Figure 6-25. - Horizontal aiming.
Select lighting fixtures with a beam spread greater than the area being lighted. When several units are required, good lighting overlap occurs when the edge of the beam of one fixture coincides with the aiming point of the adjacent fixture.
By examining the shape (beam spread) of the lighting pattern emitted by the fixture, you can begin the process of selecting the NEMA type of floodlight best suited for the application.
Horizontal and vertical lumen distribution is stated on each photometric test. Generally, the more concentrated the luminous intensity (candela), the tighter the beam spread; for instance, the NEMA Type 2 Power Spot ® floodlight has a beam spread of 22- degrees horizontal by 21-degrees vertical; whereas, a NEMA Type 5 has a beam spread of 77-degrees horizontal by 77-degrees vertical. The isofootcandle diagrams shown in figure 6-26 compare 1,000-watt metal halide Power Spot®
luminaries of NEMA Type 2 and Type 5 when each luminaire is aimed out a distance of twice its mounting height.
The initial footcandle level at the aiming point of different NEMA types varies a great deal; for example, assume that each luminaire is mounted at a 50-foot mounting height and aimed 100 feet (2 x MH) directly in front of its location. If you are using a NEMA Type 2 distribution, the approximate initial footcandle level at that point would be 20; however, if you are using a NEMA Type 5 distribution, the initial footcandle level would be approximately 1.5.
By understanding the intensity of the lighting pattern, you can now appreciate the need for a range of distribution patterns.
The performance specifications of each model, type, and size of luminaire are provided with the fixture or obtained from the manufacturer's ordering catalog. A working knowledge of this information will assist you in selecting and installing the correct floodlight to accomplish the job. Figure 6-27 shows a sample of manufacturer's literature for a 250- to 1,000- watt light fixture.Continue Reading