Area and point lights units


I’m looking for info about the units used for area and point light.

According to the documentation, all emitting objects that are not mesh or distant light use Radiant Power as a unit.

But what is Radiant Power in the field of lighting ? I can’t find any reference or order of magnitude to do some tests and find my marks. ( coming from V-Ray, so I’m a bit lost )

When I had a doubt about lighting in V-Ray I could use the physical camera to guess the power but it’s not possible here.

Thank you

I just patted myself on the back… I recalled watts. Low and behold I recalled correctly!

Thanks for the link @HISEROD, but I’m afraid I don’t understand this :

Lamp strength for point, spot and area lamps is specified in Watts. This means you typically need higher values than Blender Internal, as you could not use a 1W lamp to light a room; you need something stronger like a 100W lamp.

So does it mean that Watts are consumed watts ? But if so, shouldn’t we be able to specify the luminous efficiency in lm / W ?

I’m trying to light a room with some spotlights, it’s completely dark with a value under 1 000 000.

Below are some tests with various values : 100, 1000, 10000 and 1 000 000.

I could use exposure but it really lacks some camera-like controls to get some guides about exposure. And with standard values like 100, 10 stops aren’t even enough to expose properly. ( not to mention the noise ).

Maybe you need to make sure you are using metric/English units for correct scale?

Also this may help.

I already setted up my scene in centimeters from the beginning, and the dimensions are correct from what I can see.

When I was looking for a proper way to use real world camera values I’ve found this addon, but it just seems to tweak the Exposure slider in the Color Management panel. So if the addon is working correctly, I guess something must be wrong in the scene.

Yeah, while Blender just uses arbitrary units, Cycles is hardcoded to use 1 unit = 1 meter, so if your room is built at say 1 unit = 1 cm, then naturally the lights will require ridiculous ‘wattage’. Check in the units panel that Blender is set to 1 Blender Unit = 1 meter.

Yes, it is the issue.
I didn’t about this, it should be explicitely said somewhere when using Cycles, I didn’t find it in the docs.

Below a sample scene with units in meters, far better :

One important point when talking about light units in Cycles is that Cycles works with radiometric units, not photometric units like most software that is specifically for lighting design. Therefore, Watts here doesn’t mean the energy that a lamp consumes, but the actual energy of the emitted radiation.

Converting radiometric to photometric units unfortunately isn’t just a simple factor, it actually depends on the color (for example, 1KW of infrared radiation still is zero Lumen because humans can’t see it). For details, check e.g. this.


I sometimes do the inverse; I use the addon to set exposure values for a given situation (i.e. office lighting), then I tweak the interior lights to something that I would expect. Material wise, try to keep material albedo values within reason (using PBR cheat sheets), and minimize energy loss by observing the difference between reflective caustics on and off - that difference is then “cheesly made up for” by a “if diffuseRay, replace material by pure diffuse” setup. It won’t look the same (depending on glossy roughness of course), but at least it won’t go very dark in high gloss setups.

Is cycles the only renderer that doesn’t have reference absolute lighting or units alternative that make sense?

That addon should be vanilla and activated by default. It is key for anyone not PhD to figuring out lighting values and exposure. No light manufacture data can readily be plugged in anywhere.