Vision

Various medical conditions such as age-related macular degeneration (AMD), cataracts, glaucoma, retinitis pigmentosa, and diabetic retinopathy cause reductions in the functional capabilities of the eyes making visual tasks with products more difficult (VisionConection, 2006). The reduction in the following five functions seems to account for most of visual disability (Fletcher & American Academy of Ophthalmology., 1999; Jacko, Dixon, Jr, Scott,

& Pappas, 1999; Norton, Corliss, & Bailey, 2002; Schieber, 1992).

Visual Acuity: Acuity is the ability of the eyes to resolve fine details and differentiate different parts of the visual field from each other (Schiffman, 2000). There are various forms of acuity, including detection, vernier, resolution, recognition, and dynamic acuity

(Schiffman, 2000). The most familiar is recognition acuity measured with a letter chart at the optician. Acuity is greatest in the central visual field and it is measured as the inverse of visual angle (a measure of the size of a target subtended on the retina). Measuring the maximum visual acuity thus gives the performance limit of detail perception under the measured conditions and this has implications for text at maximum contrast. However, because acuity is measured at high contrast and many real world conditions occur at lower contrast levels, recognition acuity has limited predictive value for spatial vision and form perception (Schiffman, 2000).

Contrast Sensitivity: Contrast is a measure of the difference in luminance between an object and its background. Contrast sensitivity is a measure of the minimum contrast that can be perceived at different spatial frequencies. Based on the channel model of visual form

perception (Schiffman, 2000), real world visual stimuli are analysed by Fourier analysis into a range of spatial frequencies at various contrast levels. Large objects comprise low spatial frequencies and small objects comprise high spatial frequencies. An individual’s sensitivity to contrast at these different frequencies can be reduced due to ageing and various eye

conditions, leading to reduced performance in form perception and mobility.

Sine wave gratings are used to assess an individual's contrast sensitivity at different spatial frequencies and orientations, and the results are plotted on a graph of contrast sensitivity versus spatial frequency. This graph is termed the contrast sensitivity function or CSF (Schiffman, 2000). An example of the CSF is shown in Figure 5-1. The figure shows the reduction in contrast sensitivity for medium and high spatial frequency stimuli in the 80-year age group compared to the 20-year age group. For older people, the loss of contrast sensitivity at high spatial frequencies is generally greater than the loss at low spatial frequencies. The

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shaded region of the graph shows the range of high spatial frequencies at which visual acuity is measured. This demonstrates that visual acuity measures at high spatial frequencies cannot predict performance on real world vision tasks because it does not capture contrast thresholds for larger objects of lower spatial frequencies. The CSF therefore characterises a person’s maximum contrast thresholds for various sizes of objects, and it can be used to predict visual exclusion of product controls and other form features.

Figure 5-1 Visual acuity and contrast sensitivity with age

Colour Perception: The eyes are equipped with two types of photoreceptors known as rods and cones. In the human eye, there are three types of cones that are sensitive to short, medium and long wavelengths of light. The operation of these three types of cones gives rise to colour perception. People exhibit colour blindness if any of these cones are missing or defective, leading to a loss of discrimination of the full spectrum of colour. Despite the name, complete colour blindness, where there is no perception of colour, is extremely rare. There are two main forms of colour blindness: red-green and blue-yellow. This means that for red-green colour blindness, a person cannot distinguish colours between red and green in the colour spectrum and for blue-yellow colour blindness, a person cannot distinguish from the yellow to the blue part of the colour spectrum. Colour discrimination is also known to decrease with age. Colour blindness does not cause significant problems provided that foreground and background colours are of sufficient contrast to be detected. However, if colour is used to display information then the possible range of colour blindness and colour confusions

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becomes an important consideration. Therefore, the maximum range of colours discriminated becomes the threshold for colour perception.

Central and peripheral field of view: Various conditions can cause a reduction in the useful field of view. The central visual field can be obscured or the peripheral visual field can be reduced resulting in 'tunnel vision'. Partial combinations of central and peripheral loss are also possible. Loss of central field of view is more difficult to accommodate because acuity is greatest in the central field for colour vision. For design evaluation, the maximum extent of the central field of view is the important measure. The size and layout of interface features can be evaluated to determine if they fall easily within the users' field of view when

performing essential tasks. By grouping controls and designing them in close proximity, the product would not demand a large field of view for search and detection of the relevant controls.

Depth perception and stereopsis: Stereopsis is the ability to perceive depth based on the combination of two slightly varying images transmitted to the brain by the spherical geometry of the eyes. Depth information is also obtained from motion through space. People with loss of depth perception can have difficulty operating in a three dimensional environment.

Movement is affected and products that require spatial manoeuvres to access controls could be especially difficult.

Figure 5-2 Visual capability losses

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The diagram in Figure 5-2 symbolically demonstrates the effects of these losses of acuity, contrast, central field, peripheral field, available field and colour range in functional vision.

The reality is that there may be simultaneous multiple loss, which results in a superposition of these losses on functional vision. Common visual tasks include detecting and reading text on the product chassis and displays, detecting symbols and graphics, and detecting features of the product against the background of the chassis. Design parameters such as size, shape, colour, and contrast all constitute visual demands on the user.