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  • Published 14 Jun 2022

Lumen units for Light strength

It would be a great help for Archiviz users to be able to use Lumen as unit for Light strength as Watts are not so representative in real life situations.

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Development Status: Open

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marin1
14 Jun 2022

3 Comments

Lauloque

I think the reasons as to why Lm isn't an obvious choice in a software like Blender (or at least Cycles if I understand) is well summarized here:

Mazay --- Oct '19

You need to take in account color of light to calculate correctly.
One lumen of green light equals to 1/683 watts or 1.146mW
White light is less visible than green so it would have bit higher watts.
(...)

troy_s --- Oct '19

The reason that Blender uses Watts / Joules is because the energy in a path tracing engine is a physics unit of radiant flux . A luminous flux based measurement is perceptual, and has no place in a light transport system as it varies with the spectral composition of the light. There would be all sorts of adverse side effects trying to use a perceptual based energy inside of a path tracing engine.

singularity --- Nov '19

From a physical point of view a light power measure of W / sr (or Watts per stereo radiant) would make most sense, because you are independent from the shape of the original light source (e.g. point, spot or sun). It simply gives the power radiated power on a square radiant area on the unit sphere and can be compared easily. However from an artistic point of view this may not be the favourite choice as Mazay pointed out, but the alignment on empiric data is hard as well, because the perception is still slightly different for each individual.

Source: Why Watt as light value? - User Feedback - Blender Developer Talk

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thinsoldier

There are at least 2 addons that address those problems. It is possible for the Blender devs to investigate using those existing solutions.

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Lauloque

I am not saying it is impossible, I am saying a proper implementation is not that simple.

Though, it all depends on how you want to fix the problem.

If you just want the values to display lumens and do not care about the rendering being physically accurate to what lumens are, then you can just multiply the W by 683 and display the result labelled as Lm. It's simple and that's what many addons do like this one:

GitHub - s-leger/blender-light_luminous_intensity: Provide light luminous intensity (candela and lumen) for lights

Eventually you could at least use the actual formula instead of a huge approximation:

Φ V(lm) = P (W) × η (lm/W)

If you want the render engine to interpret it accurately, it's not just a unit conversion, it's changing how the path tracer needs to treat light.

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