Exposure mismatch between Lumen and Pathtracing

Development Rendering
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I’m using UE5 for archviz rendering. I have realized that there is a exposure mismatch between Lumen and Pathtracing. I am already aware that lumen tends to render a bit yelowish, and that Pathtracing have more light bounces and light spreads more evenly throghout the scene.
But this is something different. When I render the viewport using the same exposure for Lumen and Pathracing (I have locked EV values), I get a good result with a matching exposure.

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But when I render the same viewport but with a exposure compensation of 1.5, I start to get difference between Lumen and PT:

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An even more noticeable when going higher exposure compensation, 3.0, for instance:

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Any idea on what it is happening, or what can I do to be consistent when offsetting the inicial exposure?

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I do have a few guesses on that front:

  1. Lumen’s contact AO that’s darkening the corners is (on default scalability) basically just painted-in SSAO that’s not radiometrically accurate (AFAIK) so that’s probably one part. Try disabling it (it’s pretty superfluous at high scalabilities) and see what that gets you.
  2. The surface cache is radiometrically lossy by nature, so if there are any issues with your lumen scene coverage that’s going to result in a lack of bounce.
  3. The lumen team talked about this a bit, but lumen (at least as of 5.0, maybe updated since) isn’t strictly physicalized to radiometric units, so if you’re using physically correct values, that won’t give exactly the same results as the values the engine would like.
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