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What is to our knowledge a novel infrared thermometer (IRT) for remote measurement of the temperature rise (-20-100 K) above the variable ambient (270-320 K) of a distant object is described. A radiation-balancing method is successfully extended to the near-ambient temperature range by variation of the temperature of a built-in blackbody, until the radiation from it equals the radiation from the source, so that its temperature is proportional to that of the source. Another feature believed to be novel is simplifying the design by elimination of the need for cooling the blackbody for subambient temperature range by use of a second blackbody, strategically located, which is heated to achieve radiation balance. Detailed theoretical analysis is given, showing that the IRT can measure remotely the total emissivity or even the electric current or voltage. Resistive inserts are proposed for improving the accuracy of current measurement. A method is proposed for simultaneous remote measurement of absolute temperature and emissivity by variation of the heating current and the aperture of the blackbody for radiation balancing in two bands so that prior knowledge of the object's emissivity is not needed.

For radiometric or photometric instruments using collecting optics, the étendue or collecting efficiency varies with distance for near objects, causing significant errors. Variation in effective collecting area as well as in solid angle with distance has to be considered. Errors for a thin lens and a Cassegrainian system are compared. Moving the secondary for focusing greatly reduces this variation. The ratio of primary to secondary focal length also plays an important role. Proper choice of this ratio can minimize the variation in the field of view, improving the depth of focus. Calculations are useful for extending the range of instruments or for calibration by near sources.