accounting for irradiance due to ultraviolet in rtrace


@Lars: do you mean like this? rcalc -e '$1=($1*0.33+$2*0.33+$3*0.33)' or rcalc -e '$1=0.4*($1*0.265+$2*0.67+$3*0.065)'

None of the above... :wink:

I recommend setting all three channels to the same value. E.g. if a diffuse surface reflects 50% of UV radiation, I'd use

void plastic diffuse50
5 .5 .5 .5 0 0

Of course you must have defined what wavelength range to consider. And apply that to all materials, sources, and interpret the results. Then, the result of any channel can be taken, or, in rcalc terms, $1=$1 and $1=$2 and $1=$3 should all be valid.

? the factor of 0.4 comes from dividing the wavelength spectrum of UVR (120-400nm) to PAR (400-700nm)...

I did not understand this, but probably you do not want this...

So maybe to explain things, Radiance is a raytracer that outputs results in watts, which implies that you define some wavelength range. To make life easier for lighting applications, it calculates three ranges in parallel, and most people use this for red, green and blue. What I recommended is, as you have only one range that you are interested in, to set the first channel to UV. This will be the only channel you are interested in. Only to be on the safe side, set the second and third channel to the same values, to avoid strange effects due to weighting. Still you use only one channel, and no multiplications using rcalc.

Ah, and please get a copy of Rendering with Radiance. It will be very helpful!

Cheers, Lars.

Hi Robert,

Radiance doesn't explicitly define wavelength ranges or anything of the sort. By convention, the three channels provided are typically used for RGB data in a color space that is close to (but unfortunately not exactly equal to) the CCIR-709 primaries used by sRGB. This is the assumption if no other information is given in a Radiance RGBE image.

However, you can redefine these channels to mean whatever you like. Since Radiance treats them independently in its calculations, as it has no wavelength-dependent simulations, it really doesn't matter. You will of course need to define your materials differently with appropriate reflectances and transmittances for UV instead of visible, but the rest should just work.

The important caveats to remember are that for IR radiation, surface temperature matters, and Radiance does not computer or keep track of that. In the case of UV, Radiance does not simulate phosphorescence, which may or may not matter to you.



From: Robert Ong <>
Date: October 23, 2011 6:56:01 PM PDT

Hi Lars,

Thanks for the feedback.

Do you mind if I ask how to define the wavelength range in Radiance? UV spectrum is normally starts from 120 to 400nm.

Thanks once again.

Kind Regards,