Design For The Correct Light Levels

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:

Activity Illuminance
(lx, lumen/m2)
Public areas with dark surroundings 20 – 50
Simple orientation for short visits 50 – 100
Areas with traffic and corridors – stairways, escalators, lifts – storage spaces 100
Working areas where visual tasks are only occasionally performed 100 – 150
Warehouses, homes, theaters, archives, loading bays 150
Coffee break room, technical facilities, ball-mill areas, pulp plants, waiting rooms, 200
Easy office work 250
Class rooms 300
Normal office work, PC work, study library, groceries, show rooms, laboratories, check-out areas, kitchens, auditoriums 500
Supermarkets, mechanical workshops, office landscapes 750
Normal drawing work, detailed mechanical workshops, operation theaters 1000
Detailed drawing work, very detailed mechanical works, electronic workshops, testing and adjustments 1500 – 2000
Performance of visual tasks of low contrast  and very small size for prolonged periods of time 2000 – 5000
Performance of very prolonged and exacting visual tasks 5000 – 10000
Performance of very special visual tasks of extremely low contrast and small size 10000 – 20000

Calculating Illumination

Illumination can be calculated as

E = Φl Cu LLF / Al                           (2)

where

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

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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