The first, most basic purpose of stage lighting is to ensure that the audience can see the actors, as well as any props or background that’s significant. Lighting has to be bright enough and in the right place so people from every part of the theatre can see the action in adequate detail.
The reason stages aren’t only lit from the front but top, sides, back and even the floor, is to give it depth and dimensionality. Lighting only from the front would make the stage look flat. The use of shadow and contrast emphasizes particular shapes and forms, as well as the layered aspect of a stage setup. This is particularly important in plays when shadows and other dark enclaves emphasize fear and mystery.
Just as movies use close up shots to focus the audience’s attention on a particular part of the action, plays use lights to focus your attention. Lights tell you where you’re supposed to look and what person or part of the stage is the most important at that moment.
Lights give key clues to the setting of a scene, like what time of day it, for instance, or the atmosphere of a particular room. Harsh electrical lights conjure up the feel of a modern doctor’s office, while a scene from a historical play would use soft lighting reminiscent of candle light.
Directors always build the composition of any on stage scene carefully, placing scenery, props and actors in just the right spots to achieve the overall visual effect desired. Lighting is an essential part of that, making only the desired portions of the stage visible, in proportion to their importance.
Lights have a significant effect on the mood and style of a play. A scary scene, for instance, might have lights from below to make the actors’ faces look harsh and strange. A fantasy sequence might feature multi-colored dancing lights.
Lighting changes emphasize the pace and rhythm of a play. A quickly moving, chaotic story would have constantly shifting lights to reflect the shifting action. Sustained lighting indicates a sustained mood and more slowly unfolding scene.