A NEW METHOD FOR INVESTIGATING TEMPERATURE FIELDS*)

The yield of luminescence, i.e., the ratio of the total light output of a luminophore to the total amount of supplied energy, depends on the temperature of the luminophore (moreover, above a certain temperature—for different phosphors different—the luminescence yield decreases as the temperature rises). Recently a report appeared on the use of this effect for recording and photographing temperature fields (the authors call this method “thermography”). For this purpose, phosphors especially sensitive to temperature effects are needed: when the temperature is raised by \(1^\circ\text{C}\), the change in the brightness of the glow must be at least 5%. The author of the paper under review used chiefly ZnCdS phosphors activated with Ag and a small amount of Ni (of the order of \(10^{-4}\%\)). The nickel content determines its temperature range, in which the given phosphor can be used. Investigations showed that, for such luminophores, the luminescence yield is comparatively small, so that thermography with the aid of luminophores requires very powerful sources of ultraviolet rays.

Two methods of thermography have been developed: contact and projection. In the first method, the luminophore is applied in the form of a powder to the surface under investigation, and this surface is uniformly illuminated with ultraviolet light. The distribution over the surface under study of the brightness of the visible luminescent glow gives a visual picture of the temperature distribution. If quantitative analysis is required, the resulting glow is photographed and, by measuring the density of blackening of individual areas of the photographic plate, the temperature distribution is found. With ordinary photographic materials, a temperature difference of \(1^\circ\text{C}\) corresponds to a difference in blackening densities of about 0.08, which is easily detected with an ordinary photometer. Fig. 1 shows the temperature distribution obtained in this way—

) F. Urbach, Photographical Journal 90B*, 109 (1950).

temperatures on a thin phosphorescent screen, against which a jet of compressed air strikes. In the dark areas of the photograph the temperature is higher (compression of the jet, friction), in the light areas it is lower (adiabatic expansion). After photographing the jet the screen was graded. For this purpose it was applied to a body of known temperature and then photographed (at several temperatures). The densities of blackening of the negatives in these photographs were measured, and from them a graph was constructed of the dependence of the density of blackening in the photograph of the screen on the temperature of the latter. Knowing such a characteristic of the screen, one can take photograph 1 and construct a network of isotherms (Fig. 2). For this purpose, next to the negative under study there is placed a stepped optical wedge—a plate with bands of different, but previously known, blackening density. Both negatives—the one under study and the optical wedge—are printed on one sheet of high-contrast photographic paper. Such contact printing is performed several times, with different exposure times. Obviously, the contour of the image corresponds to a line of equal blackening density of the negative, i.e., it represents an isotherm. It remains to determine what temperature each of these isotherms corresponds to. This can be done by means of the optical wedge described above. Noting which least transparent band is still visible on the photographic paper at a given exposure, we thereby determine the density of the negative along the image contour. Using the graph for the density of the negative as a function of temperature, we find from this the temperature along the contour. The resulting network of isotherms is shown in Fig. 2; the numbers on the isotherms indicate the temperature in degrees Celsius.

Fig. 1

Fig. 2.

It is possible not to apply phosphor to the body under investigation, but to lay on it a special luminescent tape. Such tapes are calibrated beforehand at a prescribed intensity of ultraviolet irradiation. The sensitivity of the method is about \(0.2^\circ\text{C}\).

In another variant of thermography—the projection method—the luminescent screen is placed at some distance from the body under investigation.

and is heated by the latter’s thermal radiation. Fig. 3 shows the arrangement of one of the early installations of this type. A teapot filled with water, heated to \(100^\circ\) C, is photographed so that the maximum intensity of its radiation falls at wavelengths of about 7 microns. The image of the object under study is focused on a screen by means of a mirror. The more highly heated parts of the teapot emit more energy than the colder parts. The corresponding places on the screen heat up and therefore seem dark; on the contrary, the colder parts of the object appear light on the screen. The image formed on the luminescent screen is photographed with an ordinary camera. To protect against scattered ultraviolet light, a filter absorbing ultraviolet is placed in front of the objective.

Fig. 3.

In thermography, high-contrast paper is used, and any nonuniformity in the illumination of the screen caused not by temperature but by other factors has a strong effect on the quality of the image. Therefore a special filter—a “mask”—is used, the negative photograph of the screen obtained in the absence of heated objects. This filter is placed over the main negative during printing. The photographic paper records the difference in the blackening of the filter and the negative, and therefore irregularities of the screen are not noticeable in the finished print. One and the same filter—“mask”—serves for printing a very large number of pictures. In this way a photograph of a human hand has been obtained in Fig. 4. It was taken using the radiation of the hand itself at room temperature.

In developing phosphors for thermography, a substantial difficulty was encountered. If a phosphor is highly sensitive to temperature effects, then as the temperature rises its brightness quickly falls and it becomes, in this way, only a meter of temperature. But here an interesting property of some phosphors comes to the rescue. The maximum on the curve of the dependence of luminescence output on temperature, as the intensity of excitation increases, shifts toward higher temperatures. By varying the intensity of the exciting ultraviolet light, one can vary the range of operating temperatures of a given phosphor, causing it to respond with the same maximum sensitivity at higher temperatures. The method of thermography requires improvement in two directions: increasing sensitivity and obtaining higher resolving power. Sensitivity can be increased by incre-

increasing the aperture of the diaphragms, using broad beams. But then the image of the object on the screen becomes blurred and the resolving power decreases. Another cause of image blurring is no longer optical, but purely thermal—the thermal conductivity of the screen. The resolving power is also limited by yet another circumstance, analogous to noise in electronics: the different sizes of the phosphor grains and the nonuniformity of their distribution over the screen; however, the “mask” filtering method overcomes this difficulty to some extent.

Fig. 4.

In comparison with thermoelectric methods for measuring temperature and determining the intensity of infrared rays, the thermographic method has lower sensitivity and, moreover, greater inertia. The inertia of the method depends on the thickness and material of the screen. For the thinnest of the screens used, the time required for formation of the image is on the order of two seconds. A significant advantage of the method is the possibility of simultaneously displaying a considerable area of the object, with the entire picture becoming visible at once, without any kind of complicated and expensive scanning, as, for example, in radiolocation (radio vision). A very substantial study of the spectral characteristics of the method for purposes of infrared spectroscopy remains to be carried out.

M. G.