Emitters do not emit any light when the Area Light is hidden (eye / camera icon OFF):
Emitters do light the scene when the Area Light is enabled (eye / camera icon ON) but the Area Light itself does not need any intensity for this to work and can be placed anywhere in the scene.
All other lighting (e.g. environment) has been disabled in the scene.
Area Light settings: these do not actually matter. It just has to be enabled for the emitters to work:
Sample file attached.
Attribution for the bug to BamBam on the Discord forum.
His Cornell box test file is also attached.
1 Solution
Further to this...
This is Cycles (or Eevee, same result) using Principled shaders instead of Uber and the Area Light is disabled. The scene is therefore lit ONLY by the Cubes. Result is as one would expect.
Switching back to RPR v2.0.52 with the Blender Emission node instead of Uber and the Area Light is still disabled (in fact, just delete it from the scene!).
The result is as you would expect. The scene is lit entirely by the emission cubes.
And now the bug.
Changing back to the RPR Uber Emission shader with the Area Light disabled (delete it if you want)... the result is that the scene is NOT lit at all by the emission cubes. Result is not as one would expect from a light source such as two emissive cubes. Therefore the Uber emission shader has a bug (which for some reason requires an Area Light set at intensity 0 to be present and enabled in the scene for the emissive surfaces to emit any light - very weird... how would any user know to do this?).
New file attached setup with Blender nodes instead of Uber... except for the emission material.
