This publication is not intended as a definitive guide to visual impairment. The following pages are intended to give some examples of the effects of some of the more common conditions.
There are about 1 million people in the UK whose vision is such that they could be registered as 'blind' or 'partially sighted'. About three quarters of these people are over retirement age and over half live alone. About 75% have sufficient vision to read a newspaper headline with appropriate correction (e.g. spectacles) and good illumination.
It is estimated that some 10% of the population of Europe has a significant disability, but it is less often recognised that a high proportion have more than one impairment. Disabilities are not additive, they tend to multiply one another. A significant section of the population has impairments which reduce their ability to effectively use standard consumer products. Impairments such as hearing difficulties, limited dexterity, reduced strength or hand tremors are often not considered when products are designed. The effect of multiple impairments varies from individual to individual, but can be summarised as the multiplication rather than the addition of the separate impairments.
In a person aged 60 with 'normal' eyesight, about one third as much light reaches the retina as in a person aged 20. Therefore older people often have problems operating controls when there is low illumination. If a person has limited dexterity as well then the problems become more difficult.
Older people adapt at a slower rate to changes in the ambient illumination, which can give problems when the illumination on a control panel is greatly different to what is being viewed or controlled.
Myopia (shown above, left), or short-sightedness, is when the image is focussed in front of the retina. Good vision can be obtained by correction with concave lenses in spectacles or contact lenses.
Hypermetropia, or long-sightedness, is when the image is focussed behind the retina. Convex lenses give good vision.
Astigmatism literally means without point focus. It may be either myopic, hypermetropic or 'mixed' - in every case the image is elongated or distorted. Astigmatism can occur in any meridian; oblique astigmatism causing more problems than regular (horizontal or vertical) astigmatism. It is corrected with cylindrical lenses.
The term 'accommodation' is used to indicate the ability of the eye to change its focus, and thus get a clear image on the retina. The gradual decline in accommodation is called presbyopia (shown above, right), and is inevitable with age. Presbyopia commonly occurs after the age of 40, and bifocal or varifocal spectacles are often required. Multifocal spectacles may necessitate the user moving their head to get a focussed image, and thus the task may take longer. High myopia is a degenerative condition which affects about 200,000 people in the UK; this compares to about 5 million who are 'normal' myopes.
Macular degeneration accounts for about half of all registerable visual impairment in the UK. It is particularly common among older people.
The macula is at the centre of the retina and is used for detailed activities such as reading and recognising faces, and is used to detect colours.
Age-related macular degeneration usually involves both eyes, although they may not be affected at the same time. With the 'dry' form there is a gradual failure of the photosensitive cells of the retina. Sometimes there is a rapid fluid or blood leakage in the macular area which eventually causes ÔdisciformÕ degeneration Ð this is called 'wet' degeneration.
In general, magnification and high levels of illumination will assist in reading and other near vision tasks.
Having cataracts is like looking through a dirty windscreen - if the sun is behind you, your view is reasonably good, but if the sun is in front of you, then your view can be seriously impaired.
A cataract is one or more opacities of the crystalline lens. Cataracts can form at any age, but most often develop as people get older. In younger people they can be present at birth or result from an injury, certain drugs, radiation or conditions such as diabetes.
The opacification of the lens may occur in different ways, so that the light rays which reach the retina may be split, causing multiple images. Age related cataracts are often yellow or brown causing loss of sensitivity to blue.
The usual treatment is to remove the lens surgically and replace it with a plastic lens. Because there is no accommodation, spectacles are always needed for some purposes.
About 2% of the population in the UK is affected by diabetes which means that the body cannot cope normally with sugar and other carbohydrates in the diet.
Diabetic retinopathy is the largest single cause of registerable visual impairment among those of working age in the UK. The probability of a visual impairment is greatly increased if there is poor control of the diabetes.
In diabetic retinopathy the fine network of fragile blood vessels in the retina may leak or become blocked, causing local loss of function.
One of the side effects of diabetes is poor circulation which can often result in poor tactual sensitivity which means that few diabetics can read Braille.
'Tunnel vision' can be associated with a late stage of glaucoma or some forms of retinitis pigmentosa.
With glaucoma the pressure inside the eye is raised. This damages the fragile head of the optic nerve where it enters the eye causing classic loss of nerve fibres.
Retinitis pigmentosa covers a group of hereditary disorders which affect the retina. One effect is problems in low illumination and very slow light adaptation, and often problems reading displays at the red end of the visible spectrum.
With tunnel vision, it is often possible to read small print but not large print.
The most common forms of colour blindness are inherited and are associated with the inability to discriminate red and green wavelengths. Because these defects are inherited as recessive traits, the incidences are much higher in UK males (c. 8.0%), who possess a single X-chromosome, than in females (c. 0.5%), who possess two. Total colour blindness is extremely rare.
The cones which congregate in the centre of the retina perceive colour. In order to be stimulated, they require a greater intensity of light than the rods which occur more frequently in the peripheral retina and are more sensitive to movement. As a result it is particularly difficult for elderly people to detect colours when the illumination is low (e.g. driving at night).
With the design of control systems it is important not to rely on colour alone for differentiation. Although many people who have problems with distinguishing colours will be able to differentiate coloured controls, they are likely to take more time doing so.
The visible spectrum is from approximately 400nm (violet) to 800nm (red). In classical red/green confusion, the observer cannot distinguish certain wavelengths between red and green.
It is very important to avoid using red and green when offering choices.
Someone with a loss of sensitivity to some wavelengths might have great difficulty interpreting this correctly.