ON THE USE OF INTEGRATING SPHERE REFLECTOMETERS FOR MEASURING THE SOLAR ABSORPTANCE OF SELECTIVE SURFACES

Solar absorptances are often derived from the reflectance values measured with devices which make use of integrating spheres. Instruments of this type can be operated in two different ways, according to whether spectral or integrated reflectances are measured. Though solar absorptances derived from spectral data are inherently more accurate, some commercially available reflectometers are found which are conceived for measuring integrated rather than spectral reflectances. Since these instruments are often provided with radiation sources which do not match the solar spectrum and/or detectors showing a spectrally selective response, they could in principle be affected by errors when dealing with selective surfaces. With the aim of giving an error estimate, the theory of the measurement was analyzed with,reference to some selective samples well characterized for their reflectance properties. The results werethengivenan experimental validation by using an integrating sphere equipped with different sources and detectors.