For a street having 15 m width, what is the spacing between lighting units to produce average lux of 6·0 ? (Take lamp size as 6000 lumen, coefficient of utilization as 0·5, and maintenance factor as 0·9)
- A30 mCorrect
- B15 m
- C45 m
- D60 m
Solution
Concept
Illumination design by the lumen method
Formula
E = F × CU × MF / (S × W)
Step-by-step solution
1. S = F × CU × MF / (E × W) = 6000 × 0.5 × 0.9 / (6.0 × 15). 2. = 2700 / 90 = 30 m.
Explanation
The average illumination on the road surface equals the useful lumens from each lamp divided by the area lit per lamp (spacing × width).
Why the other options are wrong
15 m, 45 m and 60 m would give an average illumination of 12, 4 and 3 lux respectively.
Shortcut
S = F · CU · MF / (E · W).
Common mistake
Forgetting the maintenance factor.
Correct answer: A) 30 m