# Radiance value of uniform sky

(The previous message doesn't seem to show up properly, and is therefore re-sent here without the attachment. Sorry for the duplication.)

Dear all,

I am trying to obtain irradiance value on the surfaces of building blocks under daylight but encounter the following problems. Could you shed some light on how they can be solved?

1. Uneven distribution of the uniform sky

I produced a uniform sky using the u option under the gensky function. As measured from the fisheye view of the sky (both angular and hemispheric), however, the radiance of the sky slightly decreases towards the edge of the circle. Is there a way to improve this situation? Here are the parameters that I used for rendering the sky:

> Rpict vf view.vf ab 0 aa 0 ar 128 ad 4096  as 1024 sky.oct

2. Different value obtain in each run

I tried to render some building blocks (with homogeneous color and reflectance) and then obtain the irradiance of a point on the ground with rtrace. Different values returns even if the inputs are identical. The discrepancy can be as much as 4%. Like the command line printed above, ambience parameters had been set at their maximum values. Is there a way to obtain irradiance values with a smaller margin of error?

Thank you very much for your help!

Regards,
Kam

Hi Kam,

I produced a uniform sky using the –u option under the gensky function. As measured from the fisheye view of the sky (both angular and hemispheric), however, the radiance of the sky slightly decreases towards the edge of the “circle”. Is there a way to improve this situation?

What I believe you are seeing is the gradual transition at the horizon between the sky and ground radiance produced by a (steep) power-law blending function in skybright.cal. This introduces a slight deviation from the mathematically 'perfect' distribution which might be noticed in certain situations. You can prevent this happening by using a slightly modified version of cal file available here:

http://www.iesd.dmu.ac.uk/~jm/pickup/skybright_nomix.cal

Simply place where it will be found by RAYPATH and rename 'skybright.cal' in the gensky output to 'skybright_nomix.cal'. This will work for all sky luminance distributions. If uniform is the only 'perfect' sky that you need, then the easiest way is create it manually:

# uniform brightness sky (B=1)
void glow sky_glow
0
4 1 1 1 0

sky_glow source sky
0
4 0 0 1 180

Re: Q2 - Have a look at the "ambient accuracy parameter" thread in December 2006 (especially Greg's comment).

Regards,

-John

PS. The current skybright.cal (2.7) is a later version than the one I modified (2.6), but I didn't note any difference in the code other than the RCS header.

···

-----------------------------------------------
Dr. John Mardaljevic
Senior Research Fellow
Institute of Energy and Sustainable Development
De Montfort University
The Gateway
Leicester
LE1 9BH, UK
+44 (0) 116 257 7972
+44 (0) 116 257 7981 (fax)