Autocorrelation of Diatoms as a Function of Depth of Focus

Abstract

The use of matched spatial filters in the identification of diatoms on 35mm transparencies has been previously reported [1,2]. In these papers the diatoms were said to play an important role in water pollution monitoring. Also that their identification by humans requires a trained taxonomist and is a tedious process. While the use of spatial filtering has been shown to work, one would like to avoid, if possible, the use of transparencies as input to the hybrid optical processor and, instead, work directly with the microscope slides. This method, however, introduces new problems. One problem being speckle. That is, the coherent illumination of a microscope slide introduces a speckle-noise background that degrades the diatom image. Preliminary results on the speckle problem have already been obtained and reported [3]. Another problem is that of depth of focus. A study of this problem and how the autocorrelation signal changes as a function of depth of focus is now presented.