[Radiance-general] Mkillum Question

[Greg Ward]

Hi Terry,

The reason for the change is because the glass primitive takes into  
account internal reflections, which use the transmission value to  
compute transmittance as a function of angle.  In contrast, the illum  
primitive doesn't account for these, which are reintroduced using the  
winxmit.cal pattern.  This pattern is normalized to 100%  
transmittance, and must have normal transmittance as a multiplier on  
the material.

I don't know how to make it clearer than that -- I realize it's a bit  
confused, but it is correct.

-Greg

> From: "Terrance McMinn" <T.McMinn at curtin.edu.au>
> Date: October 16, 2005 9:28:09 PM PDT
>
> Why is the visible light transmittance used with the glass  
> primitive while the visible light transmission value is used in the  
> illum primitive? See below example from chapter 1 pp 29 & 32 of RWR
>
> For Pilkington Optifloat Clear 3mm glass the Visible Light  
> Transmission is 89% (http://www.pilkington.com.au/resources/ 
> pilkingtonperformancedatasingleglazing.pdf)
>
> The transmissivity is calculated using:
>     icalc /usr/local/lib/ray/src/cal/cal/trans.cal
>     Tn=0.89
>     tn
>     $1=0.969269687
>
> Therefore for a glass definition you would use something like:
>     mod glass window_glass
>         0
>         0
>         3 0.97 0.97 0.97
>
> But when you use mkillum (Rwr chapter 1, page 32 with alteration of  
> the transmission value from 0.88 to 0.89 and transmittance from  
> 0.96 to 0.97) the illum uses the visible light transmission value.
>     mod glass window_glass
>         0
>         0
>         3 0.97 0.97 0.97
>
>     skyfunc brightfunc window_dist
>     2 winxmit winxmit.cal
>     0
>     0
>
>     window_dist illum window_illum
>     1 window_glass
>     0
>     3 0.89 0.89 0.89
>
>     window_illum polygon window
>     0
>     0
>     12 ...
>
> Regards
>
> Terry Mc Minn
>