The visual mechanism is a sensitive tool which enables us to see under a wide range of lighting conditions, from near darkness to glaring light. However, the process of adaptation to extreme light levels is gradual, and is most efficient when the lighting, too, changes gradually. When light levels change drastically, visual adaptation lags behind. For instance, when the light is switched off it takes a while before we can once again discern objects, and when a very strong light is turned on we are temporarily blinded.
The pupil is partly responsible for the eye's to changes in lighting levels by expanding and contracting to regulate the amount of light which reaches the retina.
A photo-chemical process which takes place within the
photoreceptors
enables vision under poor lighting - scotopic vision. A pigment called Rhodopsin which is stable in low light disintegrates under strong light. The
photoreceptors
responsible for
scotopic vision
- the
rods
- become active under poor lighting, and are rendered inactive under normal lighting, when the cones, the other type of photoreceptors, take over. When lighting levels are low again, the pigment will slowly rebuild. It takes a few minutes in the dark for enough pigment to build up in the
rods
to enable us to see once more, but complete dark adaptation takes about 40 minutes.
The eye exposed to bright light with contracted iris (top). The light with low light exposure with expended iris (bottom).
Vision is obviously not possible in the complete absence of light, yet the threshold of sight, when the eyes are fully dark adapted is amazingly low. There are two separate thresholds of
visibility
which affect us. The one is the faintest stimulus at which our senses are capable of registering, and at which we can say "I can see". The other threshold has to do with change: how much must the stimulus be modified in order for us to register a difference.
Dark adaptation graph, indicating the time it takes the eye to adjust from bright light to total darkness.
