Colour Blindness: Its not all Black and White

When people hear the term "colour blind" they often assume a world of black and white. However, there is a spectrum of colour deficiencies ranging from a decreased ability to distinguish hues to the complete inability to perceive colour.

The human retina is composed of two types of light-sensitive cells: rods and cones. It is the cones that are responsible for colour vision. The three types of cone cells contain different pigments which are activated when they absorb light energy; each cone being uniquely sensitive to a specific wavelength.

Anomalous trichromacy is the most common type of colour deficiency, affecting approximately 6% of men and 0.4% of woman. Impairment in hue discrimination results when one of the three cones has a decreased sensitivity. These individuals will struggle with discerning between either red-green hues or blue-yellow hues, depending on which cone is affected. Individuals with this diagnosis may not even be aware of their deficiency because it can have such a minimal affect on daily living.

Dichromacy is a more severe deficiency where one of the three cones is totally absent. Colour vision is reduced to the point where these individuals see no perceptible difference between red, orange, yellow, and green. All of these colours appear identical to the 2.4% of males and 0.03% of females who fall into this category.

Finally, the only true "colour blindess" is monochromacy where two or three of the cones are totally absent. There is a complete inability to distinguish colours and therefore these individuals perceive only variations in brightness. This is the least common type of colour deficiency affecting 0.00001% of the population.


Dr. Kristyn Pozzer


Millcroft Shopping Centre
2080 Appleby Line, Unit E6
Burlington, Ontario, L7L6M6
Tel: (905) 319-1066