The simplest approach to estimating was developed by Souka and Safat (1966) and later adopted by the American Society of Heating, Refrigeration, and Air Conditioning (ASHRAE), and is known as the “ASHRAE incidence modifier.”
This function has the benefit of needing only one parameter, , but suffers from a discontinuity at and low accuracy at high incident angles. Some authors suggest not applying this function for incident angles where (Abella et al., 2003). Figure 1 compares ASHRAE model results for two values of .
Figure 1. IAM Model Results for the ASHRAE model.