--- title: "Light sources and specular reflection principle" subject: "Industrial Power Distribution and Illumination" chapter: 6 key: "semester_6/industrial_power_distribution_and_illumination:light-sources-specular-reflection" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/light-sources-specular-reflection site: BCT Engineering --- # Light sources and specular reflection principle Industrial Power Distribution and Illumination · Chapter 6: System Components for Industrial Illumination - Asked in: 2069 Bhadra, 8 marks - 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/light-sources-specular-reflection - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-6 ## Question List and explain briefly various types of light sources and explain the specular reflection principle of street lighting system. ## Short answer Light sources include incandescent, fluorescent, CFL, HID and LED. Specular reflection on wet roads produces elongated bright patches. Lamps are spaced so patches overlap, forming a continuous bright background against which objects appear as dark silhouettes for motorists. ## Answer #### Types of light sources 1. **Incandescent (filament) lamps:** a tungsten filament heated to about 2500–2800 °C glows. Includes GLS lamps and **tungsten-halogen** lamps (halogen gas allows higher filament temperature and longer life). Efficacy 10–25 lm/W, excellent colour rendering (Ra ≈ 100), cheap but short life (1000–2000 h). 2. **Low-pressure discharge lamps:** - **Fluorescent tube (FTL)** and **compact fluorescent lamp (CFL):** mercury vapour discharge produces UV, which phosphor coating converts to visible light. 50–90 lm/W, life 8000–15000 h, need ballast. - **Low-pressure sodium (LPS):** monochromatic yellow light, very high efficacy (up to 180 lm/W) but very poor colour rendering. 3. **High-intensity discharge (HID) lamps:** **high-pressure mercury vapour** (35–60 lm/W), **high-pressure sodium** (80–140 lm/W, golden-yellow), **metal halide** (70–110 lm/W, white light, good CRI). Used for streets, high-bay factories and floodlights. 4. **Solid-state lighting – LED:** semiconductor p-n junction emits light when forward biased. Efficacy 100–200 lm/W, life 25,000–50,000 h, instant start, dimmable, no mercury. Now the most used source for all applications. 5. **Others:** induction lamps (electrodeless, very long life), neon lamps for signs. #### Specular reflection principle of street lighting When a road surface is **wet or smooth**, it behaves partly like a mirror (specular reflection): the angle of reflection equals the angle of incidence. A motorist then does not see the road as uniformly lit; instead, each lamp produces a bright **elongated patch (a "T"-shaped streak)** on the road between the lamp and the driver, and the road between patches looks dark. The specular reflection principle uses this effect: lamps are arranged so that the bright reflected patches of successive lamps **overlap and form a continuous bright background**, against which objects (pedestrians, vehicles) are seen in **dark silhouette**. ```text lamp lamp lamp O O O |\ |\ |\ | \ bright | \ bright | \ ==|==\=patch====|==\=patch====|==\=== road \ \ \ \______________\_____________→ driver's eye ``` Design rules: - Use a fairly **small spacing** and **low mounting height** so the bright patches join; the patch is long in the direction of travel. - Luminaires are **mounted over the road** (on brackets/outreach) and aimed along the road so that maximum light reaches the driver's direction of view. - Use **cut-off or semi-cut-off luminaires** to limit glare to the driver. - The principle works for smooth/wet roads; on rough dry roads diffuse reflection dominates and the diffusion (lumen) principle is used instead. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the advantage of the halogen gas in a tungsten-halogen lamp? A. It converts UV to visible light B. It allows a higher filament temperature and longer life C. It removes the need for a filament D. It makes the light monochromatic yellow Answer: B. Halogen gas allows a higher filament temperature and longer life than a plain GLS lamp. (Key point: Halogen lamps) 2. How does a fluorescent tube produce visible light? A. A heated tungsten filament glows B. A p-n junction emits light when forward biased C. Mercury vapour discharge produces UV which a phosphor coating converts to visible light D. Sodium vapour emits yellow light directly Answer: C. Mercury discharge gives UV; the phosphor coating converts it to visible light. A ballast is needed. (Key point: Fluorescent lamp principle) 3. Which lamp has very high efficacy (up to 180 lm/W) but very poor colour rendering? A. Metal halide B. Incandescent C. High-pressure mercury D. Low-pressure sodium Answer: D. Low-pressure sodium gives monochromatic yellow light with very high efficacy but very poor colour rendering. (Key point: Choice of street-lamp types) 4. In the specular reflection principle of street lighting, how are objects on the road seen? A. As bright objects on a dark road B. As dark silhouettes against a continuous bright background of overlapping patches C. Only by diffused light from the sky D. Without any contrast Answer: B. Bright reflected patches of successive lamps overlap to form a bright background, against which pedestrians and vehicles appear in dark silhouette. (Key point: Specular reflection principle) ## Related in Industrial Power Distribution and Illumination - [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 - [Light sources and indoor lighting schemes](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Alight-sources-indoor-schemes) — Industrial Power Distribution and Illumination, chapter 6 - [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× - [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 - [Outdoor lighting purposes and street lighting](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aoutdoor-lighting-street-methods) — Industrial Power Distribution and Illumination, chapter 8, asked 2× - [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 ## Related in other subjects - [Specular reflection](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Aspecular-reflection-expression) — Computer Graphics, chapter 8, asked 4× - [Ambient, diffuse and specular light](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Aambient-diffuse-specular-expressions) — Computer Graphics, chapter 8, asked 2× - [Street lighting importance and design factors](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Astreet-lighting-importance-design-factors) — Transportation Engineering II, chapter 1 - [Total internal reflection as light conduit](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Atotal-internal-reflection-light-conduit) — Propagation and Antenna, chapter 6