot noise, as well as device noise, on the performance of a direct-detection spatial tracking system is investigated for arbitrary detector arrays, assuming linear loop operation. The performance of quadrant detectors as a function of background radiation and detector radius is then analyzed and compared to some performance bounds. Tracking systems based on avalanche photodetectors (APD) and PIN detectors also are compared. The effects of non-focal-plane processing, focus error, and pupil walk on tracking performance are investigated. Experimental results of a 2.4 kHz two-axis tracking loop operating at low signal power using an APD quadrant detector are presented.

Perception is the integration of various sensor outputs into a unified vision of the world. A major part of research in machine vision has been limited to the use of individual sensors for building machine vision systems. The use of multisensor data is advocated for object recognition systems, and an algorithm for fusion of intensity and range data for segmentation is proposed. The algorithm consists of two steps. First, the initial seed segmentation is achieved by using the most dominating sensor at a given time. For this purpose the distributions of the intensity and range data are considered, and the image is segmented recursively by using the most significant peak in both histograms. Second, the initial segmentation is refined by using region merging; the regions are merged if the combined strengths of range and intensity boundaries are low. The experimental results for synthetic and real scenes are presented.

A solid-state microscope (SSM) for quantitative microscopy in image cytometry has been designed to optimize spatial, photometric, and spectral resolution. The SSM is an optoelectronic device for scanning and viewing microscopic objects in the visible light spectrum. Using Kohler illumination, pulsed light is transmitted through the microscope sample and focused by a single objective lens. The objective magnifies and projects the image onto a large-area charge-coupled device (CCD) located at the intermediate focal plane. Flexibility in scanning of the sample and optical considerations require a CCD array of >1000 X1000 picture elements, with each element having a sensing area of approximately 7 Am X7 um. The signals from the picture elements are directly digitized and mapped on a one-to-one basis into a large-frame memory at a rate of approximately 20 Mbytes/s. The entire digital image is continuously displayed in real time on a monochrome monitor at a rate of >60 Mbytes/s. The images stored in the frame memory can also be accessed by an external image processing system for quantitative measurements. The main features of the SSM are its simple optical path, high resolution, large field of view, image display of the image on a monitor with overlay graphics for object labeling, direct access to any part of the digital image, different scanning modes includ-ing full-frame scanning and time delay integration, spectral imaging, and large dynamic (sensitivity) range.

An active research area in optical computing is finding new optical devices to implement Boolean functions used in conventional computers. Reversible nonlinear interface devices have a picosecond response time and hence produce a very fast half-adder circuit. These devices can be used in the implementation of an optical CPU.

The white-light point spread function (PSF) of the human eye is computed from monochromatic results obtained by a hybrid optical-digital procedure. The method is based on the linear recording of monochromatic aerial short-term retinal images of a point test. From these data, the monochromatic PSF is computed, and the wave aberration of the human eye is retrieved from the actual PSF by means of a phase retrieval method. The white-light PSF is generated by a digital image processing system from the monochromatic wave aberrations corresponding to three different wavelengths. The procedure proposed here allows a more complete evaluation of the optical image quality in the human eye and can be used in a variety of practical applications. As an example, the methodology is used to obtain objectively information on the reflecting layers in the retina for different incident wavelengths.

In my editorial entitled "Three Dawn Two to Go," which appeared in the June 1988 issue of Optical Engineering, I announced that I would step down as Editor after two more years. Now, one year later, I still have a year and a half to go. Where did the extra half year come from? The answer is simple; my sentence was extended an additional six months as punishment for some poor editorial writing. (Actually, I was asked to stay on for six months to help ease the transition for my successor, who shall remain unnamed for the time being.)

The Spectroscopy Source Book and the Solid State Physics Source Book both contain material from the McGraw-Hill Encyclopedia of Science and Technology. They are two of several recently released source books on different topics in science. Each begins with an introduction that is actually an introduction to the relevant science.