The Wavelength Diversity Of Speckle

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Abstract
Under monochromatic illumination, objects with a scale of roughness or texture grossly on the order of the wavelength appear to a viewer to be speckled. The occurrence of these speckles, which are rapid intensity variations in the image, often leads to a substantial reduction in the apparent resolution of the imaging system. While the exact speckle structure is, in principle, deterministically related to the specific textural detail of a particular object, still its effect is often deleterous since other feature details are often greatly obscured in the speckled image. The distinction between textural and feature details is rather subjective, and so it may be helpful to describe some specific cases. Thus, holograms of typical biological specimens of microscopic size are usually so speckled in reconstruction as to render them impractical. Boundaries, cell walls and the like, are completely lost in the speckle. And one almost never uses a laser illuminator in a microscope due to the attendant speckle and interference fringes, unless there is an overriding need for the mono-chromaticity, e.g., as in flourescence systems. Some feeling for the effect of speckle can be gained if the reader will glance ahead to the illustrations in Figs. 1, 2, and 3. So for many purposes speckle is troublesome and unwanted, that is the position we take in this paper; however, it is worth noting, as an aside, that others have reported useful applications of speckle in metrology and in surface texture analysis.

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