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 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 also called a "hair" light because it is used to create the appearance of physical separation between the subject's head and background.

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 also called a "hair" light because it is used to create the appearance of physical separation between the subject's head and background.

A typical studio lighting configuration will consist of a fill source to control shadow tone, a single frontal key light to create the highlight modeling clues on the front of object facing the camera over the shadows the fill illuminates, one or more rim/accent lights to create separation between foreground and background, and one or more background lights to control the tone of the background and separation between it and the foreground.

There are two significant differences between natural lighting and artificial sources. One is the character of the fill and the other is more rapid fall-off in intensity. In nature skylight fill is omni-directional and usually brighter from above. That "wrap around" characteristic is difficult to duplicate with a directional artificial source.