[Radiance-general] Radiance-general Digest, Vol 166, Issue 33

[Greg Ward]

Hi Valerie,

You are correct about irradiance units, which are watts/meter^2.  That was my mistake.

If you wish to add up irradiance, I would still recommend doing this with rtrace rather than rsensor, unless you have a specific spherical sensitivity in mind.  You can give rtrace any hemisphere orientations you like and average the results using "total" or your own program.

However, the light arriving on a plane *is* irradiance, which is a sum over the hemisphere of incoming directions with a cosine weighting.  If you were to give rtrace a set of different directions for irradiance, then you would arrive at something more like total incident radiation at a point, which is an unusual quantity.  What do you need this value for, exactly?

Cheers,
-Greg

> From: Valerie Tan <vlt26 at cornell.edu>
> Date: December 28, 2017 1:55:24 PM PST
> 
> Hi,
> 
> In your example, you test the irradiance at a single point in only one direction (up) which is not what I am trying to do unfortunately. If I want to calculate the energy arriving at a point on a plane, calculating irradiance from a single direction isn't realistic. Thus, I asked about RSENSOR which you recommended that I use a long time ago. I am thinking that for each point, I call RTRACE with the view direction -vd changing in equal increments and then add the irradiances. For example, the view directions for a very rough calculation would be in spherical coordinates/degrees: (1, 0, 0), (1, 90, 0), (1, 180, 0), (1, 270, 0), (1, 0, 45), (1, 90, 45), (1, 180, 45), (1, 270, 45), (1, 0, 90). 
> 
> Also, I am confused about "a list of RGB irradiance values (watts/sr/m^2)." This looks like radiance, and reputable sources state that the units for irradiance are (W/m^2). 
> 
> Thank you! 
> 
> Best, 
> 
> Valerie
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