Vision Impairment and Blindness
Adaptive Technologies
Summary of vision loss
Some important terms
Some types of vision loss
Summary of Vision Loss
When considering equipment for use by a person who has a vision impairment, it is crucial to be aware of the nature and severity of the condition. Importantly, most people who have a serious visual loss have some useful vision. While this is valuable, it can also involve complexities not faced by those who have little or no sight.
A person’s level of vision loss is often erroneously referred to as a percentage. For example, “He has 15% sight.” This is not very helpful in determining what difficulties the individual faces and what equipment is likely to be most useful. Vision loss is not simply a reduction of vision. The specific condition which causes the loss will affect what areas of visual performance will be compromised.
Some important terms
Visual acuity is a measure of distance vision. A person who has 6/6 vision as measured with a standard eye chart is said to have normal vision (a measure of 20/20 is equivalent in feet). That is, the person sees what is normal at that distance. If the acuity figure is 6/9, the person sees at 6 metres what is normal for nine metres etc. A person with acuity of 6/60 must be ten times closer than someone with normal vision to see the same object with the same level of clarity.
Near vision is the eye accommodating to focus at distances between 25cm and 60cm. This is important for reading and other activities involving objects close to the eyes. It is normal for people who have good vision to require reading glasses somewhere between 45 and 50 years of age. Various conditions, however, can severely compromise ability to read and to perform other tasks requiring good near vision. The technical options discussed on this site can help with these tasks, but do not replicate full vision. A person may be able to read, for example, 12pt print with an aid in a clinical setting. In day to day life factors such as fatigue, glare or insufficient light can be significant problems.
Visual field is a measure of how much can be seen while the eye is stable. A normal visual field is approximately 160 to 170 degrees. Field of view can be seriously limited by some conditions. A constricted field poses major difficulties for moving about, as many objects will not be seen. Another issue is that the periphery of the eye provides vision in dim light. Therefore, a person who has seriously constricted visual fields will have little or no vision at night.
Photophobia is sensitivity to light. It affects many people who have a significant vision loss.
Legal blindness in Australia is determined by the following criteria. Corrected vision (with spectacles) in the best eye is less than 6/60 or visual field binocularly is less than 15 degrees. A person who is legally blind is entitled to various government benefits.
Some types of vision loss
Diabetic retinopathy can occur if a person suffers from diabetes. Blood vessels in the retina may haemorrhage and areas of the retina where this occurs die. Nature and extent of vision loss will depend on location and severity of the damage. As well as vision loss, the damage will commonly result in photophobia.
Retinitis pigmentosa (or retinal dystrophy) is an inherited condition which results in progressive loss of visual fields. It will often ultimately result in total blindness, but rate of deterioration between individuals is highly variable. As well as “tunnel vision”, problems include poor adaption when going from light to dark and vice versa. Good illumination is required for tasks such as reading but, almost paradoxically, photophobia is a major problem.
Macular dystrophy results in loss of central vision. There is reduced acuity, resulting in significant problems with reading and other tasks requiring good central vision. Colour perception is also affected.
