The algorithm of overcast estimation is based on approximation of the solar irradiance, taking into account changes of the solar constant, optical mass of the atmosphere, light absorption and dispersion in the atmosphere. Overcast is estimated on a deviation of an approximating curve from the measured values.
The CMP 3 pyranometer is an instrument for measuring the solar irradiance. The thermopile sensor construction measures the solar energy that is received from the total solar spectrum and the whole hemisphere (180 degrees field of view). The output is expressed in Watts per meter square. The CMP 3 pyranometer is designed for continuous indoor and outdoor use. The pyranometer is intended for shortwave global solar radiation measurements in the spectral range from 300 to 2800 nm.
Operating temperature range is −40 ºC to +80 ºC and the stability is better than 1 % per year.