The incidence effect (IAM, for “Incidence Angle Modifier”) corresponds to the decrease of the irradiance really reaching the PV cells’ surface, with respect to irradiance under normal incidence, due to reflections increasing with the incidence angle.

IAM curve

In practice, the following formula gives a good approximation of the IAM performance factor (FIAM), it uses only one parameter (β):
FIAM = 1 – β x (1/Cos i – 1), with i = incidence angle on the plane.
For single-glazed windows, the usually accepted value for β is of the order of 0.1. But in a PV module, the lower interface, in contact with the cell, presents a high refraction index and measurements on real crystalline modules actually indicate a value of β = 0.05.
IAM loss varies case-by-case and is normally included in the main solar energy simulation programs.