Design Nomenclature / Light & Optics

Form shadow

Also called attached shadow, body shadow

The region of an object turned away from the light — unlit because of its own orientation, not because anything blocks the source. Its boundary is the terminator, its edge softens in proportion to the source's angular size, and almost everything in it arrives as indirect light.

A sphere in section with its value steps named around the curve: specular, diffuse highlight where the normal points at the key, halftone, the terminator where the normal is perpendicular to the light, and beyond it the form shadow — all of it on the object itself, none of it thrown onto anything.

Not to be confused with

Cast shadowThe shape an occluder subtracts from another surface's illumination. Unlike a form shadow it has a projected outline governed by the geometry between blocker and receiver: it sharpens as the two approach contact and softens as they separate, regardless of the object's own shading.
Core shadowThe darkest band within a form shadow, lying just past the terminator, where the surface has turned fully away from the key but has not yet swung around far enough to face whatever is bouncing light back. Its presence is what stops a shaded sphere reading as flat.

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