Distribution Of Light

Distribution Of Light

Ever since street lights were first introduced in the suburbs there has been development in the distribution of light by street lamps all of which have tended to increase the efficiency of illumination. The distribution of light about a bare lamp depends upon the position of burning and shape of the filament.

While it is true that any type of globe surrounding a lamp will reduce the total light output of the unit, it is also true that proper equipment reduces glare, improves appearance and protects the light from the elements. This enclosing glassware was first opal, but the more recent tendency has been toward the rippled type because it presents a more pleasing appearance and gives certain sparkle and animation to the light. The, amount of light absorption depends upon the density of the glass and will vary from 10 to 30%.

light intensity for each lamp power
In the early days it was realized that for the purpose of street lighting, all the light going above the level of ,the lamp was void. Hence the need of a reflector was realized. One of the earlier types of reflectors which was greatly used throughout the suburbs was the radial wave type reflector. This consisted simply of a metal plate with waves in it and a shiny finish on the surface of the reflector where the light from the lamp struck. The purpose of the waves was to scatter the light over a wider street surface than the flat disc would. The radial wave type after introduced was used in the suburbs to the almost exclusion of other types. The only developments in this type were: the perfect,ion of a porcelain enamel finish on the reflector and improvements in insulation around the socket.

Gradually this type is being replaced by either a new reflector type known as the "eternalite" unit or "eternalite globe" lamps in which the, reflectors are entirely porcelain.

Refractors are combinations of pieces of glassware each molded into a series of prisms which refract the light and redirect it toward the surface of the street. There are two classes of distribution with refractors; namely, symetrical and non-symetrical distribution. The non-symetrical refractors, in addition to redirecting the upward right down, direct most of it on the actual road surface rather than on Sidewalks, lawns, etc.

This control of the distribution af light has been a great development in street lighting. Two types of refractors are the dome and bowl types. The dome type is simpler and is more widely used in the suburbs.

Brackets are used to hold the fixtures to the line poles and at first were very simple in design. Gradually, in attempt to beautify the fixture, the, bracket became more and more ornate. The tendency in recent years, on the other hand, is toward a plain but substantial bracket . At present in the suburbs there are two predominant types - the bent bracket and the straight bracket.

Both are shown in the accompanying picture with the mounting heights also given. Experience has taught us that within certain limits the higher we place the source of light the better the illumination. The spacing of lamps bears a definite relation to the mounting height. The empirical rule accepted by most engineers is that the spacing between lamps must not exceed night times the mounting height.

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