Determine the incidence angle modifier using the ASHRAE transmission model.
ashraeiam calculates the incidence angle modifier as developed in , and adopted by ASHRAE (American Society of Heating, Refrigeration, and Air Conditioning Engineers) . The model has been used by model programs such as PVSyst .
Note: For incident angles near 90 degrees, this model has a discontinuity which has been addressed in this function.
aoi (numeric) – The angle of incidence between the module normal vector and the sun-beam vector in degrees. Angles of nan will result in nan.
b (float, default 0.05) – A parameter to adjust the modifier as a function of angle of incidence. Typical values are on the order of 0.05 .
IAM (numeric) – The incident angle modifier calculated as 1-b*(sec(aoi)-1) as described in [2,3].
Returns zeros for all abs(aoi) >= 90 and for all IAM values that would be less than 0.
 Souka A.F., Safwat H.H., “Determination of the optimum orientations for the double exposure flat-plate collector and its reflections”. Solar Energy vol .10, pp 170-174. 1966.
 ASHRAE standard 93-77
 PVsyst Contextual Help. http://files.pvsyst.com/help/index.html?iam_loss.htm retrieved on September 10, 2012