[Radiance-general] [help] how to get short-wave and long-wave solar radiation value?

[Lars O. Grobe]

Hi Ji Zhang!

> May I ask whether it's possible to "isolate" the contribution of
> short-wave and long-wave solar radiation using Radiance?
>
> One of our colleagues wants to get these values for the calculation of
> "mean radiant temperature" for the purpose of analyzing the implications
> of urban geometries on thermal comfort.
>
> Correct me if I'm wrong, the irradiation value obtained in Radiance is
> the total solar radiation energy received on a given surface.
>
> So, is this "total" value encompasses the full spectrum of solar
> radiation, including the visible part and the short-wave and long-wave
> parts that are not visible to human eyes?
>
> If it is, then, is there a way to single out the contribution from
> short-wave and long-wave radiation separately?
>
> Clarification and advices are deeply appreciated!

You are (almost) completely free to choose what wavelength range to 
squeeze into any of the three channels that we usually refer to as RGB. 
So instead of calling the first channel "RED", you could call it 
"Shortwave". As long as you use the three channels for the same 
wavelength ranges when defining materials, sources, and interpreting 
results, that is fine for radiative transfer of energy.

However I would advice not to make use of the possibility of rendering 
all these wavelength ranges at once by making use of the three channels. 
It would be better to set all three channels to the same wavelength 
range, so work with "neutral grey" lights and surfaces. The reason is 
that there are some (few) parts of Radiance's code where the channels 
are weighted according to human perceiption assuming RGB-like wavelengths.

Another question is how to find how much of the absorped short-wave 
solar irradiation may get emitted as long-wave radiation by a surface. 
Radiance does not consider such "shift" effects. So IR radiation by 
surfaces heating up in the sun would not be reflected.

Cheers, Lars.