Glossary: Key / fill light
In classical Hollywood cinema, the brightest illumination (and casting the strongest shadows) in the scene is called the keylight. The keylight often corresponds to the motivating light source in the setting. Illumination from a source less bright than the key light, commonly used to soften deep shadows in a scene, is called the fill light.
From Rear Window
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Frame 1: Keylight on Jeff (motivated by the lamp in
the foreground)
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Frame 2: Fill light on Lisa (the keylight comes from
the right side of the image; a fill light is directed at Lisa's face
to soften the shadow)
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