| Watts per lamp | 25 | 40 | 60 | 160 | 150 | 250 |
| Effective lumens per lamp | 95 | 160 | 250 | 420 | 630 | 1090 |
| Lumens per watt | 3.8 | 4.0 | 4.2 | 4.2 | 4.2 | 4.3 |
The size of the units is a matter of choice since six 400-lumen units are equal to four 600-lumen units in illuminating power, etc. In deciding upon the proper size of lamps to use, consideration must be taken of the outlets if the building is already wired. In general the fewest units consistent with good distribution will be the most economical. The table shows the lumens effective for ordinary lighting with Mazda lamps and clear high-efficiency reflectors with dark walls and ceiling. Where both ceiling and walls are very light these figures may be increased by 25 per cent.
To illustrate the use of the table, take an average room 16 by 24 to be lighted with Mazda lamps to an intensity of 3.5 foot-candles. If clear Holoplane reflectors are used, the values for lumens effective on the plane may be increased 10 per cent. due to reflection from fairly light walls. The lamps in this case are to be of the 40-watt type which in the table are rated at 160 lumens. To this amount 10 per cent. is added on account of the reflectors and walls. This data applied to the formula gives:
s = 16 by 24 feet
I = 3.5
Lumens per lamp = 160
((16 × 24) × 3.5)/176 = eight 40-watt lamps.
Fig. 219.
Reflectors.
—The character and form of reflectors have much to do with the effective distribution of the light produced by the lamp. The most efficient form of reflectors are made of glass and designed to project the light in the desired direction. The illustration in Fig. 219, marked open reflector, shows the characteristic features of reflectors designed for special purposes. They are made of prismatic glass fashioned into such form as will produce the desired effect and at the same time transmit and diffuse a part of the light to all parts of the space to be lighted. The greater portion of the light is sent in the direction in which the highest illumination is desired. The reflectors are made to concentrate the light on a small space or to spread it over a large area as is desired. They are, therefore, designated as intensive or extensive reflectors and made in a variety of forms.