Abstract
This paper describes a relatively simple system for detecting in-focus objects. A nonlinear, wide-area photocell with parallel-plate electrodes is vibrated axially behind a lens. When the object is out of focus, the photocell conductance varies at the vibration frequency. When the object is in focus, the photocell conductance signal “folds over” because of the symmetry about the image plane. This “fold over” is detected by electronic circuitry. The scheme is shown to operate over a wide field of view, and the photocell conductance as a function of axial position is given for a number of idealized object patterns. An experimental unit built on these principles is described briefly.
© 1964 Optical Society of America
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