Diffuse sky radiation has little energy in the shortwave near-infrared region 1000-2800 nm, while the direct beam has significant energy in that region. Therefore, nothing affecting the direct beam total irradiance between 1000 and 2800 nm, such as atmospheric watervapor, affects a shaded pyranometer signal. Consequently, for several different water vapor concentrations, and direct normal irradiances, the same shaded signal is possible from the pyranometer. By varying total precipitable water vapor from 0.5 to 3.5 cm, this “spectral mismatch” effect can be shown to result in differences of about 0.5% in Rs (Myers et al. 2004).