ASHRAE Model

The simplest approach to estimating $IAM_{b}\left ( \theta \right )$ 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.”

$IAM_{B}=1-b_{0}\left ( \frac{1}{\cos \left ( \theta \right )} -1\right )$

This function has the benefit of needing only one parameter, $b_{0}$ , but suffers from a discontinuity at $\theta =90^{\circ}$ and low accuracy at high incident angles. Some authors suggest not applying this function for incident angles where $\theta > 80 ^\circ$ (Abella et al., 2003). Figure 1 compares ASHRAE model results for two values of $b_{0}$ .

Figure 1. IAM Model Results for the ASHRAE model.