The lamp is the second eye
One photograph, one lamp whose position is known, and a point's place in space comes back to 9e-16 m — the camera's ray through the point, the lamp's ray through the image of its shadow, and the intersection of two lines. It is triangulation with one of the two eyes replaced by a light, and it degrades exactly like a stereo pair: 5.9 mm of depth per pixel at 39° between the rays, 1 mm at 15.4°.
part 4 · light