[Radiance-general] Re: query about accurately modelling glazing

[Zack Rogers]

Hi Daniel,

>My question is this.  To what extent will my simplified way of 
>describing the transmittance affect the accuracy of my results?  
>
Defining glass using the BRTDFunc method just lets you more accurately 
define the front and back reflectance of the glass.  Defining glass with 
just the glass primitive uses a default front and back reflectance.

>I 
>don't completely understand the workings of the BRTDfunc (even after reading the manual page), and
>think I 
>would have trouble measuring any variable of glazing other than 
>transmittance of the specimens I am sampling in the real world (Rendering with Radiance 
>talks briefly of measuring glazing transmittance, 
>but not things like reflectance.  
>
I recommend looking at Optics 5 from LBNL.

http://windows.lbl.gov/materials/optics5/

 It has a very extensive database of just about every type of glass and 
glass compositions available and allows you to build up various double 
pane combinations and save it all as a Radiance input file.  Rather than 
measuring a sample in the real world and defining it in Radiance, you 
could probably find the glazing in this database and use Optics 5 to 
define it.

>
>I presume also that transmittance + reflectance (+ absorptance) of 
>glazing can never exceed 1.  In Desktop Radiance, the description of 
>Generic clear glass lbnl (clear3.rad) states a transmittance of 
>89.90% and a reflection of 82.0%.  Is my understanding fundementally 
>wrong, or is this a typo?
>  
>
I've always been a little confused how desktop radiance defines glass.  
The front RGB reflectance is actually defined with this line under 
"clear3_front"

0.07428    0.08322    0.08556

and the back RGB reflectance is defined with this line under "clear3_back"

0.07567    0.08418    0.08538

So the reflectances are roughly 8% which does obey the rule.

I am not completely clear what the other RGB values are but my guess is 
that they modify the transmittivity defined under clear3_glass somehow.  
Whenever I've manually defined glass I've used this form (from 
"glazing.cal") instead, its just a little clearer to me.

mod BRTDfunc my_glazing
    10    rrho    grho    brho
        rtau    gtau    btau
        0    0    0
        glazing.cal
    0
    18    0    0    0
        0    0    0
        0    0    0
        FRRHO    FGRHO    FBRHO
        BRRHO    BGRHO    BBRHO
        RTAU    GTAU    BTAU

    where:
        FRRHO    FGRHO    FBRHO    is front normal spectral reflectance
        BRRHO    BGRHO    BBRHO    is back normal spectral reflectance
        RTAU    GTAU    BTAU    is normal spectral transmittance

Hope this helps!

Zack

-- 
Zack Rogers
Staff Engineer
Architectural Energy Corporation
2540 Frontier Avenue, Suite 201
Boulder, CO 80301 USA

tel (303)444-4149 ext.235
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