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.
The complete lighting unit [also called the light fixture] controls and distributes the light. [Light fixtures are often referred to as "luminaires" in technical publications.]
Various types of light fixtures are designed to distribute light in different ways. These fixtures are known as:
- Direct.
- Direct-indirect.
- Indirect.
- Shielded [various types].
No single type of light fixture is appropriate in every situation. The amount and quality of lighting required for a particular workstation or task will determine which light fixture is most suitable.
Direct light fixtures project 90 to 100 percent of their light downward toward the work area. Direct lighting tends to create shadows.
Direct-indirect light fixtures distribute light equally upward and downward. They reflect light off the ceiling and other room surfaces. Little light is emitted horizontally meaning direct glare is often reduced. They are usually used in "clean" manufacturing areas.
Indirect light fixtures distribute 90 to 100 percent of the light upward. The ceiling and upper walls must be clean and highly reflective to allow the light to reach the work area. They provide the most even illumination of all the types of fixtures and the least direct glare. Indirect light fixtures are usually used in offices.
Shielded light fixtures use diffusers, lenses and louvers to cover bulbs from direct view; therefore, helping to prevent glare and distribute light.
- Diffusers are translucent or semi-transparent [see-through] covers made usually of glass or plastic. They are used on the bottom or sides of light fixtures to control brightness.
- Lenses are clear or transparent glass, or plastic covers. The lens design incorporates prisms and flutes to distribute light in specific ways.
- Louvers are baffles that shield the bulb from view and reflect light. The baffles can be contoured to control light and decrease brightness. Parabolic louvers are specially shaped grids that concentrate and distribute light.