A Full-radiator Lamp designed to replace the Tungsten Strip Lamp as a Pyrometric Standard

Abstract

TUNGSTEN strip lamps are used extensively as standard sources of luminance in photometry and in particular for the calibration of optical pyrometers. They have been shown to be stable within the limits of visual optical pyrometry1 provided they are well designed and certain precautions are taken with the operating conditions such as the ambient temperature, orientation of the plane of the filament with respect to the optic axis, direction of flow of the current, etc. However, when we try to exploit the higher potential accuracy of photoelectric pyrometry it is soon found that the conventional tungsten strip lamp imposes severe limitations on the reproducibility of reading because of the aforementioned defects2. The use of the more sensitive photoelectric method also reveals variations in surface luminance previously undetected3. These variations become more intense as the filament becomes thermally etched, and after prolonged use the crystal growth produces regions of varying reflectivity plainly visible when the filament is cold. At incandescence, however, the consequent variations in luminance are not detectable with the visual optical pyrometer. Scanning such a filament photoelectrically shows variation of the order of 2 per cent and it is also observed that there are differences in the polarization of the light emitted from different parts of the filament. These effects will be discussed in more detail in a paper to be published. In order to overcome these difficulties and provide ourselves with a uniform disk of light independent of surface variations in the filament we are proposing to use a full radiator to replace the tungsten strip in a lamp assembly.