Info on windows (transmittance, U-factor, etc.)
    https://www.commercialwindows.org/vt.php
    
 Solar radiation values definition/relations
    https://www.ammonit.com/en/wind-solar-wissen/solarmessung

Computing DNI/DHI from GHI and solar position: 
    https://plantpredict.com/algorithm/irradiance-radiation/