Pneumatic Detector of Infrared Radiation

An infrared-radiation detector based on measuring the pressure of a gas in an enclosed space was first proposed by Hayes and was successfully used by Terenin and Yaroslavskii² in their infrared spectrometer. The principle of operation of such a detector is as follows: when infrared radiation illuminates a blackened porous plant powder, a certain quantity of the gas adsorbed on the powder is desorbed, as a result of which the gas pressure in the enclosed space rises slightly. The changes in gas pressure are converted by means of a microphone into electrical oscillations, which are then amplified in the usual way.

G. A. Zahl and M. J. Golay³ constructed, on the same principle, a pneumatic detector with an optical method of observation instead of an electrical one. However, a detector of a somewhat modified design proved to be more sensitive, namely the following. Inside a massive metal block a channel was drilled, closed at one end by a window transparent to infrared rays, and at the other by an optically polished glass wedge. The radiation, having passed through the window, fell on a blackened collodion film 500 Å thick; in front of the wedge, at a distance of several microns from it, there was placed a second, identical but unblackened film, approximately 300 Å thick. The reflection coefficient of this film for the green mercury line 5461 Å is of the order of 4%, i.e., the same as the reflection coefficient of glass with a refractive index of 1.5. Thus, the system consisting of the glass wedge and the film, when illuminated by the green Hg line, should give a sharp interference pattern. The mechanism of action of the detector is as follows: infrared radiation absorbed by the blackened film slightly heats the air in the chamber. As a result of the resulting increase in pressure, the transparent film is deformed. The first result of this is a change in the intensity of illumination of the film; then a system of curved interference lines appears and, under continued thermal action, Newton’s rings.

For practical use an instrument was built consisting of 61 cells of the type described. The instrument was placed at the focus of a 60-inch parabolic searchlight mirror. The approach of a radiating object is detected by the appearance of Newton’s rings. For example, an airplane at a short distance gives 20 or more Newton’s rings.

The instrument described was built on assignment from a military agency for detecting airplanes. It turned out, however, that it is considerably inferior to radar. In particular, the authors state that rain, fog, or snow limit the action of any thermal device for detecting airplanes.

The authors promise to describe scientific applications of the pneumatic detector described in another article.

E. Shpol’skii

Cited Literature

1. H. V. Hayes, Rev. Sci. Justr 7, 202 (1936).
2. A. Terenin and N. Yaroslavskii, Izvestiya AN SSSR, physical series.
3. Harold A. Zahl and Marcel J. E. Golay, Rev. Sci. Instr. 17, 511 (1946).