Determining Direct component and Internally Reflected Component.

Hello Everybody,
Another simple question coming your way.
In my grid calculations (at the workplane) which I talked about in my earlier message I am calculating illuminance levels for a clear sky without sun.

Hi Ashu,

Here comes a weakly composed response, owing to my vague understanding of things, but I think I'm right about this.

With -ab 0, only points directly illuminated by the SUN will register in your rtraces. The luminance of the sky (clear and overcast) is not factored in until the indirect calculation.

I believe this explains why with -ab 0 you don't get satisfactory results with overcast skies. So, I don't know if it's possible to determine "direct component" of sky brightness. But if anyone knows a workaround, you're sure to find out here...

Rob Guglielmetti
[email protected]


On Thursday, October 9, 2003, at 06:34 PM, Arvinder Dang wrote:

To calculate the direct component separately at the points I think i would have to specify the ambient bounces as 0.This would remove the IRC from the total. However if I do this I get all my answers as 0. Is this due to the sky type (which�I dont think should be since some light from the sky should reach the points directly) or there is something wrong in the parameters I have specified. I remember getting results with -ab 0 when I did the calculation in Desktop Radiance.
Please guide me as to what I am doing wrong. Is there some other way to seperate the direct and IRC (say making reflectances 0).


For a sky without sun (whatever the sky type), setting -ab 1 for
an illuminance(*) calculation will give the direct sky component
of daylight illuminance. For the inter-reflected component, set
higher -ab values and subtract the values you got for -ab 1.
And, of course, the ambient value should be zero.

For the direct sky calculation, all opaque materials should be
set to diffuse only. Otherwise you may get some specular reflections
happening at -ab 1.

-John Mardaljevic

(*) Of course, we first compute irradiance and convert to illuminance.


Dr. John Mardaljevic
Senior Research Fellow
Institute of Energy and Sustainable Development
De Montfort University
The Gateway
+44 (0) 116 257 7972
+44 (0) 116 257 7981 (fax)

[email protected]