night vision

By Dr Samuel Strauss.

How’s your night vision technique? A sharp eye and optimal night vision can pick up that traffic sooner and increase your margin of safety.

Optimal vision is critical to every aspect of safe flying. We need it for the recognition and identification of distant objects including other aircraft, structures close to the ground, and birds. Good vision is also necessary to perceive the details of shape and colour, to judge distances, relative object movement, and to read flight deck displays, charts and flight plans. Night vision is unique in that it functions differently than day vision. Effective night vision requires special skills and knowledge.

The retina is the inner most and light sensing part of the eye. It contains a very small area called the fovea which senses maximal visual clarity and colours. The fovea works well under moderate to high levels of illumination. It however fails under low intensity light such as at night.

The non-central, peripheral part of the retina perceives light at low levels of illumination. It can actually perceive light at one thousandth the illumination needed by the fovea. Sometimes pilots complain that they may see an object at night only to have it disappear as they look directly at it. What happens is that they shift from peripheral dark- adapted vision to central day light vision. This part of the eye is not able to detect objects at low intensity. Another location of the retina which cannot see at all is the nearby "blind spot" where the optic nerve enters the retina. Looking at objects off centre about 15 degrees will correct that loss of vision in this area.

Before the peripheral part of the eye can see efficiently, it must for undergo dark adaptation. This is a vitamin A dependent photochemical process that occurs in each eye. It usually takes about twenty to thirty minutes to fully dark adapt, but can be lost rapidly when exposed to bright light.

Hypoxia, low oxygen in the blood, will reduce light perception especially in low light conditions. This results from a high cabin altitude, such as with loss of normal cabin pressure. It also results from smoking cigarettes which reduces the oxygen carrying capacity of your blood.

Fatigue, such as occurs with jet lag, also reduces night vision. This may be due to hormonal factors or be a function of eye muscle fatigue which interferes with the ability of yours eyes to focus under certain lighting conditions.

Think about the "Night Vision Checklist" before embarking on you next night mission:

  • Get dark adapted prior to starting night flying duties and reduce or eliminate unnecessary flight deck bright light sources.

  • Keep glasses and windscreen clean.

  • Look off to the side by 15 degrees while doing your outside scanning.

  • Close one eye if a temporary bright light source cannot be avoided.

  • Properly use supplemental oxygen in case of loss of cabin pressure.

  • If you smoke, get help to quit.

  • Make sure your diet contains adequate sources of vitamin A.

  • Adapt to jet lag and get adequate rest prior to scheduled night missions.