# Mkillum Question

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
Faculty of Built Environment, Art and Design Curtin University of Technology GPO Box U 1987 Perth 6845 Western Australia
Email: T.Mcminn@curtin.edu.au
CRICOS Provider Code: 00301J

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@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
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
3 0.97 0.97 0.97

skyfunc brightfunc window_dist
2 winxmit winxmit.cal
0

window_dist illum window_illum
1 window_glass
0
3 0.89 0.89 0.89

window_illum polygon window
0
12 ...

Regards

Terry Mc Minn

Hello Greg,

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.

The missing link in the documentation! Quite clear!

Many thanks.

Regards

Terry Mc Minn
Faculty of Built Environment, Art and Design
Curtin University of Technology
GPO Box U 1987 Perth 6845
Western Australia
Email: T.Mcminn@curtin.edu.au
CRICOS Provider Code: 00301J