Illuminance - Recommended Light Levels

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)

where

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:

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:

Calculating Illumination

Illumination can be calculated as

E = Φ_l C_u L_LF / A_l                           (2)

where

E = illumination (lux, lumen/m²)

Φ_l = luminance per lamp (lumen)

C_u = coefficient of utilization

L_LF = light loss factor

A_l = 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 C_u = 0.6 and L_LF = 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).