In addition to the obvious role of making actors visible in a darkened theatre, good lighting design can help establish the mood and setting of a play. In addition, lighting can be used to draw focus to a particular actor or area of the stage.
A variety of lighting instruments can be used to light the stage. These range from the ultra-bright, state-of-the-art Source 4's to the more traditional PAR cans. Spotlights, strip lights and projectors can also be used by a lighting designer. In some newer theatres, the lights can be operated by remotes or by computer, but for the most part they are operated by a variety of lighting control boards.
Gels are thin sheets of colored plastic used to change the color of the light being shined on the stage. Gels come in all colors of the rainbow, as well as in what are called frosts. These are transparent gels that are textured and used to diffuse or soften the light.
Gobos are cut-out metal stencils placed over the light source in order to cast shadows. Gobos can be used to create a variety of effects such as clouds, sun-dappled forest floors, windows, doors, foliage, fire and many more.
In addition to on-the-job learning, there are a variety of programs all over the country to study lighting design. You may choose a BA, BFA, or MFA program at a college or university near you.