Is there a formula

If I want to match an emission shader to a real light (point or area), is there a formula to convert watts to emission value? At the moment, I’m re-rendering to get the best match, but would be nice to have a simple formula to do it.

There is a reason for this. Sometimes in preview I use real lights for a light test using a material override (so can’t use the emission), and then switch to geometry without the override for the final render, and so want to be able to match the final lighting to the light test.

EDIT. For info, using a 10 watt area light my current scene is coming in at around 18.5W (i.e. multiplier of 1.85) to match an emission strength of 100, but will this be consistent?

Hi Roken,

Unsure of the formula but perhaps the Photographer 4 addon may help in the area. You could inquire with your issue with the author as he’s very knowledgeable regarding Blender lighting.

Cheers and stay safe!

Thank you. I agree with observations that it isn’t a rounded 2.0, but it seems I may be around correct with 1.85, and it does from that thread suggest that it scales at that value.

lights= watts

sun and emission=watts/m(squared)

https://docs.blender.org/manual/en/latest/render/shader_nodes/shader/emission.html#emission