Illuminance E as a measure of luminous flux incident per surface unit A.
Illuminance (E) describes the quantity of light incident on a surface. It is the ratio of the incident luminous flux (Φ) to the size of the surface (A). The unit of measure of illuminance is lux (lx). This is defined as lumens per square metre (1lx = 1lm/m2). Illuminance recedes with the square of the distance from the light source (photometric law of distance).
Good to know:
Calibrated lux meters should be used to correctly measure illuminance. The measuring head must be aligned parallel to the measuring surface and ideally rest on it. Because the photometer measures all the light incident at this point, care must be taken that the person taking the measurement does not block any luminaire in the measuring field!
| Workspaces and activities | Illuminance lx |
|---|---|
| Tea points, canteens and break rooms | 200 |
| Office: filing and copying | 300 |
| Office: writing, reading and data processing | 500 |
| Office: technical drawing by hand | 750 |
| Archives | 200 |
| Sales area | 300 |
| Checkout areas | 500 |
| Control, fine work | 1000 |
Source: ASR A3.4 Lighting and Visual Contact, Committee for Workplaces, ASTA Management, BAuA, www.baua.de
The importance of daylight was specifically emphasised in the updated DIN EN 12464-1 from 2011. In addition, it is repeatedly pointed out that lighting should be controllable or adjustable.
The Committee on Workplaces supervised by the German Ministry of Labour and Social Affairs also reflects the state of scientific knowledge for occupational safety and health in the setting up and operating of workplaces:
For light visual tasks, sufficient visual performance is achieved with low illuminance levels, whereas complex visual tasks need high illuminance levels. For example, 20 lux is a bottom limit at which people's facial features can only just be distinguished.
The illuminance generated by two or more light sources can be added together. For example, if one luminaire in an office generates illuminance of 500lx on a desk and a second one does the same, then the illuminance on the desk is 1000lx if both luminaires are operated.
