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document Edition 9: Accessibility

Vision and Visual Disabilities – An Introduction

By Gerd Waloszek, SAP User Experience, SAP AG – June 7, 2005; updated: May 21, 2013

Basics of Vision | Visual Impairments | Overview of Visual Impairments | Further "Visual Traps" | A "Visual" Checklist | References

Vision is our dominant sense. Computers and computer software make even more prominent use of the visual sense than human beings. Therefore, one should know that a considerable fraction of the human population has visual impairments and what the causes for the common impairments are. As most visual impairments have physiological causes, I begin this article with a selection of physiological facts about vision.


Basics of Vision


Light is an electromagnetic radiation that emanates from light sources or is reflected from non-radiating objects. We perceive light by our eyes and process and interpret it in our brain. There are different dimensions or aspects of light:

  • Physical dimensions: Frequency (wavelength), intensity (amplitude), saturation (spectral composition), contrast (intensity differences)
  • Perceptual dimensions: Brightness – primarily related to intensity; hue (tone, tint) – primarily related to frequency (wavelength)
  • Psychological dimensions: warmness/coldness, moods induced by colors, color associations and cultural differences with respect to these

The visible light spectrum lies between 380 nm (violet) and 760 nm (red). Light travels at a constant velocity of about 300.000 km/s; it takes 1.25 sec to the moon, nearly 8 minutes to the sun, 4.2 years to the nearest star (Alpha Centauri), and about 2.2 million years to the Andromeda galaxy.

Lens, Iris, and Eye Ball – The Optical System

The eye's lens and iris form an optical system similar to the one used in a photo camera. The lens projects an upside down image onto the back of the eye ball where the retina is located. The retina is a thin neural tissue containing light sensors – it is the analogue to the film in a camera or sensor in a digital camera.

Figure 1: The human eye (from National Eye Institute)

The eye's lens is flexible and can change its shape in order to focus objects at different distances – this focusing process is called accommodation. Muscles can move the eye balls to point to a target – this process is called convergence.


Figure 2a: Accommodation


Figure 2b: Convergence

The iris acts as an f-stop mechanism that regulates the amount of light that is transmitted through the eye to the retina.

Malfunctioning of the optical system may cause different types of visual impairments – see below for details.

Light Sensors

The retina in our eyes contains two basic types of light sensors (photoreceptors), which form independent visual systems that are dedicated to special tasks:

  • Rods are primarily used for brightness and motion perception, as well as for night vision
  • Cones are primarily used for color and daylight vision

Each eye has about 5.000.000 cones and 120.000.000 rods. People can distinguish about 200 different hues and about 100 levels of gray.

Schematic rod and cone

Figure 3: Schematic rod (left) and cone (right) (from Sensation & Perception)

Depending on the functioning or malfunctioning of these receptors, there are different types of visual impairments. I will return to that below.


The rod and the cone systems exhibit different overall sensitivities to light. This impacts our ability to see by daylight and by night: Cones are primarily used for daylight vision (photopic or light vision), while rods are primarily used for night vision (scotopic or dark vision) – probably because there are so many more rods than cones.

The sensitivity of rods depends on the wavelength of the perceived light (the maximum sensitivity for rods is at 505 nm = green). But as there is only one rod type available in the eye, solely brightness information can be extracted from the rods.

Cones have a spectral sensitivity distribution that is analog to that of rods. But as there are three cone types, each having a different distribution, color information can be extracted from the cones (theory shows that any combination of three sufficiently different color distributions allows for color vision):

  • One cone type is most sensitive to violets (the maximum sensitivity is at 445-450 nm)
  • Another one is most sensitive to greens (the maximum sensitivity is at 525-535 nm)
  • A third cone type is most sensitive to yellow-greens (the maximum sensitivity is at 555-570 nm)

Distribution of relative sensitivity

Figure 4: Distribution of relative sensitivity for cones (from Sensation & Perception)

Rods and cones also have different overall spectral sensitivity distributions. The maximum overall sensitivity for cones is at a wavelength of 555 nm = yellow-green, while the maximum sensitivity for rods is at 505 nm = green. This difference can be experienced in twilight, when colors change in relative apparent brightness (known as the Purkinje shift).

Distribution of relative overall

Figure 5: Distribution of relative overall sensitivity for rods and cones (from Sensation & Perception)

Distribution of Light Sensors

The light sensors are not evenly distributed across the retina. Cones are concentrated at the center of the retina, called macula or macula lutea (yellow spot). At the center of the macula is the fovea centralis, which has the highest cone density and therefore is critical in visual perception (ca. 150.000 receptors per square mm at maximum): As sharp vision is restricted to the fovea, our eyes are in steady movement to focus targets and to give us the impression of a "sharp" environment. Cones are fairly thinly distributed over the periphery of the retina (ca. 10.000 receptors per square mm).

Rods are absent from the central area of the retina. Their distribution reaches its peak at an angle of about 20 degrees from the center of the retina (ca. 160.000 receptors per square mm) and decreases to about half of it at about 60 degrees.

Distribution of rods and cones

Figure 6: Distribution of rods and cones in the human retina (from Sensation & Perception)

Visual Acuity

Visual acuity depends very much on the distribution of cones in our retina: Where cones are plentiful, acuity is good; where cones are sparse, acuity is poor. Maximum acuity is concentrated on a visual angle as small as one minute of arc; this comes up to the size of a quarter seen at a distance of 81m. Acuity drops rapidly down to the periphery.

Distribution of visual acuity

Figure 7: Distribution of visual acuity (from Sensation & Perception)

Motion Perception

Rods dominate in the periphery of the retina. As rods are more sensitive to motion than cones, peripheral vision is more sensitive to movements than the center of the retina. Thus, animated graphics that flicker in the periphery distract us even more as if they were in the focus.

Neural Pathways

The neurons from each eye form the two optic nerves that transmit visual information to the brain. Both optic nerves come together at the optic chiasm, where the information from the left sides of both eyes is combined and directed to the left side of the brain; the information of the right sides of the eyes is directed to the right side of the brain. As the images in the eyes are upside down, the information from the left half of the visual field is actually directed to the right side of the brain and vice versa. This combination of visual information is important for depth perception.

Behind the optic chiasm, the optic nerve is called optic tract; it continues to the lateral geniculate nucleus (shown as cycles in figure 8) and from there to the visual cortex, those areas of the brain, that process visual information. Figure 8 shows "cuts" in the visual pathways (from A to G); these account for different vision field losses.

The visual pathways from the eyes to the brain

Figure 8: The visual pathways from the eyes to the brain (from Guide Dog Association Website)


Visual Impairments

Most visual impairments have physiological reasons. Some of them originate from the lens alone, some from its physical relation to the eye ball, others from the sensors, that is from malfunctions of rods or cones, or the neural pathways and processing.

See also Overview of Visual Impairments for a quick overview.

Blindness and Low Vision

Blindness means that people cannot see or recognize anything. This includes cases where people may have a "global" brightness impression only. Blindness can be congenital, or appear later during the life span– then caused by accidents, diseases, or the aging process (e.g. diabetic retinopathy).

From a software accessibility point of view, cause and nature of blindness are irrelevant for what has to be done to improve accessibility for blind users. Therefore, I focus on different cases of low or impaired vision. Low vision is a visual impairment, not corrected by standard eyeglasses, contact lenses, medication, or surgery, that interferes with the ability to perform everyday activities (definition by National Eye Institute).

Impairments Caused by the Optical System

The shape of the eye ball is important for receiving sharp images. If the eye ball is too long (myopic eye) the light rays do not focus on the retina but before it: we are nearsighted and have to move the target, for example, a book, close to our eyes. If the eye ball is too short (hypermetropic eye), the light rays focus behind the retina: we are farsighted and have to move the target away from our eyes. Often, our arms are not "long enough" for getting a sharp image. As the flexibility of the lens decreases with age, most people become farsighted in their early forties.

Further seeing problems may arise from distortions (e.g., astigmatism) or darkening of the lens (cataracts, glaucoma). Most of these problems – caused by the optical system of the eye – can be remedied by glasses that correct the lens, or by lens surgery.


Cataracts are a cloudiness that is formed in the lens of the eye and may cause poor vision. Most cataracts result from aging and long-term exposure to ultraviolet light. Cataracts get worse over time – the clouded areas become larger and denser. The time taken for this to happen varies from a few months to many years. Cataracts are the leading cause of vision loss among adults of 55 and older.


Color Deficiencies – Malfunctioning of the Cone System

Some people show drastic deficiencies in their ability to discriminate colored stimuli. These people are called colorblind, although this term is too strong for most of the deficiencies. According to the trichromatic color theory, which is based on the fact that our color vision uses three cone types, malfunctions in the cone system are responsible for five types of color deficiencies:

  • Monochromats I: People with no functioning cones; people with this deficiency have the following problems:
    • Lack of color vision – the rods can distinguish only levels of gray (analog to black-and-white images)
    • Day-blindness – the daylight is too bright for them (and the rod system) and causes pain
    • Low visual acuity – the area of highest sensor density, the fovea, contains no rods
  • Monochromats II: People with only one variety of the cones functioning in addition to the rods. These people also see colors only as variations in intensity, that is analog to unicolored images.
  • Dichromats: People with only one malfunctioning cone system
    • Protanopia: Malfunctioning in the red cone system; typically only two (yellow, blue) or three colors (yellow, blue, purple) can be distinguished – yellow comprises red, orange, yellow, and green, blue coincides with blue and purple
    • Deuteranopia: Malfunctioning in the green cone system; green cannot be distinguished from certain combinations of red and blue; this is the most common type of color deficiency
    • Tritanopia: Malfunctioning of the blue cone system; longer wavelengths appear as red and the shorter ones as bluish-green; this color deficiency is very rare

Mild instances of color deficiencies are called "anomalous trichromatism" and are fairly common. Typically, these people do not act exactly like a dichromat, but need more red (protoanomaly) or green (deuteranomaly) than a color-normal individual to match colors. More than 8% of the male and about 0,04% of the female population have some sort of color anomaly or deficiency.

Ishihara color blindness test chartsIshihara color blindness test charts Ishihara color blindness test charts

Figure 9: Ishihara color blindness test charts

Photo demonstrating normal vision    Photo demonstrating glare sensitivity

Figure 10a-b: Normal vs. day-blindness (glare sensitivity) (from former Retina International Website)


Other Receptor-Related Impairments


Night-blindness is a lack of the ability to see at night. It sets in if light intensity goes below a certain level, for example, at the twilight level.

Night-blindness is caused by malfunctions of the rod system. The cone system alone is not sensitive enough for night vision (cones comprise only about 1/25 of the number of rods). The rods may be affected in many ways, one common example is a breakdown of the pigments (retinitis pigmentosa).

Photo demonstrating night-blindness

Figure 11: Night-blindness (middle) vs. normal sight (left and right) (from former Retina International Website)


  • Retina International: (no longer available)

Loss of Central Vision – Macular Degeneration

Macular degeneration is damage to or breakdown of the macula and thus a loss of the central vision. Color vision may also be diminished, although peripheral vision and night vision usually remain unaffected.

Macular degeneration may be the result of aging processes in the eye (age-related maculopathy, ARM, also called senile macular degeneration); there are also some other forms of macular degeneration, which are inherited and not associated with aging.

Normal vision - scene in a beer garden

Loss of central vision blends out the menu

Loss of central vision blends out the waitress' face

Figure 12a-c: Loss of central vision blends out the object that a person wants to fixate – here either the menu (middle) or the face of the waitress (bottom); normal vision is shown at the top (from former Retina International Website)


Retinitis Pigmentosa (Retinal Dystrophy)

Retinitis pigmentosa (RP) is a congenital inherited disease, which causes the breakdown of pigment in the retina (both rods and cones degenerate). It usually starts with night-blindness; as the condition worsens, people may also have difficulty seeing in dim light. Eventually, RP may progress to loss of peripheral vision, which leads to tunnel vision. Color vision may also be affected.


Tunnel Vision

Tunnel vision is like seeing the world through a small tube. This makes it very hard for people to maintain orientation in their daily lives. Two common causes of tunnel vision are glaucoma and retinitis pigmentosa. In the latter case, tunnel vision may be combined with night-blindness.

Demonstration of tunnel vision    The same view for normal vision
Demonstration of tunnel vision    The same view for normal vision

Figure 13a-d: Tunnel vision (left) makes it difficult to orient in the environment, for example to find the doors of a train (from former Retina International Website)


  • Retina International: (no longer available)

Neural-Related Impairments


Glaucoma is often caused when the fluid in the eye does not drain away fast enough. The built up pressure damages the optic nerve and prevents visual information from reaching the brain. Glaucoma typically affects peripheral vision first (tunnel vision), and can cause total blindness if not treated at an early stage.


Vision Field Loss (Neurological Vision Impairment)

Visual information is sent from the eye through the optic nerve to the brain. When the optic nerve or those parts of the brain that are used for seeing are damaged, parts of or even the whole vision is lost. Which part of the vision field is missing, depends on the affected neural pathway(s). The eyes may still work normally.

Vision field loss can have many causes, such as strokes/CVA (cerebral vascular accident), brain tumors, post surgery implications, or head injury (e.g. caused by car accidents).



Overview of Visual Impairments

The following two tables provide an overview of the most common visual impairments. The first table covers impairments that are related to malfunctions of sensors. The second table includes impairments caused by defects in the optical or neural system or by diseases.

Sensor-Related Impairments

Loss of Central Vision
Loss of Peripheral Vision
Low Acuity
Color Deficiencies
Synonyms   Tunnel vision     Glare sensitivity Color blindness
Affected Sensors Cones Rods Cones Rods Cones Cones (one or more types affected)

Macular degeneration (age-related or inherited)

Inverse retinitis pigmentosa

Retinitis pigmentosa (RP)

Usher syndrome (RP combined with hearing loss)

See also Glaucoma

Coincides with loss of central vision Retinitis pigmentosa Monochromats (all cone types affected)



Table 1: Overview of sensor-related visual impairments

Visual Impairments Caused by the Neural or Optical System or by Diseases

Lens Clouding
Loss of Vision Field
Loss of Peripheral Vision
Focusing Problems, Distortions
Synonyms     Cortical blindness Tunnel vision  
Remarks Typically remedied by glasses or contact lenses Can be remedied by lens surgery Can include total loss of vision Can lead to total loss of vision Can lead to total loss of vision
Affected Parts Lens, eye ball Lens Optic nerve (visual pathway), brain Optic nerve (close to lens) Blood vessels (arteries) in retina




Cataracts Damages in the neural pathways or in brain regions dedicated to vision Glaucoma Diabetic retinopathy (caused by age-related diabetes)

Table 2: Overview of further visual impairments


Further "Visual Traps"

Finally, I list two more issues that are relevant for "visual" accessibility – not only for visually impaired users but also for users with normal vision.

Contrast Sensitivity

The contrast sensitivity function for red and green is greatest for 10 point text (spatial frequency = 1 line pair/mm = 8 Hz/degree) at normal viewing distance (46 cm) and about 14 point text at arm length (65 cm).

For blue the greatest sensitivity is around a spatial frequency of 1 Hz/degree The sensitivity itself is about one tenth of that of red and green. This is due to the scarcity of blue cones in the retina and to the absence of a local yellow-blue opponent mechanism. (From Jackson et al.)


Figure 14: The primaries blue, red and green in normal face and bold face on white and black backgrounds


Red and blue colors are perceived at different depths. This effect is due to the chromatic aberration of the eye (light of different wavelengths is refracted differently).

Blue on Red
Red on Blue

Figure 15: Foreground and background color combinations to avoid!

Because chromostereopsis can be quite disturbing visually, avoid displaying both vivid red and blue together!


A "Visual" Checklist

The following checklist is a first proposal for check items, which help to ensure that a Website or software application conforms to the accessibility guidelines with respect to visual aspects.

Do not rely on color coding alone

Colorblind people as well as blind people cannot recognize the difference


Provide redundant in formation such as titles, descriptions, images (with descriptions)

WCAG CP 2.1 (P1),
S508 (a)


Offer font scaling mechanisms

People with low visual acuity may need to use larger fonts for reading

People with good vision may want to scale the font down for smaller screens, such as 800x600

Use sufficient background-foreground contrast for text Low contrast makes reading text harder, especially for people with low vision

WCAG CP 2.1 (P2)

Computer Generated Color

Avoid blue text Visual acuity is only about 1/10 for blue than for other colors; therefore, avoid delicate blue structures, such as small blue text Computer Generated Color
Avoid certain color combinations for foreground and background

Certain color combinations should be avoided:

  • Red text on blue background (chromostereopsis – the text and the background are perceived in different depths)
  • Combinations that cannot be differentiated by color blind users: red-green, blue-yellow



Computer Generated Color

Table 3: Checklist for vision-related accessibility issues




  • Core, S., Porac, C., & Ward, L. M. (1978). Sensation and Perception. Academic Press: New York.
  • Jackson, R. & MacDonald, L., & Freeman, K. (1994). Computer Generated Colour: A Practical Guide to Presentation and Display. Wiley: New York.

Regulations and Guidelines

Color Blindness and Design for Visually Impaired Users, Color Palettes, ...

Visual Impairments in General

Specific Visual Impairments

Educational Resources (Including for Kids)


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