Fill Light is a secondary light source used to reduce the harshness of shadows created by the key light, softening contrast and revealing detail in darker areas of the frame without eliminating shadows entirely. It is one of the three foundational lights in the classic three-point lighting setup used across photography, film, and video production.

The fill light is typically positioned opposite the key light and set to a lower intensity, often half the brightness or less. Its purpose is not to create its own shadows or serve as a primary light source but rather to gently lift the shadow areas created by the key light, making them less impenetrable while preserving the overall directionality and mood established by the key. The ratio between key and fill light intensity, known as the lighting ratio, determines the overall contrast and dramatic quality of the image. Higher ratios with minimal fill create high-contrast, moody imagery, while lower ratios with stronger fill produce softer, more evenly lit results.

In AI image and video generation, understanding fill light helps creators describe lighting setups more precisely in prompts. Referencing fill light or three-point lighting in generation instructions gives models context about the intended lighting structure, helping produce outputs with more intentional and cinematically grounded illumination rather than flat or randomly lit scenes.