Statistics of the contrast of coherent images.

Contrast optimization, also known as image sharpening, is a method that can be used to estimate phase errors in coherent images. However, the contrast measure of a coherent image is a random variable because of the speckle present in coherent images. The variance of this measure puts a limit on the ability of contrast optimization to focus an image. We derive the probability distribution function of the most common contrast measure, the sum of the pixel intensities raised to a power. These statistics are then verified by a number of speckle simulations and compared with measured statistics from synthetic aperture sonar images. The developed statistics can be used as a tool to understand and improve the method of contrast optimization as well as assess its performance for a given imaging system. They can also be used to predict the effect of certain image processing operations on the contrast.

Noise-free contrast improvement with a low frequency polarizing filter: a practical evaluation.

Contrast improvement in a coherent image enhancement system by very low spatial frequency attenuation is shown, demonstrating possible errors incurred by incorrect attenuation. A novel way to achieve a continuous range of low frequency attenuation is introduced. The variable attenuation is produced by constructing the spatial filter from polarizing material and altering the relative polarization between the object transparency illumination and the spatial filter. This technique gives the observer control over a continuous change in contrast enhancement and greatly increases the versatility of low frequency spatial filtering. Coherent imaging systems are usually contaminated by noise, and a simple way to reduce the noise content in the image is shown.