Measuring Units Light Level – Illuminance

Illuminance is measured in foot candles (ftcd, fc, fcd) in the Imperial system or lux in the metric SI system.

  • one foot candle = one lumen of light density per square foot
  • one lux = one lumen per square meter
  • 1 lux = 1 lumen / sq metre = 0.0001 phot = 0.0929 foot candle (ftcd, fcd)
  • 1 phot = 1 lumen / sq centimetre = 10000 lumens / sq metre = 10000 lux
  • 1 foot candle (ftcd, fcd)  = 1 lumen / sq ft = 10.752 lux

Recommended Indoor Light Levels

The outdoor light level is approximately 10000 lux on a clear day. In a building in the area closest to the windows the light level may be reduced to approximately 1000 lux. In the middle area it may be as low as 25 – 50 lux. Additional lighting is often necessary to compensate low levels.

According EN 12464 Light and lighting – Lighting of workplaces -Indoor work places, the minimum illuminance is 50 lx for walls and 30 lx for ceilings. Earlier it was common with light levels in the range 100 – 300 lux for normal activities. Today the light level is more common in the range 500 – 1000 lux – depending on activity. For precision and detailed work the light level may even approach 1500 – 2000 lux.

Recommended light levels for different types of work spaces are indicated below:

(lx, lumen/m2)
Public areas with dark surroundings20 – 50
Simple orientation for short visits50 – 100
Areas with traffic and corridors – stairways, escalators, lifts – storage spaces100
Working areas where visual tasks are only occasionally performed100 – 150
Warehouses, homes, theaters, archives, loading bays150
Coffee break room, technical facilities, ball-mill areas, pulp plants, waiting rooms,200
Easy office work250
Class rooms300
Normal office work, PC work, study library, groceries, show rooms, laboratories, check-out areas, kitchens, auditoriums500
Supermarkets, mechanical workshops, office landscapes750
Normal drawing work, detailed mechanical workshops, operation theaters1000
Detailed drawing work, very detailed mechanical works, electronic workshops, testing and adjustments1500 – 2000
Performance of visual tasks of low contrast  and very small size for prolonged periods of time2000 – 5000
Performance of very prolonged and exacting visual tasks5000 – 10000
Performance of very special visual tasks of extremely low contrast and small size10000 – 20000

Calculating Illumination

Illumination can be calculated as

E = Φl Cu LLF / Al                           (2)


E = illumination (lux, lumen/m2)

Φl = luminance per lamp (lumen)

Cu = coefficient of utilization

LLF = light loss factor

Al = area per lamp (m2)

Example – Illumination

10 incandescent lamps of 500 W (10600 lumens per lamp) are used in an area of 50 m2. With Cu = 0.6 and LLF = 0.8 illumination can be calculated as

E = 10 (10600 lumens) (0.6) (0.8) / (50 m2)

  = 1018 lux


Luminance is the only basic lighting parameter that is perceived by the eye. It describes on the one hand a light source’s impression of brightness, and on the other, a surface and therefore depends to a large extent on the degree of reflection (color and surface).

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