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Thanks, Andy! Greg Ward also provided the following info, which is worth looking into if you're still interested in representing an IGU as a single polygon (as I am):

The reason the transmittance-to-transmissivity formula looks so complicated is because it takes into account the many internal interreflections that occur to sort out the transmissivity. In this sense, it is not quite the right formula for a multi-paned material, but there is a way to "fake it" if you know the normal reflectance of your system as well as the transmittance.

The file ray/src/cal/cal/trans2.cal contains the following formulae to approximate index of refraction and transmissivity from Rn and Tn:

tn = Tn / (1 - Rn); F = Rn / (1 + tn*tn); n = (1 + sqrt(F)) / (1 - sqrt(F));

"Just use the computed tn and n values for a 'glass' material based on your manufacturer's data, and it should be reasonably close. It would be interesting to compare this approximation to the better one produced by the glaze script or angle-based glazing measurements.

Cheers, -Greg

We will likely implement Greg's trick above in the OpenStudio-to-Radiance translator, for a variety of reasons.

Thanks, Andy! Greg Ward also provided the following info, which is worth looking into if you're still interested in representing an IGU as a single polygon (as I am):

The reason the transmittance-to-transmissivity formula looks so complicated is because it takes into account the many internal interreflections that occur to sort out the transmissivity. In this sense, it is not quite the right formula for a multi-paned material, but there is a way to "fake it" if you know the normal reflectance of your system as well as the transmittance.

The file ray/src/cal/cal/trans2.cal ray/src/cal/cal/trans2.cal contains the following formulae to approximate index of refraction and transmissivity from Rn and Tn:

tn = Tn / (1 - Rn); F = Rn / (1 + tn*tn); n = (1 + sqrt(F)) / (1 - sqrt(F));

"Just use the computed tn and n values for a 'glass' material based on your manufacturer's data, and it should be reasonably close. It would be interesting to compare this approximation to the better one produced by the glaze script or angle-based glazing measurements.

Cheers, -Greg

We will likely implement Greg's trick above in the OpenStudio-to-Radiance translator, for a variety of reasons.

Thanks, Andy! Greg Ward also provided the following info, which is worth looking into if you're still interested in representing an IGU as a single polygon (as I am):

The reason the transmittance-to-transmissivity formula looks so complicated is because it takes into account the many internal interreflections that occur to sort out the transmissivity. In this sense, it is not quite the right formula for a multi-paned material, but there is a way to "fake it" if you know the normal reflectance of your system as well as the transmittance.

The file ray/src/cal/cal/trans2.cal contains the following formulae to approximate index of refraction and transmissivity from Rn and Tn:

tn = Tn / (1 - Rn); F = Rn / (1 + tn*tn); n = (1 + sqrt(F)) / (1 - sqrt(F));

"Just use the computed tn and n values for a 'glass' material based on your manufacturer's data, and it should be reasonably close. It would be interesting to compare this approximation to the better one produced by the glaze script or angle-based glazing measurements.

Cheers, -Greg

We will likely implement Greg's trick above in the OpenStudio-to-Radiance translator, NOTE: See Andy's latest update to his answer to this post for a variety of reasons.

details on this; it's not the magic bullet.