<Li> Lens or opening--the gap in the housing where the light is intended to come out . </Li> <Li> Reflector--behind or around the light source in such a way as to direct more light towards the lens or opening . </Li> <P> Additional features will vary depend on the exact type of fixture . </P> <P> Most theatrical light bulbs (or lamps, the term usually preferred) are tungsten - halogen (or quartz - halogen), an improvement on the original incandescent design that uses a halogen gas instead of an inert gas to increase lamp life and output . Fluorescent lights are infrequently used other than as worklights because, although they are far more efficient, they are expensive to make dimmed (run at less than full power) without using specialised dimmer ballasts and only very expensive models will dim to very low levels . They also do not produce light from a single point or easily concentrated area, and usually have a warm - up period, during which they emit no light or do so intermittently . However fluorescent lights are being used more and more for special effects lighting in theaters . High - intensity discharge lamps (or HID lamps), however, are now common where a very bright light output is required--for example in large follow spots, hydrargyrum medium - arc iodide (HMI) floods, and modern automated fixtures . When dimming is required, it is done by mechanical dousers or shutters, as these types of lamps also cannot be electrically dimmed . </P>

Cues provide information regarding changes in lighting that occur during a performance