Abstract

The performance of a given microscope arrangement is usually specified in terms of its resolution limit, in practice the radius of the Airy disk. Merely ensuring that the angular subtense of the objective’s Airy disk is brought by the eyepiece near the resolution limit of the eye does not mean that the performance of the eye–microscope system has been optimized. Targets near the resolution limit of the objective will produce luminance distributions in the image that are of the order of size of the Airy disk but of low contrast. Such targets, as well as larger targets of low contrast, will be detectable with the eye only if the visual contrast threshold is exceeded. The latter is highly dependent on the visual angle and increases sharply in the vicinity of the resolution threshold of the eye. Increasing the angular subtense of the feature at the eye by increase of eyepiece magnification, while decreasing the field of view and luminance, can serve to make visual detection more secure by ensuring that the spatial characteristics of the target match those for which visual contrast sensitivity is highest. Numerical aspects of this phenomenon are considered.

© 1972 Optical Society of America

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