Absolute solar transmittance interferometer for ground-based measurements.

The absolute solar transmittance interferometer measures absolute solar radiance at the Earth's surface. The instrument is based on a Fourier-transform spectrometer that utilizes a liquid-nitrogen-cooled InSb detector and appropriate optical bandpass filters. The recorded solar spectra are calibrated against National Institute of Standards and Technology traceable lamps and a blackbody source. The spectral range addressed by this instrument is from 1950 to 10100 cm(-1) at a resolution of 2 cm(-1). The optical design of the instrument and the experimental methods are discussed. A discussion of the uncertainties involving the instrument and the calibration sources is presented. Initial measurements from several sites are compared with atmospheric model calculations.