Stage lighting instrument in the context of Back light

A light beam or beam of light is a directional projection of light energy radiating from a light source. Sunlight forms a light beam (a sunbeam) when filtered through media such as clouds, foliage, or windows. To artificially produce a light beam, a lamp and a parabolic reflector is used in many lighting devices such as spotlights, car headlights, PAR Cans, and LED housings. Light from certain types of laser has the smallest possible beam divergence.

A parabolic (or paraboloid or paraboloidal) reflector (or dish or mirror) is a reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis. The parabolic reflector transforms an incoming plane wave travelling along the axis into a spherical wave converging toward the focus. Conversely, a spherical wave generated by a point source placed in the focus is reflected into a plane wave propagating as a collimated beam along the axis.

Parabolic reflectors are used to collect energy from a distant source (for example sound waves or incoming star light). Since the principles of reflection are reversible, parabolic reflectors can also be used to collimate radiation from an isotropic source into a parallel beam. In optics, parabolic mirrors are used to gather light in reflecting telescopes and solar furnaces, and project a beam of light in flashlights, searchlights, stage spotlights, and car headlights. In radio, parabolic antennas are used to radiate a narrow beam of radio waves for point-to-point communications in satellite dishes and microwave relay stations, and to locate aircraft, ships, and vehicles in radar sets. In acoustics, parabolic microphones are used to record faraway sounds such as bird calls, in sports reporting, and to eavesdrop on private conversations in espionage and law enforcement.

Stage lighting is the craft of lighting as it applies to the production of theater, dance, opera, and other performance arts. Several different types of stage lighting instruments are used in this discipline. In addition to basic lighting, modern stage lighting can also include special effects, such as lasers and fog machines. People who work on stage lighting are commonly referred to as lighting technicians or lighting designers.

The equipment used for stage lighting (e.g. cabling, dimmers, lighting instruments, controllers) are also used in other lighting applications, including corporate events, concerts, trade shows, broadcast television, film production, photographic studios, and other types of live events. The personnel needed to install, operate, and control the equipment also cross over into these different areas of "stage lighting" applications.

A floodlight is a broad-beamed, high-intensity artificial light. It can provide functional area lighting for travel-ways, parking, entrances, work areas, and sporting venues to enable visibility adequate for safe task performance, ornamental lighting for advertising, façades, monuments, or support perimeter security. Floodlights are often used to illuminate outdoor playing fields while an outdoor sports event is being held during low-light conditions. More focused kinds are often used as a stage lighting instrument in live performances such as concerts and plays.

Floodlights may also be used to add effects to buildings at night; this is commonly termed facade lighting or—less precisely—architectural illumination.

Ellipsoidal reflector spot (abbreviated to ERS, or colloquially ellipsoidal or ellipse) is the name for a type of stage lighting instrument, named for the ellipsoidal reflector used to collect and direct the light through a barrel that contains a lens or lens train. The optics of an ERS instrument are roughly similar to those of a 35 mm slide projector.

There are many types of ERS that are designed for the myriad applications found in the entertainment industry. ERS instruments come in all shapes and sizes. Each particular model of ERS has its own set of characteristics. Generally, ERS instruments are the most varied and utilized type of stage lighting instrument. ERS may also be referred to as Profile Spotlights (especially in Europe) because the beam can be shaped to the profile of an object.