When talking about the quality of light in a working environment, the question 'How much light?' is the first one to ask. There are recommendation as to what the minimal illuminance should be. The table below shows some examples from the CIE Code for Interior Lighting:
| Category | Recommended illuminance [lux] | Example | ||
|---|---|---|---|---|
| General areas that are not in permanent use or that do not demand a high visibility of the task | 20 | 30 | 50 | public areas |
| 50 | 100 | 150 | orientation during short stop | |
| 100 | 150 | 200 | rooms that are not in permanent use (storage rooms) | |
| General lighting for indoor work | 200 | 300 | 500 | details easy to see (operation of machinery) |
| 300 | 500 | 750 | normal visual tasks (offices) | |
| 500 | 750 | 1000 | details difficult to see (quality control) | |
| Tasklighting for highly demanding work | 750 | 1000 | 1500 | demanding task undertaken for an extended time (watchmaking) |
| 1000 | 1500 | 2000 | extremely fine details (microelectronic assembly) | |
| >2000 | special tasks (surgery) | |||
These recommendations are derived from visibility tests. They are applicable for mid-aged persons, medium reflectances in the visual environment and for normal priority tasks. Should the conditions differ, then the illuminances will have to be adjusted.
The following the table shows correction factors for cases where the normal recommeneded illuminance levels can not be applied:
| Case | Correction factor | ||
|---|---|---|---|
| -1 | 0 | +1 | |
| Age of work force [years] | 40 | 40...55 | >55 |
| Reflectance of near surrounding | >0.7 | 0.3...0.7 | <0.3 |
| Speed and accuracy of perception | unimportant | important | very important |
For each of these cases, the correction factors are summed up. If their total is -3 or -2, the lower illuminance is used. For a weighting factor of -1, 0, or +1, no correction needs to be applied. A total of +2 and +3 means the conditions are rather poor, so more light is needed. Most countries have their specific interpretations of these CIE recommendation. They can be found in the national codes of practice.
In rooms that have multiple use such as a general office in which technical drawings are made as well, it is not necessaryto have the higher recommended illumiance in the entire room. Instead, the room can be split up into different areas. Also, it is often wasteful to produces the required lux level with a general lighting system. Localised task lighting can be far more efficient and people often prefer it because they can adjust it individually.
