# Converting transmittance to transmissivity for IGUs

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?