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Colour Blindness

Mon, 07/16/2012 - 14:42 -- admin

Colour blindness is the inability to see colours, either whole or in part.

Colour Blindness

What is colour blindness?

As the name suggests, colour blindness is the inability to see colours, either whole or in part. Colour vision is made possible by the presence of cones
in the eye. The Cones are found on the retina. There are 6 to 7 million cones divided into “red” cones, Green cones and blue cones providing the eye's
colour sensitivity. The cones are maximally concentrated in the fovea centralis (located in the macula), and is a 0.3 mm diameter rod-free area with very
thin, densely packed cones.

There are 7 types of colour deficiency due to cone abnormalities. People with normal colour vision are called "trichromats" because they require three primaries
to match any arbitrary sample. The trichromatic eye has three cone types, each containing a photopigment which responds to a restricted range of wavelengths.
The major colour deficiencies are abnormalities in the way cone pigment is distributed in the cones.

What are the types of inherited colour vision defects?

There are three groups of inherited colour vision defects: monochromacy, dichromacy and anomalous trichromacy. The last two groups are subdivided into red-green
and blue-yellow types of defects. Inherited red-green colour vision defects are more common in males (1 to 8 percent depending on race) than in females
(about 0.4 percent). Inherited blue-yellow defects are rare in either sex.

Monochromacy: Rod monochromats, or complete achromats, are truly "colour blind" since they cannot distinguish any hues (eg, blue, green, yellow and red).
They see only different degrees of lightness. For them, the world appears to be shades of gray, black and white. They also have poor visual acuity, aversion
to bright light and nystagmus (an involuntary, rapid movement of the eyes). Rod monochromacy is a genetic defect that is inherited from both parents.

Dichromacy is a less severe form of colour defect than monochromacy. Dichromats can tell some hues apart. Dichromacy is divided into three types: protanopia,
deuteranopia and tritanopia.

Protanopia and deuteranopia are red-green defects. Persons with red-green defects have difficulty distinguishing between reds, greens and yellows but can
discriminate between blues and yellows. Protanopes often can name red and green correctly because green looks lighter to them than red.

Males have red-green defects if they inherit a defective gene from their mother. Affected males pass the defective gene to all of their daughters but none
of their sons. Females who inherit only one defective gene are carriers of that gene. Females who inherit the gene for red-green defect from both parents
are affected.

Hereditary tritanopia is a blue-yellow defect. Persons with blue-yellow defects cannot see the difference between blues and yellows but can distinguish
between reds and greens. Tritanopia is somewhat rare and occurs equally in both sexes. Triatnopes usually have fewer problems in performing everyday tasks
than do those with red-green dichromacy.

Anomalous Trichromacy: The ability of anomalous trichromats to distinguish between hues is better than dichromats but still not normal. Red-green anomalous
trichromacy is subdivided into protanomaly and deuteranomaly. Both types are inherited in the same way as for red-green dichromacy. The severity can range
from mild to extreme. Some persons with the mildest forms may not even realise their colour vision is abnormal.

A third type of anomalous trichromacy is tritanomaly. This condition is more often acquired than inherited.

Do the elderly see colours differently from the young?

Yes. In fact it is somewhat surprising that designers worry so much about colour blindness, which is 1% (dichromatic) of the population, while they often
ignore the much larger group of elderly, who almost always exhibit visual deficits. The elderly (65+) are 12% of the population and those just beginning
to experience visual decline (50+) are 25%. Moreover, the elderly population is booming and expected to more than double in the next 30 years.

Vision declines with age in several ways, but the most relevant for colour design is the yellowing and darkening of the lens and cornea and the shrinking
pupil size. Yellowing selectively blocks short wavelength light, so blues look darker. Moreover, the elderly have difficulty discriminating colours which
differ primarily in their blue content: blue-white, blue-gray, green-blue green, red-purple, etcetra.

Aging also reduces the amount of light reaching the photoreceptors compared to the young viewer. All colours will be dimmer and visual resolution lower.
For example, a moderately bright yellow may appear brownish and dimmer blues will appear black. When designing for the elderly, use bright colours and
make sure that brightness contrast is especially high (and text larger) to help compensate for acuity loss.

Can colour vision defects be cured?

No cure exists for inherited colour vision defects since they are caused by missing or incorrect visual pigments. Acquired colour vision defects can be
corrected sometimes if the underlying cause can be treated.

Special aids have been developed to help persons with colour vision defects distinguish some of the colours that cause them trouble. These devices include
specially tinted contact lenses and eyeglasses. However, these aids do not provide normal colour vision and therefore should be used with caution.

How are colour vision defects diagnosed?

Specialised colour vision tests can easily detect colour vision defects. Pseudoisochromatic plate tests are commonly used to screen for inherited colour
vision defects. In this group of tests, a pattern of coloured dots forms a number or letter against a background of other coloured dots. Persons with normal
colour vision can discern these patterns but those with colour defects cannot.

Pseudoisochromatic Plate
As seen by normal eye
As seen by red-green colour blind eye
Pseudoisochromatic Plate 1
Pseudoisochromatic Plate 2
Pseudoisochromatic Plate 3
Pseudoisochromatic Plate 4
Pseudoisochromatic Plate 5

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