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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