The eye delivers to the brain more information about the outside world, and at a much faster rate, than any other sensory organ.
The eye has 1.2 million neurones leading from the retina to the brain, while there are only about 50 000 neurones between the brain and the inner ear. It is not surprising, then, that, when flying under Visual Flight Rules (VFR) which most PPL holders do most of the time, pilots rely overwhelmingly on their visual perception to fly safely and efficiently. If a VFR-only qualified pilot finds himself in marginal Visual Meteorological Conditions, he can very quickly get into trouble.
We will begin this chapter on vision by looking at the physiology of the eye. The word “physiology” is a technical term meaning the science of the organic functions of animals and plants. The eye is the organ of sight. In fact, it is the most sensitive of all our sensory organs.
It is the organ which receives electromagnetic waves within the visual spectrum from the external
world and passes them to the brain for interpretation into an image.
The basic structure of the eye is similar to a simple camera with an aperture called theiris, a lens, and a light sensitive film called the retina. The structure of the eye can be seen in Figure 5.2 .
The Cornea.
Light enters the eye through the cornea, a clear window at the front of the eyeball.
The cornea acts as a fixed focussing device and is responsible for between 70% and 80% of the total focussing ability of the eye. The cornea helps focus light onto the retina by bending the incoming light rays.
The Iris and Pupil.
The amount of light entering the eye is controlled by the iris, the coloured part of the Figure 5.1 The Visual Sensory System.
Figure 5.2 Structure of the Eye.
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by altering, as necessary, the size of the pupil, the clear centre of the iris. The size of the pupil can change rapidly to cater for changing light levels.
The Lens.
After passing through the pupil, light passes through a clear lens. The lens is the component of the eye which bends the light rays, and focuses them onto the retina.
The shape of the lens is changed by the muscles (ciliary muscles) surrounding it. It is the ciliary muscles which enable the final focussing of the light onto the fovea.
This change of shape of the lens under the action of the ciliary muscles is known as accommodation. The effectiveness of accommodation is influenced by the ageing process or fatigue. When a person is tired, accommodation is diminished, resulting in blurred vision.
The Retina.
The retina is a light-sensitive screen lining the inside of the eyeball. On this screen are light-sensitive cells known as cones and rods, which, when light from an object falls on them, generate a small electrical charge which passes an image of the object to the brain via nerve fibres (neurones) which combine to form the optic nerve. The image formed on the retina is inverted, but is perceived “right-way-up” by the brain.
Cones and Rods.
Cones are used for direct vision, in good light, and are colour-sensitive, capable of distinguishing approximately 1 000 different shades of colour. The rods can only detect black and white but are much more sensitive at lower light levels. As light decreases, the sensing task is passed over from the cones to the rods. This means that, in poor light levels, we see only in black or white, or varying shades of grey.
Rods are responsible for our peripheral vision.
The Fovea.
The central part of the retina, the fovea, contains only cones. Any object which needs to be examined in detail is automatically brought to focus on the fovea. The fovea is the area of greatest visual acuity on the retina.
The Blind Spot.
In each eye there is a small area in which the blood vessels supplying Oxygen to the retina, and where the nerves forming one end of the optic nerve, are concentrated.
This is the blind spot (see Figures 5.2 and 5.3). An image falling on the blind spot of an eye is not detected by the brain, but the image will almost certainly be detected by the other eye.
Figure 5.3 The image of the aircraft falls on the blind spot of one eye, but is “seen” by the other eye.
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Central Vision and Visual Acuity.
The rest of the retina fulfils the function of attracting our attention to movement and change. Only at the fovea can vision be of normal visual acuity, 20/20 or 6/6. This clear vision is termed central vision.
Each eye is equipped with muscles which enable the eyeballs to rotate in their sockets, thus enabling them to keep track of a moving object. To track an object successfully, or to focus on an object, the eyes need to move in harmony with one another. This means that the brain must co-ordinate control of the muscles of the two eyes.
Binocular vision refers to the fact that two eyes are required for a complete visual capability. We need binocular vision to create for us a three-dimensional picture of the world.
Visual acuity is a measure of central vision. Visual acuity is the capacity of the eye to determine small detail, undistorted, at a given distance. The sharpest visual acuity occurs when the object being viewed is sharply focused on the fovea.
A person with 20/20 or 6/6 vision is said to have normal visual acuity. The figures 20/40 (or 6/12) mean that the observer can only read at 20 feet what a person with normal vision can read at 40 feet (or at 6 metres what a person with normal vision can read at 12 metres).
Normal visual acuity permits pilots to detect objects clearly at safe distances. If a person’s vision with the naked eye is impaired, this will not normally prevent that person from becoming a pilot, provided normal visual acuity can be achieved by wearing spectacles or contact lenses.
Whereas normal visual acuity is required to see objects clearly at a distance, good near-vision, too, is necessary for a pilot to read instruments and maps. Being able to focus on close objects is a function of the eye’s ability to accommodate. The power of accommodation usually diminishes in middle age, but can easily be corrected by wearing reading glasses. Pilots and drivers normally wear bi-focal spectacles to allow them to see clearly at a distance and to read their instruments and maps.
The sharpness of central vision, that is the image at the fovea, drops as light falls on the retina at increasing angles from the fovea. At as little as 5° from the fovea, visual acuity drops to 20/40. That is only half as good as the visual acuity at the fovea. At approximately 25 degrees, visual acuity decreases to a tenth of its normal performance (20/200).
Limitations of Visual Acuity.
Visual acuity is limited or impaired by the following factors:
• Angular distance from the fovea.
• Physical imperfections within the visual system.
• Age.
• Hypoxia.
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• Alcohol.
• Atmospheric visibility (dust, mist etc. in the air will reduce visibility).
• Light intensity.
• Size and contours of an object.
• Distance of the object from the viewer.
• Contrast of an object with its surroundings.
• Relative motion of a moving object.
• Drugs or medication.
Night Vision.
If a person has been in bright light for a long time, the sensitivity of the eye to light is greatly reduced. Thus, passing from a brightly lit room into the dark of night has the effect of vision being severely reduced until dark adaptation takes place.
On the other hand, if the person remains in darkness for a long time, the eye becomes super-sensitive to light so that even the faintest amount of light can irritate the retina and dazzle the person concerned. The eye adjusts more quickly to this second occurrence than to the first. This is why it is especially important for pilots to allow sufficient time for dark adaption to take place before flying at night. It takes time for our eyes to adapt to darkness: about 7 minutes for the cones and 30 minutes for the rods.
As you have learnt, rods are very sensitive to poor light, but see only shades of black and white. They also give us our peripheral vision. Thus, as the fovea contains no rods, this area of best visual acuity is virtually blind in dim light conditions such as those which prevail at night. At night, to achieve maximum visual acuity, it is advisable to look slightly to one side of the object so that the light falls onto a part of the retina where there are rods. This is a good technique to use when night flying.
You can demonstrate this technique to yourself by looking at dim stars on a clear night. Though they may appear extremely faint if you look directly at them, they will be more clearly discernible if you look slightly off to one side.