A key light refers to the principal source of light in a scene. It is the most intense light on the subjects in frame.
Key light placement is KEY in scenes of film noir. For purpose of placement, think of the subject of the frame being the center of a clock. In film noir, directors tend to place sources of light at the 3 o’clock position (directly to the side) or at the 12 o’clock position (behind the subject). This positioning is called fill light. The 3 o’clock position divides the character’s face into into halves, symbolic of that person’s duality or tough decision.
The 12 o’clock position creates a halo effect, adding an ominous mood to the current scene and a mystery to the character (image below). The entire scene with the reporters in Citizen Kane is a great example of the halo effect. The halo effect is employed to move the emphasis away from the reporters and towards Kane. The audience is left with a sense of mystery surrounding the reporters, and this mystery persists for the rest of the movie.