[Radiance-general] TMY weather data as input for Gensky

[John Mardaljevic]

Chris,

Be wary that gendaylit can produce skies with massive distortions  
(e.g. huge -ve luminance cusps at low altitude) for certain  
combinations of direct, diffuse and solar altitude.  The absolute  
values for these tend to be uninteresting for daylight provision  
(i.e. on the low side), but they could foul-up an automated procedure  
and/or bias output if they are not picked up.  The distortion  
occurred in routine application of UK climate datasets, and I believe  
it has been noted with use of other climate files.  Note that, when  
it occurs, the normalisation to diffuse horizontal is still correct  
-- so you can't test for it by comparing predicted diffuse horizontal  
with the corresponding value from the climate file.  However,  
predictions for vertical illuminances (or light through a window)  
could be way out.  The only safe way to test is to send out a  
hemisphere of rays to the sky and check for any returned -ve  
luminances (replace these instances with CIE overcast).

-John

-----------------------------------------------
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