Thought I’d try and dust this godforsaken answer off once and for all.

Q: What is the unit of measurement of light energy in Cycles?

This has been asked of me far too many times, and a quick Google search reveals even more. Couple that with completely confused folks conflating watts with electrical watts, and photometric units that are completely wrong, I thought I’d step up as formal idiot and try to get a clear answer. I believe @brecht is the only person who can answer this conclusively.

To the best of my knowledge, we divide “energy” up into four classes, only two of which are relevant to a path tracer:

- Radiance
- Irradiance
- Luminance
- Illuminance

First, the difference between the terms and their “ir” counterparts. The “ir” prefix typically refers to *exitant* values. The non-“ir” values are incident. That is, in layperson’s terms, outgoing versus incoming in relation to a surface.

Second, Luminance and Illuminance are *photometric* units, meaning they are psychophysical and wound up with more complex ideas of spectra. Therefore, the terms and concepts are not relevant to this discussion.

That leaves us with radiance in Cycles. In the documentation, the unit of measurement is listed as watts per square meter, which *I believe* cannot be the appropriate definition as it would be missing the geometry aspect to calculate the energy. I *believe* the correct unit here is watts per square meter per steradian, which is the traditional unit of measurement for radiance by all accounts.

So, once and for all, is the base unit of light energy indeed the radiance based watt per square meter per steradian @brecht?

Thanks so much.

Additional Question: Given there are different classes of lights, it also stands to reason that the units of energy vary between them. Is this the case?