# Description of complex glass systems (geometry + material)

Hi Marija, hi Jack

Some glass types I want to simulate are not in the IGDB, so I will
describe the glasses with the measurements-method,
but I am not really sure I have understood this method properly. There
are those precisions that make radiance so great, but sometimes also
difficult to learn. Could you tell me if this would be then right as
follows?

This is the hole double glazing geometry description:

#window geometry:

double_glazing_a polygon window
0
0
12 0 -0.18 0
0 -0.18 3.34
4.13 -0.18 3.34
4.13 -0.18 0

Then I should measure the interior pane (a laminated float glass with an
interlayer) and separately the exterior pane (a 4 mm float glass). The
hole input data are then:

Red: 700nm -- rf1_r rb1_r rf2_r rb2_r t1_r t2_r
Green: 546nm -- rf1_g rb1_g rf2_g rb2_g t1_g t2_g
Blue: 436 -- rf1_b rb1_b rf2_b rb2_b t1_b t2_b

Then substitute these in the formel you said and this would be the
complete description of the double glazing material:

#window material:

void BRTDfunc double_glazing_a
10
if(Rdot,cr(fr(rf1_r),ft(t1_r),fr(rf2_r)),cr(fr(rb2_r),ft(t2_r),fr(rb1_r)))
if(Rdot,cr(fr(rf1_g),ft(t1_g),fr(rf2_g)),cr(fr(rb2_g),ft(t2_g),fr(rb1_g)))
if(Rdot,cr(fr(rf1_b),ft(t1_b),fr(rf2_b)),cr(fr(rb2_b),ft(t2_b),fr(rb1_b)))
ft(t1_r)*ft(t2_r)
ft(t1_g)*ft(t2_g)
ft(t1_b)*ft(t2_b)
0 0 0
glaze2.cal

0
9
0 0 0
0 0 0
0 0 0

I also try to understand how this calculation system is related to the
physical process. So I have another question.
If I measure with a spectrometer the complete normal reflectance and
transmittance of a double glazing, the interior reflectances are already
in the total measurement. So I understand that, also when in the
physical reality the total reflectance of a double glazing could be the
input data for the simulation, the calculation system of radiance
calculates only right if you specify the interior reflectances and the
calculation take place within the BRTD function. Am I right on that?

Thanks again,

Marina

Hi Marina,

Yes you want to describe the glass geometry, that is one polygon
representing the multiple layers of the physical makeup. The material is
what should represent the behavior. If you take the output from Optics
and run it through optics2rad you will get a single BRTDfunc material
description. One thing to be careful of is that this is dependent on the
correct orientation (normal) of the glass geometry. I believe that the
normal is supposed to be pointing "into" the interior of the building.
One way to check normal orientation in radiance is to assign
(temporaryily) a "glow" material to the glass geometry and view it. The
sides that are "black" are the back sides of the geometry, the sides
that are not black are the positive normal side.

Regards,

-Jack de Valpine

Marija Velickovic wrote:

···

Hi Marina,

1.glazing geometry

Always define glazing object as a polygon, no matter how many layers
of glass it contains in real world.
Double clear, laminated and other glazings differ only in BRTDfunc
(light transmittance and reflectance distribution function), while
real glazing width is not important here.

Note that polygon face should be oriented towards building interior.

##########
2.glazing material
Since BRTD function for double glazing is different then for single
glazing, I don't suggest using of simple glass material.
Procedure we use is next:
*Measure each glass layer transmittance and reflectance data. If you
can measure their r,g,b components it is good, if not use the same
value for all 3 components.
or
*Export from IGDB (using Optics5 I suppose), each layer separately.
And write down transmittances/reflectances you have in BRTD functions.
Note that in Optics5 front side of the glazing is towards outside, so
their front and Radiance "front" are different.

So you should have next parameters for double glazing parameters after
measures or after export:
rf1 - interior pane front reflectance (towards the room)
rb1 - interior pane back reflectance (towards the exterior pane)
rf2 - exterior pane front reflectance (towards the interior pane)
rb2 - exterior pane back reflectance (towards the exterior)
t1 - interior pane transmittance
t2- exterior pane transmittance
r,g,b - red, green and blue color components
*For clear glass layers front and back reflectance are always the same
/
void BRTDfunc double_glazing
10
if(Rdot,cr(fr(rf1_r),ft(t1_r),fr(rf2_r)),cr(fr(rb2_r),ft(t2_r),fr(rb1_r)))
if(Rdot,cr(fr(rf1_g),ft(t1_g),fr(rf2_g)),cr(fr(rb2_g),ft(t2_g),fr(rb1_g)))
if(Rdot,cr(fr(rf1_b),ft(t1_b),fr(rf2_b)),cr(fr(rb2_b),ft(t2_b),fr(rb1_b)))
ft(t1_r)*ft(t2_r)
ft(t1_g)*ft(t2_g)
ft(t1_b)*ft(t2_b)
0 0 0
glaze2.cal

0
9
0 0 0
0 0 0
0 0 0
/

In glazing definition function
if(Rdot,cr(fr(rf1_r),ft(t1_r),fr(rf2_r)),cr(fr(rb2_r),ft(t2_r),fr(rb1_r)))
means:
-if light is incident to the front side of the glazing then
reflectance is cr(fr(rf1_r),ft(t1_r),fr(rf2_r))
-else (light is incident to the back side) reflectance is
cr(fr(rb2_r),ft(t2_r),fr(rb1_r))

Functions for double glazing reflectance and transmittance are defined
in *glaze2.cal *script which is provided with Radiance:
-cr(refl1, trans,refl2) - is reflectance distribution for double
glazing as function of light incident angle. refl1 is reflectance of
first glazing pane, refl2 of the second one, and tran1 is
transmittance of the first glazing pane
-ft(t) - transmittance distribution for single glazing as function of
light incident angle. For double glazing total transmittance is
ft(t1)*ft(t2)

Hope this helps,
Marija
De Luminae <http://www.deluminaelab.com>

------------------------------------------------------------------------

_______________________________________________