Light Level or Illuminance, is the total luminous flux incident on a surface, per unit area. The work plane is where the most important tasks in the room or space are performed.
Illumenance is measured in foot candles (ftcd, fc, fcd) (or lux in the metric SI system). A foot candle is actually one lumen of light density per square foot, one lux is one lumen per square meter.
Common light levels outdoor at day and night can be found in the table below:
|Very Dark Day||10||107|
The outdoor light level is approximately 10,000 lux on a clear day. In the building, in the area closest to windows, the light level may be reduced to approximately 1,000 lux. In the middle area its may be as low as 25 - 50 lux. Additional lighting equipment is often necessary to compensate the low levels.
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.
The table below is a guidance for recommended light level in different work spaces:
|Public areas with dark surroundings||20 - 50|
|Simple orientation for short visits||50 - 100|
|Working areas where visual tasks are only occasionally performed||100 - 150|
|Warehouses, Homes, Theaters, Archives||150|
|Easy Office Work, Classes||250|
|Normal Office Work, PC Work, Study Library, Groceries, Show Rooms, Laboratories||500|
|Supermarkets, Mechanical Workshops, Office Landscapes||750|
|Normal Drawing Work, Detailed Mechanical Workshops, Operation Theatres||1,000|
|Detailed Drawing Work, Very Detailed Mechanical Works||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
I = Ll Cu LLF / Al (1)
I = illumination (lux, lumen/m2)
Ll = lumens per lamp (lumen)
Cu = coefficient of utilation
LLF = light loss factor
Al = area per lamp (m2)
10 incandencent 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
I = 10 (10600 lumens) (0.6) (0.8) / (50 m2)
= 1018 lux