[Radiance-general] Re: Units

[Greg Ward]

Beware to those who attempt to read the Radiance source code -- it is 
not meant for human eyes!  I have trouble interpreting it, myself.  The 
best I can say for it is that it compiles well, most of the time....

Though it's true that gensky uses a different efficacy for the sky 
conversion, it only does this when the zenith brightness is being 
computed from turbidity, which is an iffy proposition at best.  If you 
use either the -b or -B options to enter your own zenith radiance or 
irradiance (from TRY data or on-site measurements), it is YOUR 
responsibility to convert from whatever was measured to the right units 
for your simulation.  If you took a luminance measurement on a gray 
sky, then you should divide by 179.  If your sky has some color to it, 
you should take this into account as Chas recommends.

-Greg

> From: Charles Ehrlich <ckehrlich at yahoo.com>
> Date: July 16, 2004 9:56:39 AM PDT
>
A caution regarding gensky, luminous efficacy factors and spectral 
conversions. 
   
The physical skydome is strongly blue-colored.  This matters if you are 
attempting to represent the brightness of objects relative to sources 
of light with different spectra, and/or relative to room surfaces with 
different reflectance spectra. 
   
Blue light is more efficacious, i.e., it both has more energy and 
appears brighter.  The lumens per watt conversion factor of 179 is 
really only valid for pure white light (I think I've got that right?).  
If you look into the source code for gensky, you will see that a 
different lm/watt factor was used to derive the zenith brightness (203 
??).  But this efficacy is not tied to the color of the skydome as 
modeled by the user in the input file in the skyfunc "glow" entity 
below:

skyfunc glow sky_glow
0 0
4 0.75 0.9 1.2 0

sky_glow source sky
0 0
4 0 0 1 180
 
I do not know your specific application, but if the appearance of the 
sky is not important for your simulations, you might consider changing 
the skydome glow color component to be equal R G B factors (1 1 1) so 
that you do not convolve different efficacy factors and spectral 
influences on your final result.  Alternatively, you could re-write 
gensky so that it adjusts the luminance of the skydome (by adjusting 
the lm/watt factor) in response to the color of the skyglow primitives.
   
In a quick study of this effect several years ago, we found errors as 
much as 20%--which is small potatoes for daylight simulation purposes, 
but troubling nevertheless because it is (arguably) an internal 
consistency error, not a modeling error.
 
The real problem is that the international unit of luminous flux is 
defined in terms of a human perceptual factor rather than on a 
radiometric unit, imho.  This results in skydome brightness 
measurements in candellas per meter squared (instead of watts/meter 
squared) whose conversion factor depends upon the color of the 
light source. 
 
-Chas