This section is a review of visual impairments, their causes and symptoms and includes some statistics relating to the visually impaired population and computer use by the visually impaired. This is followed by a review of existing simulations of visual impairments.
A visual impairment is a reduction in either a person's visual fiel or visual acuity. The visual field is defined in the Dictionary of Eye Terminology [1 as cited in 2] as the "Full extent of the area visible to an eye that is fixating straight ahead." Visual acuity is defined as an "Assessment of the eye's ability to distinguish object details and shape, using the smallest identifiable object that can be seen at a specified distance (usually 20ft. or 16in.)" [1 as cited in 2].
A Snellen Test is a common way of measuring the severity of visual impairments. It involves reading a chart "imprinted with lines of black letters graduating in size from smallest on the bottom to largest on top"  from a measured distance. "The Snellen fraction is a ratio, for instance 20/20 or 20/100 (metric equivalent 6/6, 6/30), measuring the acuity of a person's eyesight compared to a standard observer with good normal acuity" . Normal vision is defined to be 20/20 Snellen, which means an observer "can resolve 2 target features at 20 feet" .
Euroblind  define blindness as, "Individuals are registered blind if they have:
a) a visual acuity of less than 3/60 Snellen or
b) a visual acuity of between 3/60 and 6/60 Snellen and a considerable contraction of their field of vision or
c) a visual acuity greater than 6/60 Snellen and a field contraction covering majority of the field."
Euroblind  define partial sight as, "Individuals are registered partially sighted if they have:
a) a visual acuity of between 3/60 and 6/60 Snellen and a full field of vision or
b) a visual acuity of between 6/60 and 6/24 Snellen and a considerable contraction of their field of vision or
c) a visual acuity up to 6/18 Snellen, or better, with a gross field defect."
In the 'normal' eye, parallel light rays are focussed onto the retina when the eye is relaxed. Divergent light rays (from a close object) are focussed on the retina by a process of accommodation where the ciliary muscle contracts to make the lens of the eye more globular . The retina is a layer of "[l]ight sensitive nerve tissue in the eye that converts images from the eye's optical system into electrical impulses that are sent along the optic nerve to the brain" [1 as cited in 2]. The macula is a small specialised area at the centre of the retina and "is responsible for the most detailed vision and colour discrimination" . Figure 2.1 show the structure of the eye and some of the areas that can cause of loss of vision:
Myopia is also known as short-sightedness.
Cause: Refractive error. Parallel light rays are focussed in front of the retina when the eye is in its relaxed state. This is because the eyeball is too long for the natural converging power of the cornea and lens .
Symptoms: The eye can see close objects clearly whist distant objects appear blurred .
Hyperopia is also known as hypermetropia or long-sightedness.
Cause: Refractive error. Parallel light rays are focussed behind the retina when the eye is in its relaxed state. This is because the eyeball is too short for the natural converging power of the cornea and lens .
Symptoms: The eye can see distant objects clearly whist close objects appear blurred .
Cause: Refractive error. Irregular curvature of the cornea results in uneven focus in different parts of the visual field. "[T]he eye cannot compensate by accommodating" .
Symptoms: Objects at all distances appear blurred .
Cause: Refractive error. With age, the lens gradually loses its elasticity and even when the ciliary muscle contracts, the lens no longer adjusts its shape.
Symptoms: The eye's ability to accommodate and change focus is reduced .
There are two types of macular degeneration:
Cause: Wet macular degeneration "results in a build-up of fluid under the retina. This causes bleeding and scarring which leads to sight loss" . Dry macular degeneration is caused by "pigmentary changes at the macula" .
Symptoms: Macular degeneration usually "occurs in both eyes, but it may be asymmetrical" . In the early stages central vision may be disturbed or blurred. Straight lines may seem wavy and objects may become distorted or appear an unusual size or shape. Peripheral vision is unaffected. . "Eventually, central vision may be completely lost"  resulting in a "blank patch or dark spot" . "This central area of visual distortion or loss moves as the patient tries to look around it" .
Cause: "[T]he optic nerve is damaged at the point where it leaves the eye" . This may be caused by increased intracocular pressure (pressure of the fluid inside the eye) or by weakness in the optic nerve . In the normal eye, the ciliary body (a layer of cells behind the iris) secretes a watery fluid called aqueous. This fluid then flows through the pupil before leaving the eye via the trabecular meshwork (drainage channels between the back of the cornea and the iris) .
Primary open angle or chronic glaucoma occurs when these drainage channels become slowly blocked and the eye pressure rises, causing damage to the optic nerve. The field of vision gradually becomes impaired . Acute angle closure glaucoma occurs "when there is a sudden and more complete blockage to the flow of aqueous"  caused by the "apposition of the lens to the back of the iris" .
Symptoms: Reduction in the peripheral field of vision (tunnel vision).
Cause: A cataract is the opacification (clouding) of all or part of the lens which prevents light passing through. Cataracts often form slowly with age. The lens turns from being clear to yellow and then eventually to brown .
Symptoms: Sensitivity to glaring light. Blurred or misty vision, multiple images, double vision or ghost images. Print appears hazy. Colours may appear faded, and contrast is limited .
Cause: diabetic retinopathy affects the fine network of blood vessels within the retina. It is usually graded in three main stages according to how severe it is .
Symptoms: Peripheral vision will be initially preserved. Some central vision loss may occur. As the condition progresses to Proliferative diabetic retinopathy, pigment or blood in the vitreous may cause blurred or patchy vision with floaters or blobs .
Cause: retinitis pigmentosa is the name given to a group of hereditary eye conditions that affect the retina's ability to respond to light .
Symptoms: Reduction in the peripheral field of vision (tunnel vision) and difficulty in seeing in poor light .
Hemianopia is also known as Hemianopsia.
Cause: Damage to the brain as a result of stroke, tumour or trauma .
Symptoms: Loss of vision in one half of the visual field .
"Colour blindness is the reduced ability to discriminate between different wavelengths of light" .
Cause: The retina contains three types of cone cells. The short-wave (blue), medium-wave (green) and long-wave (red) cone cells each contain a different photosensitive pigment. They have peak sensitivities in the blue, green and red wavelengths of the visible spectrum. colour blindness is the result of deficiencies in the cone photopigments, which can be classified into three main types .
The RNIB  state that "[t]here are about 1 million people in the UK whose vision is such that they could be registered as 'blind' or 'partially sighted'. This is supported by statistics published by Euroblind  (Table 2.1) and the Department of Health  (Table 2.2).
|Total Registered Visually Impaired||Total Identified Visually Impaired||Total UK Population|
|Aged 0-4||Aged 5-17||Aged 18-49||Aged 50-64||Aged 65-74||75 and over||All Ages|
|Registered Partially Sighted||585||4,230||15,315||12,935||16,640||105,525||155,230|
Data published by the American Foundation for the Blind  shows that the number of people with a visual impairment who use a computer on a regular basis is significantly lower than those with no visual impairment (Table 2.3).
|Estimated number of youths and adults, aged 15 and older, who use a computer on a regular basis: United States, 1999.||Total number Number who use a computer on a regular basis|
|No Visual Impairment||166,108,000||84,621,000 (51%)|
|Limitation in seeing||7,326,000||979,000 (13%)|
|Severe limitation in seeing||1,536,000||102,000 (7%)|
|Not severe limitation in seeing||5,790,000||877,000 (15%)|
A number of websites have images showing how real-life scenes and text might appear to people with a visual impairment (see Appendix A). Interactive simulations of visual impairments are described in the sub-sections below:
The Vision Simulator  is an interactive simulation which shows how a scene or photo may appear to someone with a visual impairment. There is a choice of ten impairments which can be selected by clicking a link at the side of the simulation. A slider allows the user to adjust the severity of the condition. Three images are used to simulate different focal lengths.
The WebAim Low Vision Simulator  simulates viewing a website with glaucoma, macular degeneration and cataracts. Keyboard shortcuts allow the user to select the impairment, zoom in and out, and set the eye position.
The Vischeck Colour Blindness Simulator  simulates the effects of colour blindness on the appearance of websites. Users can enter a URL, and the tool provides a link to a modified copy of the page.
Fork In The Road  and VINE  supply sets of goggles that allow the wearer to experience a range of visual impairments.
The causes and symptoms of myopia, hyperopia, presbyopia, astigmatism, glaucoma, retinitis pigmentosa, macular degeneration, diabetic retinopathy, hemianopia, cataracts and colour blindness were reviewed. These impairments do not generally result in complete blindness, but instead result in part of the visual field becoming distorted or obscured. According to the RNIB , "[a]bout 75% [of people with a visual impairment] have sufficient vision to read a newspaper headline with appropriate correction (e.g. spectacles) and good illumination."
Existing simulations of visual impairments include static images, interactive computer simulations and simulation goggles.
© Stephen Ratcliffe. 2005
Department of Computer Science
University of York