Vision

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The human eyes work by allowing light to enter through the pupil. The lens is used to focus light rays onto the retina, which is the surface at the back of the eye that is sensitive to light.

The retina consists of two types of receptor cells known as rods and cones. Rods allow us to differentiate between shades of black and white while cones allow us to see colour. Most cones are concentrated in a particularly sensitive region of the retina called the fovea. This region enables us to see the greatest detail. The receptor cells use the optic nerve to transmit signals to the brain, which interprets the signals from both eyes to construct the image we see. The combined image from both eyes assists depth perception.

Common conditions such as short and long-sightedness can occur due to a misshapen lens or eyeball, and can be corrected by glasses. However, as the eye muscles weaken, or the lens becomes stiffer, the eye can no longer adapt to focus on objects at different distances.

Because of ageing and various eye conditions, the structure and function of the eye can also deteriorate, which can result in blurry vision, central or peripheral loss of visual field, and reduced contrast sensitivity. Colour blindness is mainly inherited as a genetic defect, although the ability to distinguish between blue and green can decrease with age.

The visual system serves various functions, four of which are discussed in the context of designing products.

• Visual acuity is the ability to see fine details of objects
• Contrast sensitivity is the ability to discriminate between different brightness levels
• Colour perception is the ability to distinguish between different colours
• Usable visual field is the ability to use the whole of the visual field to perceive detail in the area being looked at and the surrounding area

Visual acuity is the ability of the eye to see fine detail. Being able to see details depends on several factors

• The ability of the lens to focus the light given the viewing distance
• The smallest feature or space that the retina can resolve (for example, text size or stroke thickness)
• The clarity of the fluid and membranes that the light passes through to reach the retina
• The ambient illumination and the contrast between the foreground and background

Reading text is the most common task with a high acuity demand. The matrix opposite shows various combinations of text size and font style. Consider the following when choosing text style

• The ease with which characters within a font can be recognised depends on the thickness and spacing of the lines that make up a character, together with the uniqueness of the character's shape
• Legibility and reading comfort are both reduced for underlined, italicised or decorative font styles
• Ensuring that characters remain unique after being reflected or rotated can help those with dyslexia

Visual tasks requiring acuity also include identifying graphics and symbols, reading signage and recognising faces.

Colour can be described by its hue, saturation and brightness (how much light is apparently reflected). Contrast sensitivity is the ability to perceive the difference in brightness between a foreground colour and a background colour. It is related to the size, distance and illumination of the object to be detected. Maximum contrast occurs with white on black or vice versa.

Higher contrast levels result in a greater likelihood of detection by people with low contrast sensitivity. For example, the ability to distinguish number keys from the body of a mobile phone depends on their contrast, the viewing distance, the size of the text and the ambient illumination.

Visual acuity is related to contrast, and low contrast text will be more difficult to discriminate than high contrast text. In addition, controls on products need to be of sufficient contrast against product chassis to be easily detected. Contrast sensitivity is important for activities such as detecting and reading text, moving around in the environment, and detecting the outlines of buildings, roads, and pavements.

The diagrams opposite show some different foreground and background colours for text and the corresponding images viewed with a reduced brightness contrast. Note how a sharp colour distinction helps to discriminate between the foreground and background when the brightness contrast is reduced.

Colour conveys information about the physical world. Loss of colour discrimination (or colour blindness) is the inability to distinguish between various colours. The notion that people with colour blindness cannot perceive colour is inaccurate, and in fact total loss of colour perception is extremely rare. A person who is colour blind cannot effectively distinguish between different colours in the colour spectrum.

The most common form of colour blindness is red-green colour blindness, where the ability to distinguish between colours from red to green in the colour spectrum is reduced. These colour confusions can make interaction with products difficult if colour alone is used to provide information.

The diagram opposite shows various foreground and background colour combinations (top) and the corresponding simulated appearance for a red-green colour blind person (bottom). Note that images that have similar brightness contrasts can disappear when viewed by someone with colour blindness.

With increasing age and various eye conditions, the usable field of view can change. This loss can either start from the centre of the visual field (central field loss) or from the outer edge of visual field (peripheral field loss).

The central visual field is used for focusing and perceiving detail. When the central field is obscured, tasks that require perception of detail (such as reading) become very difficult. People generally adapt to compensate for this loss and attempt to use the peripheral visual field. However, this part of the retina is less sensitive, resulting in a loss of acuity and contrast sensitivity. The same guidance given for acuity and contrast will help users with central field loss.

A reduction in the usable peripheral visual field results in tunnel vision and can affect mobility. A rail ticket machine with widely spaced controls can result in problems when viewed with a peripheral loss. The lower figures indicate a proposed redesign of the same machine where the continuity between the required action areas has been emphasised, resulting in a more usable interface.

The ‘apparent’ size of an object is directly proportional to the distance it is away. Successfully visual perception depends primarily upon the brightness contrast, colour contrast and ‘apparent’ size of the detail to be perceived, the ability of the eye to successfully focus given the viewing distance and the ambient illumination. The environmental factors are now considered in further detail

The level of ambient illumination, together with the spatial relations between the light source, the product, and the user, impact on the ability to perceive visual detail. Examples highlighting this include reading the text on the back of a television, or reading a road sign whilst facing the sun.

Glare can cause significant difficulties when reflective surfaces such as a screen are viewed from a certain angle related to the sun or light source. Consideration of reflectivity, distance and viewing angle is particularly significant when the position of the product or user is fixed or constrained, such as when viewing a road-sign.

• Consider potential issues with glare, based on
  • The surface finish of the item being looked at
  • The intensity of the light source
  • The spatial position of the surface, the viewer, and the light source with respect to the angle of view
• Contemplate enabling users to control the intensity, position, and angle of the lighting sources to best suit their own vision ability and the task
• Think about using colours to help convey information but ensure they are supplemented by presenting the information in alternative ways (e.g. use shapes and text)
• Consider using colour contrast to help make things stand out, but ensure there is also sufficient brightness contrast
• Check that a product remains usable when its image is converted to grey scale
• Think about assisting those with vision impairments by supplementing information through auditory or tactile means, with due consideration for information overload
• Arrange the design so that it remains visible and usable for those who have some loss of visual field
• Think about minimising exclusion by ensuring the design remains usable while wearing a vision impairment simulator

• Choose text of suitable size, font and brightness contrast for comfortable reading given the likely viewing distance and potential difficulties caused by ambient lighting
• Be careful with the use of text on a patterned or picture background, with due consideration for legibility
• Avoid italicised or decorative font styles for blocks of text or signs
• Carefully consider the line thickness, line spacing and overall size when designing graphical symbols or logos
• Note that common practice is to use a serif font for large blocks of text, and a sans serif font for signs, labels or headings

For further guidelines on printed text, see RNIB (2006).[ RNIB (2006) See It Right: Making information accessible for people with sight problems. Royal National Institute for the Blind, London. ISBN: 1858787041 ]

Vision ability level (in increasing order)

Ability level is measured with any desired vision aids.

No demand

• The user is not required to perceive anything by sight

Low demand: The user is required to have sufficient ability to do things like:

• Read a newspaper headline (≈ 72 pt text)
• Recognise a friend at arm's length away

Moderate demand: The user is required to have sufficient ability to do things like:

• Read a large print book (≈ 16 pt text)
• Recognise a friend across the room

High demand: The user is required to have sufficient ability to do things like:

• Read ordinary newsprint (≈ 9 pt text) without difficulty
• Recognise a friend across the road without difficulty

Demand levels assume that any desired vision aids will be used.