Illuminance - Recommended Light Level
Working activities and light levels - required illuminance.
Light Level or Illuminance is the total luminous flux incident on a surface per unit area. The area - the work plane - is where the most important tasks in the room or space are performed.
Illuminance can be expressed as
E = Φ / A (1)
E = light intensity, illuminance (lm/m2, lux)
Φ = luminous flux - the quantity of light emitted by a light source (lumen, lm)
A = area (m2)
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
Outdoor Light Levels
Common outdoor light levels at day and night:
|Very Dark Day||10||107|
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 works the light level may even approach 1500 - 2000 lux.
Recommended light levels for different types of work spaces are indicated below:
|Public areas with dark surroundings||20 - 50|
|Simple orientation for short visits||50 - 100|
|Areas with traffic and corridors - stairways, escalators and travelators - 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|
|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|
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).