About one quarter of the human brain is involved in visual processing, more than any other sense. Arguably the most closely studied of the five main senses. More is known about vision than any other vertebrate sensory system.
To see clearly at different distances the eye makes two adjustments:
- A change in the refractive power of the lens to enable the eye to focus, and
- A change in the convergence of the eyes - binocular vertege.
For the brain to make judgement of depth certain cues are used:
- Binocular vision (two eyes), people who are monocular make adjustments which compensate for the loss of one eye.
- The relation, size and clarity of the object.
- The relative movement and texture of the object.
Distance Estimation and Depth Perception
Cues to distance estimation and depth perception are easy to recognise when pilots use vision under good illumination. As the light level decreases, the ability to judge distance accurately is degraded and the eyes become more vulnerable to illusions. Pilots can judge distance at night if they understand the problems in obtaining accurate cues to distance estimation and depth perception. A pilot normally uses subconscious factors to determine distance where either single or a variety of cues is used. Accurate estimates of distance can be gained if the pilot is aware of the factors to be aware of. Cues to distance or depth perception are either monocular or binocular.
The human being is able to focus both eyes on a single object. This is called stereoscopic vision. Each eye sees an object at a slightly different angle (binocular cues). The images seen are merged together and the human being sees a three dimensional object.
Stereoscopic vision does not play a major role in depth perception over a distance of 12 m, beyond this range other static and dynamic cues are used.
A pair of healthy human eyes has a total field of view of approximately 200 degrees horizontally - about 120 degrees of which are shared by both eyes, giving rise to what's known as binocular vision - and 135 degrees vertically, (though these values tend to decrease with age). This is due to the fact that both of our eyes are positioned more or less on the front of our heads, as opposed to the sides.
Binocular cues depend on the slightly different view each eye has of an object. Consequently, binocular perception is of value only when the object is close enough to make a difference in the viewing angle of both eyes. When flying, most distances outside the cockpit are so large that the binocular cues are of little value. Binocular cues operate on a more subconscious level than the monocular cues.
Several monocular cues aid in distance estimation and depth perception such as:
- Geometric perspective.
- Motion parallax.
- Retinal image size.
- Aerial perspective.
Geometric Perspective Where an object appears to have a different shape when viewed at varying distances and from different angles. The types of geometric perspective are explained in the following paragraphs.
Parallel lines, such as railway lines, tend to converge as distance from the observer increases.
A problem where distant objects appear elliptical due to their distance from the viewer. For example, a lake may look elliptical from a distance but the real shape is revealed as the distance to the lake reduces.
Binocular vision is not essential for flying - there are one eyed airline pilots.
Considered the most important cue in depth perception. Motion parallax is the apparent, relative motion of stationary objects when viewed by an observer moving across the landscape. Near objects appear to move past; distant objects seem to move in the direction of motion or remain fixed. The rate of apparent movement depends on the distance the observer is from the object. For example, when driving a Go-cart the ground underneath appears to be moving fast; when flying at altitude the ground underneath seems to move slowly. Motion parallax can cause problems to pilots taxiing:
A pilot who changes from a low cockpit height aircraft will taxy at a specific speed. The ground movement outside is one cue he takes his taxiing speed from. If the pilot changes to a high cockpit aircraft he will tend to taxy too fast as he uses the relative speed of the ground as his cue for the taxy speed.
Retinal Image Size
An image focused on the retina is perceived by the brain to be of a certain size. The factors that aid in determining distance using the retinal image are explained below:
Known Size of Objects
The nearer an object is to the observer, the larger its retinal image. The brain adapts to estimate the distance of familiar objects by using the size of their retinal image. The diagram below shows how this method is used. A structure projects a specific angle on the retina, based on its distance from the observer. If the angle is small, the observer judges the structure to be at a greater distance. A larger angle indicates to the observer that the structure is close. In the case below, the observer can judge the distance from the object by the relative size. If no experience exists, an object's distance would be determined primarily by motion parallax.
Increasing or Decreasing Size of Objects
Using common sense, if the retinal image:
- Increases in size - the object is moving nearer the observer.
- If the retinal image decreases - the object is further away. If the retinal image is constant, the object is at a fixed distance.
Comparison of objects, such as an airport with an aircraft flying, will help to determine the relative size and apparent distance of the object from the observer. Objects associated together are judged to be at approximately the same distance. In the diagram below, an aircraft that is observed near an airport is judged to be in the traffic pattern and, therefore, at approximately the same distance as the airport.
Terrestrial Distance of Objects Used to Determine Distance.
The clarity of an object and the shadow cast by it are perceived by the brain and are cues to estimating distance. To determine distance with these aerial perspectives, most pilots use the areas discussed below:
- Fading Colours or Shades: Objects viewed through haze, fog, or smoke are less distinct and appear to be at a greater distance than they really are. If an object is seen more distinctly in clear air it appears to be closer than it actually is.
- Loss of Detail or Texture: Distant objects become less discrete. If a ploughed field is viewed from a distance it appears brown. As the observer closes, not only does the colour appear brown but also the ruts of the ploughing become visible.
- Position of Light Source and Direction of Shadow: All objects cast a shadow if lit. The direction of the shadow depends on the position of the light source. If the shadow is toward the observer, the object is closer than the light.
Interposition of Objects
Interposition cues occur when there is overlapping of objects. The overlapped object is considered further away.
G-FIND must be the closer aircraft as it obscures the other aircraft behind.
The healthy state of the eye when fully relaxed. Parallel rays of light are focused on the retina.
Myopia (Short Sightedness)
In short sightedness the eye is longer than normal and this results in an image focusing in front of the retina. Accommodation (focusing) by the lens cannot overcome this deficiency.
Distant objects will be out of focus, with close up vision being satisfactory. To correct short sightedness a concave lens is used.
Hypermetropia (Long Sightedness)
In long sightedness a shorter than normal eye results in the image being focused behind the retina.
Close up vision is blurred yet long distance vision is usually clear. To correct long sightedness a convex lens is used.
Close up vision deteriorates with increasing age. Hardening of the lens in people over 40 results in a mild form of long sightedness. This is known as presbyopia. Difficulty in reading fine print in poor light is normally the first sign of the onset of presbyopia. Half Moon spectacles are used to correct the defect; corrections for middle and distant vision can be made by using bifocal, trifocal or even quadrifocal lenses.
Hardening of the lens may also result in clouding of the lens. This clouding is associated with cataract formation. Pilots with early cataract problems may see an eye chart, but can have difficulty in bright light. Due to the scattering of light as it enters the eye this sensitivity may be disabling under certain circumstances. Any clouding of the eye should be investigated immediately.
An optical defect caused by abnormalities to the surface of the cornea or lens. In a healthy state the cornea is spheroidal, like a football. The astigmatic cornea is oval shaped, like a rugby ball. Errors caused by astigmatism can be corrected by a cylindrical lens.
Spectacles Variable focus lenses are an alternative to bifocal or multifocal lenses. There is no clear demarcation between upper distance vision to near vision correction in the lower portion of the lens. Distortion also occurs near the periphery of vision. Because of these problems varifocal lenses are not advised for use in flying.
Contact lenses provide better peripheral vision and are not subject to misting. There are a some problems associated with flying with contact lenses. The cornea does not have its own blood supply and obtains oxygen from the ambient air, the contact lens may starve the cornea of the oxygen required. Mild hypoxia and dehydration, caused by the low humidity on the flight deck also increase the potential for corneal damage. Cabin decompression can result in bubble formation. Rubbing of the eyes may dislodge a contact lens.
Before a medical certificate can be annotated approving the wearing of contact lenses the applicant must provide a report from an ophthalmologist or contact lens practitioner. If all requirements are met then the use of contact lenses is approved; the certificate usually carries an annotation stating that a pair of ordinary spectacles must be carried in flight whilst the contact lenses are being worn.
Bifocal contact lenses, for the correction of presbyopia, are unsuitable for flying.
During a rejected take-off in a B747 the flight engineer lost visual co-ordination between the throttles and EPR gauges and advised the captain that the number three engine was losing thrust. The cause of the engineer's action was attributed to his multifocus lens spectacles which he was wearing for the first time.
The above example shows the importance of wearing the correct spectacles and the time it takes the eye to adapt to them.
Radial keratotomy is a surgical procedure that creates multiple radial, spoke-like incisions on the cornea of the eye to produce better visual acuity. Glare sensitivity can be a complication of the procedure which may be troublesome at night. Other complications include fluctuating visual problems because of corneal swelling and increased susceptibility to injury. Possible long-term complications of this procedure are unknown and no pilot should undergo the treatment.
Colour Vision and Colour Blindness
People with normal colour vision can distinguish up to 120 different colours and over 1000 differing shades of these colours. 8% of the male population and 1% of the female population cannot distinguish between red and green. There are 4 types of red/green colour blindness:
- Protanopia: Blue-green appears grey; red-purple appears grey
- Protanomalia: Blue-green appears an indistinct grey; red-purple appears an indistinct grey
- Deuteranopia: Green appears grey; purple-red appears grey
- Deuteranomalia: Green appears an indistinct grey; purple-red appears an indistinct grey
Total colour blindness is rare but can be found in both males or females. Two types of total colour blindness are known:
- Typical: The person has no colour discrimination and sees everything as black or white. If a person suffers from this type of colour blindness then they usually suffer from other types of visual impairment.
- Atypical: A condition where only very clear colours can be discerned.
Colour vision is affected in people who do not have colour blindess by:
- Yellowing of the cornea and lens due to old age.
- Smoking and alcohol.
Normal colour vision is not essential for flying. However, there is a need to be able to distinguish between red, green and white lights in order to comply with:
- The rules of the air by night.
- Light signals from the ground.
- Aerodrome signs and markings.
- The changing colours associated with glass cockpit displays.
Empty Field Myopia
When flying at altitude, at night or above cloud where there is no definite pattern of earth or sky to focus upon, the eye adopts a resting focus of 1 - 2 metres away. Distant objects have to be relatively large to be seen. Effort is needed to refocus the eye on infinity. In order to see objects outside the flight deck the eyes should be focussed on objects such as the wing tips or clouds.
Glare is caused when flying above a layer of cloud or flying into a low sun. The brightness contrast outside and inside the cockpit can make it difficult to read the instrument panel. Photochromatic lenses are now commonly used by pilots but these adapt to ambientlight slowly. A problem may occur when changing from bright light to relatively darker backgrounds eg sunlight to cloud. Pilots should be aware of the possibility of ending up flying blind without realising.
Cumulative damage to the retina can occur over a number of years due to glare. Good quality sunglasses give protection by filtering out both blue and ultra violet light.
Bright flickering light can cause epileptic type fits. Helicopter passengers have suffered fits because of the rotor blades turning in bright sunlight andcausing a flicker effect. An individual may feel uneasy or suffer discomfort in this flicker environment. Precautions can be taken by a sufferer such as wearing sunglasses. It is possible that the warning symptoms of mental unease or discomfort can last for a few minutes, but this cannot be relied upon. The following suggestions are made with reference to flickering light:
Pilots If a pilot suffers from flickering light effect:
- Wear sunglasses.
- Turn away from the sun to reduce the flicker effect.
- Land immediately.
Passengers Affected passengers are usually on the sunny side of the aircraft. To reduce flicker effect:
- Wear sunglasses.
- Cover adjacent windows.
- Cover or close the eyes.
- Move to a seat which is not affected by the sun.