The DISC Model uses empirical relationships between the global and direct clearness indices to estimate the direct beam component.
The user supplies hourly average measured global horizontal data. Computations are based on the solar geometry for the hour and clearness indices.
The DISC model was authored by Dr. E. Maxwell.
A Quasi-Physical Model for Converting Hourly Global Horizontal to Direct Normal Insolation, NREL Technical Report (1987)
An Hourly Diffuse Fraction Model with Correction for Variability and Surface Albedo, Solar Energy (1998)