The "lumen" method is the accepted procedure used in the design of general lighting installations, especially those having a number of symmetrically positioned outlets. This method assumes an average intensity of illumination over a given area. The cal culations involved represent a simple short-cut procedure, but allow for all variables affecting the illumination, such as type of lighting unit, general proportions of room, interior colourings and maintenance conditions.
(a) Choose the foot-candle illumination required from reference to the available tables of preferred intensities or the re quirements of the task.
(b) Select the type of lighting unit best adapted to the location, namely, as to whether it be a concentrating reflector for a small area ; a wide spreading reflector for broadcast light ; a diffusing globe or bowl for the office, etc.
(c) Determine the proper spacings of outlets, which will in dicate the preferential mounting heights. As a general guide the spacing of ceiling fixtures will be closely equal to the ceiling height, and for general space lighting will result in approximately i oo square feet of floor area per lighting unit.
(d) Determine the size of lamp to produce the necessary foot candles with the following formula : Area per Outlet X Foot-Candles Lumens of Lamp= Depreciation Factor X Coef. of Utilization In this formula the depreciation factor must be estimated according to the cleanliness of the location, the light re flection character of the surroundings and their colours. For the average case this factor may be taken as 0.75, meaning that 25% of the generated light will be lost through general depreciation. The coefficient of utilization is the ratio between the light reaching a working plane and the total light generated by a lamp. Tables published by lighting equipment manufacturers will give these data, this particular factor averaging about o.5. When the above formula is used, one will obtain the lumens desired for each lamp or more explicitly for each outlet. Then by comparing the calculated lumens per outlet with the lumens produced by each various size of lamp as given in the lamp manufacturers' catalogues, one immediately arrives at the proper size or wattage of lamp bulb to install. The relation ship between the useful quantity of light on a working plane, and illumination in foot-candles on the same surface will be understood when noting that the area in square feet X foot-candles =useful lumens, or that r lumen per square foot produces i foot-candle.