Figure 6-14. - Streetlight manufacturer's literature.

footcandle values per 1,000 lamp lumens. Each footcandle value must be multiplied by 50 to obtain the correct footcandle value on the isofootcandle diagram. This ratio of actual lamp lumens divided by 1,000 is known as the lamp factor (LF). Note that the lamp factor allows a curve to represent the distribution from more than one lamp wattage; for example, from 250-and 400-watt lamps.

Maintenance Factor

Lighting efficiency is seriously impaired by blackened lamps, by lamp life, and by dirt on the reflecting surfaces of the luminaire. To compensate for the gradual loss of illumination, you must apply a maintenance factor (MF) to the lighting calculations.