3 Visual impairments

3.2 Blindness, partial sightedness and colour-blindness

There are approximately two million people in the UK who have a sight problem. The Royal National Institute for the Blind (RNIB) defines someone as having a sight problem if they are unable to recognise someone across the road or they have difficulty reading newsprint even when wearing glasses.

When discussing visually impaired people it is important to distinguish between partially sighted people (people having low vision) and people with no useful vision (blindness). This distinction is important because these two groups interact with information and devices in very different ways.

Visual impairments can be caused by many different conditions. Some conditions are associated with ageing, while others are congenital (present at birth). Different visual impairments have different effects on a person's vision: some impairments cause increased sensitivity to light, others result in a restricted or partially occluded field of view, such as lack of central vision, or tunnel vision. Other conditions cause rapid and uncontrollable eye movements which make it difficult to see clearly.

The following activity is designed to give you a little insight into the problems faced by people with certain types of visual impairment.

Activity 8

For this activity you will need the ActiveX control: Shockwave player 10.1 from Adobe Systems incorporated.

Visual Simulation

Click here to run the Visual Simulation, follow the instructions and perform the tasks. The feedback to the tasks provides some guidelines for designers but this isn't really the purpose of asking you to run the simulation. Rather, I want you to gain a little insight into what it is like to have a visual impairment and use a website.


Of course, simulations like this can only give you a very little idea of what it is like to have a visual impairment. As I stated above, there are a variety of different impairments, and within each class of impairment, a range of severities. Still, a very little idea is better than nothing.

People who are colour-blind cannot distinguish between certain ranges of colour. The most common form is red-green colour-blindness. People with this do not see these colours in the same way as most people do, and cannot distinguish between them. Approximately 1 in 10 men and 1 in 200 women are affected by red-green colour-blindness.

Activity 9

Look at Figures 1(a) and 1(b). What can you see?

An image of Ishahara plates for testing colour-blindness

Figure 1: Ishihara plates for colour-blindness

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If you have normal colour vision, you should see the numeral 5 in Figure 1(a) and 8 in Figure 1(b). People with certain types of colour-blindness will not see any numerals in Figure 1(a), and/or the numeral 3 in Figure 1(b).

These figures are examples of ‘Ishihara plates’, which may be used in assisting the diagnosis of different types of colour-blindness.