Abstract
A reciprocal-imaging optical system for monitoring the position of a specular reflecting surface (e.g., the back surface of a photographic film) is described. The technique involves imaging an illuminated pattern onto the reflecting surface, and reimaging the reflection back through the same optical system onto the original pattern. If the first image is in focus on the reflecting surface, then by optical reciprocity, the returned image is congruent with and superimposed upon the original source pattern. As the reflecting surface moves out of the image plane, some of the returned light falls beyond the input pattern and is intercepted by the monitoring system. Thus, intercepted light is nominally zero with the mirror in the image plane, and increases with displacement in either direction away from focus. The case of a uniformly illuminated circular source pattern imaged by a circular lens is analyzed, and the intercepted light as a function of mirror position is determined both analytically and experimentally. The critical parameters in the design of the system are identified and the effects of optical misalignment are discussed.
© 1968 Optical Society of America
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