Outdoor lighting, flood and street calculation

Outdoor lighting is artificial lighting of open areas – roads, building facades, monuments, sports grounds, yards and parking areas – for safety, security, night work, advertising and decoration. The two main calculations are for floodlighting and street lighting.

Floodlighting calculation

Floodlighting means lighting a large surface with projectors giving concentrated beams.

1. Select illumination E (lux) based on the surface reflectance and surroundings.
2. Area A = length × height of the surface.
3. Total lumens required:

F = E × A × DF × WLF / CU

where CU = coefficient of utilisation (beam lumens/lamp lumens, 0.3–0.6), DF = depreciation factor (1.3–1.5), WLF = waste light factor (1.2–1.5). 4. Number of projectors N = F / (lamp watts × efficacy). 5. Beam spread θ = 2 tan⁻¹(half-height / distance).

Example: 20 m × 10 m face, E = 150 lux, CU = 0.5, DF = 1.3, WLF = 1.2, 150 W LED floodlights of 110 lm/W.

F = 150 × 200 × 1.3 × 1.2 / 0.5 = 93 600 lm
Lamp lm = 150 × 110 = 16 500 lm
N = 93 600 / 16 500 = 5.7 → 6 floodlights

Street lighting calculation

(a) Lumen method (average illumination):

E_av = F × CU × MF / (S × W)
S    = F × CU × MF / (E_av × W)

F = lamp lumens, S = pole spacing, W = road width, MF = maintenance factor.

(b) Point-by-point method (minimum illumination): for a lamp of intensity I at height h, a point at horizontal distance x gets

E = I cos θ / d² = I h / (h² + x²)^1.5

Add contributions of adjacent lamps (usually the two nearest) midway between poles to check minimum and uniformity.

Example: Road 8 m wide, 150 W HPSV lamps (15 000 lm), CU = 0.4, MF = 0.8, E_av = 15 lux, h = 9 m.

S = 15 000 × 0.4 × 0.8 / (15 × 8) = 40 m
S/h = 40/9 = 4.4  → too high; take S = 35 m
E_av = 15 000×0.4×0.8/(35×8) = 17.1 lux