With low key lighting, props are usually the only source of lighting in a scene. Instead of studio lighting, the director relies on lamps to emphasize how little of an effect light has on the scene and add mystery to characters. The use of lamps can portray a little bit of purity or good character that is left in a character; surrounded by darkness, the lamp struggles to keep up its “good character.” When the lamp flickers, it is the point where a character begins their descent into the criminal underworld. Shown in the image below, a character can also turn a lamp on or off, indicating a change in their tactics or overall motive.
Lamps can also be used in a comedic or more lighthearted way in film noir. In the following clip from The Big Sleep, a lamp is used to signify Humphrey Bogart getting an idea. The light especially sticks out in a film that’s dark for the most part.