About Vision ?

The Basic Function of the Eye

In order to understand vision, it is useful first to know a little about the anatomy of the
eye (see Figure 4). The basic structure of the eye is similar to a simple camera with
an aperture (the iris), a lens, and a light sensitive surface (the retina). Light enters the
eye through the cornea, then passes through the iris and the lens and falls on the
retina. Here the light stimulates the light-sensitive cells on the retina (rods and cones)
and these pass small electrical impulses by way of the optic nerve to the visual
cortex in the brain. Here, the electrical impulses are interpreted and an image is
perceived.

The Cornea

The cornea is a clear ‘window’ at the very front of the eye. The cornea acts as a fixed
focusing device. The focusing is achieved by the shape of the cornea bending the
incoming light rays. The cornea is responsible for between 70% and 80% of the total
focusing ability (refraction) of the eye.

The Iris and Pupil

The iris (the coloured part of the eye) controls the amount of light that is allowed to
enter the eye. It does this by varying the size of the pupil (the dark area in the centre
of the iris). The size of the pupil can be changed very rapidly to cater for changing light
levels. The amount of light can be adjusted by a factor of 5:1.

The Lens

After passing through the pupil, the light passes through the lens. Its shape is
changed by the muscles (cillary muscles) surrounding it which results in the final
focusing adjustment to place a sharp image onto the retina. The change of shape of
the lens is called accommodation. In order to focus clearly on a near object, the lens
is thickened. To focus on a distant point, the lens is flattened. The degree of
accommodation can be affected by factors such as fatigue or the ageing process.

When a person is tired accommodation is reduced, resulting in less sharp vision
(sharpness of vision is known as visual acuity).

The Retina

The retina is located on the rear wall of the eyeball. It is made up of a complex layer
of nerve cells connected to the optic nerve. Two types of light sensitive cells are
found in the retina - rods and cones. The central area of the retina is known as the
fovea and the receptors in this area are all cones. It is here that the visual image is
typically focused. Moving outwards, the cones become less dense and are
progressively replaced by rods, so that in the periphery of the retina, there are only
rods

Cones function in good light and are capable of detecting fine detail and are colour
sensitive. This means the human eye can distinguish about 1000 different shades of colour.

Rods cannot detect colour. They are poor at distinguishing fine detail, but good at detecting 

movement in the edge of the visual field (peripheral vision). They are much more 

sensitive at lower light levels. As light decreases, the sensing task is passed from the 

cones to the rods. This means in poor light levels we see only in black and white and 

shades of grey.

At the point at which the optic nerve joins the back of the eye, a ‘blind spot’ occurs.

This is not evident when viewing things with both eyes (binocular vision), since it is

not possible for the image of an object to fall on the blind spots of both eyes at the

same time. Even when viewing with one eye (monocular vision), the constant rapid

movement of the eye (saccades) means that the image will not fall on the blind spot

all the time. It is only when viewing a stimulus that appears very fleetingly (e.g. a light

flashing), that the blind spot may result in something not being seen. In maintenance

engineering, tasks such as close visual inspection or crack detection should not cause

such problems, as the eye or eyes move across and around the area of interest

(visual scanning).

Factors Affecting Clarity of Sight

The eye is very sensitive in the right conditions (e.g. clear air, good light, etc.). In fact,

the eye has approximately 1.2 million nerve cells leading from the retinas to the area

of the brain responsible for vision, while there are only about 50,000 from the inner

ears - making the eye about 24 times more sensitive than the ear.

Before considering factors that can influence and limit the performance of the eye, it

is necessary to describe visual acuity.

Visual acuity is the ability of the eye to discriminate sharp detail at varying distances.

An individual with an acuity of 20/20 vision should be able to see at 20 feet that which

the so-called ‘normal’ person is capable of seeing at this range. It may be expressed

in metres as 6/6 vision. The figures 20/40 mean that the observer can read at 20 feet

what a ‘normal’ person can read at 40 feet.

Various factors can affect and limit the visual acuity of the eye. These include:

• Physical factors such as:

• physical imperfections in one or both eyes (short sightedness, long

sightedness),

• age.

• The influence of ingested foreign substances such as:

• drugs,

• medication,

• alcohol,

• cigarettes.

• Environmental factors such as:

• amount of light available,

• clarity of the air (e.g. dust, mist, rain, etc.).

• Factors associated with object being viewed such as:

• size and contours of the object,

• contrast of the object with its surroundings,

• relative motion of the object,

• distance of the object from the viewer,

• the angle of the object from the viewer.

2.6.5 Each of these factors will now be examined in some detail

Physical Factors

 Long sight - known as Hypermetropia - is caused by a shorter than normal eyeball

which means that the image is formed behind the retina (Figure 5). If the cornea and

the lens cannot use their combined focusing ability to compensate for this, blurred

vision will result when looking at close objects.

A convex lens will overcome long sightedness by bending light inwards 

before it reaches the cornea.

 Short sight - known as Myopia - is where the eyeball is longer than normal, causing

the image to be formed in front of the retina (Figure 6). If the accommodation of the

lens cannot counteract this then distant objects are blurred. 

A concave lens will overcome short sightedness by bending light 

outwards before it reaches the cornea.

Other visual problems include:

• cataracts - clouding of the lens usually associated with ageing;

• astigmatism - a misshapen cornea causing objects to appear irregularly shaped;

• glaucoma - a build up in pressure of the fluid within the eye which can cause

damage to the optic nerve and even blindness;

• migraine - severe headaches that can cause visual disturbances. 

 Finally as a person grows older, the lens becomes less flexible meaning that it is

unable to accommodate sufficiently. This is known as presbyopia and is a form of

long sightedness. Consequently, after the age of 40, spectacles may be required for

near vision, especially in poor light conditions. Fatigue can also temporarily affect

accommodation, causing blurred vision for close work.

Foreign Substances

Vision can be adversely affected by the use of certain drugs and medications, alcohol,

and smoking cigarettes. With smoking, carbon monoxide which builds up in the

bloodstream allows less oxygen to be carried in the blood to the eyes. This is known

as hypoxia and can impair rapidly the sensitivity of the rods. Alcohol can have similar

effects, even hours after the last drink. 

Environmental Factors

Vision can be improved by increasing the lighting level, but only up to a point, as the

law of diminishing returns operates. Also, increased illumination could result in

increased glare. Older people are more affected by the glare of reflected light than

younger people. Moving from an extremely bright environment to a dimmer one has

the effect of vision being severely reduced until the eyes get used to less light being

available. This is because the eyes have become light adapted. If an engineer works

in a very dark environment for a long time, his eyes gradually become dark adapted

allowing better visual acuity. This can take about 7 minutes for the cones and 30

minutes for the rods. As a consequence, moving between a bright hanger (or the

inside of an aircraft) to a dark apron area at night can mean that the maintenance

engineer must wait for his eyes to adjust (adapt). In low light conditions, it is easier to

focus if you look slightly to one side of an object. This allows the image to fall outside

the fovea and onto the part of the retina which has many rods.

 Any airborne particles such as dust, rain or mist can interfere with the transmission

of light through the air, distorting what is seen. This can be even worse when

spectacles are worn, as they are susceptible to getting dirty, wet, misted up or

scratched. Engineers who wear contact lenses (especially hard or gas-permeable

types) should take into account the advice from their optician associated with the

maximum wear time - usually 8 to 12 hours - and consider the effects which extended

wear may have on the eyes, such as drying out and irritation. This is particularly

important if they are working in an environment which is excessively dry or dusty, as

airborne particles may also affect contact lens wear. Goggles should be worn where

necessary.

The Nature of the Object Being Viewed

Many factors associated with the object being viewed can also influence vision. We

use information from the objects we are looking at to help distinguish what we are

seeing. These are known as visual cues. Visual cues often refer to the comparison

of objects of known size to unknown objects. An example of this is that we associate

small objects with being further away. Similarly, if an object does not stand out well

from its background (i.e. it has poor contrast with its surroundings), it is harder to

distinguish its edges and hence its shape. Movement and relative motion of an object,

as well as distance and angle of the object from the viewer, can all increase visual

demands.

Colour Vision

Although not directly affecting visual acuity, inability to see particular colours can be

a problem for the aircraft maintenance engineer. Amongst other things, good colour

vision for maintenance engineers is important for:

• Recognising components;

• Distinguishing between wires;

• Using various diagnostic tools;

• Recognising various lights on the airfield (e.g. warning lights).

Colour defective vision is usually hereditary, although may also occur as a temporary

condition after a serious illness.

Colour-defective vision (normally referred to incorrectly as colour blindness) affects about 

8% of men but only 0.5% of women. The most common type is difficulty in distinguishing 

between red and green. More rarely, it is possible to confuse blues and yellows.

There are degrees of colour defective vision, some people suffering more than

others. Individuals may be able to distinguish between red and green in a well-lit

situation but not in low light conditions. Colour defective people typically see the

colours they have problems with as shades of neutral grey. 

Ageing also causes changes in colour vision. This is a result of progressive yellowing

of the lens, resulting in a reduction in colour discrimination in the blue-yellow range.

Colour defective vision and its implications can be a complex area and care should be

taken not to stop an engineer from performing certain tasks merely because he

suffers from some degree of colour deficient vision. It may be that the type and

degree of colour deficiency is not relevant in their particular job. However, if

absolutely accurate colour discrimination is critical for a job, it is important that

appropriate testing and screening be put in place.