In lighting design terminology, a lighting instrument is an instrument that produces lighting with controllable features. All lighting instruments share the same four basic components. The box, or housing, is a container made of metal or plastic which wraps around the lighting device and prevents light from spilling in undesired areas. The light source is the lamp from which light emits. A Lens or opening is the front of the container where light comes out. A reflector is placed behind or around the light source to direct more light towards the opening.
Lighting's most basic function is to brighten the stage so the characters can easily be seen by audience members. The technical term for this function is called illumination. Lighting can also set a scene's tone or mood. For instance, a bright red light that washes over the stage might convey a hazardous or seductive tone. A lighting cue can signal action or an event to occur onstage, which is useful if the event marks a significant point in the play. Lighting can reveal form by altering the perception of things onstage, namely objects. This effect makes backdrops seem more three-dimensional. Moreover, lighting can bring attention to or pull attention away from something. Location and time can be established. Light blues can represent an afternoon day and dark blue and purple can indicate night time. Lastly, composition can reveal or frame certain areas of the stage.
Intensity, color, pattern and focus are the four key qualities of lighting. Intensity is determined by how far away a lighting fixture is from the lit area, lamp strength, the fixture's design, presence of color gels or specials, the type of color to be lit, beam or angle of the lamp and the overall lighting scheme of the stage. Intensity is measured in lux, lumens and foot-candles. The second lighting quality is color. Color gels are pre-colored by the manufacturer and typically stay true to the color when lit. Color gels can be altered slightly by strengthening or weakening the light source. Color temperature is measured in Kelvins. Certain lamp fixtures, such as borderlights and cyclorama lights, can aid in the mixing of color gels and create multi-colored effects. The third quality, pattern, refers to the shape, clarity and evenness of a lighting fixture's output. A lamp's mounting position, reflector's shape and size, and lens characteristics affect its pattern. The last lighting quality, focus, refers to where the lamp is directed or pointed.
A popular lighting special is a gobo. A gobo is a disk with a cutout pattern that casts a shadow when a spotlight shines through it. There are three types of gobos: metal, glass, and full color and dichroic gobos. A metal gobo is created by cutting a pattern into sheet metal. Metal gobos are ideal for larger, more abstract patterns. They are the cheapest way of making patterns designed for the stage. Glass gobos contain etched patterns that are created using aluminum or another comparable coating to block the light. Complex patterns benefit from a glass gobo. Glass gobos can crack if heating precautions are not followed. Full color and dichroic gobos allow projection of colored images.