Irradiance of Horizontal Quartz-Halogen Standard Lamps.
Spectral irradiance calibrations often require that irradiance standard lamps be oriented differently than the normal calibration orientation used at the National Institute of Standards and Technology and at other standards laboratories. For example, in solar measurements the instruments are generally upward viewing, requiring horizontal working standards for minimization of irradiance calibration uncertainties. To develop a working standard for use in a solar ultraviolet intercomparison, NIST determined the irradiance of quartz-halogen lamps operating in the horizontal position, rather than in the customary vertical position. An experimental technique was developed which relied upon equivalent lamps with independent mounts for each orientation and a spectroradiometer with an integrating sphere whose entrance port could be rotated 90° to view either lamp position. The results presented here are limited to 1000 W quartz-halogen type lamps at ultraviolet wavelengths from 280 nm to 400 nm. Sources of uncertainty arose from the lamps, the spectroradiometer, and the lamp alignment, and increased the uncertainty in the irradiance of horizontal lamps by less than a factor of two from that of vertical NIST standard lamps. The irradiance of horizontal lamps was less than that of vertical lamps by approximately 6 % at long wavelengths (400 nm) to as much as 12 % at the shortest wavelengths (280 nm). Using the Wien radiation law, this corresponds to color temperature differences of 15.7 K and 21.3 K for lamps with clear and frosted envelopes, respectively.
National Institute of Standards and Technology, Gaithersburg, MD 20899-0001.
Edward A
EA
Early
National Institute of Standards and Technology, Gaithersburg, MD 20899-0001.
Ambler
A
Thompson
color temperature; lamp orientation; quartz-halogen lamps; spectral irradiance; standard lamps; ultraviolet