The tinted bulbs in residences or the coloured glass ware in these and other locations, in conjunction with interior paints, may modify the spectral quality to a visible degree. The human eye is most sensitive to the yellow-green colour at about 5600 Angstroms wave length. However, no practical source, ex cept the sodium vapour lamp, produces such monochromatic light and hence the eye is accustomed to function well under any reasonable range from the blue-white of the north sky to the flame tints of decorative filament lamps, or of the kerosene lamp. So far in man's development of artificial lighting he has, for reasons of economy, done little with colours although for esthetic or "mood-conditioning" reasons much can be expected. Most animals or insects do not perceive colour as do humans and there is a wide variation in colour perception among individuals so that any applied lighting usually deviates from the yellowish-white colour only insofar as to provide decorative effects.
In certain operations like engraving, sculpture, or work on three-dimensional objects, a reasonable amount of shadow emphasizes relief and improves visual acuity. When reading a printed page the visibility depends upon the contrast between black type and white paper, and shadowless illumination is indi cated. In the case of decorative lighting, the choice and direction of shadows is very important. Also in inspections of surfaces the shadows produced by grazing light will emphasize depressions and spots, as the automobile headlight emphasizes the irregulari ties in the road surface. In common practice the larger the light giving area the fewer the shadows, hence the indirect lighting system using much of the ceiling as a secondary source results in the closest approach to shadowless diffusion. Any large areas of dark shadow towards which the eye may glance at occasional times will result in fatigue if the vision be shifted to a well lighted space afterward. In general, shadows must fall away from the body, hence the stronger light sources are placed above and be hind and usually to the left of the individual.
The material possessing the best reflection (highest reflection coefficient) is magnesium car bonate which returns better than 95% of the incident light. It is pure white. Light cream coloured ceiling paints or papers are easily available with reflecting coefficients from 8o to 90%. A light green, very light ivory, or light yellow wall colour has a reflection coefficient on the order of 4o to 50%, while browns, olive greens, and medium blues reflect about io to 20%. It is
highly important to choose light coloured interior paints or finishes since these if properly selected may easily double the available illumination on a book or desk top. In accordance with the typical gradation of an outdoor landscape whereof the grass or the immediate foreground possesses roughly a io% reflection coefficient, the trees and lower horizon perhaps 2o%, and where the sky represents the brightest part of the field of view, it be comes natural to grade the colours in a room so that the lower portions of the walls may reflect up to 20%, the upper walls about 40%, and the ceiling 8o% or greater. Glossy surfaces are to be avoided especially as these result in specular images or bright spots of high contrast. A mat, depolished, or eggshell surface is preferable. Shiny machinery or glazed book paper are deleterious to comfortable vision.
The success of any lighting installation depends greatly upon a knowledge and practice of a few essentials of service and main tenance.
From among the several thou sand types and sizes of incandescent lamps available for general lighting service it is wise to choose carefully the style and the type fitted to the particular service. For example, in the indirect floor lamps the bulb is designed to be burned base downward and will have a much shorter life in the reverse position. Lamps for projection purposes usually fail prematurely if tilted more than some io degrees from the vertical. For especially hot locations as in bake-ovens or boiler rooms, one should choose the lamp with special basing cement that will not loosen, or where the bare bulb is exposed to rain and snow there should be employed the temperature resisting or hard glass bulb. On portable extension cords use the rugged shock resisting design, or in the mill or vehicle subject to continuous rapid vibration select the "mill type" lamp.
The voltage figure usually marked on the bulb should correspond exactly with the voltage supplied to the lamp socket, hence one should guard against lamps of higher voltage rating since these will burn dim and yellow, or guard against losses in the inadequate wiring of the building,—a very prevalent fault which reduces the socket voltage and causes continuous power losses to the owner. A proper lighting installa tion should be designed to have no greater than 2% voltage varia tion from the standard of the lamp rating.