A NEW INFRARED RAY RECEIVER

The journal Optika1 describes a new infrared ray receiver which, according to the author, has very low inertia and receives, without attenuation, signals modulated at a frequency of 20,000 Hz. The receiver described is a 0.01-mm-thick tungsten ribbon placed in a pumped-out bulb with a window made of KRS-5 crystal (diameter 40 mm, thickness 3 mm). The tungsten ribbon is heated by an electric current to a high temperature (2000° K). The radiation of the ribbon is collected by a lens ($f = 20$ mm, diameter 25 mm) on a selenium vacuum photocell. Infrared rays from the radiation source (a black body at 331° K; $\lambda_{\max} = 8.1\,\mu$; radiation density 14.6 W/cm²; aperture 0.02 cm²) were collected by a front-silvered mirror ($f = 120$ mm, diameter 60 mm) onto a diaphragm 2 mm in diameter. With the aid of a second similar mirror the diaphragm was projected, in a ratio of 1:1, onto the incandescent tungsten ribbon. The angle between the incident and exiting rays was about 35°. A modulating device was placed in front of the diaphragm.

Under the action of infrared rays, the surface temperature of the tungsten ribbon rises (under the experimental conditions by approximately 0.5°); correspondingly, its radiation increases, and consequently so does the photocurrent, which is fed to an amplifier tuned in resonance with the frequency of modulation of the infrared rays. The constant component of the photocurrent is cut off, and the magnitude of the current at the output of the amplifier, measured by a galvanometer, will therefore be proportional to the intensity of the flux of infrared rays. The author gives the following data characterizing the new receiver: the energy flux on the mirror was $8.7 \cdot 10^{-3}$ W; of these rays, $1.4 \cdot 10^{-3}$ W was absorbed by the ribbon, as a result of which its temperature rose by 0.58° C. The flux of visible radiation from the portion of the ribbon on which the infrared rays were focused was 8.0 W, and the energy of the modulated radiation was $1.4 \cdot 10^{-2}$ W. Correspondingly, a radiation flux of $5.5 \cdot 10^{-2}$ W fell on the photocell, and the modulated—

... — \(9.7\cdot 10^{-6}\) W; the latter produced in the photocell a current of \(1.7\cdot 10^{-7}\) A. The author considers that, with this photocell, the sensitivity threshold of the instrument is \(3.6\cdot 10^{-4}\) W. The highest permissible modulation frequency may be estimated from the following considerations. Infrared radiation penetrates into a tungsten strip to \(10^{-5}\) cm. Taking the density of tungsten \(\gamma=19\ \text{g}/\text{cm}^3\) and the specific heat \(c=0.008\ \text{W}\cdot\text{s}/\text{g}\cdot\text{deg}\), we find from the equation for thermal modulation \(f_{\max}=A/4K\), where \(A\) is the specific radiation \(=63\ \text{W}/\text{cm}^2\) and \(K=\gamma\cdot c\cdot d\); \(f_{\max}=10^7\) Hz. It is interesting to note that in this instrument, unlike fluorescence and phosphorescence, there is a conversion of long-wave radiation into shorter-wave radiation. The author indicates that the increase in the surface temperature of the strip can be observed not by a change in its visible radiation, but by a change in the thermocurrent. It may be expected that in this case the threshold sensitivity will be of the order of \(1\cdot 10^{-6}\) W.

K. Vulfson

1. P. E. Weber, Optik 6, 290 (1950). ↩