Version 6.00 of SCATMECH added Rigorous Coupled Wave (RCW) analysis of gratings to its suite of capabilities, through the classes RCW_Model and Grating. Because gratings inherently have BRDFs that behave like delta functions, and because delta functions cannot be numerically integrated, the class RCW_BRDF_Model was created, which casts the theory into a form acceptable to MIST. In short, it spreads each of the diffraction peaks into a finite cone with a half-angle given by the parameter alpha. Version 7.00 of SCATMECH added RCW code for cross gratings, though the classes CrossRCW_Model and CrossGrating. In a similar fashion, the class CrossRCW_BRDF_Model was created to allow its use by MIST.
To calculate diffraction efficiency using MIST, one can start by examining the example code grating.mist. In order to detect a specific diffraction order, we need to calculate at what angles it will occur:
It then calculates the diffraction efficiency by evaluating the effective BRDF in the diffracted direction (given by thetas and phis), correcting for the solid angle over which the light is spread:
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Latest MIST Version: 4.10 (October 2017)