Illuminance - Recommended Light Levels

Recommended light levels - illuminance - for some common types of working activities

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.

Measuring Units Light Level - Illuminance

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.

  • 1 lux = 1 lumen / sq meter = 0.0001 phot = 0.0929 foot candle (ftcd, fcd)
  • 1 phot = 1 lumen / sq centimeter = 10000 lumens / sq meter = 10000 lux
  • 1 foot candle (ftcd, fcd)  = 1 lumen / sq ft = 10.752 lux

Common Light Levels Outdoor

Common light levels outdoor at day and night can be found in the table below:

Condition Illumination
(ftcd) (lux)
Sunlight 10,000 107,527
Full Daylight 1,000 10,752
Overcast Day 100 1,075
Very Dark Day 10 107
Twilight 1 10.8
Deep Twilight .1 1.08
Full Moon .01 .108
Quarter Moon .001 .0108
Starlight .0001 .0011
Overcast Night .00001 .0001

Common and Recommended Light Levels Indoor

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:

Activity Illumination
(lux, lumen/m2)
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

Calculating Illumination

Illumination can be calculated as

I = Ll Cu LLF / Al       (1)

where

I = illumination (lux, lumen/m2)

Ll = lumens per lamp (lumen)

Cu = coefficient of utilization

LLF = light loss factor

Al = area per lamp (m2)

Example - Illumination

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

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