PRINCIPLES OF INTERIOR ILLUMINATION

BY A COMMITTEE

J. R. CRAVATH, CHAIRMAN

WARD HARRISON

ROBERT ff. PIERCE

PART I. ELEMENTS OF DESIGN

As the subject of illumination units and calculations is treated in a separate lecture only those parts of this subject of immediate practical application to design will be taken up here, and no attempt will be made to explain the derivation of units, or the terms or diagrams here mentioned in connection with calculations.

CALCULATIONS

Measurement and Expression of Light Output from Sources. One of the first things necessary in illumination calculations for interiors. is a knowledge of the light output or luminous performance of various sources of light available for lighting the interior in question. In connection with the light output of a source it is important that we should know: (a) how the light is distributed from the source, that is, the candle-power distribution or intensity in various directions; (b) the flux of light in lumens or mean spherical candle-power; and (c) the brightness per unit area of the source of light.

Candle-power Distribution.-The polar coördinate curve, Fig. 1, is the common means of expressing the intensity of candle-power of light in various directions from a source. Such a curve (in which the candle-power is shown by the distance of the curve from the reference point or light source) gives at a glance a good idea of the characteristics of light distribution from the source, provided the distribution of light is symmetrical around a vertical axis. If it is not symmetrical, of course, several curves plotted from candle-power readings in different planes are necessary.

The practising engineer should be an industrious student and. collector of curves of this kind.

Light Flux.-The total output or flux of light in lumens (which is 12.57 times the mean spherical candle-power) is sometimes graphically expressed by a Rousseau diagram but more frequently by numerals showing the lumens emitted in different zones together with the total lumens.

The mathematical derivation of light flux from the polar coördinate curve is out of the scope of this lecture except that one short-cut method of great practical convenience for quickly determining the light flux in any zone or zones from a common polar co

ördinate curve should be mentioned. The method is based on the principle that on a polar coördinate curve the light flux in various zones is proportional to the length of a perpendicular line drawn from the candle-power curve at the middle of the zone to the vertical axis. If we take the sum of the perpendicular distances for 10-deg. zones (such as AB plus CD plus EF etc., in Fig. 1) from the curve to the vertical as measured from the center of each 10-deg. zone (measuring these distances by the same scale as the candle-power scale of the curve) and add 10 per cent. to this sum, the result will be the total lumens in the zones under considera

tion. The quick way to get this sum is by the use of a strip of paper and a sharp pencil. Starting at a marked zero point measure the perpendicular distance from the curve to the vertical 10-deg. zone at the 5-deg. point (AB Fig. 1) marking it on the strip. Then with the last mark as a starting place measure the distance for the second zone, CD from the vertical to the curve at the 15-deg. point, and so on adding each perpendicular distance for every 10 degrees to the one before, over the whole 180 degrees. Then by using the candle-power scale of the curve to measure the total length of the slip of paper so measured off and adding 10 per cent., the numerical value of the lumens emitted in any zone or for the entire sphere o to 180 degrees is quickly ascertained. Obviously the same method applies to any one or more of the 10-deg. zones into which the sphere is divided by this method, so that the lumens can be thus determined for any one or more 10-deg. zones.

The brightness over the area of the source of light (or of the source of light with its enclosing equipment such as a globe or reflector) is of much importance in connection with the hygiene of the eye in designing interior illumination. Such brightness has been expressed in many units, such as candles per square centimeter, candles per square inch, candles per square foot, etc., but practice is rapidly settling to the new unit approved by our Society, namely, the "lambert" and its 1000th part, the millilambert. The latter is about equal to the brightness of white blotting paper when illuminated with 1.25 foot-candles. Table 1 shows the relation of various brightness units.

Luminous Output of Bare Light Sources.-Although in good practice in the lighting of interiors, the lamps are seldom used bare without reflectors, shades or globes of any kind, it is nevertheless of fundamental importance to the engineer to know the luminous output of the various sources of light without auxiliary equipment. Then he can proceed with his calculations by allowing the proper percentage of loss for whatever equipment is used around the lamps.

The luminous output of different kinds of lamps per unit of input has been rapidly changing during the past few years owing to improvements in the art and will probably continue to change so that any data given here must be taken with the idea that they must be revised from various reliable sources at frequent intervals.

Table II shows the lumens and the lumens per watt for a number TABLE II. LUMENS OUTPUT OF AMERICAN TUNGSTEN INCANDESCENT LAMPS JULY 1, 1916

of the commonest sizes and types of tungsten filament incandescent lamps, new, as made and used in the United States, August, 1916, when operated at a voltage giving an average rated life of 1000 hours. From this it is seen that the lumens per watt range from 7.5 for the 10-watt size to 18 for the 1000-watt size.

Gas mantle burners, new, and properly adjusted range in specific output from 200 to 325 lumens per cubic foot of gas per hour in sizes giving 400 to 3000 lumens. These figures vary with the composition of the gas and many other factors.

The amount of light obtained from the old-fashioned open flame burner gas jet depends upon the richness of the gas in certain hydrocarbons which produce a yellow flame in the open jet. This quality is commonly known as the candle-power of the gas and was at one time the common standard by which gas was rated. With the gas mantle, however, the candle-power according to the old standards has nothing to do with the light output of the burner which in this case depends on the composition of the gas.

The efficiencies of lamps burning acetylene, Blau gas, alcohol, kerosene and gasoline vary considerably, depending upon the design of the burner, the purity of the illuminant and the conditions of supply. The following figures have been actually obtained under favorable conditions, but do not necessarily represent the maximum obtainable. On the other hand, the average results in the case of kerosene and gasoline are probably much below the stated values.

Kerosene lamps in particular suffer a considerable decrease in efficiency during burning.

The older carbon filament incandescent lamp gave a specific output of from 2.5 to 4 lumens per watt.