# Reflection and Refraction

This routine computes losses due to optical effects at the module level, namely reflection and refraction losses on the sunny-side glass superstrate. This is known as the incidence angle modifier (IAM). Three models are supported: *Sandia, ASHRAE, and Tabular*.

# Sandia IAM

This model computes the incidence angle modifier coefficients based on the Sandia method.

## Inputs

## Outputs

## Algorithm

*Θ*is treated here. Furthermore, it was observed that the best fit of the IAM coefficients for both FSLR anti-reflective coating (ARC) and regular non-ARC modules shows an oscillation below incidence angles of 34°, as shown in

_{Eff,D}**Figure 32**.

### Figure 32. Comparison of Custom IAM Profile with Sandia Polynomial, Showing Oscillation for Incidence Angles Lower than 34°

To avoid this artifact, the polynomial is clipped to force the IAM to unity, as follows:

## Reference

King, D. L., Kratochvil, J.A., Boyson, W.L., Measuring Solar Spectral and Angle-of-Incidence Effects on Photovoltaic Modules and Solar Irradiance Sensors. Sandia National Laboratories, Albuquerque, NM, September 1997.

# ASHRAE Direct Beam IAM

This model uses the incidence angle modifier coefficients based on the ASHRAE parameterization.

## Inputs

## Outputs

## Algorithm (Direct Beam Component)

1.) Compute the direct beam incidence angle attenuation coefficient as a function of the incidence angle.

# Tabular IAM

This model uses incidence angle modifier coefficients defined in a table in the Module file.

## Inputs

## Outputs

## Algorithm (Direct Beam Component)

1.) Calculate the direct beam incidence angle modifier by doing a cubic spline interpolation on IAM tabular values.

*μ*is greater than 1, set it to 1.

_{IAM,B}