Lighting is a very significant device of theatre. It is not just a decoration or a put-on---something superficial and imposed from the outside. Lighting can make or break a performance. In fact, its role in theatre is functional and it participates semantically and semiotically in the theatrical meaning-effect. Lighting determines the stage-space, develops the spectacle, defining and limiting it. Its subtle shades create the differential pattern of theatrical meaning. The colour of it has obvious symbolic connotations too.
To give an example from 20th century theatrical practice, lighting in Samuel Beckett's theatre is an elementary tool of communication. In the tiniest play of the world, Breath, Beckett writes down the absolutely exact configurations of light by using a hypothetical range to talk about its increase and decrease. In a play like Not I, there is no full human subject, but a mouth and it is the spotlight that effectively renders this reduction. In Play, the lights become a character--that of the interrogator. The lights keep flashing from one urned face to the other to extract compulsive speech from him or her. The light in theatre makes not just the audience but also the actors captive. Beckett's meticulous insistence on a grey light on stage is a definitive articulation of a post-war light, hazed with the traces of bombardment and devastation.