Abstract
Optical pyrometry is extended to the temperature range between 400°C and 500°C, by using a dark-adapted eye as the photometric comparison instrument. It is demonstrated experimentally and theoretically that in this range of temperature the accuracy of the scotopic optical pyrometry is not lower than that of pyrometry at higher temperatures where photopic vision is used. By means of a specially constructed extended-field blackbody, it is ascertained that the lowest temperature of the blackbody perceived by the scotopic eye as light emitting, is 391.5°C. This temperature, while reproducible within a few degrees by the same observer, seems to be subjected to a systematic shift with the age of the observer. In accordance with theoretical considerations, the advantage of a blue filter in optical pyrometry is advocated instead of the usual red filter.
© 1957 Optical Society of America
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