The physical model for the incident angle modifier is based on Snell’s and Bougher’s laws, and was published by De Soto et al. (2006). Our presentation here includes correction to a few errors present in that paper. The first step is to calculate the angle of refraction () using Snell’s law.
, where is the index of refraction of the cover glass and the 1 in the numerator is the index of refraction of the air.
The incident angle modifier at an angle, , is a ratio between the transmittance at that angle and the transmittance when normal to the sun :
A good approximation of the transmittance of the module cover is given by:
where is the glazing extinction coefficient (1/meters) and is the glazing thickness (meters).
can be determined by .
DeSoto et. al lists the following typical input parameters for PV modules:
The resulting IAM function is plotted below:
De Soto, W., S. A. Klein and W. A. Beckman (2006). “Improvement and validation of a model for photovoltaic array performance.” Solar Energy 80(1): 78-88.