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11.7.3 Vision, Disorientation, Illusion


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Dark Adaption

When the light intensity entering the eye changes, the eye adapts, adaption takes some time and the elapsed time increases with age. There are two adaption methods used, the first is an adjustment by the pupil to allow more or less light into the eye, this is a reasonably crude adjustment and has variation capacity of only 30 times, while the eye has a working range of over a million to one.

The second process involves sensing passing from the cones to the rods. This is a progressive action. This occurs as low light vision with the rods involves a pigment called visual purple (rhodopsin). This pigment is bleached in bright light and takes time to adapt to lower light levels. During the adaption time the cones cease to function.

A secondary consideration of using rods for vision is that due to their position around the eye the centre of the eye becomes blind and it becomes important to use peripheral vision (the 'off-centre" method of identifying objects at night). This means not looking directly at visual targets, to allow the rods to perform the sensing task.

Note also in the diagram below that to become fully adapted takes about 40 minutes and younger subjects are able to sense (or see) in a lower light intensity than older subjects.

For dark adaption it was originally believed that red light was best and wartime crews prepared in a red lit ready room. The red light was also believed to be less distinctive in the cockpit when viewed from the outside. Following World War II, studies found low colour-temperature white was only marginally less effective and acceptable for civil operations.

A more recent trend has been a move to brighter white light as it causes less drowsiness and fatigue. Brighter cockpit lights also provides protection to the crews vision during lightning storms, where full dark adaption can disrupted in a fraction of a second, leaving the crew temporarily blinded.

For light, general aviation aircraft, the standard cockpit light is still a red flood light with some instrument pillar or back lights. The general aviation pilot should therefore pre-flight and prepare early, preferably during daylight before avoiding bright lights prior to flight.

A secondary consideration is the use of red colours on charts which disappear under red light conditions. To minimise any potential problems, charts should be studied under normal light and cross checked with a shielded torch in flight.