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. [Illustration: 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. =Choice of Reflector.=--Where the light from a single lamp must spread over a relatively great area, it is advisable to use an _extensive_ form of reflector. This reflector is applicable to general residence lighting, also uniform lighting of large areas where low ceilings or widely spaced outlets demand a wide distribution of light. Where the area to be lighted by one lamp is smaller, the _intensive_ reflector is used. Such cases include brilliant local illumination, as for reading tables, single-unit lighting or rooms with high ceilings as pantries or halls. Where an intense light on a small area directly below the lamp is desired, a _focusing_ reflector is used. The diameter of the circle thus intensely lighted is about one-half the height of the lamp above the plane considered. Focusing reflectors are used in vestibules or rooms of unusually high ceilings. Type Height above plane to be lighted Extensive 1/2 _D_ Intensive 4/5 _D_ Focusing 4/3 _D_ _D_ = distance between sides of room to be illuminated.