Icaroscope

An icaroscope is a telescope-like nonlinear optical device that enables viewing of both very bright and dark objects in the same image simultaneously.^[1] The problem the icaroscope was designed to solve was observing enemy aircraft approaching with the sun behind them, when the bright sun in a clear sky dazzles the observer and masks aircraft near the sun's disc.^[1]^[2] In the icaroscope, the scene is not viewed directly; instead it is briefly projected onto a screen coated with a special phosphor, and this screen is then shown to the viewer.^[2] The specific silver-activated zinc-cadmium sulphide phosphor has a short afterglow even in areas saturated by the full brightness of the sun.^[2] By rapidly exposing the phosphor, allowing it to decay for around 5 ms, and showing it to the viewer, the effect is to attenuate the brightness of the sun's disc by about 500 times, allowing details near it to be clearly seen. The icaroscope repeats this process at a rate of 90 Hz, permitting continuous observation.^[2]

Development of the icaroscope was carried out during the Second World War at the Institute of Optics by Brian O'Brien, Franz Urbach, and other researchers.^[1] The device is named for Icarus, the mythological figure known for flying too close to the sun.^[1]

[1]
Stroud, Carlos (2019). "Chapter 16: Nonlinear Optical Device (Icaroscope) at the Institute of Optics, University of Rochester During the Second World War". In Rhodes, William (ed.). Quantum Photonics: Pioneering Advances and Emerging Applications. Vol. 217. Springer. p. 551. ISBN 978-3-319-98400-1. One such effort involved the development of phosphors that would emit visible light upon excitation by infrared radiation. This project was carried out under the immediate supervision of Franz Urbach ... These phosphors led to the development of various infrared and low-light image converters that were crucial to the U.S. war effort. But an additional application used the nonlinear transfer characteristics of these phosphors ... This device became known as the Icaroscope, named after the Greek tragic hero Icarus. Ironically, this same group also developed an instrument called the "Seebackascope" which allowed an attacking pilot to accurately align his airplane along a path coming exactly from the direction of the sun.
[2]
O'Brien, Brian (1946). "A brightness limiting phosphor telescope with selective action on the brighter portions of an image". Journal of the Optical Society of America. 36 (12): 709. A telescope-like device with selective brightness limiting properties ... makes possible simultaneous observation of the sun and surrounding sky without being dazzled by the high brightness of the solar disk itself. An objective, especially designed to reduce troublesome flare, forms an image on a transparent phosphor screen exhibiting short period afterglow. The screen is viewed through an eyepiece and erecting system. Between objective and screen and between screen and eyepiece are two sector disk shutters rigidly mounted on the same motor shaft, but with the sector openings out of phase. Thus the screen is not visible while being illuminated, but is viewed a half-cycle later, after the illumination has ceased. This is repeated at the rate of 90 cycles per second ... the phosphor is especially chosen for short afterglow with saturation ... a silver activated zinc-cadmium sulphide having proven most satisfactory. As typical of icaroscope performance the rendition of all object brightnesses up to that of the sky at the sun's limb (object brightness approx. 50 lamberts) will give substantially normal contrast, yet the phosphor image of the solar disk (object brightness approx. 5 × 10⁵ lamberts) will appear only about 20 times brighter than that of the surrounding sky.

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