Meaning of RGB values taken from "rvu" command? and calculating IR?

Dear Radiance Users,

I’m a new user of Radiance. The questions that I will ask now might be very simple for you but I got confused.

I have a 3D model of a street in Sketch Up and exported it to Radiance to calculate how much radiation is reflected back and how the materials will affect the radiation within the street canyon. To learn the reflected value, I created small surfaces with a specific height in the middle of the street.

To produce a sky, gendaylit program is used with -E and -O 1 option because I have only global horizontal irradiance value for my site measured by a pyranometer. (gendaylit.exe 6 1 13:00 -E 904 -O 1 -a 51.1800 -o -1.0300 -m -15.0). Then, I combined my files (material.rad, sky.mat, objects.rad) with oconv command to render a scene.

This is one of my materials,


void plastic textured_dark_terra_cotta_brick
5 0.392 0.163 0.087 0 0
void alias myroof textured_dark_terra_cotta_brick.

After that, rvu and/or rpict command is used to see the irradiance value of a specific point.

rvu -vf views/Scene_1.vf -i -aa 0.15 -ab 7 -ad 4096 -ar 1024 -as 2048 mymodel.oct
rpict -vf views/Scene_1.vf -i -aa 0.15 -ab 7 -ad 4096 -ar 1024 -as 2048 -x 2240 -y 2240 mymodel.oct > myScene1.hdr
ximage -ov myScene1.hdr.
By pressing right button and enter it gives an irradiance value of the pixel on the displayed screen.

Question 1: Ok, I got the irradiance value of the pixel but When I look at the terminal, there are three values (RGB). what are the meanings of these three channels? Which one is the irradiance value.

(at (2.27286 1.96969 1.84) (5.40378)
value (122.94 83.069 67.404) (1.66e+04L))

Question 2: and How can I learn IR value of a specific point with these commands? or what should I do to calculate it?

I would be very grateful if you are able to answer the questions.
I look forward to hearing from you.Thank you very much in advance.


Hi Muhammed – welcome to the community.

Your use of Radiance is already fairly sophisticated. In answer to your first question, the RGB values given simply correspond to the three channels you have used in your material values. Since you told gendaylit to produce radiometric totals ("-O 1"), it uses the same radiometric quantity for all three channels. It is up to you in this case how you interpret RGB. One sensible approach is to assume that they evenly divide the radiometric spectrum and simply average the three values.

For your second question, there are a few ways to go about it. The simplest and fastest is to employ the rtrace program with the -I+ option, which computes irradiance at the specified XYZ coordinates and surface normal orientation. If you have the point positions and normals you are interested in, simply feed these to the standard input of rtrace and it will produce the RGB irradiance values on its output.

I hope this helps.

Dear Greg,
Thank you very much for your reply. It is a pleasure for me to be a part of the community.

Regarding the first question, everything is much clearer right now. Thank you very much again for your answer. I think I should use “ 0.265Rr + 0.670Rg + 0.065*Rb” formula to get the average of the three values. May I ask If it is correct?

And for the second question, I read something about the meaning of the three channels of rtrace output. It was said that red is infrared, green is visible and blue is ultraviolet. On another webpage, it was said that red is infrared, green is photometric and blue is whole spectrum. I could not find out which one is correct. Ok, the common part of these two examples is that red channel shows the IR. I would like to learn the true answer from you. May I ask which channel gives what kind of result?

Best Regards,

That formula is designed for photopic response, which is appropriate to human vision. If you are looking for something closer to the energy you might get from a photovoltaic panel, then these coefficients would not be appropriate. (I’m not sure what the right ones would be, however – perhaps someone else can answer to that.)

The three RGB channels in Radiance can be defined however you like. The default definition uses the CIE (x,y) chromaticities in src/common/color.h for the three channel primaries. Others have used the three channels in other ways, which is why you are seeing inconsistent recommendations on the web. There is no “default IR channel” or the like. They can be assigned any way you choose – just be aware that your input files must be consistent with your chosen definiton.