When it is necessary to access lighting objectively and quantify the lighting ambience of a space, i.e. Low, soft or strong lighting, the concept of illuminance is used. For instance, the luminous intensity which is provided on a single gray wall by a lamp of identical power decreases according to inverse of the square of the distance. The illuminance, in other words the quantity of light incident upon this surface, is a function of the distance to the source (d), its luminous intensity (I), and the angle that exists between the incident light which reaches the material and the perpendicular to the surface.
The illuminance is expressed in terms of lux (lumen/m2). This is a means of comparison which lighting engineers often use to describe the functionality of lighting.
Common Light Levels Outdoor
Common light levels outdoor at day and night can be found in the table below:
Condition | Illumination (lux, lumen/m2) | |
Direct Sunlight | 100000- 500000 | |
Full Daylight | 10000-40000 | |
Overcast Day | 20,000 | |
Twilight | 400 | |
Full Moon | 100 | |
Quarter Moon | 0.01- 20 | |
Starlight | 0.001-10 | |
Overcast Night | 0.0001 |
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 |
