So let us firstly look at the fundamentals of colour vision and the abnormalities thereof. In later follow-up articles we shall examine the role of colour vision in aviation tasks.
A person with normal colour vision possesses three distinct colour receptive pigments in the retina-of the eye. The first is responsive to mainly the red end of the visible spectrum, the second to mainly the green portion of the spectrum, and the third to mainly the blue end of the spectrum. The sensitivities of the three pigments overlap to a large degree so that when a "colour normal" individual is exposed to a light of a given colour each of the three pigments will be stimulated to some degree. Such an individual will possess the ability to discriminate between very large numbers of different colours. This is called normal TRICHROMATIC colour vision.
The genetic coding for the body's production of the first and the second pigments is carried on the X chromosome, which also happens to be one of the chromosomes that determine the sex of the individual the other being the Y chromosome. Males have one X and one Y chromosome, while females have two X chromosomes in each living cell of their body. The code for the third pigment is carried on a non-sex (autosomal) chromosome. It has recently been determined that the genetic codes (or "genes") for the first and second pigments actually lie next to each other on the X chromosome and that they are structurally very similar.
When an individual has an abnormality in the structure of the responsible gene for one or other of the colour pigments that individual will have some degree of abnormal sensitivity to colour. The exact loss of sensitivity will depend on how "abnormal" the gene is and on which gene is involved. If the gene is that one whose role is the production of the first (red sensitive) pigment, that individual will be relatively insensitive to light in the red end of the spectrum and this puts the person into the PROTAN group. If the abnormality is in the gene for the second (green sensitive) pigment, the individual is a DEUTAN, and will have reduced sensitivity to colours in the green part of the spectrum. For the third gene the associated abnormality makes the individual a TRITAN who will have reduced sensitivity to blue colours. Tritans form a rare group and will not be discussed in any detail.
In each of these three groups there is a wide range of severity of the defective function of the appropriate pigment involved. So, for example, Protans range from the very mild Protanomal through to the severe Protanomal. Where there is no detectable red sensitive pigment function that person is called a Protanope. The same applies to the other two pigments, resulting in the following classification of colour vision abnormalities:
PROTANS: Protanomaly (mild to severe), Protanopia
DEUTANS: Deuteranomaly (mild to severe), Deuteranopia
TRITANS: Tritanomaly (mild to severe), Tritanopia
The vast majority of colour defective pilots belong to the PROTAN and DEUTAN groups and by far, most of them are males. Together, these two groups account for almost TEN percent of males in Caucasian populations. Deutan and Protan males pass on the abnormal genes to their daughters, who in turn transmit the gene to their
sons. Females are affected far less frequently because they would need to carry two abnormal colour pigment genes (one on each of their two X chromosomes) before the condition could be “expressed”.
If you are one who has failed the colour vision tests in your pilot (or other) medical examination, I suggest it should be of some interest to you to find out the type of colour vision abnormality that you have. As a rough guide, if you think the red and amber lights on traffic signals look very similar at a distance and if the green signal looks almost white from a distance, then you are probably a DEUTAN. If on the other hand the red looks much darker than the amber signal you are probably a PROTAN. There are, however, sophisticated tests available now to accurately diagnose the type and severity of colour vision defect that you have and I strongly recommend you should seek to have these done so that you are fully in the picture, so to speak.
The reason for this is that there is a very big difference between the protan and deutan group in more than their colour discrimination abilities. The protan group have an additional problem called "reduced luminosity". This means that all protans, regardless of their severity, have reduced ability to see not only the "redness" of a light, but to even SEE the light. In effect, a red light seen by a "colour normal" at, say, two miles may be invisible to a protan until the light is very much closer. This problem of reduced luminosity is not found in the DEUTAN group. Reduced luminosity, as such, is a totally different problem from that of reduced colour discrimination and obviously places the Protan group in a somewhat more difficult position than the Deutan group, with regard to the colour vision in aviation debate.
Next month I will be discussing in some detail the aviation environment as it relates to the uses of colour in various aircraft and airport displays. Following that I will relate some of the intricate manoeuvres employed by the Department of Aviation over the years to cloud the issue of colour vision and I will report on progress of my own appeal to the Administrative Appeals Tribunal to have night flying restrictions lifted from my license. This appeal is currently being fought by the Department with "tooth and nail" so to speak, but on very shaky ground, I might tell you. Anyhow, more on that at a later date.