Hi All,

Merry Christmas and Happy New Year!

I am a Master's thesis student and I am a beginner in Radiance. I would like to

thank Alex Jacobs for LEARNIX and tutorials, and Greg Ward for the effort he put

into Radiance. I apologize in advance if there is something wrong with my

English or if I wrote something stupid related to Radiance.

For the purposes of my work, I need to calculate illuminance level in one point

of working plane in a simple office (for clear sky and sunny day), for different

luminaires output levels, different slat angles of exterior venetian blinds, and

for different times of the day (i.e. for different solar angles). I am not

interested in for image rendering, just for the illuminance calculation. I am

stuck with window glass modeling. This is what I got from Optics5 for 2 layers

glazing system (1. clear glass 6mm ; AIR gap 12mm ; 2. clear glass 4mm):

void glass GlzSys1_glass

0

0

3 0.832 0.884 0.859

void BRTDfunc GlzSys1_front

10

0.136 0.147 0.146

0.764 0.811 0.789

0 0 0

.

0

9 0 0 0 0 0 0 0 0 0

void BRTDfunc GlzSys1_back

10

0.139 0.148 0.148

0.764 0.811 0.789

0 0 0

.

0

9 0 0 0 0 0 0 0 0 0

And this is what I got from Window6 for the same glazing system :

Angle 0 10 20 30 40 50 60 70 80 90 Hemis

Vtc : 0.798 0.798 0.796 0.791 0.779 0.748 0.675 0.521 0.261 0.000 0.695

Rf : 0.144 0.144 0.144 0.147 0.157 0.185 0.255 0.405 0.664 1.000 0.230

Rb : 0.146 0.146 0.146 0.149 0.159 0.188 0.258 0.411 0.674 1.000 0.233

When I make a model of complex fenestration system with exterior venetian

blinds, I have two different cases :

Case 1. When sun rays don't pass directly between the venetian blind slats,

just indirectly from slats (reflected from slats).

Question 1. Because sun rays don't pass directly trough window glass, I suppose

that all the light that reaches the window glass is diffuse (light is reflected

from ground, sky and slats). If this is so, which is the correct value for the

definition of window glass material? Is it perhaps Vtc (visible tansmittance)

value 0.695 for Hemis (hemispherical) multiplied by 1.09 to get the

transmissivity value, or maybe mean Vtc value for angles from 0 to 90

[(0.798+0.798)/2+(0.798+0.796)/2+...+(0.261+0.000)/2]/9=0.639 also multiplied

by 1.09 to get the transmissivity, or maybe some other value?

I have noticed the following :

The second row of the BRTDfunc has the values: 0.764 0.811 0.789.

If we multiply these values with 1.09, we get: 0.832 0.884 0.859,

which are the transmissivity values for glass. Also, if we put these values into

the expression:

0.265*R + 0.670*G + 0.065*B, we get:

0.265*0.832 + 0.670*0.884 + 0.065*0.859 = 0.868595. If we divide this value

with 1.09, we get value 0.797, which is the Vtc value at normal incidence (0).

Question 2. Let us assume that correct Vtc (transmittance) value is 0.695. Is

this the right way to calculate RGB transmissivity for glass:

0.832*(0.695/0.798) 0.884*(0.695/0.798) 0.859*(0.695/0.798),

which is equal to:

0.725 0.770 0.748

Question 3. This may be a stupid question. Theoretically, if I should get the

same results in the following two cases:

1. I create the distribution for the window with mkillum; window glass is

defined with transmissivity values:

0.725 0.770 0.748

I compile a new scene octree that includes the old one and adds the new illum

window (new .rad file) to it. Then, I calculate lux level on working plane with

rtrace.

2. I don't create window polygon (there's nothing, i.e. there is air). But

instead, I can scale RGB values for light solar with the glass transmissivity

values. For example:

Istead:

void light solar

0

0

3 2.72e+06 2.72e+06 2.72e+06, I put:

void light solar

0

0

3 2.72e+06*0.725 2.72e+06*0.770 2.72e+06*0.748, i.e.:

void light solar

0

0

3 1.972e+06 2.094e+06 2.035e+06

I compile a scene with changed .mat file, and then I calculate lux level on

working plane with rtrace. Of course, I will try to calculate this, but I'm

definitely interested in the opinion of experts.

Question 4. How can I scale the material of sky, if it is defined with:

void brightfunc skyfunc

2 skybr skybright.cal

0

7 1 3.76e+00 3.72e+00 2.98e-01 -0.489041 -0.851114 0.190903

skyfunc glow skyglow

0

0

4 .85 1.04 1.2 0

I guess that is not allowed to scale the RGB values .85 1.04 1.2, because of

the equation: 1 = 0.265*R + 0.670*G + 0.065*B.

Maybe I can scale the values from skyfunc: 3.76e+00 (zenith brightness) and

3.72e+00 (ground plane brightness)?

Case 2. When one part of the sun rays pass directly between the venetian blind

slats, and the second part indirectly from slats.

This is perhaps the worst case for modeling. Let us assume that exterior

venetian blinds have 35 slats. If I want to make a proper model of window glass

polygon, I suppose that I need to replace one glass polygon with 69 glass

polygons. 34 are directly exposed to the sun and have some values for RGB

transmissivity (which depend from sun angle). Other 35 are in shade and have

some other values for RGB transmissivity (transmissivity for diffuse light). Of

course, I need to do some trigonometry calculations. This model of window glass

is valid only for one light source (solar source sun), but not for

skyglow source sky. This seems very complicated, and because of that would be

great if I could do what I asked in questions 3.2 and 4.

Queation 5. I just need a short comment of paragraph above.

Case 3. No venetian blinds - just window glass.

Let us assume that octree (scene) includes 3 light sources (sun, sky and

ground), and sun incidence angle is 70 degrees. If I make a model of window

glass with RGB transmissivity:

0.725 0.770 0.748 (transmittance=0.695),

the model will be correct for sky and ground, but not for sun, because of

transmittance for angle of 70 degrees is 0.521.

Thus, whatever value that I take for RGB transmissivity, I make a some mistake,

when I calculate illumination.

Question 6. Does this problem can be solved in one of two ways:

1. I scale values for sun, sky and ground, in the manner described in questions

3.2 and 4.

2. I compile an octree that includes an office, only one source of light (sun),

and a model of window glass with transmissivity that corresponding to the sun

angle. For this octree, I calculate illumination (I1).

Then, I compile a new octree that includes a same office, two sources of light

(sky and ground), and a model of window glass with transmissivity that

corresponding to the sky and ground. For this octree, I calculate illumination

(I2).

In the end, I calculate I = I1 + I2, which actually represents the total

illumination.

I apologize for the long mail.

Thank in advance, Milan Cakanovic