Illuminance is a measure of luminous flux striking a given surface, i.e. it is the quotient of lumens and area. A luminous flux of 1000 lumens beamed uniformly on a surface measuring 5 sq. m., for example, gives an illuminance of 200 lux.
The performance of the human eye is highly dependent on the level of illuminance within the field of vision. But general performance and motivation also improve with increasing illuminance, while the frequency of mistakes decreases.
In the individual case the level of Illuminance required will depend on the tasks involved and the precision of visual performance required. Adequate illuminance at the workplace is dependent on the choice of luminaires and lamps, correct calculation of the number of luminaires required, and installation in an efficient configuration.
The required levels of illuminance are defined in the various national standards and guidelines (e.g. EN 12464, ÖNorm 1040, SEV 8912).
The DIN standard, for example, prescribes mean illuminance as an average figure for both time and place, and also defines minimum illuminance which must be maintained at all times and at all points within the interior. The levels of illuminance quoted in the DIN standard are graded as follows: 3, 5, 10, 20, 30, 50, 100, 200, 300, 500, 750, 1000, and 1500 lx.
Service illuminance is always quoted relative to the position of the plane to be illuminated. Horizontal illuminance, for example, is an expression of the level of illuminance on a horizontal work plane, while vertical illuminance relates to a vertical plane. In the individual case the latter may also apply to such planes as a drawing board placed at an angle of 70°.
Cylindrical illuminance s the average value for vertical illuminance beamed from all directions around a single point (on a vertical axis).
A comparison between cylindrical and horizontal illuminance provides a rough assessment of the relationship between diffuse light and directional light. Good shadow conditions, which are important for three-dimensional vision and orientation, are the product of a reasonable balance between these two forms of illuminance (between 30 and 70%). Direct light alone creates harsh shadows, whereas purely diffuse lighting generates a flat and monotonous effect without any shadows at all (cf. contrast).
The following is a list of typical values for illuminance in our everyday lives:
| Horizontal illuminance outdoors: | ||||
|---|---|---|---|---|
|
Type |
Illuminance [lx] |
|||
|
Sunny summer's day |
60.000 bis 100.000 |
|||
|
Dull summer's day |
bis 20.000 |
|||
|
TDull winter's day |
bis 3.000 |
|||
|
Night with full moon |
bis 0.25 |
|||
|
Starlit sky |
bis 0.01 |
|||
| Typical illuminance in interiors: | ||||
|---|---|---|---|---|
|
Type |
Illuminance [lx] |
|||
|
Office |
300 bis 1000 |
|||
|
Industrial plant |
100 bis 500 |
|||
|
Living room |
100 bis 300 |
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|
Cellar |
50 bis 100 |
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