--- title: "Street lighting principles and flood design" subject: "Industrial Power Distribution and Illumination" chapter: 8 key: "semester_6/industrial_power_distribution_and_illumination:diffusion-specular-flood-procedure" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/diffusion-specular-flood-procedure site: BCT Engineering --- # Street lighting principles and flood design Industrial Power Distribution and Illumination · Chapter 8: Outdoor Lighting Design - Asked in: 2078 Chaitra, 8 marks - Video: [ILLUMINATION ENGINEERING - PART -12 - FLOOD LIGHTING - IMPORTANT TERMS & PRBOLEMS](https://www.youtube.com/watch?v=xOpYxgnD9kg) — zenmurali Sachithanandam - 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/diffusion-specular-flood-procedure - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-8 ## Question Explain diffusion principle and specular reflection principle of street lighting. State the design procedure of flood lighting system. ## Short answer Diffusion principle: rough road reflects light uniformly; E = I h/d³. Specular reflection: smooth road creates bright streak; objects appear as silhouettes. Floodlight design procedure: survey object, set illumination level, compute area, position projectors (0.7–1×height), choose beam spread, select factors (CU, DF, WLF), calculate total lumens, pick lamp wattage, arrange for uniformity. ## Answer Street lighting works on two principles: the road is seen either by **diffused** light from a rough road or by **specular** (mirror-like) reflection from a smooth road. #### Diffusion principle - Luminaires with diffusing reflectors send light **downward** on the road; about 30–40% of light falls on the road. - The rough road surface reflects light **equally in all directions**, so it appears uniformly bright from every viewing angle. - Illumination at a point P at horizontal distance x from a pole of height h: E = (I / d²) cos θ = I h / d³, with d = √(h² + x²). - Lamps are placed so the dip in illuminance midway between poles is small. - Suitable for rough, dry roads, pedestrian areas and residential streets. #### Specular reflection principle - On a smooth or wet road, light is reflected like a mirror. The lamps are placed so that light reflected **towards the motorist** forms a bright streak/background on the road ahead. - Objects on the road (vehicle, person) appear as **dark silhouettes** against this bright background, so they are easily seen. - Luminaires direct light at high angles (near 75–80° from the vertical) along the road. - Needs fewer, more widely spaced lamps; but works well only on straight, smooth roads and may cause glare. ```text Lamp Driver's eye ● ◉ \ incident reflected / \ / ════\════ bright patch ══/════ road ``` #### Design procedure of a floodlighting system 1. **Study the object:** size, shape, colour and reflection factor of the surface; surroundings (bright/dark). 2. **Select illumination level (E):** from tables, based on surface reflectance and surrounding brightness. 3. **Find the area (A)** to be lit: A = length × height. 4. **Choose location and distance of projectors:** generally a distance of 0.7–1 times the height of the building, outside the line of sight of viewers. 5. **Select the beam spread:** narrow beam for long throw, wide beam for short throw. Beam spread needed = 2 tan⁻¹(half-size / distance). 6. **Decide factors:** coefficient of utilisation or beam factor (0.3–0.6), depreciation or maintenance factor, waste light factor (1.2 regular / 1.5 irregular surfaces). 7. **Total lumens required:** F = E × A × DF × WLF / CU (or E × A × WLF / (BF × MF)). 8. **Select lamp size and type:** lumens per lamp = W × efficacy. 9. **Number of projectors:** N = F / lumens per lamp; round up. 10. **Arrange and aim** the projectors so beams overlap and give uniform brightness; check glare and spill light. 11. **Design supply, control and cabling** (timers, switches) and check cost. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a rough road look uniformly bright in the diffusion principle? A. It reflects light equally in all directions B. It absorbs all light C. It reflects light only towards the driver D. It acts like a mirror Answer: A. Diffuse reflection scatters light in all directions, so brightness is similar from every viewing angle. (Key point: Diffusion principle) 2. In the specular principle, how are objects on the road seen? A. Bright against a dark road B. Dark silhouettes against a bright streak of reflected light C. Not visible D. In diffused light only Answer: B. Reflected light towards the motorist forms a bright background and objects block it. (Key point: Specular principle) 3. Which is the total lumen formula in the floodlighting design procedure? A. F = E x A x DF x WLF / CU B. F = E x A x CU C. F = E / (A x DF) D. F = A x DF / E Answer: A. Illuminance times area is increased by depreciation and waste light and divided by CU. (Key point: Flood design lumens) 4. How are projector distance and beam spread chosen? A. Distance 0.7-1 times building height; beam spread = 2 tan^-1(half-size/distance) B. Distance 10 times height; beam fixed C. Placed in the viewers' line of sight at 1 m D. Beam spread equals the lamp wattage Answer: A. Projectors sit about 0.7-1 times the height away, out of the viewers' sight, with the beam angle set by geometry. 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