Lighting creates the 2D pattern of contrast the brain interprets to recognize 3D objects in photographs. In an in-person viewing experience, the brain relies on stereoscopic vision, parallax, shifting focal in addition to the clues created by the highlight and shadow patterns the light on the object creates. When viewing a photo, the brain tries to match the patterns of contrast and color it has seen to those other sensory memories.

To differentiate that role from that of "key" modeling when a modeling source moves behind the object, it is typically called a "rim" or "accent" light. In portrait lighting, it is also called a "hair" light because it is used to create the appearance of physical separation between the subject's head and background.