Lux tenebrae
Two coherent lamps over a floor, and black stripes wherever both reach: the second light made parts of the floor darker than one lamp alone.
Add a second light, and parts of the floor go dark.
Two lamps side by side throw waves across a floor. Where the crests of one meet the troughs of the other they cancel, and the floor goes black: a fan of dark stripes spreading down the frame from between the lamps. On those stripes the floor is darker than it would be under either lamp alone.
Once a loop the second lamp fades out. The stripes hold until it is about half gone, then fill in, and with one lamp the floor is an even grey with nothing on it but the waves themselves, rings travelling outward. As the second lamp comes back, the stripes cut back in exactly where they were.
The frame is a lit field, not a dark one: this is the one piece in the series where the darkness is the subject rather than the ground. Nothing reaches white.
Rendered offline from a single closed-form shader by the same module code a live page would run. No camera, and nothing leaves the machine.