The Suitability of Vacuum Tungsten Blackbody Lamps as Secondary Standard Sources in Photoelectric Pyrometry

The suitability of vacuum tungsten blackbody lamps as secondary standard sources in photoelectric pyrometry has been investigated. It has been found that although the sources are stable when operated for long periods at temperatures up to 1625 °C they will not maintain calibration when physical strain is introduced into the tungsten tube or supports, for example, by vibration of the lamp. All changes detected in calibration of lamps have been accompanied by changes in their electrical resistances, and therefore this parameter can be used as a monitor of calibration stability. Variations in resistance at the gold point of 0.1% produce significant changes in calibration. The effect of pin temperature on the lamp resistance and spectral radiance temperature are given. When precautions are taken in handling, mounting and storage to minimise the introduction of strain in the lamps the uncertainty in their calibration at the gold point is less than ±0.1°C.