I have been studying light units, intensities and such in order to provide a tutorial for artists. I also want to convert my LightPack to LuxCore and I'm trying to correctly match lighting intensities.
I am a bit confused by the default 17 lm/W luminous efficacy in BlendLuxCore.
I was wondering if it was a typo, as the only information I could find about white light luminous efficacy was from a comment in Cycles IES code:
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//The D65 standard illuminant has a Luminous efficacy of 177.83
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// Scale should be set to 179.0: the value of 179 lm/w is the standard luminous
// efficacy of equal energy white light that is defined and used by Radiance. It
// can be used here to produce the same output of Radiance.
// More information here: http://www.radiance-online.org/pipermail/radiance-general/2006-April/003616.html
- Cycles and Lux lights match each other (except spotlights) with identical Power, and Efficacy and Gain of 1.
- Without tonemapper, LuxCore Sun is approximatively a Cycles sun light with a Strength of 90 000. 90 000 is pretty close to the Sun illuminance of 98 000 lux, so this would somewhat validate that LuxCore outputs absolute irradiance in Lux.
- Cycles Sun unit is in Watts/m2 (according to Brecht), not lux (lumen/m2), so Cycles Sun should be set around 1050 (according to Wikipedia). Which means that if you want to match local lights between Cycles and LuxCore, you need to use an efficacy of 90 000/1050 = 85,7 lm/W. Which isn't 179 lm/W, but at least not as far as 17 lm/W.
Johnson Martin did some interesting math at some point to find out the Sun strength in Cycles should be set to 441 W/m2, which would give 204,1 lm/W.
Unfortunately his webpage isn't enable anymore. https://www.blendernation.com/2017/06/1 ... ss-cycles/
- Could it be that the 17 lm/W is a value taken from Wikipedia about Lighting Efficiency (or Overall Luminous Efficacy), which takes in account the luminous efficacy of radiation and the wall-plug efficacy? https://en.wikipedia.org/wiki/Luminous_ ... efficiency
If so, isn't it incorrect, as there is no notion of electrical consumption in CGI and Power should only be about luminous efficacy of radiation?
- I read the code Dade shared about the Power and Efficacy being divided by the color.Y value. viewtopic.php?t=942
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emittedFactor = gain * color * (power * efficency / color.Y());
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emittedFactor = gain * color;
In fact, Power is radiant flux which is not weighted by the luminous efficiency function. So it is expected that a green light feels brighter when using Power. If you divide the (Power*Efficiency) by color.Y, doesn't it mean that you are weighting it by the color luminance?
- That being said, I did some tests comparing using Gain instead of Power and Efficacy, and couldn't notice any difference depending on the colors. The difference are that the Spotlights are very slightly darker, and area lights are much darker. Point lights stay identical.
Is the color luminance weighting not working?
I hope someone can shed a light on these questions
Thanks!