So you can add each light individually and recreate what they look like when they’re all on. But what if you’ve got light pollution? Originally I did this with a closed bathroom door and a towel along the door seam. What if you’ve got a bathroom window, or an open door?
I took a starting shot with no lights on, but WITH hallway pollution. Then, one shot per six light sources (3 bulbs, a bedlamp, my cell phone flashlight, and another flashlight). Each of those shots also had the light pollution.
But hey! Light pollution + a light source is still linear addition. So with that light pollution establishing shot, you can use linear subtraction.
First I add a layer to the hallLights. Then I subtract a duplicate of the hallLights layer. Then add another layer, and subtract again. Add subtract add subtract. The final result is identical to the photo where all 6 lights were on! (7 lights, if you count the light pollution.)
What’s cool about light is that, in linear space, it literally just adds together. I’ve known this in 3D for a while, but it’s quite different to try it out in the real world. Here’s two pics from my bathroom. The first: with 3 light bulbs on, plus my cell phone light and a flash light. The second: each of those lights shot individually, and all the photos set to linear dodge in Photoshop. Except for tiny errors (camera wiggle, me getting in the shot) they produce identical images.