Without the use of specially designed and accurately made auxiliaries, the waste of light in street illumination is greater than it is in any other place. In interior lighting, the walls, ceiling and floor of a room act both as a limit to the path of light and as reflectors which return a large portion of it into the area of utility. In the street, however, there is only one natural surface that is available either as a limit or as a reflector, namely, the ground and its useful effect is very slight. Lateral limits corresonding to the walls of a room are, of course, impossible if we except such approximations as the walls of buildings along the street.

Waste above the Horizontal Line through the Lamp

Light above a horizontal line drawn through a street lamp is useless for purposes of ground illumination. About 50 per cent. of the light given from a bare lamp is emitted above the horizontal; and hence, if the lamp be used without reflecting or refracting equipment, this upward emitted light is wasted.

Limitations of Reflectors

Reflectors for outside use cannot extend the light rays sufficiently to eliminate dark spots between lamps, unless the spacing is prhibitively close. The ratio of effective illumination being only 2 or 2½ times the mounting height.

Suitability of Refractors

The principle of refraction offers a practicable solution for this purpose. A Refractor bends light by transmission through a transparent medium designed to act as a lens.

It is the only auxiliary to a lamp that will give an efficient lateral extension of illumation.

The Three Divisions of the Problem

The basic considerations which go to make up Scientific Lighting practice are three in number.

• The light must be generated efficiently.
• The light must be distributed efficiently over the area to be illuminated.
• The installation must be so designed that the highest efficiency of vision be obtained.

The Three Requirements

(1) Economic Light Production. The gas filled type electric lamps are extremely well suited to this service. This type of lamp is not only efficient, but is available in many sizes, it is simple and easy to operate and requires a minimum of maintenance.

(2) Efficient Distribution. Considerations of economy dictate that the lights should be placed as far apart as possible with due regard to the results to be obtained. This means that the light of the lamp should be redirected in a manner which will spread it far up and down the street.

(3) Avoidance of Glare. If adequate illumination between units is to be obtained without unduly close spacing, the lighting units must give a lateral light distribution, the maximum candle power being at angles from 70 to 80 degrees from the vertical. Naturally, the candle power of the light beams will be high, and the blinding effect of glare from the beams will be serious unless provision is made for overcoming this effect. There are two ways of minimising this. One, to use high mounting for the light units, and the other by increasing the size of the source so that the intensity covers a comparatively large area, thus reducing the intrinic brilliancy of the source. This is accomplished by covering the lamp with a refracting and diffusing globe, which then becomes the virtual light source.

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