Electro-optical Pyrometer*)

Described below is an optical disappearing-filament pyrometer that uses electron-optical conversion of infrared radiation into visible radiation. Such a pyrometer makes it possible to measure temperatures in the range \(350—700^\circ\mathrm{C}\), whereas ordinary methods of optical pyrometry have a lower limit of about \(650^\circ\mathrm{C}\).

*) C. R. Barber and E. C. Pyatt, J. Sci. Instr. 27, 4 (1950).

The electro-optical converter is made in the form of a cylinder of Pyrex glass with flat end windows. On one window there is a silver-cesium photocathode treated with oxygen; parallel to it, at a distance of 0.5 cm, is a fluorescent willemite screen, observed with the aid of a magnifying eyepiece through the second window. Electrons released from the cathode under the action of radiation are accelerated by a voltage of 6000 V and form on the screen a visible image corresponding to the image on the cathode. The photocathode is sensitive to the near infrared radiation up to 1.3 μ, i.e. far beyond the limit of sensitivity of the eye.

To supply the electro-optical converter, a vibrator converter is used, operating from a 12 V battery and consuming 350 ma.

The optical arrangement of the pyrometer is shown in the figure. A narrow strip of paper 1 mm wide, illuminated by an automobile bulb (12 V, 24 W), replaces in this arrangement the filament in an ordinary optical pyrometer with a disappearing filament. In view of the fact that only part of the lamp radiation is reflected from the strip of paper, the temperature of the lamp filament is considerably higher than that being measured. The latter circumstance is very important, since at a low temperature of the filament the ambient temperature noticeably affects it and the calibration of the pyrometer becomes unreliable. The image of the illuminated strip of paper, observed with the aid of the electro-optical converter, is superimposed on the image of the object. As in an ordinary optical pyrometer, the current through the bulb is varied until the strip disappears against the background of the object. To reduce the amount of scattered light, the strip of paper is illuminated through a narrow slit. A red filter improves the color matching of the strip and the object. Without a filter, the strip of paper appears yellow-green against the background of the object, since, in addition to the electronic image, the direct image of the strip reaches the fluorescent screen, as a result of which it is impossible to make the strip disappear.

Calibration of the pyrometer was carried out with the aid of a black body, whose temperature was measured with a platinum—platinum-rhodium thermocouple.

When measuring temperatures above 500°C, the instrument can be used in an un-darkened room. Below 500°C it is possible to work in a partially lit room; however, for an observer exposed to daylight, 1–2 minutes of dark adaptation are required. At temperatures below 420°C the image becomes

dim, and measurements require a darkened room and prolonged dark adaptation. At \(350^\circ\mathrm{C}\), 5–10 minutes are required in order to see the image, and 15–20 minutes before satisfactory readings can be obtained. At any temperature, however, the maximum spread of readings for six measurements did not exceed \(5^\circ\mathrm{C}\), and the mean of the readings of two observers differed by less than \(1^\circ\mathrm{C}\).

I. L.