For example, a delicately etched sheet of thin metal would produce the illusion of light falling through a cell’s bars, or sunlight dappling through tree leaves, or a cross of light on a scenery flat…

Funny, for me, coming from a film rather than stage background a gobo is more like a flag, for blocking off light, and a cookie (or coocoloris) is the one with holes in it to cast interesting shadows.

Ceci n'est pas un seing.