How to Design Efficient Street Lighting-Part-4

(F) Lighting Pollutions

  • Light pollution is an unwanted consequence of outdoor lighting and includes such effects as sky glow, light trespass, and glare. 
  • 30 to 50% of all light pollution is produced by roadway lighting that shines wasted light up and off target.

(1) Glare:

  • Glare is the condition of vision in which there is discomfort or a reduction in the ability to see significant objects. Glare affects human vision and it is subdivided into four components, Disability Glare, Discomfort Glare, Direct Glare and Indirect Glare.
  • By origin
  1. Direct Glare
  2. Indirect (reflected) Glare
  • By effect on people
  1. Disability Glare
  2. Discomfort Glare
  • Disability glare:
  • Disability glare is the glare that results in reduced visual performance and visibility.
  • Since disability glare reduces the ability to perceive small contrasts.
  • It can impair important visual tasks in traffic such as detecting critical objects, controlling headlights, and evaluating critical encounters, making glare a potential danger for road users.
  • LED light sources can provide very high luminance lev­els which may cause glare. For this reason, LED lamps are commonly equipped with diffusers to reduce this luminance.
  • Disability glare may vary for dif­ferent individuals and it can be calculated objectively.
  • In a particu­lar illuminated environment, the human eye will be able to detect differences in luminance down to a certain threshold. This threshold can be compared for a situa­tion in the same environment when a source of glare is added. By comparing these thresholds, the threshold increment can be derived.
  • Discomfort glare:
  • Discomfort glare is the glare producing discomfort. It does not necessarily interfere with visual performance or visibility.
  • As vertical light angles increase, discomforting glare also increases
  • Discomfort glare, on the other hand, is a subjective phenomenon and there is no method for its Rating.
  • Although the 9-point De Boer scale (ranging from “1” for “unbearable” to “9” for “unno­ticeable”) is the most widely used in the field of auto­motive and public lighting.
  • Direct Glare:
  • Direct glare is caused by excessive light entering the eye from a bright light source. The potential for direct glare exists anytime one can see a light source. With direct glare, the eye has a harder time seeing contrast and details.
  • A system designed solely on lighting levels, tends to aim more light at higher viewing angles, thus producing more potential for glare.
  • Exposed bright light source, for example a dropped lens cobra head or floodlight causes of direct glare.
  • Direct glare can be minimized with careful equipment selection as well as placement.

  • Figure illustrates two examples of exterior lighting that results in glare. 

  •  Fig shows how full cutoff luminaries (Shielded Luminaires) can minimize this direct glare. In exterior applications, use fully shielded luminaires that directs light downwards towards the ground.
  • Indirect Glare:
  • Indirect glare is caused by light that is reflected to the eye from surfaces that are in the field of view – often in the task area.
  • Indirect Glare can be minimized with the type and layout of lighting equipment. Direct the light away from the observer with the use of low glare, fully shielded luminaries.
  • As the uniformity ratio increases (poor uniformity), object details become harder to see.
  • For roadway lighting, good uniformity shows evenly lighted pavement. However, to meet small target visibility criteria, a non uniform roadway surface may be better.
  • There should be a balance between uniform perception and detecting objects on the road. Also, emphasis is put on horizontal surface uniformity. In reality, vertical surfaces may require more lighting in order to improve guidance.
  • How to Reduce Glare:
  • Glare and light trespass are more concern when installing floodlights.
  • Use shielded Light should be use to reduce Glare.
  • Higher mounting heights can more effective in controlling spill light, because floodlights with a more controlled light distribution (i.e., narrower beam) may be used, and the floodlights may be aimed in a more downward direction, making it easier to confine the light to the design area.
  • Lower mounting heights increase the spill light beyond the property boundaries. To illuminate the space satisfactorily, it is often necessary to use floodlights with a broader beam and to aim the floodlights in directions closer to the horizontal than would occur when using higher mounting heights.
  • Lower mounting heights make bright parts of the floodlights more visible from positions outside the property boundary, which can increase glare.

(2) Sky glow:

  • Sky Glow is brightening of the night sky caused by outdoor lighting.
  • Light that is emitted directly upward by luminaries or reflected from the ground is scattered by dust and gas molecules in the atmosphere, producing a luminous background. It has the effect of reducing one’s ability to view the stars in Night. 

  • How to Reduce Sky Glow
  • While it is difficult to accurately model sky glow, at this point it is presumed that the most important factors are light output and lamp spectral characteristics, light distribution from the luminaire, reflected light from the ground, and aerosol particle distribution in the atmosphere.
  • If the quantity of light going into the sky is reduced, then sky glow is reduced. Thus, to reduce sky glow by
  • By using full cutoff luminaires to minimize the amount of light emitted upward directly from the luminaire.
  • Reduce Lighting Level.
  • Make practice to Turn off unneeded lights
  • Limited Lighting hours in outdoor sales areas, parking areas, and signages
  • Installing Low-Pressure Sodium light sources, which allow astronomers to filter the line spectra from telescopic images.

 (3) Light Trespass:

  • Light trespass is condition when spill (Unwanted or Unneeded) light from a streetlight or floodlight enters a window and illuminates an indoor area.
  • How to Reduce Trespass
  • Select luminaries, locations, and orientations to minimize spill light onto adjacent properties.
  • Use well-shielded luminaries.
  • Keep floodlight aiming angles low so that the entire beam falls within the intended lighted area.

Difference between full cutoffs and fully shielded:

  • The full cutoff has is luminaries that have no direct up light (no light emitted above horizontal) and 10% of light intensity between 80° and 90°.
  • The term full cutoff is often substituted for the term fully shielded.
  • The both terms are not equivalent. Fully shielded luminaires emit no direct up light, but have no limitation on the intensity in the region between 80° and 90°
  • Luminaires that are full cutoff, cutoff, semi cutoff, and non cutoff , may also qualify as fully shielded.

  • There is also a confusing assumption that a luminaire with a flat lens qualifies as a full cutoff luminaries. While this may be true or not in some Lighting Fixtures case.

  • Fully shielded means, a lighting fixture constructed in such a manner that the bulb should be fully recessed into Fixture so that all light is directed downward below the horizontal.
  • The fixture is angled so the lamp is not visible below the barrier (no light visible below the horizontal angle).

(G) Selection of Luminas:

(1) Types of Lighting Source

  • Street Lights are mostly Low-pressure sodium (LPS), High-pressure sodium (HPS), Metal halide and Light emitting diodes (LED).
  • LPS is very energy efficient but emits only a narrow spectrum of pumpkin-colored light that some find to be undesirable.
  • LPS is an excellent choice for lighting near astronomical observatories and in some environmentally sensitive areas.
  • HPS is commonly used for street lighting in many cities. Although it still emits an orange-colored light, its coloring is more “true to life” than that of LPS.
  • Where it’s necessary to use white light, there are metal halide and LEDs.

  • High-pressure sodium lamps should be used for expressways, main roads, secondary roads and branch roads.
  • Low-power metal halide lamps should be used in mixed traffic roads for motor vehicles and pedestrians in residential areas.
  • Metal halide lamps can be used for motor vehicle traffic, such as city centers and commercial centers, which require high color identification.
  • Metal halide lamps, CFL lamps are used at Pedestrian streets in industrial areas, sidewalks in residential areas, and sidewalks on both sides of motorway traffic.
  • LED streetlights are more durable, longer lasting, efficiency, dimmable capacity and cost effective than traditional lights.
  • LED also enhances public safety by delivering superior visible light while providing the environmental advantage of using less energy.

(2) Color Rendering Index (CRI):

  • CRI Measures the ability of the artificial light to show or reproduce the colors of the road or objects on the road, relative to a natural light source.
  • The natural light source (the sun) has CRI of 100. The higher
  • This index the better the visibility will be. For all types of road CRI ≥ 70 is recommended.
  • Efficacy
  • At the low end LED efficacy starts at 70 lumens per watt (lm/W) and reaches as high as 150 lm/W.
  • While the mean efficacy for outdoor area fixtures is slightly lower than common indoor fixtures such as troffers and linear lighting about 100 lm/W for area lights compared to about 110 lm/W for troffers and linear fixtures this difference is not significant. It may be the result of outdoor area lights requiring more precise luminous intensity distributions and other factors unique to outdoor lighting.

(3) Fixture Protection:

  • When using sealed road lighting, the protection level of the light source cavity should not be lower than IP54.
  • For roads and places with dangerous environmental pollution and heavy maintenance, the protection level of the light source cavity should not be lower than IP65.
  • The degree of protection of the lamp electrical appliance cavity should not be lesser than IP43.
  • Lamps with excellent corrosion resistance should be used in areas or places with high levels of corrosive gases such as acid and alkali in the air.

(H) Effective Road Lighting:

  • Sufficient illumination.
  • Good uniformity.
  • No Glare.
  • Low consumption.
  • No Color Temperature abnormalities
  • No Zebra effect
  • Shielded lighting to ensure light is pointed downwards
  • Completely uniform illuminance.
  • No requirement for over lighting to obtain sufficient average illumination.
  • Absence of glare.
  • Absence of low angle radiation that causes sky glow.
  • Control of light trespass.
  • High redundancy.

Effective Road Lighting 



Proper pole height & spacing

 Provide uniform light distribution

Proper Luminaire aesthetics

 Blends in with the surroundings
Good maintenance

Reduce problems in lightning

High lamp efficiency

 Minimize energy cost

Life of Luminaire

Reduce lamp replacement cost
Good color rendering

 Helps object appear more natural

Proper light distribution

 Provide required light on roads

Cost effectiveness

 Lowers operating cost

Minimizing light pollution & glare

 Reduce energy use

Effective Energy-efficient Street Lighting Systems (NYSERDA, 2002)



Proper pole height and spacing Provides uniform light distribution, which improves appearance for safety and security Meets recommended light levels Minimizes the number of poles, reducing energy and maintenance costs
Proper luminaire aesthetics Blends in with the surroundings
High lamp efficacy and Luminaire efficiency Minimizes Energy cost
Life of the luminaire and other components Reduces lamp replacement costs
Cost effectiveness Lowers operating cost
High Lumen Maintenance Reduces lamp replacement costs
Good color rendering Helps object appear more natural and pleasing to the public Allows better recognition of the environment, improves security
Short lamp Re strike Allows the lamp to quickly come back after a power interruption
Proper light distribution Provides required light on the roads and walkways
Proper Cutoff Provides adequate optical control to minimize light pollution
Minimizing light pollution and Glare Reduces energy use
Automatic Shutoff Saves energy and maintenance costs by turning lamps off when not needed

Minimum Value of Street Light Designing


Min Value


Lumens Per Watt

80 To 140



50 To 60Hz

Power Factor

  More than 95


 < 20%
Life Hours

70,000 hours

Color Temperature

4000K  To  5000K


More than 75

Beam Angle / Beam Pattern

 Type 2,3,4,5
Operating Temperature

(-)25°C To (+)50°C

Working Humidity

10% To 90% RH
IP Rating




Optic Lens Material

High Polycarbonate (PMMA)
Forward Current



IP65 – Aluminum Alloy and PC Lens


 18.23″ X 13.58″ X 4.57″


 15.30 lbs – 34.39 lbs

10 Years



How to Design Efficient Street Lighting-Part-3

(C) Lighting Factor:

(1) Maintenance Factor (Light Loss Factors) (MF)

  • The Maintenance Factor (Light loss factor) is the combination of factors used to denote the reduction of the illumination for a given area after a period of time compared to the initial illumination on the same area.
  • The efficiency of the luminaire is reduced over time. The designer must estimate this reduction to properly estimate the light available at the end of the lamp maintenance life.
  • Luminaire maintenance factors vary according to the intervals between cleaning, the amount of atmospheric pollution and the IP rating of the luminaire.
  • However, it is proposed to consider maintenance factor of not less than 0.5 for LED Road lighting installations for IP66 rated luminaires.
  • The maintenance factor may range from 0.50 to 0.90, with the typical range between 0.65 To 0.75
  • These maintenance factor values shall be adopted for the purposes of producing the lighting simulation design.
  • The maintenance factor is the product of the following factors.
  • LLF = LLD x LDD x EF
  • Mostly We consider Maintenance factor from 0.8 to 0.9
  • We have to choose Maintenance factor carefully by increasing maintenance factor 0.5 the spacing of pole increasing 2 meter to 2.5 meter
Maintenance Factor Max. Spacing of Pole (Meter)
0.95 43
0.9 40.5
0.85 38
0.8 36

(a) Lamp Lumen Depreciation Factor (LLD)

  • As the lamp progresses through its service life, the lumen output of the lamp decreases. This is an inherent characteristic of all lamps. The initial lamp lumen value is adjusted by a lumen depreciation factor to compensate for the anticipated lumen reduction.
  • This assures that a minimum level of illumination will be available at the end of the assumed lamp life, even though lamp lumen depreciation has occurred. This information should be provided by the manufacturer. For design purposes, a LLD factor of 0.9 to 0.78 should be used.

(b)Luminaire Dirt Depreciation Factor (LDD).

  • Dirt on the exterior and interior of the luminaries and to some on the lamp reduces the amount of light reaching the roadway.
  • Various degrees of dirt accumulation may be anticipated depending upon the area in which the luminaire is located. Industry, exhaust of vehicles, especially large diesel trucks, dust, etc, all combine to produce the dirt accumulation on the luminaries.
  • Higher mounting heights, however, reduce the vehicle-related dirt accumulations.

LDD factor of 0.87 to 0.95 should be used. This is based on a moderately dirty environment and three years exposure time.

(c) Equipment Factor (EF).

  • Allows for variations inherent in the manufacture and operation of the equipment (i.e., luminaries, system voltage and voltage drop).
  • It is generally assumed to be 95%.

(2) Coefficient of Utilization (CU):

  • Coefficient of Utilization is the ratio of the luminous flux from a luminaire received on the surface of the roadway to the lumens emitted by the luminaire’s lamps alone.
  • Coefficient of Utilization should be maximum.
  • Coefficient of Utilization differs with each luminaire type, and depends upon mounting height, road width, and overhang.
  • The coefficient of utilization (K) should be over 30% or the utilance above 40% for the road, highway, square or enclosure. Luminaires or floodlights should not by placed far from the area to be lit or, where appropriate, light projection beyond the useful zone should be minimized (K = average maintained illuminance multiplied by the surface calculation and divided by the lumens installed).

Various Factors

Type Luminaries Dirt Depreciation Luminaire Lumen Depreciation Total Light Loss Factor
LED 0.9 0.85 0.765
HPS 0.9 0.9 0.81
LPS 0.9 0.85 (0.7 for 180W) 0.765 (0.63 for 180W)

Light Loss Factors

Type of Lamp Laminar Dirt description Light Loss Factor
HPS 0.88 0.74
Induction 0.88 0.62
LED 0.88 0.72

Maintenance factors

Cleaning intervals (months) Pollution category
High Medium Low
12 0.53 0.62 0.82
18 0.48 0.58 0.8
24 0.45 0.56 0.79
36 0.42 0.53 0.78
Maintenance Factors for 36 month cleaning interval
Factors IP5X IP6X
Pollution category Pollution category
Low Medium High Low Medium High
LMF 0.88 0.82 0.76 0.9 0.87 0.83
LLMF 0.89 0.89 0.89 0.89 0.89 0.89
MF 0.78 0.73 0.68 0.80 0.77 0.74


(E) Lighting Uniformities

(1) Lighting Uniformities

  • Uniformity is a description of the smoothness of the lighting pattern or the degree of the intensity of bright and dark areas on the road.
  • Uniformity is a measure of how evenly distributed the light on the road is, which can be expressed as Overall Uniformity (UO) and Longitudinal Uniformity (UL).
  • The uniformity ratio shall not exceed 4:1 and preferably should not exceed 3:1 except on residential streets, where 6:1 may be acceptable.

(a) Overall uniformity:

  • In design, the overall uniformity (UO) is expressed as a ratio of the minimum to the average luminance on the road surface of the carriageway within the calculation area.
  • UO=Lmin / Lave

  • It is a measure of how evenly or uniformly illuminate on the road surface.
  • A good overall uniformity ensures that all spots and objects on the road are sufficiently lit and visible to the motorist.
  • The industry accepted value for UO is 30 to 0.40.

(b) Longitudinal uniformity:

  • The longitudinal uniformity (UL) is expressed as the ratio of the minimum to maximum luminance along the center line of a lane within the calculation area.
  • UL=Lmin / Lmax.
  • Longitudinal uniformity is a measure to reduce bright and dark bands of light appearing on road lit surfaces. The effect can be somewhat hypnotic and present confusing luminance patterns.

  • It is a measure to reduce the intensity of bright and dark banding on road lit surface.
  • A good level of longitudinal uniformity ensures comfortable driving conditions by reducing the Pattern of high and low luminance levels on a road (i.e. zebra effect).
  • It is applicable to long continuous roads.

 Combination of Overall Uniformity and Longitudinal Uniformity:

  • The picture on the left shows a road with good UO while the picture on the right has low level of UO. The Road is more visible in the road with higher UO. Having higher UO allows the motorist to see the road clearly and anticipate potential road hazards (e.g. open manholes, road excavations, sharp objects on the road, people crossing the street).
  • The picture on the right shows a road with low level of UL demonstrating the ‘Zebra Effect’ while the picture on the left has high level of UL without ‘Zebra Effect’.
  • The ‘zebra effect’ can cause discomfort to motorists, posing a risk to road safety. Ensuring good level of uniformity can reduce the luminance level needed.

Lighting Levels

Category Eave ( LUX) Emin  LUX) Uniformity ratios
Emax : Emin Eave : Emin
Express & Main street 30 15 3:01 2.5:1
Suburban shopping street 25 10 5:01 3:01
Subsidiary street 15 10 5:01 3:01
Other streets 15 5 10:01 5:01


 Lux Level

Road Classification Area Classification Average  Lux Uniformity Ratio (Aver./Min.)
Arterial (Minor & Major) Commercial 12 3 to 1
Intermediate 9
Residential 6
Collector (Minor & Major) Commercial 8
Intermediate 6 4 to 1
Residential 4
Local Commercial 6
Intermediate 5 6 to 1
Residential 3
Alleys Commercial 4
Intermediate 3 6 to 1
Residential 2
Sidewalks (Roadside) Commercial 3 3 to 1
Intermediate 6 4 to 1
Residential 2 6 to 1
Pedestrian Ways 15 3 to 1


Illumination for Intersections

Functional Classification Average Maintained Illumination at Pavement by Pedestrian Area Classification in Lumen Uniformity
High Medium Low Eavg/Emin
Major/Major 37 28 19 32
Major/Collector 31 24 16 32
Major/Local 28 22 14 32
Collector/Collector 26 19 16 43
Collector/Local 23 17 11 43
Local/Local 19 15 9 65


Illumination for Pedestrian Areas

Maintained Illuminance Values for Walkways
Area Classification Description E avg (Lux) EV min (Lux) E avg/Emin
High Pedestrian Conflict Mixed Vehicle and Pedestrian 22 11 43
Areas Pedestrian Only 11 5 43
Medium Pedestrian Pedestrian Areas 5 2 43
Conflict Areas
Low Pedestrian Rural/Semi-Rural Areas 2 1 108
Conflict Areas Low Density Residential (2 or fewer dwelling units per acre) 3 1 65
Medium Density Residential (2.1 to 6.0 dwelling units per acre) 4 1 43
Pedestrian Portion of Pedestrian/Vehicular Underpasses Day 108 54 43
Night 43 22 32

 (2) Surround Ratio (SR):

  • Road lighting should be illuminate not only the road, but also the adjacent areas so motorists can see objects in the periphery and anticipate potential road obstructions (e.g., a pedestrian about to step onto the road).
  • The SR is the visibility of the road’s periphery relative to that of the main road itself.
  • As per industry standards, SR should be at least 50.
  • Figure show how road lighting should illuminate both the main road and its periphery.

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