Where:

N = number of floodlights

LL = initial lamp lumens

CU = utilization of the floodlights

From the utilization data (fig. 6-27), you can find that the utilization for the HLX luminaire aimed at two mounting heights across an area 160 feet or four mounting heights wide is 38 percent. The initial lumens for the 1,000 watt lamp are 140,000 lumens, obtained from the manufacturer's literature. Substituting in the formula,

fc = (2)(140,000)(0.38) / (160)(160) = 4.2fc

The maintained light level is obtained by multiplying the initial light level by the maintenance factor.

fc = (4.2)(0.75) = 3.15 fc

Using the isofootcandle diagram, we obtain point by point footcandle values: for example, the center of the area occurs just inside isofootcandle line E. From the initial footcandle table, the 1,000-watt HLX at 40 feet produces 3.1 footcandles at line E and 7.8 at line F. Since the point is approximately one fourth of the distance between the two isofootcandle lines, the value will be about 4.0 footcandles. With the two floodlights contributing, the value in the center will be 8.0 footcandles. Note that the corners of the area will have very little light. That is why two or more floodlights are recommended at perimeter locations.

Another design method that will yield sufficient accuracy is the quick selector design method. The general layout considerations shown in figure 6-28 should be followed. The watts per square foot obtained from the graph in figure 6-29 produce an average lighting level accurate to within 20 percent of desired value. That is close enough, since the difference between the luminaire requirement obtained from the graph and the number that will actually be needed to satisfy the physical requirements of the job involve adjustments greater than 20 percent. It is not unusual, for instance, to need two poles instead of one or to require three luminaries per pole instead of two. This calculation method should not be used for sports lighting or where the poles are set back from the area to be lighted.

Before determining the number of luminaries, you should work out the size of the area to be lighted. Also, you should determine the maintained illumination level. The following rules of thumb provide some guidelines to help in these decisions.

1. From figure 6-21, you find that the minimum average footcandles recommended for industrial yard/material handling is 5.

2. Read up the left side of the graph in figure 6-29 until you come to 5. Follow this line across until you intersect the dark diagonal line representing Lucalox^® .

3. By reading straight down from this intersection to the value at the bottom of the chart, you find 0.095 lamp watts/square foot of the area is required to light the yard to 5 footcandles.

4. Area to be lighted is (160)( 160) = 25,600 square feet.

5. Multiply 25,600 by 0.095 = 2,432 lamp watts.

2,432 is more than two 1,000-watt Lucalox^® lamps

2,432 is approximately equal to six 400-watt Lucalox^® lamps

2,432 is approximately equal to ten 250-watt Lucalox^® lamps

6. By using the general layout considerations, you

will find that the most economical floodlight installation will use the 400-watt Lucalox

^® lamps, mounted on 40-foot poles, as shown below.

2X + 2X = 4X = 160 feet

X = 160 feet / 4 = 40 feet MH.

SECURITY LIGHTING

Requirements for security lighting at activities will depend upon the situation and the area to be protected. Each situation requires careful study to provide the best visibility that is practical for guard duties, such as identifying personnel and vehicles, preventing illegal entry, detecting intruders, and investigating unusual or suspicious circumstances.

The type of security lighting may be either the continuous or the standby type. The continuous type, as the name implies, is on all the time during the hours of darkness. The standby type is activated either manually or automatically when suspicious activity is detected.