IRRADIANCE OF HORIZONTAL QUARTZ-HALOGEN STANDARD LAMPS

Spectral irradiance calibrations often require that irradiance standar d lamps be oriented differently than the normal calibration orientatio n used at the National Institute of Standards and Technology and at ot her standards laboratories. For example, in solar measurements the ins truments are generally upward viewing, requiring horizontal working st andards for minimization of irradiance calibration uncertainties. To d evelop a working standard for use in a solar ultraviolet intercomparis on, NIST determined the irradiance of quartz-halogen lamps operating i n the horizontal position, rather than in the customary vertical posit ion. An experimental technique was developed which relied upon equival ent lamps with independent mounts for each orientation and a spectrora diometer with an integrating sphere whose entrance port could be rotat ed 90 degrees to view either lamp position. The results presented here are limited to 1000 W quartz-halogen type lamps at ultraviolet wavele ngths from 280 nm to 400 nm. Sources of uncertainty arose from the lam ps, the spectroradiometer, and the lamp alignment, and increased the u ncertainty in the irradiance of horizontal lamps by less than a factor of two from that of vertical NIST standard lamps. The irradiance of h orizontal lamps was less than that of vertical lamps by approximately 6% at long wavelengths (400 nm) to as much as 12% at the shortest wave lengths (280 nm). Using the Wien radiation law, this corresponds to co lor temperature differences of 15.7 K and 21.3 K for lamps with clear and frosted envelopes, respectively.