Figure 6-21. - Recommended intensities for specific night activities.
The beam spread can be described in degrees or by NEMA types (table 6-2). The beam spread is based on the angle to either side of the aiming point where the candlepower drops to 10 percent of its maximum value. The lamp and floodlight NEMA type is given in the upper left-hand corner of each isofootcandle diagram.
The NEMA type should only be used as a reference. It does not describe the shape of the light pattern the floodlight produces or the peak illumination level (footcandles). Symmetrical floodlights have the same horizontal and vertical beam spread and are classified with one NEMA number. Asymmetrical beam spreads have a horizontal (H) and a vertical (V) designation. The horizontal value is always given first.
Example: NEMA TYPE ( H / 7 ) x ( V / 6 )
The size of the area to be illuminated has a direct effect on determining the number and spacing of the poles. The suggested area that can be covered by a single pole is four times the mounting height. That is known as the "2X-4X" rule (fig. 6-22).
Areas lighted from interior poles or other central locations (fig. 6-22A) can be more economical, butContinue Reading