[Radiance-general] Problem with getting correct irradiance from sky dome

[Claus B. Madsen]

Dear List,

I have problems getting trustworthy irradiances in a scene where I am 
using a 180 degree light source as a sky dome. For testing purposes I 
have created the following very simple setup: a groundplane with normal 
(0 0 1), and a 180 degree source with direction vector (0 0 1), the 
radiance of the source is RGB=(1 1 1) (unity radiance). I set the camera 
to look straight down on the groundplane and render using rpict -i to 
get irradiances. In theory such a source should result in an irradiance 
at the groundplane of exactly pi, but when I render I get 
approx.RGB=(4.77  4.77  4.77)

Any ideas for explanations? Any ideas for how to correct this?

Best,
Claus




The scene.rad file:
------------------------------------
# Materials
void plastic perfectlambert
0
0
5 1.0 1.0 1.0 0 0

#Geometry
!genbox perfectlambert groundplane 10000 10000 0.5 | xform -t -5000 
-5000 -0.25


# Illumination environment: a 180 degree unity radiance skydome
void light skyradiance
0
0
3 1 1 1

skyradiance source TheSky
0
0
4 0 0 1 180.0
---------------------------------------------------------------------------------


The render parameters:

rpict  -i -x 1024 -y 1024 -ab 4 -aw 1 -ps 1 -dt 0.0 -dr 3 -dp 8192 -ds 
0.01 -dj 0.9 -dc 0.8 -u -vf scene.vp




And the scene.vp file:
---------------------------------
rview -vtv -vp 0.0 0.0 10.0 -vd 0.0 0.0 -1.0 -vu 0 1 0 -vh 45 -vv 45
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