Illuminance

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