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:
| Condition | Illumination | |
|---|---|---|
| (ftcd) | (lux) | |
| Sunlight | 10000 | 107527 |
| Full Daylight | 1000 | 10752 |
| Overcast Day | 100 | 1075 |
| Very Dark Day | 10 | 107 |
| Twilight | 1 | 10.8 |
| Deep Twilight | 0.1 | 1.08 |
| Full Moon | 0.01 | 0.108 |
| Quarter Moon | 0.001 | 0.0108 |
| Starlight | 0.0001 | 0.0011 |
| Overcast Night | 0.00001 | 0.0001 |
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:
| 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 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 |
| 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).
Related Topics
• Physiology
Human physiology vs. air quality, comfort temperatures, activity and metabolic rates. Health effects of gases adn polutions like carbon monoxide and more.
Related Documents
Absorbed Solar Radiation
Solar radiation absorbed by various materials.
Color Mixing Theory
Subtractive and additive color mixing.
Heat Gain from Lights
Heat contributed by lights may have major impact on air-condition systems.
LENI - Lighting Energy Numeric Indicator
Energy consumption of lighting systems
Lights vs. Power Installation
Lights vs. power installations in common types of buildings and rooms.
Luminous Efficacy
The visible light produced by light sources.
Materials - Light Reflecting Factors
Light reflecting factor materials.
Optical Distance Law
Geometric dilution of light vs. distance.
Refractive Index common Liquids, Solids and Gases
Some common fluids and their refractive index.