AUTOMATED MULTIFILTER ROTATING SHADOW-BAND RADIOMETER - AN INSTRUMENTFOR OPTICAL DEPTH AND RADIATION MEASUREMENTS

The multifilter rotating shadow-band radiometer is a ground-based inst rument that uses independent interference-filter-photodiode detectors and the automated rotating shadow-band technique to make spectrally re solved measurements at seven wavelength passbands (chosen at the time of manufacture between 350 nm and 1.7 mum) of direct-normal, total-hor izontal, and diffuse-horizontal irradiances. This instrument achieves an accuracy in direct-normal spectral irradiance comparable with that of tracking radiometers, and it is more accurate than conventional ins truments for the determination of the diffuse and total-horizontal spe ctral irradiances because the angular acceptance function of the instr ument closely approximates the ideal cosine response, and because the measured direct-normal component can be corrected for the remaining an gular acceptance error. The three irradiance components are measured w ith the same detector for a given wavelength. Together with the automa ted shadow-band technique, this guarantees that the calibration coeffi cients are identical for each, thus reducing errors when one compares them (as opposed to measurements made with independent instruments). O ne can use the direct-normal component observations for Langley analys is to obtain depths and to provide an ongoing calibration against the solar constant by extrapolation to zero air mass. Thus the long-term s tability of all three measured components can be tied to the solar con stant by an analysis of the routinely collected data.