Calibration of incandescent lamps for spectral irradiance by means of absolute radiometers.

A method for calibrating incandescent lamps for spectral irradiance by means of absolute radiometers is described in which a secondary radiometer is calibrated spectrally against absolute radiometers and then used in conjunction with a series of filters to calibrate the lamps. Considering both narrowband and wideband filters, an extensive mathematical error analysis is performed. The use of narrowband filters (20-25-nm halfwidth) is found to be advantageous because very little information is required on the spectral distribution of the lamp being measured. The most serious source of error is a wavelength shift in the measured spectral transmittances of the filters, especially at shorter avelengths; for example, at 400 nm, a wavelength shift error of 1 nm can cause an error approaching 3%. It is estimated that the accuracy of spectral irradiance measurements made using the method described here will vary between +/-1 and +/-0.5% from ~350 to 800 nm. Measurements on 500-W quartz-bromine spectral irradiance standards are described. With such lamps, only four or five narrowband filters are required to cover the spectral range from the near UV to the near IR. The measured and calibration values agreed to ~ +/-0.5% on average with a maximum difference of ~1%.