Luminance Ratios

In most lighting standards, the famous "1:3:10" rule of thumb is often quoted. The principle is based on the idea that the luminance in the visual field of someone who's doing a static task, must remain in reasonable ratios in order to prevent glaring situations caused by a heavy contrast, hence impairing visual performances. It is recommended that luminance ratios do not exceed the following values:

The adjacent and non-adjacent surfaces can be delimited by two cones of 60 and 120 degrees respectivly, as shown on the image below.

Field of vision

Field of vision

This rule of thumb should not be used with the same rigour in any situation, though. The nature of the light source has a lot of impact on perception as well. A daylight and an artificial ligth situation will not have the same effects on people. People are more likely to tolerate a higher level of glare in a daylit environment. It has been found that a strong correlation exists between the prefered luminance ratios and the visual interest of a scene [Loe, 1994]. That is to say that the more interesting a scene is rated, the higher the tolerated luminance ratios will be. That tends to prove that when we are sitting next to a window, we would rather tolerate a high amount of daylight and enjoy the view outside than drawn the blinds down and use artificial lighting. We can also add that it seems that the experienced glare sensation is correlated with the nature of the task that is carried out.