Colour blindness refers to an inability to distinguish between some of the colours that others can perceive as different. It is often the result of a genetic deficiency, but can also result from eye, nerve, or brain damage, or exposure to particular chemicals.
As is briefly explained in How the Eye Works, the retina contains primarily two types of cells – rods and cones. Cones are responsible for colour perception. There are three types of cones, and each is sensitive to different wavelengths. The receptors are often referred to as – green, red, and blue,- though this distinction is not entirely accurate.
Genetic colour blindness is typically caused by a mutation on the X-chromosome, though the mapping of the human genome revealed that mutations on 19 different chromosomes can all cause colour blindness. Colour blindness can be present from birth, or appear later in life, and it may be stationary or progressive.
8% of males and 0.5% of females are colour blind in some way. This disproportionate amount of males can be attributed to the fact that males have only one X-chromosome. If this chromosome is mutated they will be colour blind, whereas for females, both of their two X-chromosomes must be mutated in order for them to be colour blind. Beyond genetic causes, retinal or brain damage from Shaken Baby Syndrome can induce colour blindness, as well as other trauma or accidents that produce swelling in the occipital lobe. Exposure of the retina to ultraviolet light can also cause colour blindness.
Colour blindness is typically classified as either acquired or inherited. Inherited, or congenital, colour blindness can come in three forms – monochromacy, dichromacy, or anomalous trichromacy. Monochromacy is total colour blindness, where two or three of the cone pigments are missing. Dichromacy is less severe, where only one of the three cone colour mechanisms is not properly functioning and colour is reduced to two dimensions. Protanopia is caused by the absence of red photoreceptors, deuteranopia – the most common form of dichromacy – is caused by the absence of green photoreceptors, leading to a difficulty and discriminating red and green, and tritanopia – the rarest of the three – is caused by the absence of blue retinal receptors. Finally, anomalous trichromacy occurs when one of the cone types is altered, which results in an impairment, but not a loss, of three-dimensional colour vision.
The most common test used to diagnose colour blindness is the Ishihara colour test, which involves plates of coloured spots. It has, however, been criticized for only using numerals, which makes it difficult to diagnose young children. As a result, alternative tests have been developed which utilize simple shapes such as squares and circles.
Colour blindness may affect one’s ability to work in particular occupations. For instance, an 1875 train crash in Lagerlunda, Sweden was attributed to the colour blindness of the engineer, which led to widespread testing of railway and shipping employees. Some countries, including Romania and Turkey, refuse to grant driver’s licences to those who are colour blind. The justification for this is that colour blind individuals cannot identify colour-related traffic signals. Pilots, likewise, may not be granted licenses, or may be granted restricted licenses if they are colour blind. Restrictions may include prohibition of night flying, and flying by colour signals, which in turn will make it virtually impossible for these individuals to work for an airline.