Hi Tim,
Seems a good answer to your question depends a bit on hearing some more details on your material.
There are quite a few degrees of freedom left for the answer:
Polycarbonate itself is quite close to be ideal transparent, - and therefore used for many optically clear and mechanically robust applications from CD/DVD discs to aircraft windows. A polycarbonate single pane (straight out of manufacturing, no dirt, no scratches) would be best described by Radiance "glass".
When used in double-walled extrusion panes (product name LEXAN and others, which actually may have more than two internal walls for structural reasons), the resulting light distribution is more complex than a straight-through scattered beam plus a reflected beam: Due to multiple internal reflections, the outgoing light is cone shaped. This may cause unpleasant glare, if material orientation, sun position and viewer are aligned "correctly" (and there's a good chance for that).
This light distribution is more complex than what "trans" can do. In Radiance, it requires either a traditional cal file with a functional model or the new BSDF material. The latter has the advantage that the illumination calc may be more efficient. If you just want to analyse the "correct" alignment between sun-material-viewer, the cal file is pretty simple. A more detailed and energy-correct model is a more complex. So far, so hoopy, that's the situation for the *clear* standard material.
However, during the extrusion, Titaniumdioxid (TiO2 or anything else that is stable and scattering) may be added to the Polycarbonate base, leading to a varying degree of light scattering of the finished material. This may range from "a bit" of scattering to nearly opaque, depending on the amount of TiO_2 per volume.
With an increase of TiO2 , the light distribution is increasingly dominated by the scattering within the material. And the influence of the inter-reflections between the walls decrease and so does the cone-shaped, glare creating part of the outgoing light distribution. Depending on TiO2 amount and internal structure, the material is then mostly forward scattering, and can be described by "trans".
Btw, the TiO2 had been used to get the Sydney2000 stadium roof working without glare. The original plans with clear material would have created an unpleasant glare for folks in many rows.
So, the way in Radiance depends on your specific extrusion profile and material, plus your intended use.
best regards
Peter
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pab advanced technologies Ltd, http://www.pab.eu