[Radiance-general] gendaylit

[John Mardaljevic]

Roberto,

Use of gendaylit for overcast sky conditions would be wrong because  
the 'overcast' skies (note plural) produced by the Perez All-Weather  
model do not correspond to the strict CIE definition, i.e. L_alt =  
L_zenith(1 + 2sin(alt))/3.

Actually, gendaylit gives you a range of overcast types depending on  
the absolute diffuse horizontal illuminance.  The ratio between zenith  
and horizon luminance seems to increase with higher values for  
horizontal illuminance.  As several users have noted, gendaylit bombs  
out with a warning:

Warning : skyclearness or skybrightness out of range ;
  Check your input parameters

for certain input values.  The minimum diffuse horizontal illuminance  
that works appears to be 8603 (lux).  Try:

gendaylit 7 1 12 -L 0 8603

gendaylit 7 1 12 -L 0 8602

The diffuse horizontal illuminance upper limit for an 'overcast' sky  
(i.e. direct normal illuminance = 0) is somewhere 80kLux and 90kLux.   
Which is, of course, improbably high for any sort of overcast sky.

-John

-----------------------------------------------
Dr. John Mardaljevic
Reader in Daylight Modelling
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