Light Projector

a lighting device that uses an optical system to concentrate the luminous flux from a lamp in a limited solid angle. If the average brightness of a light source located at the focus of the optical system (reflector) is equal to L, then the maximum luminous intensity of the projector on its optical axis is I₀ = kLA; here A is the area of the projector’s light aperture (the projected area of the reflector on a plane perpendicular to the optical axis) and k is a loss factor, which in practice ranges from 0.6 to 0.75. Projectors are characterized both by the quantity I₀ and by the value of the plane angle of radiation α₁₀, within which the luminous intensity is reduced to 0.1 I₀. Depending on their function, projectors are classified as searchlights, floodlights, or beacons.

Searchlights, which have achieved wide military use, employ paraboloidal reflectors up to 3 m in diameter to produce conical beams. In searchlights with the most powerful light sources—high-intensity carbon arc lamps—I₀ has a value of up to 10⁹ candles and α₁₀ is no more than 0.5°.

Floodlights are used to illuminate open areas, such as railroad tracks and stations, quarries, construction sites, airfields, and docks. They are also used to light the fronts of buildings and motion-picture and stage sets. Axially symmetrical glass and metal reflectors ranging from 0.25 m to 0.6 m in diameter are used, as are ellipsoidal reflectors, which provide a fan-shaped beam of light. The value of I₀ in a floodlight using incandescent lamps ranges from 10⁵ to 10⁷ candles, and α₁₀ in the vertical and horizontal planes ranges from 12° to 3° and from 40° to 20°, respectively. Floodlights use all modern light sources up to 50 kilowatts in power.

Light beacons are used to transmit information by flashes of light or to establish position, as with a lighthouse. In the first case, the beacons are equipped with paraboloidal reflectors ranging from 0.25 to 0.4 m in diameter and gas-discharge light sources, including carbon-arc lamps. In the second case, the beacons are only slightly different in design from searchlights. The optical systems of beacons utilize not only mirror-type reflectors but also annular (disk-shaped) and belt (cylindrical) Fresnel lenses.

Further improvement in all types of light projectors implies an increase in the manufacturing precision of the optical devices, a reduction in overall mass, and utilization of more powerful light sources.