Light sources and specular reflection principle

Types of light sources

1. Incandescent (filament) lamps: a tungsten filament heated to about 2500–2800 °C glows. Includes GLS lamps and tungsten-halogen lamps (halogen gas allows higher filament temperature and longer life). Efficacy 10–25 lm/W, excellent colour rendering (Ra ≈ 100), cheap but short life (1000–2000 h).
2. Low-pressure discharge lamps:
   • Fluorescent tube (FTL) and compact fluorescent lamp (CFL): mercury vapour discharge produces UV, which phosphor coating converts to visible light. 50–90 lm/W, life 8000–15000 h, need ballast.
   • Low-pressure sodium (LPS): monochromatic yellow light, very high efficacy (up to 180 lm/W) but very poor colour rendering.
3. High-intensity discharge (HID) lamps: high-pressure mercury vapour (35–60 lm/W), high-pressure sodium (80–140 lm/W, golden-yellow), metal halide (70–110 lm/W, white light, good CRI). Used for streets, high-bay factories and floodlights.
4. Solid-state lighting – LED: semiconductor p-n junction emits light when forward biased. Efficacy 100–200 lm/W, life 25,000–50,000 h, instant start, dimmable, no mercury. Now the most used source for all applications.
5. Others: induction lamps (electrodeless, very long life), neon lamps for signs.

Specular reflection principle of street lighting

When a road surface is wet or smooth, it behaves partly like a mirror (specular reflection): the angle of reflection equals the angle of incidence. A motorist then does not see the road as uniformly lit; instead, each lamp produces a bright elongated patch (a "T"-shaped streak) on the road between the lamp and the driver, and the road between patches looks dark.

The specular reflection principle uses this effect: lamps are arranged so that the bright reflected patches of successive lamps overlap and form a continuous bright background, against which objects (pedestrians, vehicles) are seen in dark silhouette.

 lamp          lamp          lamp
  O             O             O
  |\            |\            |\
  | \  bright   | \  bright   | \
==|==\=patch====|==\=patch====|==\=== road
      \             \             \
       \______________\_____________→ driver's eye

Design rules:

• Use a fairly small spacing and low mounting height so the bright patches join; the patch is long in the direction of travel.
• Luminaires are mounted over the road (on brackets/outreach) and aimed along the road so that maximum light reaches the driver's direction of view.
• Use cut-off or semi-cut-off luminaires to limit glare to the driver.
• The principle works for smooth/wet roads; on rough dry roads diffuse reflection dominates and the diffusion (lumen) principle is used instead.