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.
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 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
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.
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 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 Theaters | 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 = 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
I = 10 (10600 lumens) (0.6) (0.8) / (50 m2)
= 1018 lux