REFLECTORS. The light from most illuminants is dis tributed in a manner such that under most conditions it cannot be efficiently employed without the use of reflectors, or other media for re-directing the light into useful angles. Reflectors in common usage to-day not only control the direction of the light, but also serve to shield the eyes from the brilliancy of the bare light source, and diffuse the light to produce a comfortable illumination. While all materials have to some degree the qualities of reflection, only a few can be used efficiently as accessories for primary light sources. Consideration of permanence, transmission, opacity, character of surface and reflection factor are necessary in deter mining the fitness of materials for use as reflectors.
The materials ordinarily used for reflectors are classed as (I) translucent, in which a part of the incident light is reflected and part transmitted or absorbed, and (2) opaque, in which part of the light is reflected and the rest absorbed; both types are neces sary depending upon the character of the light distribution. The three types of light distribution from reflectors used for general interior illumination are classed as (I) direct, (2) semi-indirect and (3) indirect, a classification based upon the percentage of emitted light below or above the horizontal plane through the centre of the light source. Direct reflectors shed more than half of the emitted light below the horizontal ; semi-indirect reflectors throw more than half above the horizontal with a small amount below ; and indirect reflectors throw all of the light upward to the ceiling which reflects it downward.
The shape of the candle-power distribution curve of a reflector is within the control of the designer because of the diverse types of reflection characteristics available. Where very accurate control of the light is necessary, polished-metal, mirrored-glass, or pris matic-glass surfaces are usually employed. These media have a
reflection characteristic similar to a mirror where the angle of re flection is equal to the angle of incidence. Automobile headlight reflectors, searchlight and floodlight projectors, and industrial high mounted luminaires are typical of the equipment requiring this type of reflector. An inconsiderable corrugation of surface of this type produces a slight diffusion of the light and yet permits of a very accurate control of the light distribution. Dull-finished metal surfaces, of which unpolished aluminium is typical, have some of the light-directing characteristics of the polished surface but more diffusion. This type of surface is commonly used for small local lighting reflectors and in some types of floodlights and spotlights. The inherent low cost, ease of manufacture and light weight make spun aluminium popular for many uses.
Rough or mat-finish reflecting surfaces permit of little or no actual control of light, since a beam of light striking a mat-finish surface will be reflected in all directions. Block magnesium car bonate is typical of several surfaces falling in the classification, although it lacks the necessary permanence, soils easily, and is difficult to clean. Porcelain enamelled metal combines the diffusing characteristics of mat-finish surfaces with some of the light control characteristics of gloss surfaces. Porcelain-enamelled reflectors are popular for industrial lighting purposes because of the ease of cleaning them when they get dirty. The cost is low because the metal shell can be formed by spinning, or in the press, and the porcelain enamel finish put on afterwards. Opal glass has the same general reflection characteristics as porcelain enamel and also transmits some light—a feature desirable for some locations. (See ILLUMINATING ENGINEERING; TELESCOPE; MIRROR; MICRO