Emitters: radiant power vs luminous power

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Emitters: radiant power vs luminous power

Postby lance_uppercut » Fri Feb 23, 2018 9:48 pm

lance_uppercut Fri Feb 23, 2018 9:48 pm
I'm using Octane Standalone, and I'm hoping someone could clarify a few questions about how Octane deals with emitter power. Unfortunately the official documentation seems unclear on the units of power being used:

- The documentation suggests that the 'power' value is the real world wattage of the light source. Is Octane expecting the electrical power the lamp consumes (i.e. the 100 Watt light bulb noted in the manual), or is it the total Radiant Power (in Watts) that is being emitted? These are very different things. If it's wattage consumed, then I should enter in the texture field, a 'wall-plug efficiency' value of about 2.5% (0.025) for an incandescent bulb, which is also the Octane default. But if it's radiant power emitted, then the value should be somewhat higher, about 6.5%, since i'm only looking at the proportion of radiant energy that is in the visible spectrum.

- a few posts by bpeg4d note that a factor of 1/683 should convert Luminous Power (Lumens) to Radiant Power (Watts). This would support the assumption that we're talking about Radiant Power. Wikipedia's article https://en.wikipedia.org/wiki/Photometry_(optics)#Watts_versus_lumens on photometry also gives this conversion factor, but notes that is a value for an ideal monochromatic source at 555 nm (green), which produces 683 lumens per watt and was selected to define 100% luminous efficacy. But for a typical LED, fluorescent, or incandescent source, there can be a dramatic difference in luminous efficacy. Does Octane take care of this based on the blackbody emitter's output colour? If I only know that my luminaire produces 800 lumens, then by entering an efficiency value of 1/683, I take it that I'm basically telling octane that the radiant power going out into the scene is 1.1713 Watts, and that all of it is this particular green...is this correct?

- How are emitters with 'surface brightness' scaled with respect to their area? I have below an example scene where two emitters in separate boxes each have identical emitter materials, except that the one on the right is 'surface brightness'. The emitters are both 1 square metre, have not been scaled in the placement node, the camera has a linear response and no gamma applied. Assuming that 'surface brightness' means watts per square metre, these should be the same brightness, but clearly they're not. In fact, the one on the right needs to be 2.2 times brighter to look the same.

Octane_surface_brightness.png
Comparison of two 1m x 1m plane emitters. One on the right is surface brightness, otherwise identical to the one of the left.


- IES files contain all sorts of information about a luminaire's distribution, but also the luminous intensity (in Candelas) in a given direction. When loaded into Octane, it looks like the values are simply normalized so that the maximum intensity is translated 1.0 or white, is that right?

- When 'surface brightness' is turned off, the total light output is supposed to be the same no matter what the emitter's area is. But from my tests it looks like Octane doesn't take into account IES or greyscale distribution patterns, which makes using accurate IES lights pretty hard. My workaround right now is to read the maximum candela intensity from the IES file, multiply by 4π to get the lumen output for an isotropic, spherical emitter, and then use that as the Lumen value (assuming that the factor above can really be used to turn that into Watts). Is there an easier way?

- Octane developers: please switch to photometric units! Support for IES sources in Standalone would also be appreciated by all us architects and designers. Trying to physically model a lamp, shade, lens, and everything is not realistic or good for performance.
Thanks!
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Re: Emitters: radiant power vs luminous power

Postby J.C » Tue Feb 23, 2021 8:42 am

J.C Tue Feb 23, 2021 8:42 am
- Octane developers: please switch to photometric units! Support for IES sources in Standalone would also be appreciated by all us architects and designers. Trying to physically model a lamp, shade, lens, and everything is not realistic or good for performance.
Thanks!


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Re: Emitters: radiant power vs luminous power

Postby Lewis » Tue Feb 23, 2021 1:35 pm

Lewis Tue Feb 23, 2021 1:35 pm
Agree and it's been asked before and discussed here in forum that value for light should be Lumens and not Watt.
Maybe is best go to Feature request and submit that to official FR channel and then let us know so we can vote also ?

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