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A Petoscope is an optoelectronic device for detecting small, distant objects such as flying aircraft. The design, as described in 1936,[1] consisted of an instrument with two parallel light paths. In each path was a collimating objective lens, a screen marked with many small, alternating opaque and transparent squares in a chequerboard pattern, and a second concentrating lens focused on a photocell. The two screens were inverted with respect to each other. This caused a small object in the instrument's field of view to produce differing signals in the two photocells, while a large object affected both light paths equally. The difference between the two signals was amplified and used to raise an alarm. At the beginning of World War II, the device was adapted for use in proximity fuses for bombs.[2]

The inventor was Alan S. Fitzgerald of Wynnewood, Pennsylvania, U.S.A.,[3][4] a Research Associate in Electrical Engineering at Swarthmore College.[5]


  1. ^ "Twin 'Eyes' Scan Sky for Planes", Popular Mechanics, Vol. 66, No. 2, ISSN 0032-4558, Hearst Magazines, August 1936
  2. ^ Burns, R.W., "Early history of the proximity fuze (1937-40)", in Science, Measurement and Technology, IEE Proceedings A, pub. May 1993, Volume: 140, Issue: 3, pages 224- 236, ISSN: 0960-7641
  3. ^ Fitzgerald, A. S., "The Petoscope: A New Principle in Photoelectric Applications", Wireless Engineer, Iliffe Electrical Publications, 1937
  4. ^ U.S. Patent 2,016,036,was issued 1 October 1935, USPTO
  5. ^ Persifor Frazer, in Journal of the Franklin Institute, Franklin Institute (Philadelphia, Pa.), 1936