I wrote something like this in an e-mail the other day, and realized that it might be of general interest. Then I realized that I wasn't sure it was correct! In any event, here it is, and I hope it proves edifying, either for me or for someone else.
Ies2rad chooses the shape of the luminous object based on the "luminous opening" in the IES file. It will generate a surface (or two surfaces, one up and one down), a box, a flattened cylinder, or a sphere. Then it chooses one of the mappings for the photometry from source.cal: boxcorr, cylcorr, flatcorr, or corr.
design machine group, architecture department, university of washington
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