The Simple Definition
One lux = one lumen per square metre. If a 10,000 lumen light illuminates a 1,000 sq.m area uniformly, the illuminance is 10,000 ÷ 1,000 = 10 lux. In reality, light is not uniformly distributed, so photometric calculations are needed. Lux levels drop as you move away from the pole (light falloff) and they decrease with higher mounting heights.
IS 1944 Lux Standards for Indian Roads
The Bureau of Indian Standards IS 1944 specifies minimum maintained average horizontal illuminance for roads: Category P1 (high-volume arterial roads, urban expressways): 20–30 lux. Category P2 (collector roads, main urban roads): 15–20 lux. Category P3 (local distributor roads): 10–15 lux. Category P4 (local access roads, residential): 7–10 lux. Category P5 (very low activity roads, village lanes): 3–7 lux. Municipal engineers and PWD inspectors use these standards during project evaluation.
How to Measure Lux on a Road
Professional measurement uses a calibrated lux meter at 0.85m height from the road surface (approximate eye level for seated occupants of vehicles). Measurements are taken at a grid of points - typically a 2m × 2m or 5m × 5m grid between poles. Average all readings for maintained average illuminance. The uniformity ratio (minimum lux ÷ average lux) must be at least 0.25–0.4 for road safety. A smartphone lux meter app can give approximate readings for verification.
What Lux Feels Like in Real Life
1–3 lux: barely visible, can see shapes but not faces or obstacles. 5–10 lux: good for village lanes, you can walk safely and see potholes. 15–25 lux: comfortable road driving at 40–60 kmph, faces visible. 30–50 lux: bright - suitable for intersections, pedestrian crossings, petrol pumps. 100+ lux: very bright - construction sites, factory floors, sports grounds. 300–500 lux: broadcast sports lighting. 500–1000 lux: operating theatre, precision assembly.
