Bracket length, 12 feetRequired average maintained level ofillumination, 2 footcandles400-watt luminaire (50,000 lamp lumens)Solution:1. Spacing. The equation to determine correctspacing isSpacing (S) =^{(LL)(MF)(CU)}(fc)(W)Where:LL =MF =CU =fc =W =rated initial lamp lumensmaintenance factorcoefficient of utilizationillumination in footcandlesstreet width. curb to curbThe values are given for LL (50.000), MF (assume0.70). W (50). and fc (2). After a value for CU isdetermined, you can solve the equation for averagespacing.To determine the coefficient of utilization,calculate the amount of wasted light on the street side(SS) and the house side (HS) where:Ratio of HS =Transverse Distance10==0.25Mounting Height4050-1040Ratio of SS=Transversel Distance_{=}=1.0Mounting Height4040From the utilization curve in figure 6-17, the ratioof 1.0, street side,corresponds to 40 percent. and theratio of 0.25, house side, corresponds to3 percent. for atotal of 43 percent CU.Spacing can be determined asS = (50,000)(0.70)(0.43)= 150 feet.(2)(50)2. Uniformity.The uniformity of illumination is expressed interms of a ratio ofAverage fcMinimum fcIt has been determined that one-side spacing of150 feet will produce an average of 2 footcandles onthe roadway surface. The point of minimumillumination can now be determined from theisofootcandle diagram.The minimum value of the illumination can befound by studying the isofootcandle diagram and tak-ing into account all luminaries that are contributingsignificant amounts of light. Generally, the minimumvalue will be found along a line halfway, between twoconsecutively spaced luminaries. The minimum valuecan be determined by. checking the minimum foot-candle values at points P1, P2, and P3. as shown infigure 6-18.The roadway surface can be plotted on theisofootcandle curve by observing the 40-footmounting height to longitudinal and transversedistance ratios. (See fig. 6-19.) Since PI is locatedoutside the 0.02 footcandle line. it is the lowest totalfootcandle value. This value would be 0.03 fc (0.015footcandle from each luminaire).Figure 6-20 shows a perspective view of the twoisofootcandle lines that are considered whendetermining the illumination value at P1.The following factors are now applied to this“raw” footcandle value, as shown in the formula:fc min = (fc) (LF) (MF) (CF)Where:fc min = minimum point footcandlesfc = raw footcandle from isofootcandlediagramLF = lamp factorMF = maintenance factorCF = mounting height correction factorThe values are given for fc (0.03) and MF (assume0.70). The value for LF was determined earlier as 50for the 400-watt lamp. The CF factor can bedetermined from the correction chart below theisofootcandle curve in figure 6-14. The CF for a 40-foot mounting height is 0.56.6-16