[Radiance-general] Re: trans - georg vs greg vs book

[Peter Apian-Bennewitz]

Hi Brett,

regarding your trans question and to confuse you even more:
There's no such thing as "Specular transmittance" for scattering
materials. All "transmittance" values [0..1] are integrals over the BRTF
times incident radiance, with the BRTF limited by [0...infinity] . In
case of an ideal specular BRTF (ideal glass), a "specular transmittance"
is readily defined, but for all other cases it is necessary to specify
over which solid angle the BRTF was integrated (that's why there's a
*diffuse* transmittance under all circumstances, - that integral
extends over the hemisphere). Specularwise it gets all very ugly and has
been an efficient source for a lot of confusion. IMHO, the easiest way
is to stick to the BRTF and even then it is worth clearly separating
between an ideal BRTF (which may be a delta function) and
measurements/simulations (which both deal with folded, aka integrated,
BRTFs, so no delta-functions). This holds also true for "trans", whose
BRTF gets more "peakish" (delta-like) with lower roughness values.

http://www.pab-opto.de/pers/phd/apian_phd.pdf (in German, sorry for
that) lists the transmittance defs on (acrobat page numbering) page 31
and the trans BRTF on page 131.

-Peter

-- 
pab-opto, Freiburg, Germany, http://www.pab-opto.de
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