[Radiance-general] Glass transmittance

[Ali Fatoorechi]

Hi Guys,

I am fairly new to Radiance and am trying to test one of the CIE Technical
Report test 5.10 "Sky Component under a roof glazed opening".
There is only one window that is a 1m x 1m opening with 6mm clear glass
that serves a 4m x 4m x 3m room.
The formula for directional transmittance is defined in the report as the
one in this webpage:
http://naturalfrequency.com/Tregenza_Sharples/Daylight_Algorithms/algorithm_2_34.htm

The Sky Component value for the opening(assuming no glazing is used)
matches the report results but on introducing the glazing, Radiance gives
slightly lower results.
I am testing this test case against our own software too. Multiplying the
above directional transmittance to the radiance value along each ray gives
the correct result which is inline with the report.
But for some reasons I cannot get same from Radiance. I have set the
transmissivity
of the glass to 1 but am not sure what value should I use here?
In other words if I want to scale the radiance along a ray by the above
formula only, how should I define the glazing?


The window rectangle is defined as two triangles

void brightfunc bf
2 val gtrans.cal
0
0
void glass Glass1
0
0
3 1 1 1

Glass1 polygon triangle1
0
0
9 (3 coordinates here..)
Glass1 polygon triangle2
0
0
9 (3 coordinates here..)

The "gtrans.cal" file is defined as below based on the formula given on the
above link:
cosine =  Dx * Nx + Dy * Ny + Dz * Nz;
val = -.028378 + (3.156075 * cosine) - (3.058376 * cosine * cosine) -
(1.428919 * cosine * cosine * cosine) + (4.014235 * cosine * cosine *
cosine * cosine) - (1.775827 * cosine * cosine * cosine * cosine * cosine);

Regards,

Ali Fatoorechi
MBS Survey Software Ltd

*62 High Street*


*SteyningWest SussexBN44 3RD*

*T +44 (0)1903 879323M +44(0)7830751409*
ali at surveymbs.com
www.surveymbs.com
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