THE HERSCHEL EFFECT AND INDIRECT METHODS OF INFRARED PHOTOGRAPHY
There are five known methods for the photography of the infrared region of the spectrum. Two of these are direct, and three are indirect. In the indirect methods the records are made on a surface which is photographed later on a normal photographic material. The chief direct method consists in the use of photographic emulsions specially prepared, or sensitized to the infrared by means of dyes. The use of such sensitizing dyes has been discussed in the preceding chapter. The second direct method, which was chronologically the second to be used for infrared recording, depends on the fact that exposure to infrared is able to destroy the effect of a previous exposure to light of shorter wavelength. This is the so-called "Herschel effect."
Of the indirect methods, one, which was the first to be employed in recording the infrared, depends on the volatilization of a thin layer of material when it is warmed by exposure to the infrared. Another makes use of the ability of the infrared to extinguish the glow of a phosphorescent screen which has been excited by exposure to light of shorter wavelength. The third and most recent method depends on the use of the so-called electron-image tube.
Although the second direct and the first two indirect methods are of little value for photographing in the region of the spectrum to which photographic plates can be made to respond--being much less convenient and less sensitive--they do permit photography of wavelengths longer than those which can be recorded by specially sensitized plates. It will not be out of place, therefore, to discuss them somewhat in detail. Photography of the electron-image tube is of considerable interest. It permits pho