Street lighting principles and flood design

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.

 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.