INTENSITY CONSTANTS FOR ARC LAMPS 5-ampere, enclosed, direct-current arc on 110-volt circuit; clear inner, opal outer globe; no reflector; large room; light ceiling; medium walls; height from 9 to 14 feet .50 Arrangement of Lamps. An arrangement of lamps giving a uniform illumination cannot be well applied to residences on account of the number of units required, and the inartistic effect. We are limited to chandeliers, side lights, or ceiling lights, in the majority of cases, with table or reading lamps for special illumination.
When ceiling lamps are used and the ceilings are high, some form of reflector or reflector lamp is to be recommended. In any case where .the coefficient of reflection of the ceilings is less than 40%, it is more economical to use reflectors. When lamps are mounted on chandeliers, the illumination is far from uniform, being a maximum in the neighbor hood of the chandelier and a minimum at the corners of the room. By combining chande liers with side lights it is generally possible to get a satisfactory arrangement of lighting for small or medium-sized rooms.
As a check on the candle-power in lamps required, we have the following: For brilliant illumination allow one candle power per two square feet of floor space. In some particular cases, such as ball rooms, this may be increased to one candle-power per square foot.
For general illumination allow one candle-power for four square feet of floor space, and strengthen this illumination with the aid of special lamps as required. The location of lamps and the height of ceilings will modify these figures to some extent.
As an example of the calculation of the illu mination of a room with different arrangements of the units of light, assume a room 16 feet square, 12 feet high, and with walls having a coefficient of reflection of 50%. Consider first the illumination on a plane 3 feet above the floor when lighted by a single group of lights mounted at the center of the room 3 feet below the ceiling. If a minimum value of .5 foot
candle is required at the corner of the room, we have the equation (first method outlined): 2.7 foot-candles, or five times the value of the illumination at the corners of the room.
Next consider four 8-candle-power lamps located on the side walls 8 feet above the floor, as shown in Fig. 54. Calculating the illumination at the center of the room on a plane three feet above the floor, we have: The illumination at the corner of the room would be In a similar manner the illumination may be calculated for any point in the room, or a series of points may be taken and curves plotted showing the distribution of the light, as well as the areas having the same illumination. Where refined calculations are desired, the dis tribution curve of the lamp must be used for determining the candle power in different directions. Fig. 55 shows illumination curves for the Meridian lamp as manufactured by the General Electric Com pany. This is a form of reflector lamp made in two sizes, 25 or 5') candle-power. Fig. 56 gives the distribution curves for the 50 candle-power unit. Similar incandescent lamps are now being manufactured by other companies.
Table XIV gives desirable data in connection with the use of the Meridian lamp.
By means of the Weber, or some other form of portable photom eter, carves as plotted from calculations may be readily checked after the lamps are installed. When lamps are to be permanently located, the question of illumination becomes an important one, and it may be desirable to determine, by calculation, the illumination curves for each room before installing the lamps. This applies to the lighting of large interiors more particularly than to residence lighting. The point-by-point method of calculation is used for very accurate work when the system of illumination admits of this method. Other methods are often simpler and sufficiently accurate for practical work.
Dr. Louis Bell gives the following in connection with residence lighting: