[Radiance-general] Glass transmittance

[Greg Ward]

Helo Ali,

If you have the formula that accounts for the presence of the window in your opening, you should not apply it to a "glass" surface.  The Radiance "glass" type does an exact calculation of transmission and reflection as a function of incident angle, which your polynomial approximates.  If you want or need to use the formula, then switch out your glass for:

void brightfunc bf
2 val gtrans.cal
0
0

bf trans Glass1
0
0
7 1 1 1 0 0 1 1

Note that you need to use "bf" as the modifier to your trans material to see its effect.  (Your original model used "void" which ignores the brightfunc entirely.)

You should also use "cosine=Rdot;" in your cal file, or change the sign of your calculation, which will always produce a negative cosine value in this case.

Finally, you should feel free to try using the "glass" type with the correct transmission value, which you can calculate from your formula, which yields 0.87881 as the transmittance at normal incidence, and the trans.cal file, which says transmission should be 0.95712, assuming an index of refraction of 1.52:

void glass Glass1
0
0
3 0.95712 0.95712 0.95712

I'm hoping the results should be the same between the two descriptions.

Cheers,
-Greg

> From: Ali Fatoorechi <ali at surveymbs.com>
> Date: March 6, 2017 8:18:00 AM PST
> 
> 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
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