Understanding illuminance
Lux, foot-candles, and "how lit is this surface".
Why lighting designers care about lux, the foot-candle's stubborn US presence, and the bands that mean comfortable, brightly-lit, surgical.
Lumens vs lux.
A lumen (lm) is the total light a source emits. Lux (lx) is the lumens per square metre falling on a surface — illuminance. A 1000-lm bulb spread over 10 m² produces 100 lx average; concentrate it onto 1 m² and you get 1000 lx. Lumens describes the bulb; lux describes the surface. Light meters report lux; the spec sheet on a bulb reports lumens.
Foot-candles (US).
The American imperial equivalent: 1 foot-candle (fc) is 1 lumen per square foot. 1 fc ≈ 10.764 lux (the area conversion m² → ft²). US lighting codes (IES, OSHA workplace standards) typically specify foot-candles; European codes specify lux. Cross-reference by the factor of ~10.76 — close enough to memorise as "10×" for rough work.
The bands by activity.
Outdoor full daylight: 10,000-25,000 lx (overcast 1000-2000 lx; bright sun 50,000-100,000 lx). Office work: 300-500 lx recommended. Supermarket: 750-1000 lx. Detailed visual work (drawing, sewing): 1000 lx+. Surgical operating room: 10,000-100,000 lx at the operating field. Hotel corridor: 100 lx. Restaurant ambiance: 50-150 lx. Cinema aisles during the show: 0.5-2 lx.
The inverse-square problem.
Light from a point source falls off as the square of distance. Double the distance from the bulb, the lux on the surface drops to 25 %. This is why a 60 W bulb on a desk lamp produces useful illuminance — it's close — and the same bulb mounted on a 3 m ceiling is barely adequate. Lighting design for offices and homes is largely about managing this: many small sources close to surfaces beat one bright source far away.
A worked check.
A 10 m² office with a 4 m ceiling needs 500 lx desk illuminance. Two panel lights at 4000 lm each = 8000 lm total. Across 10 m² of floor: 800 lx average — looks like plenty. But account for inverse-square, fixture efficiency, surface reflectance, glare, and the bottom-of-room reality often drops 20-40 % below the ceiling-emitted total. A light meter on actual desks tells the true story; design calculations are a first pass.
Office lux target
500 lx desk-level
Lumens / area gives a first estimate.
8000 lm / 10 m² = 800 lx average
= Likely 480-640 lx after losses
Modern LED specs make it easy.
Until ~2015, light-bulb specs reported watts (a power input proxy for brightness that doesn't work for LEDs). Now lumens is the standard. A modern 9 W LED produces ~900 lm (equivalent to a 60 W incandescent's 800 lm). For room-level design, multiply the lumen count by the number of bulbs, divide by floor area, treat as an upper bound. Photo studios, retail, and surgical contexts need real photometric design; everyday rooms are forgiving enough that this approximation works.