(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).
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Various Factors |
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| 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) |
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Light Loss Factors |
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| Type of Lamp | Laminar Dirt description | Light Loss Factor |
| HPS | 0.88 | 0.74 |
| Induction | 0.88 | 0.62 |
| LED | 0.88 | 0.72 |
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Maintenance factors |
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| 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.
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Lighting Levels |
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| 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 |
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Lux Level |
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| 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 | |
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Illumination for Intersections |
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| 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 |
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Illumination for Pedestrian Areas |
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| 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.
View more at https://electricalnotes.wordpress.com/2020/10/02/3171/.
Published by Department of EEE, ADBU: tinyurl.com/eee-adbu
