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The amount of light we need varies and depends on:
- Type of task being done (such as demands for speed and accuracy).
- Type of surfaces (does it reflect or absorb light).
- General work area.
- Individual's vision.
The amount of light falling on a surface is measured in units called lux. Depending on the factors noted above, adequate general lighting is usually between 500 and 1000 lux when measured 76 cm (30 inches) above the floor.*
Examples of industrial and office tasks and the recommended light levels are in the table below.
| Public spaces with dark surroundings | 20-50 |
| Simple orientation for short temporary visits | 50-100 |
| Working spaces where visual tasks are only occasionally performed | 100-200 |
| Performance of visual tasks of high contrast or large scale | 200-500 |
| Performance of visual tasks of medium contrast or small size | 500-1000 |
| Performance of visual tasks of low contrast or very small size | 1000-2000 |
| Performance of visual tasks of low contrast and very small size over a prolonged period | 2000-5000 |
| Performance of very prolonged and exacting visual tasks | 5000-10000 |
* From: IESNA Lighting Handbook. 9th ed. Illuminating Engineering Society of North America, 2000. p. 10-13.
**Lux = Lumens (quantity of light) per square metre.
To reach proper light levels and uniform light distribution in the visual environment, many light fixtures are designed to reflect light off walls, ceilings and objects. The amount of light reflected off a surface can be measured. Suggestions for the percent of light reflected off surfaces in a typical office include:
- Window blinds (40-50%).
- Walls (50% maximum).
- Business machines (50% maximum).
- Ceiling (70-80%).
- Floor (20-40%).
- Furniture (25-45%).
The percent value refers to the amount of light that a surface reflects relative to the amount that falls on the surface.
In addition, light fixtures that are too widely spaced or wrongly positioned can create shadows. Objects between the light fixture and work being done can block the light and cast shadows. Likewise, workers sitting with their backs to windows, with light fixtures directly overhead or to the rear, cast shadows on their own work surfaces.