Why is the strength unit of a light so drastically different for Radeon ProRender?

The first image is Cycles. I had to set the area light (directly above the cube) to 1000 W to achieve this level of brightness. At this point, if I switch to Radeon ProRender, it looks like the second image below. I had to change the light intensity to 100 mW to achieve this level of brightness like the third image.

My question is, why is the light intensity so drastically different? I mean, I could understand some slight differences, but this is 1000 W vs 100 mW, a 10,000 times difference.

I don’t use Prorender, but does HDRIs produce the same difference? If so, can it be just a difference in reference exposure? If you set exposure in color management to like -6 or -8 stops (if Cycles is 0), is there a fixed conversion factor/offset you can use?

Is that a 2m default cube? How high is the Area Light above it? I’m not sure I understand… is the second ProRender image being lit by the equivalent of a 0.1W bulb? That just seems weird.

The settings are as below. It is the default cube, the area light is about 10 metres above the plane. If I set 1W, it is too bright like the screenshot. 100 mW seems adequate.

Could it be be a scaling issue? It looks like you’ve scaled the area light with S. It’s showing an X and Y scale of 100 while reporting a size of 0.1m in the Lights panel. Perhaps ProRender is affected by that? You could test it with two area lights, one scaled up with S and one scaled up using using Size under the Lights panel.

With the renderer set to ProRender and Viewport to Rendered mode, scaling the area light with “S” did gradually made the scene brighter, unlike under Cycles. I wonder if this is a bug or an intended behaviour. Either way, either ProRender or Cycles need to change its behaviour for consistency…

Applying the scale (Ctrl+A, Scale) somehow did not work correctly with a light, so I guess I have to manually enter the light’s size in the text boxes from now.

I know nothing about ProRender, but I think if you uncheck Intensity Normalisation it might behave like Cycles. AFAIK the Cycles behaviour is intended. For Area Lights the scale/size only impact shadow softness. Users set the desired Wattage and it does not vary according to size. Mesh based lights with Emission shaders work differently though.