---
title: "Outdoor lighting purposes and street lighting"
subject: "Industrial Power Distribution and Illumination"
chapter: 8
key: "semester_6/industrial_power_distribution_and_illumination:outdoor-lighting-street-methods"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/outdoor-lighting-street-methods
site: BCT Engineering
---

# Outdoor lighting purposes and street lighting

Industrial Power Distribution and Illumination · Chapter 8: Outdoor Lighting Design

- Asked in: 2075 Baisakh, 8 marks; 2073 Magh, 8 marks (asked 2 times)
- Video: [Street or Road Lighting Design Considerations](https://www.youtube.com/watch?v=PU7Ez3Wvo20) — Nelca Roco, Lighting Design
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/outdoor-lighting-street-methods
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-8

## Question

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

## Answer

**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.

```text
 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).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In the diffusion method, how is the road seen?
   A. As dark silhouettes against bright road
   B. By light reflected from the road surface in all directions
   C. By light from the sky only
   D. Only by headlights
   Answer: B. Luminaires send light down and the rough road scatters it in all directions, so it looks bright. (Key point: Diffusion method)

2. What does a driver see in the specular-reflection method?
   A. Objects as dark silhouettes against a bright reflected background
   B. A uniformly lit matt surface
   C. Objects brightly lit from above
   D. No reflection at all
   Answer: A. Mirror-like reflection forms a bright background, so objects stand out dark against it. (Key point: Specular reflection method)

3. Which is true of the specular-reflection method?
   A. Needs more lamps and works on rough roads
   B. Needs fewer lamps but suits only smooth straight roads and may glare
   C. Works best on curves and junctions
   D. Never causes glare
   Answer: B. It is economical in lamps but limited to smooth, straight roads. (Key point: Specular method limits)

4. What spacing/height ratio and mounting height are typical for street lighting design?
   A. S/h about 1 and height 2 m
   B. S/h about 3-4 and height 7.5-12 m
   C. S/h about 10 and height 1 m
   D. S/h about 0.5 and height 20 m
   Answer: B. Poles are typically 7.5-12 m high with spacing about 3-4 times that height. (Key point: Spacing/height ratio)

## Related in Industrial Power Distribution and Illumination

- [Outdoor lighting, flood and street calculation](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aoutdoor-flood-street-calculation) — Industrial Power Distribution and Illumination, chapter 8
- [Ten factors for flood, street lighting, luminaires](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aten-factors-flood-street-luminaires) — Industrial Power Distribution and Illumination, chapter 8
- [Factors for street and flood lighting design](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aflood-street-design-factors) — Industrial Power Distribution and Illumination, chapter 8, asked 2×
- [Factors in power and lighting system design](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Apower-lighting-design-factors) — Industrial Power Distribution and Illumination, chapter 5
- [Street lighting principles and flood design](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Adiffusion-specular-flood-procedure) — Industrial Power Distribution and Illumination, chapter 8
- [Street lighting a 2 km road with sodium lamps](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Astreet-lighting-sodium-2km) — Industrial Power Distribution and Illumination, chapter 8

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