The rods provide achromatic (non-colour) vision in the range of illumination levels from 10-6 to 10 cd/m2, whereas the cones provide chromatic (colour) vision in the range 0.01 to 108 cd/m2. In other words the human eye is capable of detecting illumination levels with a difference of 1014 or 100,000,000,000,000 times! However, the neural units processing the signals to the brain are not capable of transmitting a signal with this complete dynamic range.
But the point is that the eye adapts to the average light level present. This adaptation is to some extent dynamic, but when the light level changes too fast the eye becomes momentarily blinded.
Turning on the light in a completely dark room is a typical example of a sudden change in light level, affecting the eye’s adaptation time, and thus resulting in temporary blinding. The headlights from an oncoming car during night time is another example. These are examples of disability glare.
The way the eye adapts to a given light level is based on the average light level in the field of vision. This level is the so-called average adaptation level and all light sources are related to this adaptation level. That’s why the headlights on the oncoming car doesn’t blind us during daylight, as the adaptation level are much higher than during night time. The brightness of the headlights remains the same. The adaptation level is a very crucial factor when it comes to designing projected displays – it’s quite possible to make them too bright.
Note that the eye perceives different luminance levels in a logarithmic or relative manner, which means that a clearly perceivable difference in luminance equals a doubling – or halving – of the light.
A light source with a luminance of 50 nit will be perceived as clearly brighter than the one with a luminance of 25 nit. To ensure the same perceived difference in brightness the next step upwards will be a luminance of 100 nit. This method of adaptation is the reason why the eye is capable of handling such extreme differences (i.e. contrasts) in light levels.
The logarithmic characteristic of the eye’s adaptation to different light levels is very important to remember when it comes to understanding projector performance and perceived image brightness. You might spend a lot more money to increase projector specification from 2,000 to 4,000 ANSI lumens, but you won’t perceive the difference as being very significant!