Man's physical universe

xanabras

ILLUMINATION 445

steps out from a well-illuminated room, and everyone has experienced

the almost blinding sensation of coming from a dark room into a bright

light. Theatre lights are turned on and ofif gradually in order to give

the eye time to adjust to the change in illumination.

The reason eyestrain is caused by contrasts in illumination Is that the

iris of the eye, which corresponds to the diaphragm in a camera, opens

or shuts with varying light intensities, and if the muscles which control

the iris are kept constantly active, they become tired, and eye fatigue

results.

In general illumination. It is obvious that contrasting regions of

strong and faint illumination should be avoided. Equal glareless illumination

is accomplished by the proper spacing of light sources and by the

proper diffusion of the light from these sources. When direct lighting is

used, better illumination is obtained by the use of frosted-glass incandescent

light bulbs than by clear-glass light bulbs. Opal glass is used in

lamp globes and light fixtures to diffuse the light still more. Flush-type

lights employ frosted glass; and when still

better diffusion is desired,

the frosted glass is replaced by glass whose surface is molded in the

form of small pyramids; this glass is called prism or "diffusex" glass.

In selecting glass block, special attention should be given to the type

of block selected for a given exposure.

Some types of glass blocks are

designed to transmit the maximum amount of light, and they are

desirable for northern exposures; but glass block which will be exposed

to direct sunlight should be of the type which transmits the least

amount of light and diffuses it as much as possible in order to cut

down on the amount of glare.

Light may also be diffused by reflection. Unpolished or rough surfaces

reflect light in many different directions and thus scatter or

diffuse it. Indirect lighting depends upon the reflection of light from

the ceiling and walls of rooms. The color of the finish on a ceiling

determines its reflecting power. Lighter shades reflect more than

darker shades.

Light-reflection

Values

Light — almost white ... 85 per cent

Cream

Yellow

Green

Blue

70-75 per cent

55-65 per cent

35-50 per cent

10-50 per cent

Indirect lighting is best for general illumination, and yet one does

not want to illuminate every point in a large living room well enough

to provide for reading fine print or doing fine needle-work. Shaded

reading lamps provide the localized light intensity required. The

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