Glazing manufacturers and IGU vendors generally provide visible light transmittance of their wares, but Radiance takes transmissivity as input to its "glass" and "trans" materials. The formula typically used for converting transmittance (Tn) to transmissivity (tn) as published in "Rendering with Radiance" (among other places) looks like this:
tn = (sqrt(.8402528435+.0072522239*Tn*Tn)-.9166530661)/.0036261119/Tn
Axel Jacobs also found this simpler factor to work well in most cases:
tn = 1.0895 * Tn (details here, p.21)
OK so that's great, but transmissivity is the fraction of light not absorbed in one traversal of a single piece of material. And so--at long last--we arrive at the question:
Do these Tn-to-tn conversion formulae fall down when applied to a Tvis number supplied for an inherently multi-paned affair like an IGU? Hmmm?
