VISION

1.DEFINITION:

Vision is the sensory system that provides 70% of the information to the brain.

It can be defined in two visual fields:

  • Central vision

  • Peripheral vision

2. FUNCTIONAL AREAS:

  • Central vision: 160 ° orientation detection

  • Peripheral Vision: identifying discrimination 20 °

3. EXTERNAL EYE DESCRIPTION:

From the five senses, it is the sight that we use the most. However, the sight has a certain limit that we should know.

4. THE BASIC FUNCTION OF THE EYES:

In order to understand vision, it is useful first to know a little about the anatomy and the function of the eye.

The basic structure of the eye is:

  • an aperture: the iris,

  • a lens,

  • a light sensitive surface where the image forms: 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 colored 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 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.

  • 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

2. EYES LIMITATIONS:

Independently of the person's eyesight, the vision is affected by several factors:

  • Light intensity,

  • Size and shape of the objects,

  • Distance,

  • Similarities between an object and its environment,

  • Relative movement of the object,

  • Atmospheric conditions.

Since the daily vision is better ensured by the central vision and that it decreases on the periphery of the retina, the whole field of vision is not seen with same clearness.

The latter decreases quickly around the center, and to see an object with accuracy, it should be fixed right in front of the fovea.

2.1. Factors affecting the visual acuity:

a) Visual performances:

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.

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

b) Factors affecting performances:

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

These include:

  • Physical imperfections in one or both eyes (short sightedness, long sightedness, astigmatism),

  • Foreign substances (Alcohol, drugs and medications),

  • Environmental factors such as:

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

    • Size and contours of the object.

2.2. Physical factors:

  • Hypermetropia (Long sight):

    Hypermetropia is caused by a shorter than normal eyeball which means that the image is formed behind the retina. 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.

Depending on the degree of farsightedness and the age of the individual, some farsighted people may see clearly at both distance and near through the process of accommodation, or focusing of the lens in the eye.

A positive (convex) lens will overcome long sightedness by bending light inwards before it reaches the cornea.

→ Hypermetropia corrected by positive (convex) lens:

  • Myopia (Short sight):

    Myopia is where the eyeball is longer than normal, causing the image to be formed in front of the retina. If the accommodation of the lens cannot counteract this then distant objects are blurred.

    It is a very convergent eye and at rest the image of one object at infinity is formed in front of the retina. The power of the eye is decreased by interposing a divergent lens.

    This occurs when the eye is relatively too long or the refractive powers of the cornea and lens of the eye are relatively too strong. Myopia usually begins in childhood, and progressively worsens until adulthood is achieved, usually about 18 to 21 years of age.

    A negative (concave) lens will overcome long sightedness by bending light inwards before it reaches the cornea.

→ Myopia vision corrected by negative (concave) lens:

  • Astigmatism:

    Astigmatism is a focusing error, which causes asymmetric blur due to the uneven curvature of the front part of the eye called the cornea.

    Some directions in an image are more out of focus than others. An astigmatic person will see images distorted or sometimes even double.

    Certain letters may be more difficult to read than others, depending on the orientation of the lines within them. If not corrected, an astigmat will experience symptoms like eyestrain & headaches.

    A cylindrical lens is used to correct astigmatism.

2.3. 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.

2.4. Environmental factors:

  • Lighting level:

    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.

  • Particles:

    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.

    Workers 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.

2.5. 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.

2.6. 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.

  • Note:

    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.

It is important for an engineer, particularly one who is involved in inspection tasks, to have adequate vision to meet the task requirements. As discussed previously, age and problems developing in the eye itself can gradually affect vision. Without regular vision testing, aircraft maintenance engineers may not notice that their vision is deteriorating.

The use of glasses or contact lenses to correct any vision problems is perfectly acceptable and indeed they must be worn as prescribed. Frequent checks should be made to ensure the continued adequacy of any glasses or contact lenses. In addition, colour discrimination may be necessary for an individual to drive in areas where aircraft manoeuvre or where colour coding is used, e.g. in aircraft wiring.

Ultimately, what is important is for the individual to recognise when his vision is adversely affected, either temporarily or permanently and to consider carefully the possible consequences should they continue to work if the task requires good vision.