Lighting Device

any of the devices used for illumination, irradiation, signaling, or projection. They are correspondingly classified as illuminating, irradiating, signaling, and projecting devices. A lighting device usually consists of a source of optical radiation, a device that transmits the radiant flux into space in the desired direction, and a housing that joins the parts into a single device and protects the radiation source and light-transmitting system against damage and the effects of the surroundings. Lighting devices with light sources of the gas-discharge type may also be equipped with devices for starting the discharge and limiting the current.

Depending on the purpose of the lighting device, the radiation used may cover the entire spectrum or only certain parts thereof (ultraviolet, visible, or infrared). Lighting devices can be subdivided into three classes according to the extent to which the radiant flux is concentrated. There are devices with a maximum concentration of luminous flux along the optical axis (projectors), devices with a maximum concentration of luminous flux in a small volume on some segment of the optical axis (projection devices), and devices that transmit luminous flux in a large solid angle (lamps).

The transmission of luminous flux in a lighting device can be achieved through a directed reflection of light by specular reflectors of parabolic (Figure l,a), ellipsoidal (Figure l,b), or some other shape (Figure 1,c). Luminous flux can also be transmitted by a directed passage of light through Fresnel (disk or cylindrical) lenses (Figure 1,d), condensing lenses or lenses with nonspherical surfaces (Figure 1,e), or devices fashioned from combined prisms (Figure 1,f). Other means of transmitting luminous flux include the diffuse and directionally dispersed reflection of light by diffuse, enameled, or frosted reflectors (Figure 1,g) and the diffuse and directionally dispersed passage of light through milk-glass, opal-glass, or frosted-glass diffusers (Figure 1,h).

The principal characteristics of lighting devices for illuminating engineering are the luminous intensity, luminosity, and illuminance. Also of interest is the device’s efficiency, which is equal to the ratio of the usefully utilized luminous flux to the total luminous flux of the light source.