Some examples of typical horizontal light levels are as follows:
The eye can adapt across a vast range of light levels but requires some time to do so.
For example, going into a dark cinema from daylight, the eye cannot immediately adapt to the much lower light level.
In practical terms, interior lighting usually spans a range of 20 to 1.
The eye can rapidly adapt across this range. Such a range of light levels allow say, a low ambient light level with certain objects lit to a much higher level, such as displays or artwork.
This gives the display a much stronger emphasis than the background and therefore makes it highly noticeable. In an office the range of light levels would be far less, say 3 or 4 to 1, so that the eye is not easily distracted from its viewing task.
In New Zealand we use the metric system so the units used for light are the Lumen and the Lux.
Lumen are used to measure the amount of light being given out by a bulb. For example a 100 watt incandescent bulb gives off approximately 1300 Lumen.
A 50watt halogen lamp gives off approximately 950 Lumen.
Lux measure the amount of light reaching a surface. One Lux is equal to one Lumen per square meter.
Examples of the minimum Lux levels that should be achieved are :
Entrance Halls 160 Lux on the floor
Kitchen benches 240 Lux
Reading 320 Lux
The amount of light reaching a surface decreases in proportion to the square of the distance from the light.
If you double the distance from the light you get one quarter of the light, triple the distance and get one ninth.
Lighting calculations to ensure that you get the required amount of light are quite complex and take a great many factors into account. These are performed by lighting consultants and are recommended for commercial and industrial projects.
For do it yourselfers see spacing of lights for some rules of thumb that may be helpful.