Gilbert V. April

Rotation-invariant digital pattern recognition using circular harmonic expansion

A new method has been developed for rotation-invariant pattern recognition. One component of the circular harmonic expansion of the target is used in the preparation of the reference. Correlations between the input and reference objects are accomplished by FFT and multiplication in the frequency domain. In an experience with targets from an image with 192 x 192 pixels, target orientations were detected with an accuracy of approximately 0.1 degrees . This method is also suitable for optical implementation.

Properties of speckle integrated with a finite aperture and logarithmically transformed

When image plane speckle intensity integrated over a finite aperture is submitted to a logarithmic transformation, the noise becomes additive and signal independent. The first- and second-order moments of the probability distribution are derived. It is found that the logarithm of speckle noise approaches a normal distribution much faster than speckle intensity. The properties of speckle noise are different from those of film-grain noise; for example, neither Nutting’s law nor Selwyn’s law is satisfied by speckle.

Speckle statistics in four-look synthetic aperture radar imagery

Speckle noise in high-resolution synthetic aperture radar (SAR) images is generated by the coherent processing of radar signals and exists in all types of coherent imaging systems. We study the statistical properties of speckle noise in a four-look SEASAT SAR image concentrating on homogeneous areas of the image. Experimental determination of the signal-to-noise ratio and ofthe probability density of the recorded intensity is performed showing evidence of correlation between underlying looks. Based on a model taking into account the weighting and possible correlation between the underlying looks, a theoretical expression for the effective number of independent looks is proposed and the corresponding probability density function of the intensity is compared to the experimentally extracted histograms of intensity fluctuations.

Recording holograms with diagonal coding on binary spatial light modulators for pattern recognition

The performance of computer-generated holographic matched filters recorded on 128 x 128 pixel supports was studied experimentally, with the objective of recording holograms on spatial light modulators such as the Semetex SIGHT-MOD for rotation-invariant pattern recognition. A new method of diagonal coding is introduced. This method yields good results, as it allows a more accurate phase quantization. Experimental results from computer simulations show that high speed rotation-invariant recognition of simple shapes is possible with such holograms.

Information content of images degraded by speckle noise

After a homomorphic transformation has been used to transform speckle noise into additive signal-independent noise, classic techniques are used to evaluate the number of discernible gray levels and the information capacity of images degraded by fully developed speckle noise. The probability distributions that we developed earlier are used to evaluate the required probabilities of error. Expressions are derived for square and for circular apertures. The information capacity is found to have a maximum of about 0.2 bits per speckle. The spatial frequency resolution required to obtain a given signal-to-noise ratio by spatial integration is evaluated.

Surface roughness characterization of dental fillings: a diffractive analysis

The large number of new materials such as amalgams and the variety of techniques for finishing and polishing in operative dentistry has stimulated interest in simple, nondestructive methods of surface roughness evaluation. We studied an optical method based on the scattering of reflected coherent light on prepared samples of composite resins submitted to different surface treatments. The method should be able to measure the degree of flatness of the samples, thus enabling a classification procedure according to a figure of merit to be defined. The diffraction properties of such moderately rough surfaces has been correlated with mechanical profilometer measurements of the residual granular structure after polishing. Different surface treatments of composite resins result in distinctive levels of surface flatness, and it is shown that a relation between the intensity of the normalized specular reflection of a beam of coherent light and the rms surface roughness can be established for characterization purposes.

Surface roughness measurements of dental fillings by optical methods

The large amount of new materials like amalgams and techniques of finishing and polishing in operative dentistry has stimulated interest for simple non-destructive methods of surface roughness evaluation. We studied an optical method based on the scattering of coherent light upon reflection on prepared samples of composite resins submitted to different surface treatments. The method should be able to discriminate the degree of flatness for the samples thus enabling a classification procedure according to a figure of merit to be defined. The diffraction properties of such moderately rough surfaces has been correlated with mechanical profilometric measurements of the residual granular structure after polishing. Different surface treatments of composite resins result in distinctive levels of surface flatness, and it is shown that a relation between the intensity of the normalized specular reflection of a beam of coherent light and the rms surface roughness can be established for characterization purposes.

Improved 2x2 pixel computer-generated holographic matched filters for spatial light modulators

An improved computer-generated hologram recording technique for 12x128-pixel binary spatial light modulators is presented. The method is based on a diagonal coding previously introduced, but instead of using 4x4 supercells as in the previous case, the size of the supercell has been reduced to 2x2 pixels. Experimental results from computer simulations show that high-speed rotation-invariant recognition of simple shapes is possible with such holograms.