In the context of lighting design, a lighting instrument (also called a luminaire) is a device that produces controlled lighting as part of the effects a lighting designer brings to a show. The term lighting instrument is preferred to light to avoid confusion between light and light sources.
There are a variety of instruments frequently used in the theater. Although they vary in many ways they all have the following four basic components in one form or other:
• Box/Housing - a metal or plastic container to house the whole instrument and prevent light from spilling in unwanted directions.
• Light source ( lamp).
• Lens or opening - the gap in the housing where the light is intended to come out.
• Reflector - behind or around the light source in such a way as to direct more light towards the lens or opening.
Additional features will vary depend on the exact type of fixture.
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.
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.
Most instruments are suspended or supported by a "U" shaped yoke, or 'trunnion arm' fixed to the sides of the instrument, normally near its center of gravity. On the end of such, a clamp (known as a hook-clamp, C-clamp, or pipe clamp—pipe referring to battens) is normally fixed, made in a "C" configuration with a screw to lock the instrument onto the pipe or batten from which it is typically hung. Once secured, the fixture can be panned and tilted using tension adjustment knobs on the yoke and clamp. An adjustable c-wrench, ratchet (US) or spanner (UK) is often used to assist the technician in adjusting the fixture.
Most venues ensure crew and performer safety by attaching a safety cable/chain (a metal wire or chain) to the fixture. In the event that the fixture's clamp(s) were to fail, the cable would arrest the fall of the fixture before it could come in contact with a person. Some larger fixtures can weigh over 100 lb (45 kg) and are suspended very high above performers heads. Many venues place strict guidelines regarding the use of safety cables.
All lights are loosely classified as either floodlights (wash lights) or spotlights. The distinction has to do with the degree to which one is able to control the shape and quality of the light produced by the instrument, with spotlights being controllable, sometimes to an extremely precise degree, and floodlights being completely uncontrollable. Instruments that fall somewhere in the middle of the spectrum can be classified as either a spot or a flood, depending on the type of instrument and how it is used. In general, spotlights have lenses while floodlights are lensless, although this is not always the case.
The entire lighting apparatus includes the lights themselves, the physical structure which supports them, the cabling, control systems, dimmers, power supplies, and the light boards. (lighting console)
