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.”
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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.
