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Factors governing illumination level

Short answer

Required illumination depends on visual task (fine work 500-1500 lux, ordinary 300-500), size/contrast, duration, age (2-3x more), room reflectances, and room index. Execution must ensure adequate uniform light, glare control, correct colour rendering, avoid stroboscopic effect, use suitable luminaires, allow maintenance, and be energy-efficient and safe.

Explain the factors governing the amount of illumination at particular place and necessary points to be kept in mind for executing lighting schemes.

The amount of illumination needed at a place depends mainly on what visual task is done there, by whom, and for how long. A good lighting scheme must give this illumination with good quality (no glare, no harsh shadows, correct colour) at the lowest running cost.

Factors governing the amount of illumination

  1. Nature of work (visual task): fine work (drawing, watch repair, inspection) needs 500–1500 lux; ordinary work (assembly, office) 300–500 lux; corridors and stores only 50–150 lux.
  2. Size of detail and contrast: small objects, or objects with poor contrast against the background, need more light.
  3. Speed and accuracy required: fast moving parts or precision work need higher levels.
  4. Duration of work: long continuous work needs more light to avoid eye fatigue.
  5. Age and eyesight of workers: older people need roughly 2–3 times more light for the same task.
  6. Reflection factors of surroundings: light-coloured ceiling, walls and floor reflect light and raise the useful illumination; dark surfaces absorb it.
  7. Size and shape of room (room index): in tall narrow rooms more light is lost on the walls, so the utilisation factor is lower.
  8. Availability of daylight and the hours when artificial light is used.
  9. Safety and economy: hazardous areas need a minimum safe level; the level chosen must also be affordable to install and run.

Recommended levels are taken from lighting codes (e.g. IS 3646 / CIBSE / IES tables).

Points to keep in mind while executing a lighting scheme

  1. Adequate illumination: give the recommended lux level on the working plane (usually 0.75–0.85 m above floor).
  2. Uniformity: the ratio of minimum to average illuminance should be about 0.7 or more; keep spacing-to-mounting-height ratio within the luminaire's limit.
  3. Freedom from glare: shield lamps, mount them above the line of sight, use diffusers or louvres; avoid shiny surfaces that cause reflected glare.
  4. Avoid harsh shadows: use several sources and diffused light, but keep some shadow for depth.
  5. Correct colour rendering: choose lamps with suitable colour temperature and CRI for the task (e.g. high CRI for colour matching).
  6. Avoid stroboscopic effect: near rotating machines, connect adjacent luminaires to different phases or use high-frequency electronic ballasts.
  7. Suitable luminaire and mounting: dust-tight, vapour-proof or flame-proof fittings where required; easy access for maintenance.
  8. Maintenance factor: allow for dirt and lamp lumen depreciation; plan regular cleaning and group replacement.
  9. Energy efficiency and cost: use high-efficacy sources (LED, T5, HPSV), separate switching, daylight and occupancy controls.
  10. Aesthetics and safety: the scheme should look neat and the wiring must follow electrical regulations.
 Task needs ──> Lux level ──> Lamp & luminaire
     │                              │
     └─> Room data (RI, reflect.) ──┴─> Number,
                                       spacing, layout

✓ Answer checked by Claude Opus 5.5 (high effort)

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