CLASSIFICATION OF LUMINAIRES Class of Luminaire Distribution of Flux (a) Direct 90% or more in the lower hemisphere.
(b) Semi-direct More than 6o% and less than 90% in the
lower hemisphere.
(c) Mixed or General 40% to 6o% in either hemisphere.
(d) Semi-indirect More than 6o% and less than 90% in the
upper hemisphere.
(e) Indirect Over 90% in upper hemisphere.
Outdoor lighting units are classified as "floodlights" if producing a medium to wide spread of beam for ranges of projection usually within 500 feet. Shorter throws and smaller illuminated areas result from narrow beam projectors or "spot-lights." Units de signed to produce one or a few narrow beams of light or a sub stantially horizontal fan of radiation either fixed or rotating, are classified as "beacons," whereas the classification of "search lights" includes the high powered projectors where the beam is essentially a sharply defined cylinder without sensible spread.
For utilitarian service and the most eco nomical redirection of light without exact beam control, there are available the opaque reflectors classified as silvered glass, enameled steel, and treated aluminium. Having no substantial difference in efficiency, their use may be interchangeable although enameled metal reflectors give less exact redirection and hence are favoured for wide spread illumination at low to medium mounting heights, whereas the other two classes are preferable for high mounting positions. The Alzac finish recently applied to aluminium has resulted in a durable reflecting surface of high polish which in addition to its excellent efficiency, may also reflect the ultra-violet as well as the visible wave lengths.
The pressed or blown glass shades and semi-indirect bowls reflect light excellently from glazed interior surfaces, and transmit percentages of light over the general range from 5 to 40%. Opal or milk glass blown into an enclosing globe acts chiefly to diffuse the light with total resultant absorption of about 15%. Practically the only redirec tional control is secured by varying the areas of the globe as projected in various directions. Clear glass pressed into surface configurations chiefly composed of prism ribs will act as an ex cellent reflector or may control the light by the property of internal reflection or refraction.
For exact narrow beam projection as in search-lights and automobile headlights, there are employed silvered glass or silvered or chromium plated metal reflectors of shallow paraboloids; deep paraboloids; and modified elliptical contours. The typical shallow paraboloid has an acceptance angle
of some 120 degrees, while a deep paraboloid intercepts the in cident rays throughout an angle of some 200 degrees. The latter reflector may direct roughly 40% of the total light flux into a beam that spreads on the order of 6 degrees, while the shallow paraboloid projects on the order of 28% into the beam but with a lesser spread on the order of 2 to 4 degrees. Usually the best grade reflectors are ground with optical accuracy and silvered on the back side, although plated metal reflectors are suitable in such general services as locomotive and vehicle headlights and medium beam spot-lights.
Reflecting and diffusing substitutes for metal and glass have been commercialized since about 1933, con sisting of cellulose nitrates and acetates and synthetic resins, or various formaldehyde compounds known as pyralyn, lumarith, plasticele, celluloid, bakelite, beetleware, etc. These materials may be clear, diffusing, or opaque, but for interior lighting service the diffusing or ivory coloured plastics are more common. In the forms of semi-indirect bowls these materials have reflecting coefficients on the order of 5o% compared with porcelain enamel steel or white plaster on the order of 75%, and may transmit up to 50% of the incident light. Their chief usage is for exposed equipment having a low surface brightness and especially where the non-fragile feature is important. Reflectors of organic plastics are in service in buses, railway cars, and for numerous bits of decorative ornament inasmuch as the material may be tinted and can be non-combustible.
Into the appli cation of artificial lighting have come many structural glass shapes such as bricks, molding, cornices, grilles, and tiles, capable of being built into the structural elements of a building and made luminous by tubular or similar light sources behind or contained therein. Glass bricks have come into the lists of lighting material since they provide in modern structures the translucent walls or substitutes for ordinary windows, especially in sealed or air conditioned buildings. Continuous ornamental grilles of clear or etched glass have grown in popularity for friezes and moldings and since they take on the colour tints of the light sources they represent low brightness lighting units of commendable eye comf ort and decorative qualities.