[Radiance-general] subtended sun angle in gensky and gendaylit

[John Mardaljevic]

Greg/Jan,

 > So I assume you changed the value in the source code, or? But as
 > long as you are not adapting the radiance value of the light source
 > (which you don't do), you'll loose more than 10% of the flux of the
 > sun by reducing the value from 0.533 to 0.5 !!
 >
 > Is there any reason to use 0.5 instead of 0.533?

Whether this matters depends on how you determine the solar  
radiance.  Usually, it is derived from measurements of direct normal  
irradiance or illuminance (commonly found in climate files).  Rarely  
is the solar radiance or luminance measured directly.  Provided the  
solar radiance is determined in a consistent fashion from  
measurements of direct normal, then there is negligible practical  
difference as far any evaluation is concerned - the flux will be the  
same whatever the solid angle used for the sun.

In any case, the best measurements of direct normal are taken using a  
tracking device that commonly has an acceptance angle of 6deg (with,  
I recall, a fairly flat response).  This is because it is impractical  
to attempt to track the sun using anything with a much smaller angle,  
let alone an angle that matched exactly the solar disc.  So, when  
using these data from climate files etc., the difference between 0.5  
and 0.533 is made pretty much irrelevant because the sun radiance is  
being calculated from a measurement that includes a lot of  
circumsolar region also.

Intriguingly, in practical terms, this matters more for overcast  
rather than clear sky conditions (I can almost hear the gasps of  
disbelief).  For clear skies, the solar radiance dominates the  
brightness of of the circumsolar region -- so the "contamination"  
that results from basing the solar radiance on a measurement that  
includes the contribution of the circumsolar region is small.   
However, when a sky is overcast, the measurement of direct normal (in  
the climate file) is that which results from a (sunless) 6deg patch  
of sky.  Say that value is used routinely in an annual calculation  
procedure to determine the radiance of the 0.5deg sun.  Then, for  
overcast days, the solar radiance will be about ~144 times the  
radiance of the background sky (i.e. [6/0.5]**2).  So, a brightish  
sun will be a permanent feature in all your overcast skies.  If your  
evaluation is very sensitive to the magnitude of the sky and sun  
radiance or luminance (as I suspect Jan's is), then this may be an  
issue.  It's probably wise to play around with a threshold value  
below which the sun radiance is set to zero.

-John

PS.  There is some discussion on related matters in chapter 3 here:
http://www.iesd.dmu.ac.uk/~jm/zxcv-thesis/

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