Why Watt as light value?

yesterday there was a commit in the way that Eevee Lights displays the energy value its a step in the right direction but ¿why Watt?
The amount of energy that something emits per watt varies from thing to thing, its not the same a led light than a incandescence light.
The usual way to measure “light energy” are the Lumens (Lm) Luxers (Lx) or Candelas (Cd)

what type of physical light is eevee emulating?
(not sure about the veracity or pressition in the image took it from google, but thats the idea 1 watt unit on different devices are different values of visible light)


Lumens make the most sense, since it is the quantity of visible light emitted from a source

Watts are a unit of power. You can measure the electrical input of a light bulb in Watts, and you can measure the light output of a light bulb in Watts. Presumably, Blender’s units are referring to light output, since Blender’s lights don’t use electricity. It ought to be possible to support both, though, considering they convert to each other by a factor of 683.


so if i have a lamp in my hand (a real lamp in real life) that says 500Lm y could multiply 500x683 and more or less, becasuse the wavelenght could change a bit from warm to cold, i could use that result in a eevee point light to emulate the real lamp
Is that correct?

You may want to ask whoever added the changes and maybe get the reasoning behind. Blender rendering in UV and IR next?

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If I understand it correctly, Eevee lamp units have always been watts, so this change is only to make that explicit and doesn’t actually change the behavior otherwise.

Cycles and Eevee lamps still need to be unified (IIRC default Eevee lamps are 10x as bright as Cycles currently).

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Because this is now part of the units system, we should be able to add a way to display light values in Lumens (Lm) Luxers (Lx) or Candelas (Cd), by adding an enum menu of options in the Scene > Units section.

This might be a good task for a new developer to create a patch for this.


Will it be Watts for the final version? I was writing a thread but I saw this one already and we are in RC1 version and no changes on it.
The light industry is moving away from Watts to Lumens because incandescent, halogen and Led lamps use different power to create the same amount of light. So it really doesn’t make sense to keep it as Watts. Even The Emission option is another kind of value to ‘generate light’. (Edit) I just checked and even Sun looks to have another kind of value!

If it goes in the final version will mean it will be harder to change it later. But if it is just a matter of unit system choice, it would be easier to fix, I guess (no coder here). But would be better to have a combo box menu to select it close to this value.

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Blender just needs to set / agreed to a default for the representation of light intensity in the value’s of Lux and Lumens.

Like Dialux or Relux has done as well. It’s not a real representation of light intensity; but a suitable one to do the job right.

A good formula is needed to take different factors in to matter. A conversion from Watt’s/Square meter is easy. But to find a good relationship between the factors Lumens > Lux > Candela; all need to precisely interact within the Filmic color management to get a close to an accurate representation of the real world values.

Right now the representation is not accurate at all and confusing. Like; when using an IES file. It contains all of these data (Lumens > Lux > Candela), but still; after importing the IES file; none of the point light settings are adjusted correctly.

In fact, when you try to get as close to the representation of brightness of a program like Dialux; you’ll need to manually set the power to 1 Watt, size; also defined by IES file; would set it to 0 or the size of the armature etc.

Hope they fix this soon…

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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.



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.


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.
My suggestion would be to switch to W / sr and give a conversion to lumen etc. below. Are there any additions or comments to this?

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Would it be ok to use lumens and other photometric units if they are normalized by the color luminance?
Or would it still be inaccurate?

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I suspect that it would yield very “non intuitive” results due to how large the discrepancy is between the radiometric energy and the luminous efficacy curve.

It’s already “non intuitive” enough to point out that a blue light requires much more radiometric energy to appear as luminous as a green light. Also, from a practical level, managing all of this within the confines of a colour management system is extremely challenging to do correctly.