Night Vision and Visual Illusions
Why the eye changes at night
At night the retina switches from cone to rod vision. Cones give colour and fine detail but need moderate light; rods are colourless, much more sensitive, and concentrated outside the fovea. The fovea is effectively a night-blind spot, so look 10–15° off-centre to see a dim object. Dark adaptation takes 20–30 minutes and is destroyed in seconds by white light; a dim red cockpit light preserves it better.
Physiology that degrades night vision
Hypoxia is the biggest silent factor. Rod vision deteriorates from about 5,000 ft cabin altitude, long before symptoms appear, so use oxygen on night flights when available. Carbon monoxide from a cabin heater, smoking, alcohol, some medications, dehydration and fatigue all reduce sensitivity. Vitamin A deficiency is relevant but rare.
The classic illusions
Autokinesis: a single static light appears to move after a few seconds of staring - scan it. Black-hole approach: an isolated runway with no surrounding lights appears higher and further than it is, luring a low approach. False horizon, somatogravic (acceleration) illusion, somatogyral (spin) illusion, the leans and Coriolis illusion all mislead without visual reference. Runway width and upslope illusions change the perceived glidepath. Empty-field myopia makes the eye focus at about 1–2 m, so force it to relax by looking at a distant object.
Worked example
You are cleared for a visual approach to a dark runway with PAPI. On short final the picture looks high, so you descend. The PAPI shows three reds and one white - you are below path. Reasoning: the black-hole effect has made the runway appear high; the PAPI is the reliable reference. Fly two red/two white, cross-check the altimeter at the threshold, and go around if not stable by minima.
Traps
Assuming five minutes is enough dark adaptation; staring at a light instead of using off-centre viewing; turning on a white flood; ignoring mild hypoxia; chasing the visual picture against the PAPI.