A NEW METHOD OF INFRARED PHOTOGRAPHY

The known methods of infrared photography on sensitized photographic plates and with the use of the Herschel effect make it possible to record only short-wavelength infrared rays. Cerni’s “evaporography” is also suitable for long-wavelength infrared rays. This method is too complicated, however, and has not gone beyond the limits of laboratory experiments.

Heinz (Journ. de Physique VII, 293, 1946) proposes a new method of infrared photography, based on the fact that part of a thin layer of liquid, heated by infrared rays at the place where the image is formed, begins to rise upward, and the liquid circulating at this place leaves traces on the bottom of the cuvette in the form of an accumulation of particles admixed to the given liquid. Fig. 1 shows the arrangement of the experiment. Fig. 2 presents a photograph of the filament of a carbon lamp obtained by this method. Alongside it is given an ordinary photograph. Photographs of the spectrum of a mercury arc, presented in the original communication, are difficult to reproduce, since the lines there are very weak.

Fig. 1. Schematic arrangement of the experiment: A—carbon lamp, L—lens, Q—flat glass cuvette, D—suspension of soot particles in kerosene, B—currents of the heated liquid, C—layer of particles settling on the bottom of the cuvette.

Heinz obtains the best results with a suspension of aluminum powder (particle size \(\sim 0.01\ \mu\)) in amyl alcohol (1 gram per 50 cm\(^3\)) and with a soot–kerosene suspension. The latter is prepared as follows: 100 cm\(^3\) of kerosene (\(d = 0.81\)) is mixed with 15 g of soot. The liquid is heated in an Erlenmeyer flask to boiling and is filtered through hygroscopic cotton at \(60\text{–}80^\circ\). This concentrated suspension can be stored for a long time. For photographing, one takes, for example, a flat cuvette with a glass bottom \(5 \times 5\) cm and pours into it \(\sim 10\) drops of the prepared suspension and enough kerosene so that a layer \(1.2\text{–}1.5\) mm thick is obtained in the cuvette*). By careful stirring, a homogeneous emulsion is obtained. Before use, the poured solution is allowed to stand for several hours, during which the soot particles settle as a thin layer on the bottom. Before photographing, the suspension is shaken again.

*) A layer of this thickness gives the greatest sensitivity of the method.

After this, the image of the object being photographed is projected onto the surface of the liquid. After the exposure, the image formed by the particles of picein on the bottom of the cuvette is illuminated with ordinary light passed through 5–10 cm of water (to eliminate thermal effects), and is photographed on an ordinary plate.

Fig. 2. a—“infrared photograph” of a lamp filament, obtained with the aid of a kerosene–picein mixture; b—ordinary photograph of the same filament.

Heins reports that under ordinary conditions, with exposures of several seconds, the sensitivity of the method is of the same order as that of thermoelectric methods. The use of long exposures (up to 45 sec) makes it possible to register radiation fluxes 20 times weaker than those detectable by the best thermopiles with galvanometers of the highest sensitivity.

A. Ilyina