• Street illumination level in Lux (E)=(Al x (cu x mf)) / (w x d)
  • E = The illumination in Lux
  • w = Width of the roadway
  • d = Distance between luminaries
  • cu = Coefficient of utilization. Which is dependent on the type of fixture, mounting height, width of roadway and the length of mast arm of outreach?
  • Al = Average lumens, Al = (E x w x d) / Cu x mf
  • The typical value of Al is
  • 20500 lumens for 400 watts
  • 11500 lumens for 250 watts
  • 5400 lumens for 125 watts
  • The value of Al varies depending upon the type of lamp specified.
  • mf : It is the maintenance factor (Normally 0.8 to 0.9)
How Do Street Lights Work
How Do Street Lights Work

(1) Calculate Lamp watt for street Light Pole:

  • Calculate Lamp Lumen for street Light Pole having Road width of 7 meter, distance between two Pole is 50 meter, Maintenance factor is 0.9, Coefficient of utilization factor is 0.29, light pedestrian traffic is medium and Vehicular traffic is very light and Road is concrete road.
    Solution:
    From Above table Recommended of illumination (E) in Lux is 6.46 per sq. meter.
    w = 7.00 meters , d = 50 meters , mf = 0.9, cu = 0.29
    To decide Lamp Watt It is necessary to calculate Average Lumens of Lamp (Al).
  • Average Lumen of Lamp (Al)=(E x w x d) / Cu x mf
  • Al=(6.46x7x50)/(0.29×0.9)= 8662.83 Average lumen
    Lamp lumen of a 250 watts lamp is 11,500 lm which is the nearest value to 8662.83 lumen. Therefore, a 250 watts lamp is acceptable.
    Let’s Computing for the actual illumination E for 250 Watt Lamp
  • Illumination (E)=(Al x (cu x mf)) / (w x d)
  • E= (11500×0.29×0.9) / (7×50) = 8.57 lumen per sq meter.
    Conclusion:
    Actual illumination (E) for 250 Watt is 8.57 lumen per sq meter which is higher than recommended illumination (E) 6.46.
  • Hence 250 watt gives adequately lighting.

 (2) Calculate Spacing between two Light Poles:

  • Calculate Space between Two Pole of Street Light having Fixture Watt is 250W , Lamp output of the Lamp (LL) is 33200 lumens , Required Lux Level (E) is 5 lux , Width of the road  = 11.48 feet (3.5 M),Height of the pole (H) = 26.24 feet (8 M) ,Coefficient of utilization (CU) = 0.18, Lamp Lumen Depreciation Factor (LLD) = 0.8 ,Luminaries dirt Depreciation Factor (LDD) = 0.9
    Solution:
  • Luminaries Spacing (S) = (LLxCUxLLDxLDD) / (ExW)
  • Luminaries Spacing (S) = (33200×0.18×0.9×0.8) / (5×11.48)
  • Luminaries Spacing (S) = 75 feet (23 Meters)
sodium street light
sodium street light

 (3) Calculation of the allowed illumination time:

  • The allowed illumination time in hours T = k.t.1000/E.
  • Where: k = extension factor
  • t = permissible time in hours at 1000 lux, unfiltered daylight
  • E = luminance (lx)
Extension Factor
LampExtension Factor
Incandescent lamps,2.7 to 3.2
Halogen reflector lamps2.5 to 3.5
Halogen capsules2.5 to 3.5
High-pressure metal-halide1.1 to 2.1
High-pressure sodium lamps4
Fluorescent lamps1.9 to 2.7
  • Example:
  • In sunlight (100000 lux) and extension factor 1: The permissible illumination time (T) =1 x 70 x 1000/100 000 = 0.7 hour.
  • In halogen light (200 lux) and extension factor 2.3: The permissible illumination time (T) = 2.3 x 70 x 1000/200 = 805 hours.
  • In UV-filtered halogen light (200 lux) and extension factor 3.5: The permissible illumination time (T) = 3.5 x 70 x 1000/200 = 1225 hours.

 (4) Calculate Uniformity Ratio:

  • Once luminaries spacing has been decided It is necessary to check the uniformity of light distribution and compare this value to the selected lighting
  • Uniformity Ratio ( UR) = Eav /Emin
  • Eav= average maintained horizontal luminance
  • Emin = maintained horizontal luminance at the point of minimum illumination on the pavement

 (5) Energy Saving Calculations:

  • At a simplistic level, the cost of running a light is directly related to the wattage of the globe plus any associated ballast or transformer. The higher the wattage, the higher the running cost and it is a straightforward calculation to work out the running cost of lamp over its lifetime:
  • Running cost = cost of electricity in $/kWh x wattage of lamp x lifetime in hours.

 Calculate Lux Level for Street Lighting

  • The Average Lux Level of Street Light is measured by 9 point method. Make two equal quadrants between two Street light poles. on the lane of light poles( one side pole to road).
  • We have 3 points P1,P2 and P3 under the light pole then P4 & P7 are points opposite pole 1 or Point P3 same is applicable for P6 and P9 for Pole 2.
  • The average lux = [(P1+P3+P7+P9)/16]+[(P2+P6+P8+P4)/8]+[P5/4]

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