Glossary - H

hard light light that causes an object to cast a sharply defined shadow.

hazard or obstruction beacon an aeronautical beacon used to designate a danger to air navigation.

hazardous location an area where ignitable vapors or dust can cause a fire or explosion created by energy emitted from lighting or other electrical equipment or by electrostatic generation.

headlamp a major lighting device mounted on a vehicle and used to provide illumination ahead of it. Also called a *headlight*. See *multiple-beam headlamp* and *sealed-beam headlamp.*

headlight† an alternative term for headlamp.

heat extraction thermal factor the fractional lumen loss or gain due to passage of room air being returned to the plenum through the lamp compartment of the luminaire.

heavy-duty floodlight (HD) a weatherproof unit having a substantially constructed metal housing into which is placed a separate and removable reflector. A weatherproof hinged door with cover glass encloses the assembly but provides an unobstructed light opening at least equal to the effective diameter of the reflector.

hemispherical-conical reflectance, p(2π; mr) the ratio of reflected flux collected over a conical solid angle to the incident flux from the entire hemisphere. Note The direction and extent of the cone must be specified.

hemispherical-conical transmittance, τ(2π; mt) the ratio of transmitted flux collected over a conical solid angle to the incident flux from the entire hemisphere. Note The direction and extent of the cone must be specified.

hemispherical-directional reflectance, p(2π; r, φr) the ratio of reflected flux collected over an element of solid angle surrounding the given direction to the incident flux from the entire hemisphere. Note The direction of collection and the size of the solid angle element must be specified.

hemispherical-directional transmittance, τ(2π; t, φt) the ratio of transmitted flux collected over an element of solid angle surrounding the given direction to the incident flux from the entire hemisphere. Note The direction of collection and size of the solid angle element must be specified.

hemispherical reflectance the ratio of all of the flux leaving a surface or medium by reflection to the incident flux. The use of this term is deprecated. See hemispherical transmittance.

hemispherical transmittance the ratio of the transmitted flux leaving a surface or medium to the incident flux. The use of this term is deprecated.
high-bay lighting interior lighting where the roof trusses or ceiling height is greater than approximately m (25 ft) above the floor.

high-intensity discharge (HID) lamp an electric-discharge lamp in which the light-producing arc is stabilized by bulb wall temperature, and the arc tube has a bulb wall loading in excess of 3 W/cm2. HID lamps include groups of lamps known as mercury, metal halide, and high-pressure sodium.

high-key lighting a type of lighting that, applied to a scene, results in a picture having gradations from middle gray to white with comparatively limited areas of dark gray and black. Also, intense, overall illumination. In motion pictures, high-level accent lighting with strong contrast (dark deep shadows with little or no middle gray). See low-key lighting.

high-mast lighting illumination of a large area by means of a group of luminaires that are designed to be mounted in fixed orientation at the top of a high mast, generally 20 m (65 ft) or higher.

high-pressure sodium (HPS) lamp a high-intensity discharge (HID) lamp in which light is produced by radiation from sodium vapor operating at a partial pressure of about 1.33 × 104 Pa (100 Torr). Includes clear and diffuse-coated lamps.

horizontal exit† an escape route from one building to an area of refuge in another building on approximately the same level. It also is an escape route through or around a fire barrier to an area of refuge on approximately the same level in the same building.

horizontal plane of a searchlight the plane that is perpendicular to the vertical plane through the axis of the searchlight drum and in which the train lies.

hot-cathode lamp an electric-discharge lamp whose mode of operation is that of an arc discharge. The cathodes can be heated by the discharge or by external means.

house lights the general lighting system installed in the audience area (house) of a theatre, film or television studio, or arena.

hue of a perceived color the attribute that determines whether it is red, yellow, green, blue, or the like.

hue of a perceived light-source color† the attribute that determines whether the color is red, yellow, green, blue, or the like. See hue of a perceived color.

hydrargyrum medium-arc-length iodide (HMI) lamp an arc light source utilizing mercury vapor and metal halide additives to produce illumination in the 5000 to 6000 K range. Requires a ballast and ignitor system for operation.