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 manufacturers literature.
Substituting in the formula,
fc=(2)(140,000)(0.38) =4.2 fc
The maintained light level is obtained by
multiplying the initial light level by the maintenance
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
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
5. Multiply 25,600 by 0.095 = 2,432 lamp watts.
2,432 is more than two 1,000-watt Lucalox®
2,432 is approximately equal to six 400-watt
2,432 is approximately equal to ten 250-watt
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
=40 feet MH.
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