--- title: "Waste light factor and floodlight calculation" subject: "Industrial Power Distribution and Illumination" chapter: 8 key: "semester_6/industrial_power_distribution_and_illumination:waste-light-factor-floodlight" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/waste-light-factor-floodlight site: BCT Engineering --- # Waste light factor and floodlight calculation Industrial Power Distribution and Illumination · Chapter 8: Outdoor Lighting Design - Asked in: 2073 Bhadra, 2+2+4 marks - Video: [ILLUMINATION ENGINEERING - PART - 14 - THREE PROBLEMS IN FLOODLIGHTING DESING](https://www.youtube.com/watch?v=kZA2zZFlxRg) — 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/waste-light-factor-floodlight - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-8 ## Question What is Waste Light Factor? Why is it necessary to be considered in floodlighting design? With a suitable example, explain the process of floodlight calculation? ## Short answer Waste light factor (WLF) increases required lumens to compensate for light spilling beyond edges (typical 1.2–1.5). It prevents a 15–35% illumination shortfall. For a 30 m × 12 m facade needing 120 lux: F = 120 × 360 × 1.3 × 1.2 / 0.4 = 168,480 lm; using 250 W lamps (20,000 lm) yields 9 projectors. ## Answer #### Waste light factor **Waste light factor (WLF)** is the factor by which the required lumens are increased to allow for light from the projectors that **spills beyond the edges** of the surface (or passes between parts of it) and so does no useful work. Usual values: **1.2** for regular, flat surfaces and **1.5** for irregular objects such as statues and towers. #### Why it must be considered - Beams are circular or oval, while buildings are rectangular; overlapping beams at the edges always spill some light. - Without WLF, the actual illumination on the surface would be about 15–35% lower than designed, making the facade look dull and patchy. - It gives a realistic number and wattage of projectors and avoids redesign after installation. #### Floodlight calculation process 1. Decide the illumination level E (lux) from the surface colour and surroundings. 2. Find the area A of the surface. 3. Choose CU (or beam factor), depreciation factor DF and WLF. 4. Total lumens: **F = E × A × DF × WLF / CU**. 5. Choose lamp wattage and efficacy; lamp lumens = W × η. 6. Number of projectors N = F / lamp lumens (round up). 7. Choose the beam spread from the distance: θ = 2 tan⁻¹(half-height / distance), then arrange and aim. **Example:** A building front 30 m × 12 m, in a medium-bright area, needs 120 lux. Projectors at 15 m; CU = 0.4, DF = 1.3, WLF = 1.2; 250 W metal-halide lamps of 80 lm/W. ```text A = 30 × 12 = 360 m² F = 120 × 360 × 1.3 × 1.2 / 0.4 = 168 480 lm Lamp lm = 250 × 80 = 20 000 lm N = 168 480 / 20 000 = 8.4 → 9 θ = 2 tan⁻¹(6 / 15) = 43.6° (wide beam) ``` So 9 wide-beam 250 W projectors are used. Without WLF (= 1.0), F would be only 140 400 lm (N = 7.02 → 8), one projector fewer, and the surface would be under-lit. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the waste light factor? A. The fraction of lumens reaching the surface B. A factor increasing the required lumens to allow for light spilling beyond the surface edges C. The factor for dirt reducing lamp output over time D. The ratio of beam angle to throw distance Answer: B. WLF compensates for spill light that does no useful work; the dirt effect is the depreciation factor. (Key point: Meaning of WLF) 2. What are typical WLF values? A. 0.5 for flat surfaces, 0.8 for statues B. 1.0 for all surfaces C. 1.2 for regular flat surfaces, 1.5 for irregular objects like statues D. 2.0 for flat surfaces, 3.0 for statues Answer: C. Regular surfaces use about 1.2; irregular objects spill more light so about 1.5. (Key point: Typical WLF values) 3. Why must WLF be considered? A. Circular/oval beams overlap and spill at the edges of rectangular surfaces, so without it the surface would be under-lit B. It reduces the number of projectors needed C. It corrects for the colour of the surface D. It accounts for voltage drop in cables Answer: A. Neglecting spill makes actual illumination 15-35% lower than designed. 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