MTF of a Gap*

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Abstract
Subject to reasonable assumptions of geometrical optics, the point spread function is calculated for a point source in a plane radiating to a nearby parallel plane. The calculation includes the case in which the gap between the planes attenuates the signal. The modulation-transfer function (MTF) is calculated from the point-spread function. If the source is lambertian, and the receiving plane (e.g., photographic emulsion) absorbs as a more or less typical diffuse absorber, and there is a response which can be expressed as a linear function of the absorbed input, the MTF can be expressed as an integral that can be evaluated numerically. The computed curves are normalized in terms of the gap width, which is taken to be unity. The method may be applicable to a fairly wide class of image-evaluation problems.