Hi!
I'm wondering how to account for the irradiance caused by UV in rtrace.
I know by default (visible light) this is normally implemented
irradiance = 0.265*R_illum + 0.67*G_illum +0.065*B_illum
That assumes useage of the three channels for RGB. If you decide to use
another wavelength range, you have two options:
1) Use one of the three channels for that wavelength range - and then,
use the radiance covered by that channel, so no summing up of channels.
2) Set all three channels to the same. E.g. use all three with equal
values for your UV range. That means a "grey" image. Then you can use
any channel to get results. Advantage is that in some few places in the
code, useage of the three channels for RGB is assumed. This might lead
to some wrong weighting if you choose option 1), so 2) seams to be
preferable here.
Besides, laws of physics apply...
Cheers, Lars.
You also have to consider how to develop your source and material definitions. Luminaires are typically measured for output only in visible spectrum, and skies in either visible or full solar spectrum. If you have a known source definition in the visible range, and you know how much UV the same source generates per W of visible or total solar, maybe you can scale your results according to UV. Or if you know the source UV output of your source directly, you might treat it the same as if it's the Watts in the visible range for your input and output through Radiance (but do a sanity check on your results and careful that you're not off by a factor of 179 for the de facto lumens per Watt Radiance conversion). You also have to consider that surfaces might reflect or transmit UV in different proportions than for the visible or solar spectrum.
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Hi Lars and Chris,
Thank you all for constructive discussions.
I should've pointed out earlier that I'm only a novice user in Radiance.
@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)' ? the factor of 0.4 comes from dividing the wavelength spectrum of UVR (120-400nm) to PAR (400-700nm)...
@Chris: do you mean I should scale my source and material definition by 4?
My apology if I didn't grasp your points clearly.
Thanks once again for your time and attention.
Kind Regards,
Robert
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On 21/10/11 16:49, Lars O. Grobe wrote:
Hi!
I'm wondering how to account for the irradiance caused by UV in rtrace.
I know by default (visible light) this is normally implemented
irradiance = 0.265*R_illum + 0.67*G_illum +0.065*B_illum
That assumes useage of the three channels for RGB. If you decide to use
another wavelength range, you have two options:
1) Use one of the three channels for that wavelength range - and then,
use the radiance covered by that channel, so no summing up of channels.
2) Set all three channels to the same. E.g. use all three with equal
values for your UV range. That means a "grey" image. Then you can use
any channel to get results. Advantage is that in some few places in the
code, useage of the three channels for RGB is assumed. This might lead
to some wrong weighting if you choose option 1), so 2) seams to be
preferable here.
Besides, laws of physics apply...
Cheers, Lars.
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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,
Robert
···
On 24/10/11 06:34, Lars O. Grobe wrote:
Hi!
@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... 
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
0
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.
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An example might be to use a typical sky definition (e.g. using gensky or gendaylit commands), and perform all of your calculations as if you were doing standard lighting calcs. You would get output in values related to lumens (in other words, human-response-corrected visible Watts of spectral radiation). And looking into the units, lux is synonymous with lumens/m2. If you calculate 100 lumens per meter square (lux) at a point using the standard methods, and you know your light source (for example daylight under those particular sky conditions) emits X Watts in the 120-400nm range for every lumen, then your UV component would be 100X Watts of UV.
You would not multiply your material definition by 4, but typically material reflectance is given for the visible or full solar range. You would have to know (or assume) the properties for UV. For example, a piece of glass with a PVB interlayer might have a 50% transmission in the visible range, but less than 1% transmission in the UV range. Not all materials might have that much variation between visible and UV properties, but you have to be sure that the material properties you use apply to the UV spectrum.
Also consider subtleties of the light source. Different levels of cloud cover probably affect the proportion of UV in the resulting radiation at the earth's surface. Electric light sources might be easier to deal with by converting units.
@Chris: do you mean I should scale my source and material definition by 4?
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Thank you both Greg and Chris. Now, I understand the concept pretty clearly.
Kind Regards,
Robert
···
On 24/10/11 22:17, Christopher Rush wrote:
An example might be to use a typical sky definition (e.g. using gensky or gendaylit commands), and perform all of your calculations as if you were doing standard lighting calcs. You would get output in values related to lumens (in other words, human-response-corrected visible Watts of spectral radiation). And looking into the units, lux is synonymous with lumens/m2. If you calculate 100 lumens per meter square (lux) at a point using the standard methods, and you know your light source (for example daylight under those particular sky conditions) emits X Watts in the 120-400nm range for every lumen, then your UV component would be 100X Watts of UV.
You would not multiply your material definition by 4, but typically material reflectance is given for the visible or full solar range. You would have to know (or assume) the properties for UV. For example, a piece of glass with a PVB interlayer might have a 50% transmission in the visible range, but less than 1% transmission in the UV range. Not all materials might have that much variation between visible and UV properties, but you have to be sure that the material properties you use apply to the UV spectrum.
Also consider subtleties of the light source. Different levels of cloud cover probably affect the proportion of UV in the resulting radiation at the earth's surface. Electric light sources might be easier to deal with by converting units.
@Chris: do you mean I should scale my source and material definition by 4?
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