I was thinking someone else might answer this one, as a lot of folks have worked on similar things, but here goes...
Figuring out Radiance "trans" parameters is a classic challenge, and there are a few spreadsheets out there to make this a bit easier. The basic reference is section 5.2.6 from "Rendering with Radiance," attached below for your convenience. (Responders please delete attachment in your replies.)
Plugging in your parameters of:
Ts = .04
Td = .67 - .04 = .63
Rd = .18
Rs = 0 (guessing no clear reflections)
A7 = Ts/(Td+Ts) = .04/(.63+.04) = .0597
A6 = (Td+Ts)/(Rd+Td+Ts) = (.63+.04)/(.18+.63+.04) = .7882
A5 = 0 (adjust if you want to scatter your transmitted rays a bit)
A4 = Rs = 0
A1 = A2 = A3 = .18/((1-0)*(1-A6) = .18/(1-.7882) = .85 (assumes uncolored material)
So, there's your material:
void trans frosted_glass
7 0.85 0.85 0.85 0 0 0.7882 0.0597
Regarding thickness, so long as the distance between slats is 16 cm or greater, you needn't worry about it. If they are closer together, you may want to use the xform -a option to create your own array of slats. It's actually pretty simple -- genblinds isn't doing that much for you with flat slats, anyway.
TransParams.pdf (80 KB)
From: Leyla Sanati <email@example.com>
Date: August 29, 2013 10:00:45 PM PDT
Following example 1 in 5-phase method tutorial, I am trying to model venetian blinds with frosted glass as blind material. The properties of the glass are as follows:
Visible Transmittance: 67%
Clarity (small-angle scattering): 4%
To generate accurate BSDF files, I thought I should model the blind slats as trans material, but I am struggling with translating above properties into trans material modifiers.
I'd appreciate it if you helped me with writing the blind material script. Also, I am not sure where to include the glass thickness. genblinds does not seem to have an option for blind thickness.
Does the glass thickness in this case affect the BSDF results?