Timothy C. Strand

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 Lees 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 Lees 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.

Adaptive restoration of images with speckle

Speckle is a granular noise that inherently exists in all types of coherent imaging systems. The presence of speckle in an image reduces the resolution of the image and the detectability of the target. Many speckle reduction algorithms assume speckle noise is multiplicative. We instead model the speckle according to the exact physical process of coherent image formation. Thus, the model includes signal-dependent effects and accurately represents the higher order statistical properties of speckle that are important to the restoration procedure. Various adaptive restoration filters for intensity speckle images are derived based on different model assumptions and a nonstationary image model. These filters respond adaptively to the signal-dependent speckle noise and the nonstationary statistics of the original image.

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.

Optical Three-Dimensional Sensing For Machine Vision

A review of optical range sensing techniques for machine vision is given. Four basic categories of range sensing techniques are discussed: geometric techniques, time-of-flight techniques, interferometric techniques, and diffraction techniques. The basic principles are elucidated, and general comparisons are made between the groups. Representative examples are given of many different approaches. The challenge for optics and optical computing is to develop new range sensors that are fast and accurate and require little or no post-detection processing.

Multilevel volumetric optical storage

Present optical storage devices use two-dimensional recording media which do not take advantage of the ability to focus light to a small spot at a large working distance throughout the volume of a storage media. In this report we propose an approach to volumetric optical storage where the optical had reads from and writes to distinct levels of a multilevel disk structure. This approach has been successfully applied to multilevel ROM and WORM optical disk memories. We detail the physical principles behind this technology, present detailed experimental data on two-level write-once media and briefly describe four-level recording. In companion papers we describe in detail results from four-level recording and readback and multilevel ROM based media and drives.

An optical method using a laser and an integrating sphere combination for characterizing the thickness profile of magnetic media

An optical instrument developed for measuring the film thickness of magnetic media is described. The system uses a polarized-laser beam and an integrating sphere to measure the absorption by the magnetic media. The film thickness is calculated from the absorption value in real time by using a personal computer. The corresponding radial and azimuthal positions for the measurement are determined from a computerized R- theta (rotational-translational) stage. The automated instrument, which rapidly maps the film thickness of particulate and thin-film media, has provided the first full-disk image of film thickness variation. >

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.

Measuring and modeling optical diffraction from subwavelength features

We describe a technique for studying scattering from subwavelength features. A simple scatterometer was developed to measure the scattering from the single-submicrometer, subwavelength features generated with a focused ion beam system. A model that can describe diffraction from subwavelength features with arbitrary profiles is also presented and shown to agree quite well with the experimental measurements. The model is used to demonstrate ways in which the aspect ratios of subwavelength ridges and trenches can be obtained from scattering data and how ridges can be distinguished from trenches over a wide range of aspect ratios. We show that some earlier results of studies on distinguishing pits from particles do not extend to low-aspect-ratio features.

Nonstationary 2-D recursive restoration of images with signal-dependent noise

A nonstationary 2-D recursive image restoration filter that uses a nonstationary mean, nonstationary variance (NMNV) image model and minimizes the local mean square error is developed. The 2-D recursive filter adapts itself to the local image statistics and is able to do space-variant processing. The NMNV image model has a simple dynamic representation which simplifies the filter structure considerably. However, the optimal recursive filter still requires extensive computation. A suboptimal approach that uses a reduced update concept is proposed to reduce the computational efforts. With some modifications, this nonstationary 2-D recursive filter is extended to a class of uncorrelated, signal-dependent noise such as multiplicative noise and Poisson noise. The explicit filter structures and simulation results for images degraded by these signal-dependent noises are presented.