How are photon's distributed across multiple lights?

Development Rendering
1

I’m lighting a building interior scene. I started with setting up my Directional Light (sun) and Skylight, and got my interior fully lit with just the sun/ambient/bounce light. Looks really nice, exactly how it should in real life if it were only lit by daylight.

Now I want to add some interior/ceiling lights (spots) into the scene. But I notice, as soon as a few of these lights are added, my sun’s bounce light is noticeably different. It looks as though the sun’s bounce light has been greatly diminished. This is especially noticable in rooms that don’t have any of these ceiling lights, and should only be lit by the sun. They are dramatically different looking, compared to when there are no interior lights in the scene.

What I assume is happening is there are only a set amount of photo’s being scattered by lightmass. So when I add a bunch of spot lights to the scene, a large percentage of those photon’s are being used for those spot lights, with only a small percentage being allowcated to the sun light. I noticed if I turn the “Indirect Lighting Intensity” to “0” on these spot lights, my sunlight GI/bake is now looking as it should.

Is that more or less correct? And if so, is there a way I can keep the indirect lighting on the spotlights, without affecting my primary directional light?

2

Some further discoveries. If I reduce the attenuation radius, or cone angle on the spot lights such as they don’t interact with too much geometry, my indirect lighting bake goes back to normal. However, with those same settings, if I just move the lights closer to the floor, the GI bakes incorrect again.

So basically, if I want my GI to bake correctly, I need to reduce my spotlights to the point where they barely interact with any geometry around them (or turn indirect lighting off entirely)

This can’t be correct, can it? That you can’t have a scene lit primarily with indirect lighting from the “sun”, and then introduce additional spotlights that also have indirect lighting? This seems like a very basic setup, and I’m not sure where I could be going wrong.

3

Another finding. I can backup the attenuation radius of the spot light right to the point where it breaks the GI (200 works, 250 does not). But when I’ve got a working value (200), if I drop the light bake quality from High to Medium, the GI suddenly brakes again.

So some setting that’s getting boosted in the lighting quality is having an impact on whether the light bakes correctly or not