Light sources and indoor lighting schemes

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.

Lighting schemes for indoor lighting

Based on how much of the luminaire's light goes directly downward to the working plane:

By coverage, indoor lighting is also arranged as general lighting (uniform over the whole area), localised lighting (extra light on specific work points), and general plus local lighting, with emergency lighting for safety.