# ASHRAE Model

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

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

This function has the benefit of needing only one parameter, $b_{0}$ , but suffers from a discontinuity at $\theta&space;=90^{\circ}$  and low accuracy at high incident angles.  Some authors suggest not applying this function for incident angles where $\theta&space;>&space;80&space;^\circ$  (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.