If the amount of light entering the eye changes then any initial coarse adjustment is made by the iris to close or open the pupil which allows more or less light into the eye. Because the pupil has only a limited capacity a second process is required. Chemical changes which involve both the rods and cones take place.
As light intensity decreases colour discrimination of the cones is difficult. It is at this stage that the rods, which are sensitive to low level illumination take over vision from the cones. The rods contain a pigment, visual purple (Rhodopsin) which is bleached by bright light. The chemical change takes a finite time as the light decreases.
This dark adaptation time is approximately:
- 30 minutes for the rods.
- 7 minutes for the cones - the fovea contains no rods so maximum visual acuity at night is achieved by looking at objects off-centre.
Best night vision is achieved after this 30 minute adaptation period. Night vision is lost immediately when the eye is exposed to bright light. The major factors that affect night vision are:
- Hypoxia: As low as 4,000 ft night vision begins to deteriorate, by 14,000 ft it is possible that up to 40% of night vision will have been lost.
- Smoking: Carbon monoxide in tobacco smoke, forms a strong bond with haemoglobin. The carboxyhaemoglobin produced reduces the amount of oxygen that is carried by the blood. A heavy smoker can suffer from hypoxia well below 10,000 ft with a consequent loss in night vision.
Other factors include, age, alcohol intake, illness and the use of stimulants.