Light Values - Lumen instead of Watts

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Light Values - Lumen instead of Watts

Postby SamuelAB » Fri Jun 30, 2017 8:36 pm

SamuelAB Fri Jun 30, 2017 8:36 pm
I come from the architecture and lighting industry. I'd like to mention that we do not use Watts to specify the strength of light emissions, we use lumen. It would be helpful if Octane could use lumen instead or in conjunction with Watts.

Why do we not use Watts? Because lumens are produces as an efficiency ratio from Watts. For example some lamps can have 5 lumen/W or 4.2 lumen/Watt. We don't usually care about this ratio and it is calculated by taking the Watts and lumen values provided to us, it's not provide as a ration on it's own. So... we usually have no idea what the ratio is. This means we have no idea what Watt value we should choose in Octane, or even what lumen/watt ration it would use.

Please provide a lumen value and rethink the Watts value. It's a detriment to people trying to do accurate architectural renders. We don't want to guess our light values. Thanks!
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Re: Light Values - Lumen instead of Watts

Postby bepeg4d » Sat Jul 01, 2017 5:53 am

bepeg4d Sat Jul 01, 2017 5:53 am
Hi SamuelAB,
please, have a look at this other discussion for converting Lumen values in Octane
viewtopic.php?f=30&t=42275&p=313725&hilit=lumen#p313725
The key value to remember is 1/683 in Eficiency/Texture.
Happy rendering,
ciao beppe
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Re: Light Values - Lumen instead of Watts

Postby SamuelAB » Sat Jul 01, 2017 10:48 pm

SamuelAB Sat Jul 01, 2017 10:48 pm
The texture is the efficiency value? So a white texture will give 100 lumen per Watt?

This is a little ridiculous, I don't think I've seen such a cumbersome and esoteric lighting system anywhere else.

Nonetheless, thanks for the help!
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Re: Light Values - Lumen instead of Watts

Postby aufwind » Sun Jul 02, 2017 6:20 pm

aufwind Sun Jul 02, 2017 6:20 pm
as an architect i'm also quite struggling with real world ies lights in octane.

According to DIAL https://www.dial.de/en/blog/article/efficiency-of-ledsthe-highest-luminous-efficacy-of-a-white-led/
the theoretical maximum luminous efficacy is quite dependent on the spectrum. Since different types of lamps have different
spectrums, even the efficacy differs dependent on the lamp type (If I understood the article correctly).

So my question is the mentioned 1/683 ratio (at green light) to be used for all lamp types without
taking the real emitted spectrum into account still the correct approach?
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Re: Light Values - Lumen instead of Watts

Postby bepeg4d » Mon Jul 03, 2017 7:55 am

bepeg4d Mon Jul 03, 2017 7:55 am
Hi,
for any kind of light source you can have the power in Watt, and the efficiency, and it depends on the light sorce dimensions, but in Octane you can tick the Surface brightness option to remove this dependency.
Since the luminous efficacy is quite dependent on the spectrum, and since Octane is a spectral render engine, instead of an RGB node, it is far better to use a Gaussian spectrum node in the Texture/Efficiency pin:
viewtopic.php?f=9&t=23669&p=104007&hilit=gaussian#p104007
If you prefer to work in Lumen, just use the Lumen value instead of Watt, and use the 1/683 ratio in the Power value of the Gaussian spectrum node.
With this approace, you can correctly mimic any kind of light. In my opinion there is nothing esoteric here, just physics and math.
Happy rendering,
ciao beppe
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Re: Light Values - Lumen instead of Watts

Postby SamuelAB » Thu Jul 06, 2017 9:27 pm

SamuelAB Thu Jul 06, 2017 9:27 pm
Thanks for the clarification @bepeg4d. I have no idea what a Gaussian Spectrum is.

It is my understanding that I can put a Gaussian Spectrum node and set it to 0.0014641288433382 (1/683) for Efficacy. Then use lumen values in the Watts section?

Thank you!
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Re: Light Values - Lumen instead of Watts

Postby bepeg4d » Fri Jul 07, 2017 5:56 am

bepeg4d Fri Jul 07, 2017 5:56 am
Hi SamuelAB,
let's say in this way, you need to set the efficiency to 1/683 to work with Lumen instead of Watt. If you are using Blackbody emission, you can use set the 1/683 value directly in the float value of efficiency.
If you are using a Texture emission node, better to use a Gaussian spectrum in efficiency, with 1/683 in the Power value of the Gaussian node.
ciao beppe
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Re: Light Values - Lumen instead of Watts

Postby SamuelAB » Fri Jul 07, 2017 9:05 pm

SamuelAB Fri Jul 07, 2017 9:05 pm
Many thanks!!!
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Re: Light Values - Lumen instead of Watts

Postby wisemanxxx » Fri Jan 12, 2018 6:46 pm

wisemanxxx Fri Jan 12, 2018 6:46 pm
This information about using lumen values is really useful. Thank you. But I can't be sure about surface brightness. For example I use a circular surface for spot light. I set power to 850 lumen and effiency to 0.0014. But when I leave surface brightness disabled it is not looking right. Light seems very strong for 850 lumen.Surface' diameter is about 8-10 cm for example.
When I enable SB option it is very dark.
What is the right way for this situation?
I've 3ds max plugin 3.06 4.41.
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Re: Light Values - Lumen instead of Watts

Postby paride4331 » Mon Jan 15, 2018 11:01 am

paride4331 Mon Jan 15, 2018 11:01 am
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