The most important use of artificial light is the illumination of interiors. For interior illumination both the incandescent electric lamp, preferably the tungsten, and the incandescent mantle gas lamp, provide light units that combine convenience and efficiency to a high degree.
There are three systems of distribution that are used. With both gas and electric lighting, the direct, the indirect, and the semi-indirect.
The direct system is one in which the light from the lamp shines directly upon the area lighted, either with or without the help of reflecting shades. This system gives the maximum amount of light for a given expenditure of energy, but is objectionable on account of the glare of the light which either comes directly from the lamp or is reflected from polished surfaces. This objection can be partially overcome by the use of frosted lamps or translucent globes, when it becomes a modified direct system.
The indirect system is one in which the lamps are placed within two or three feet of the ceiling while opaque, bowl-shaped reflectors are placed below the lamps. The inner surface of the bowls is made of a highly reflecting material and thus the light from the lamp is reflected from the ceiling, from which it is diffused throughout the room. Less light is thrown on a given surface, a table top, for example, for the same expenditure of energy, than in the direct system, but as the eyes are not tired by the direct glare of the light, objects are seen equally well under the reduced intensity. This system does away with the harsh shadows of the direct system, the only shadows formed being soft and pleasing. One form of bowl with interior reflectors is shown in Fig. 1.
The semi-indirect system is one which is intended to com bine the advantages of both the direct and indirect systems and to avoid the objectionable features of each. The method adopted is to reflect a part of the light to the ceiling for general illumination and to direct the rest downward either directly or through translucent screens.
This system has the advantages of lighting a limited area well without concentrating all the light upon it and leaving the rest of the room unlighted.
In order to light a limited area, such as the top of a desk or table, reflecting shades are used. These are of various. types, depending upon the character of distribution desired.
Those that light but a small area are called "concentrating" shades, while the "distributing " shade spreads the light over a much larger area.
Figure 2 shows one type of shade and the change in the distribution made by its use. The dotted curve gives the distribution of light from the lamp without a shade, the radial distance of the curve at any point from the center giving the candle power of the lamp in that direction. The full-line curve shows the distribution when the shade is used, the greater part of the light being thrown downward.
FIG. 3. - Section of Lighthouse Lenses arranged to throw Parallel Raye in Four Directions
