COLOUR VISION
Defective Colour Vision in Orienteers is it an issue? Barbara Junghans, Garingal Orienteers
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ESIGNING any colour coding system, say for Orienteering maps, requires an understanding of how humans see colour. It is well known that some people are colour ‘blind’, or really, colour vision ‘impaired’. Total colour blindness is extremely rare (everything seen only in various shades of grey). Typically, colour vision impairment is only partial and there is a vast range in the severity, with by far the majority having only a mild problem. For years, some event organisers have been aware of a handful of orienteers competing at top masters levels who have now been classified by the author as severely colour vision impaired. To discover the challenges they face with the current internationally accepted colour coding scheme, read on.
What part of the eyeball ‘sees’ colour? The retina, the light-sensitive layer coating the inside of the back of the eye, has two key types of cells or photoreceptors that respond to light stimulation: the rods and cones. It is only the cones that are responsive to coloured light, and there are three subclasses of cones scattered across the back of the retina. One subclass is only receptive to light of red wavelengths, another to blue, and the remaining class is only responsive to green. It is amazing that by simply varying the intensity or the ratio of the red to green to blue light ultimately results in the rainbow of perceptions enjoy by most of us. As there are three subclasses of cone cells, it can be appreciated that there might be three ways in which colour vision can go awry. If one class of cone photoreceptors is faulty, the blend of information summated from neighbouring cones to be passed up the nerve pathway to the brain will be distorted, and the colour seen will not be the same as the colour of the original object.
What types of colour defects exist? Hence, if we also count the normal situation, there are 4 different categories of colour vision: 1. Normal: (all red wave length-receptive, green wave lengthreceptive and blue wave length-receptive photoreceptors function normally). 2. Protan: a red/green defect (the red-receptive photoreceptors are faulty or unreceptive) that is uncommon (~1-2% of men, rare in women). The defect is typically congenital and without any
other associated ocular consequences, but may be acquired from ocular or other nerve diseases. 3. Deutan: a red/green defect (the green-receptive photoreceptors are faulty or unreceptive) that is relatively common (~6% of men, 0.5% women). The defect is typically congenital and without any other associated ocular consequences, but may be acquired from ocular or other nerve diseases. 4. Tritan: a yellow/blue defect (the blue-receptive photoreceptors are faulty or inactive) that is very rare except in some eye diseases. This condition is more common in ocular diseases affecting predominantly the elderly, e.g cataract or age-related macular degeneration. However, visual acuity, and hence the ability to see fine detail is typically also severely affected, probably limiting reading Orienteering maps. The 3 classes of persons who have abnormal colour vision (loosely known as protans, deutans and tritans) exhibit a wide range of severity from very mild (referred to as having protanomaly, dueteranomaly, tritanomaly, or being protanomalous, deuteranomalous, tritanomalous respectively) to severe (referred to as having protanopia, deuteranopia, tritanopia or being a protanope, deuteranope or tritanope respectively). Persons with ‘anomalous’ colour vision only confuse colours in the lighter pastel shades, and are actually able to reliably identify the more saturated or intense versions of the colours for which they confuse the paler correlates. On the other hand, persons with the ‘opia’ defect have problems even when colours are relatively saturated. Hence, any two persons with a colour vision defect, even within the one class, will likely have differing difficulties interpreting colour coding.
How do persons with defective colour vision see the world? Figure 1 shows firstly how a person with normal vision sees the entire range of colours, and then, how the three classes of colour impaired people might see those same colours. This is possible by submitting the rainbow image to the online service www. vischeck.com. Vischeck will convert any image you upload so you can see how your image is believed to appear to persons with any one of the three classes of colour vision defect. Or, you can download a plug-in ‘filter’ if you have Adobe Photoshop installed on your computer and then apply this filter yourself to any image
Figure 1. On the left shows how a person with normal colour vision sees a colour palette. In order, progressing rightwards, are simulations of how protanopes, deuteranopes and tritanopes are believed to see the same palette respectively (according to Vischeck software). 36 THE AUSTRALIAN ORIENTEER DECEMBER 2009
