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Table 6-2. - Luminaire Designations

			Min. Beam Efficiency %
Beam Spread Degrees	NEMA Type Designation	Beam Description	Incandescent, Tungsten, Halogen	High-Intensity Discharge
10 up to 18	1	very narrow	38
18 up to 29	2	narrow	30	30
29 up to 46	3	medium narrow	46	34
46 up to 70	4	medium	50	38
70 up to 100	5	medium wide	54	42
100 up to 130	6	wide	56	46
130 and up	7	very wide	60	50

perimeter locations are also desirable to provide needed visibility at entrances and exits. in the case of perimeter poles (fig. 6-22B), if comer locations are not used, the distance from any side location to the edge of the area should not exceed twice the mounting height. If building-mounted luminaire locations are limited to only one side of the area to be lighted (fig. 6-22C), the system will be effective for a distance of only two mounting heights unless glare is not a determining factor.

According to the 2X-4X rule, the spacing is determined to be, from the corner to the first pole, two times the mounting height(X). The next pole is set four times this mounting height (X), and the CE will continue in this manner until reaching the last pole, which also is to be set two times the mounting height from the far corner. This rule can be used to calculate the minimum number of poles. For long, narrow areas, it is better to choose several short poles than one tall one, especially since pole costs increase substantially above 40 feet. It is wise to consider several alternatives, however. to determine the system with the lowest cost.

If the pole is located inside the area to be lighted, there should be at least three floodlights or two streetlights per pole. For one side perimeter mounting, there should be two floodlights or one streetlight per pole.

FLOODLIGHT AIMING

When a fixture is aimed at the surface at an angle other than perpendicular, the maximum lighting level will always occur behind the aiming point, or point of maximum candela. That is important to know when the fixtures are placed close to the base of a tall structure. In this case, the highest lighting level will occur at the base, even though the fixture is aimed at the top.

For vertical aiming, the aiming point should be two thirds to three fourths of the distance across the area or twice the mounting height, whichever is the lowest value. Higher aiming angles will not improve utilization and uniformity. (See fig. 6-23.)

The highest light level (vertical and horizontal) a floodlight can produce at a distance from the pole occurs when the maximum intensity or candlepower is aimed to form approximately a three, four, five triangle. (See fig. 6-24.) That is useful when determining pole height for area lighting or setback for building floodlighting.

Floodlights with NEMA 6 or 7 horizontal beams will effectively light an area 45 degrees to either side of the aiming line. In figure 6-25, the perimeter pole needs at least two floodlights to cover the area in all directions. Narrower beam floodlights require less separation to achieve uniform lighting.