Sandia has suggested using a 5th order polynomial function to represent angular optical losses on short circuit current (Isc) [1].

$f2\left&space;(&space;\theta_{AOI}&space;\right&space;)=b_0+b_1\theta_{AOI}&space;+b_2\theta_{AOI}^2+b_3\theta_{AOI}^3+b_4\theta_{AOI}^4+b_5\theta_{AOI}^5$

where the coefficient vector, $b$, is determined from fitting experimental data measured outdoors.

An example result of this model is shown in Figure 1.  Note that the use of a 5th order polynomial causes a slight concave-up shape at low angles of incidence.  This functional form actually results in $f_{2}$ values slightly greater than one for a portion of this range (10-45 degrees), and values greater than zero for 90 degrees.  These non-physical features are downsides of this model.

Figure 1. Sandia model for incident angle modifier (on Isc)

### References:

[1]. King, D. L., E. E. Boyson and J. A. Kratochvil (2004). Photovoltaic Array Performance Model. Albuquerque, NM, Sandia National Laboratories, SAND2004-3535.