Aberration limits for annular Gaussian beams for optical storage.

Annularly apodized beams have been suggested for use in optical storage because of their potential to go beyond the conventional spot size and depth-of-focus limits. One concern for such applications is the effects of small aberrations on beams in which the energy is concentrated in a small annular ring. We present calculations and experimental results that show that annular apodization of a Gaussian beam reduces the sensitivity to defocus as well as balanced spherical and coma aberrations. The sensitivity to astigmatism is increased by a small amount.

Three-dimensional microscopy: introduction by the feature editors.

This introduction describes the motivation behind this 10 Sept. 1990 issue featuring 3-D microscopy and provides a brief overview of the issue.

Profilometry with a coherence scanning microscope.

Coherence scanning microscopy is a new technique in high resolution imaging. It shares many of the features of confocal microscopy but uses coherence effects to enhance the lateral and longitudinal resolution rather than physical apertures. This approach has two significant implications for profilometry: the longitudinal resolution is decoupled from the lateral resolution, and interference effects can be used to further enhance the longitudinal resolution. We detail the features of coherence scanning profilometry and give some examples.

Laser diode feedback interferometer for stabilization and displacement measurements.

Active laser diode interferometers in which the interference signal is fed back to the diode current are investigated for Twyman-Green and self-coupling interferometers. The Twyman-Green interferometer is stabilized with a stabilization factor of more than 100. By using the feedback signal of either type of interferometer, displacement is measured in a linear scale over a dynamic range of 8-9,microm with a precision of 10-60 nm. The feedback signal vs displacement shows hysteresis and multistable behavior, in accordance with theoretical results.

Nonlinear optical processing with halftones: accurate predictions for degradation and compensation.

A general analysis of the halftone process for nonlinear transformations in optical signal processing is presented. The analysis considers the effects of the nonideal characteristics of the recording medium. The results predict output errors due to different parts of the recording medium characteristic curve for any nonlinear transformation. A synthesis method for a discrete halftone screen density profile is also described. This produces an optimum halftone screen density profile for any form of recording medium characteristic curve and any type of nonlinearity in the sense that it minimizes the mean-square difference between desired and degraded outputs. The results of a computer simulation for logarithmic and level slice functions are given.

Angstrom resolution optical profilometry for microscopic objects.

An instrument capable of recording the amplitude and phase of reflected light with a phase resolution of better than lambda/3000 and the lateral resolution of a confocal scanning microscope was built. The instrument is based on a commercial microscope body and uses the regular interference contrast optics. The modifications consisted of adding a coherent (heterodyne) detector and a confocal laser scanning system. Two-dimensional surface images of amplitude, slope, and profile were taken with a step height resolution of typically 0.5-2 A. The instrument is described, and its characteristics for surface profilometry are discussed.

Extended unambiguous range interferometry.

Phase measuring interferometers generally measure phase modulo 2pi. We present a system which uses fringe contrast to help determine the absolute phase in the interference image within the limits of the coherence length of the illumination. This approach obviates the need for phase unwrapping and is unaffected by surface discontinuities or by data dropout. Since the phase is determined on a point-by-point basis, the processing could be pipelined. The system is set up on a microscope interferometer and produces surface profiles over an array of 512 x 512 points. The measurement range is related to the coherence length of the source and can easily be varied from 0.5 to 2.5 microm. The resolution is limited by the 8-bit quantization of the output.

Adaptive noise smoothing filter for images with signal-dependent noise.

In this paper, we consider the restoration of images with signal-dependent noise. The filter is noise smoothing and adapts to local changes in image statistics based on a nonstationary mean, nonstationary variance (NMNV) image model. For images degraded by a class of uncorrelated, signal-dependent noise without blur, the adaptive noise smoothing filter becomes a point processor and is similar to Lee's local statistics algorithm [16]. The filter is able to adapt itself to the nonstationary local image statistics in the presence of different types of signal-dependent noise. For multiplicative noise, the adaptive noise smoothing filter is a systematic derivation of Lee's algorithm with some extensions that allow different estimators for the local image variance. The advantage of the derivation is its easy extension to deal with various types of signal-dependent noise. Film-grain and Poisson signal-dependent restoration problems are also considered as examples. All the nonstationary image statistical parameters needed for the filter can be estimated from the noisy image and no a priori information about the original image is required.

Polarization properties of the variable-grating-mode liquid-crystal device.

The principal features of the liquid-crystal molecular orientation within the variable-grating-mode liquid-crystal device have been determined as a function of the applied voltage across the cell by measurement of the polarization properties of light diffracted by the liquid-crystal birefringent phase grating.

Real-time parallel logarithmic filtering.

Parallel logarithmic filtering of multiplicatively combined image signals is achieved in real time. A liquid-crystal light valve (LCLV) is combined with a specially designed halftone screen that matches the LCLV characteristics to produce an overall logarithmic transformation. Experimental results are shown.

Optical pseudocolor encoding of spatial frequency information.

An optical spatial filtering system is described which color encodes the local spatial frequency content of an image. It is shown that partially coherent illumination has advantages over coherent or incoherent illumination in this system. Experimental results are shown which indicate that the system can be used to perform a simple type of texture-to-color conversion. The system could be used to enhance textural differences for a human observer or as a preprocessor which provides texture related information to a second image processing device.