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Outdoor lighting purposes and street lighting

Short answer

Outdoor lighting purposes: traffic safety, security, night work, sports, advertising, decoration. Street lighting methods: diffusion (direct downlight, E=I cos³θ/h², good on rough roads) and specualr reflection (mirror-like surface, objects as dark silouettes). Design factors: road class, uniformity, mounting height 7.5–12 m, spacing/height ratio 3–4, lamp type, cut-off, road width, surface reflectivity, junctions, and maintenance.

What are the major purposes of the outdoor lighting? Explain methods of street lighting and factors to be considered while designing street lighting.

Outdoor lighting is the lighting of open spaces such as roads, streets, car parks, sports grounds, building fronts, monuments and industrial yards after dark.

Major purposes of outdoor lighting

  1. Safety of traffic: drivers and pedestrians can see the road, vehicles and obstacles, reducing accidents.
  2. Security: discourages theft and crime around industrial plants, roads and public places.
  3. Working at night: loading yards, ports, construction sites and switchyards can operate after sunset.
  4. Sports and recreation: stadium and playground lighting for night games.
  5. Advertising and display: shop fronts, hoardings and signs.
  6. Decoration: floodlighting of monuments, temples and building facades for beauty and tourism.

Methods of street lighting

1. Diffusion (direct-light) method

  • Uses luminaires with diffusing reflectors that send most light downward onto the road surface.
  • The road is seen by the light reflected from the road surface in all directions.
  • Illumination at a point is found by the inverse square and cosine law: E = I cos³θ / h².
  • Gives good results on rough (matt) road surfaces.

2. Specular-reflection method

  • Uses the mirror-like reflection of a smooth or wet road surface.
  • Lamps are mounted so that light reflected from the road towards the driver forms a bright background; objects on the road appear as dark silhouettes against it.
  • Needs fewer lamps but is suitable only for smooth roads and straight stretches; it can cause glare.

Arrangements of poles: single side, staggered (both sides alternately), opposite (both sides facing), and central (twin-arm) arrangement on the median.

 Single side      Staggered        Opposite
 o   o   o        o       o        o   o   o
 ========         ========         ========
                      o       o    o   o   o

Factors considered in designing street lighting

  1. Class of road and traffic density – decides the required average luminance/illuminance (main roads ~ 15–30 lux; minor roads 4–8 lux).
  2. Uniformity: minimum/average ratio of about 0.4 or more; avoid dark patches.
  3. Mounting height: usually 7.5–12 m; higher mounting gives better uniformity and less glare.
  4. Spacing of poles: spacing/height ratio around 3–4.
  5. Type of lamp: HPSV, LED or metal halide – efficacy, life, colour and cost.
  6. Luminaire light distribution (cut-off, semi cut-off) to control glare.
  7. Road width and arrangement of poles (overhang, outreach of bracket).
  8. Road surface reflection (dry/wet, smooth/rough).
  9. Junctions, curves, bridges and pedestrian crossings need extra lamps.
  10. Maintenance factor, cost of energy, cabling and control (photocells, timers, dimming).

✓ Answer checked by Claude Opus 5.5 (high effort)

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