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Flood lighting procedure and 50 × 15 m frontage

Short answer

15 projectors of 200 W each (90 lm/W). Total lumens: 100×750×1.5×1.2/0.5 = 270,000 lm. Beam spread required ≈ 33.4°. Use medium-beam projectors.

Explain briefly the design procedure of flood lighting system. A building frontage 50m × 15m is to be illuminated by floodlighting projectors situated 25m away. If the illumination is 100 lux, coefficient of utilization is 0.5, depreciation factor is 1.5, and waste light factor 1.2, estimate the number of projectors of each of size 200W with luminous efficiency of 90 lumen/watt.

Design procedure of floodlighting

  1. Survey the object: size, shape, colour and reflection factor of the surface and the brightness of surroundings.
  2. Choose the illumination level (E) from tables (higher for dark surfaces and bright surroundings).
  3. Find the area to be illuminated: A = length × height.
  4. Fix projector location – distance about 0.7–1 times the building height, out of the viewers' sight line.
  5. Select beam spread to suit the throw: θ = 2 tan⁻¹(half-height / distance).
  6. Choose factors: coefficient of utilisation (CU), depreciation factor (DF), waste light factor (WLF).
  7. Total lumens: F = E × A × DF × WLF / CU.
  8. Lamp lumens = wattage × efficacy; number N = F / lamp lumens (round up).
  9. Arrange and aim projectors for uniform, glare-free lighting; design cabling and control.

Numerical

Given: 50 m × 15 m, distance 25 m, E = 100 lux, CU = 0.5, DF = 1.5, WLF = 1.2, 200 W lamps of 90 lm/W.

Area A    = 50 × 15            = 750 m²
Total lm  = E·A·DF·WLF / CU
          = 100 × 750 × 1.5 × 1.2 / 0.5
          = 270 000 lm
Lamp lm   = 200 × 90           = 18 000 lm
N         = 270 000 / 18 000   = 15

Vertical beam spread needed (aimed at mid-height): 2 tan⁻¹(7.5/25) = 33.4°, so medium-beam projectors are suitable.

Answer: 15 projectors of 200 W each.

✓ Answer checked by Claude Opus 5.5 (high effort)

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