Adaption

The most commonly observed adaption of the eye is that of light-dark adaption. After 30 minutes in complete darkness the eye is very sensitive and can perceive extremely low levels of light energy. The rods only are working and this state is said to be SCOTOPIC. If the general illumination level increases then the rods adapt to a state of equilibrium. The equilibrium is destroyed by any sudden change to a very different level of illumination. Sudden light in a darkened room dazzles one and differences in lightness cannot be seen. Entering a cinema performance during daylight hours has the opposite but similar effect.

If the level of illumination rises far enough, the scotopic state, where only the rods are working and only shades of grey can be seen, ceases and the eye enters the MESOPIC state. In this there is enough light for the less sensitive cones to start operation and colour is perceieved. This is the region of twilight vision. A further increase in light level leaves the rods contributing much less to the totle level of information reaching the brain and the colour sensitive cones provide most of it, in the region known as daylight vision or the PHOTOPIC state.

The last state of adaption deals not so much with the quantity of light as do the three states we have considered but rather with the quality or chromaticity of the light. This state is known as CHROMATIC adaption and is perhaps the most complex.

When the eye is exposed for some time to tungsteen light which is predominantly red-yellow the violet cone receptores are not much effected as they are not being overworked. The green receptors are doing more work and become a little more desensitised while the red receptores which are the most activated become the most desensitised. This gives a relative gain in sensitivity to violet and blue.