[Radiance-general] Radiance value of uniform sky

[John Mardaljevic]

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
0
4 1 1 1 0

sky_glow source sky
0
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)

jm at dmu.ac.uk
http://www.iesd.dmu.ac.uk/~jm