A rim light is a light source positioned behind and to the side of a subject, directed toward the camera at a low angle, that creates a thin line of illumination along the subject's edge - their hair, shoulder, or profile - separating them visually from the background. Also called a back light or separation light, rim lighting is the third element of the classic three-point lighting setup, providing the edge definition that prevents subjects from merging into dark backgrounds.

The visual effect of rim lighting is immediately recognizable and strongly associated with cinematic production quality. The bright edge along a subject's silhouette creates a halo-like separation that adds dimensionality, depth, and a polished, deliberately lit quality to the image. The color and intensity of the rim light can be varied for different effects - a warm golden rim suggests sunlight coming through a window behind the subject; a cool blue-white rim creates a moonlit or studio quality; a colored rim can integrate the subject into an environmental light source in the scene. Rim lights can be positioned to illuminate only one side of the subject for a more dramatic, asymmetric effect, or placed symmetrically to create a dual-rim setup that outlines the full silhouette. The strength of the rim relative to the key light affects how prominent the separation effect is.

Referencing rim lighting in AI generation prompts reliably produces the characteristic edge separation effect. Descriptions like "strong rim light separating subject from dark background," "warm backlight creating rim illumination on hair and shoulders," or "cinematic three-point lighting with visible rim light" communicate this specific lighting quality and tend to generate imagery with the polished, dimensionally lit look that rim lighting characteristically produces.