JAA Manual of Aviation Medicine

aviation ophthalmology

Colour Vision

Colour Vision aids in detection and identification of objects in the visual scene. Colour is a quality of the mind given to light of a certain spectral composition in a certain state of ocular adaptation. Psychologically, colour can be described by the three qualities hue, saturation and lightness. These have psychophysical counterparts which can be given colorimetric figures in order to characterize the colour in question.

The early use of colour in sea and land traffic was limited by the techniques available to produce light of sufficient saturation and brightness. Therefore only red, yellow (white) and green signals were adopted and their significance is today so deeply rooted in us all they cannot be exchanged. This is unfortunate, since all people do not perceive colour in the same way and exactly these hues give rise to separation difficulties. Although attempts have been made to minimize the use of colour contrast as the sole characteristic of a stimulus, colour are still used to such an extent that some applicants have to be rejected for safety reasons.

Colour vision physiology

The person with a normal colour sense is called a normal trichromat. This person perceives as light electromagnetic radiation of wavelengths between about 400nm (violet) and 700nm (deep red). Maximum spectral sensitivity is at 555nm (yellow-green). A normal trichromat can discriminate between more that 100 hues in the spectrum; the wavelength discrimination varies somewhat in the spectrum. By adding saturation and lightness differences, several hundred thousand different colour can be discriminated. Stimulus variable which affect colour perception are the angular subtense, duration and brightness.

Normal colour vision is made possible by the presence of three different kinds of cones with each one light absorbing pigment.

Colour vision deficiencies

The congenital, hereditary colour vision deficiencies are of different quality and severity. More than 99.9% of them affect the perception of red, red-purple, green and blue-green. 

Monochromacy of achromatopsia means total absence of colour perception. These rare disorders exist in several forms; the most common is combined with low visual acuity, nystagmus and photophobia.

For a person with dichromacy, some hues are completely desaturated and impossible to distinguish from each other and from neutral grey. Wavelength discrimination is severely disturbed.

 Anomalous trichromacy is a less pronounced defect. Subjects with such an anomaly show, compared to normal, increased thresholds for saturation and wavelength discrimination in certain spectral regions.

 Congenital dichromacies and anomalous trichromacies exist in the following forms:

 The red-green defects are inherited as X-linked recessive disorders and are fairly common: 8-10% in Caucasian men, in women about 0.5%. In men, deuteranomaly is most frequent: about 5% the other three red-green defects affect approximately 1%. The yellow-blue defects are rare, about 1 in 50.000.

 Protanopia and deuteranopia have been shown to be caused by the absence of one of the retinal cone pigments: the presence of not more that two pigments only makes possible the perception of two hues. The lack of the long wavelength sensitive pigment in protanopes results in lost sensitivity to deep red light which is perceived as black. In protanomaly and deuteranomaly, an abnormal pigment has replaced a normal one. Also the protanomalous subjects have reduced sensitivity to long wavelength light. The reason for the tritan defects is supposed to be alterations in the short wavelength sensitive pigment.

 Protanopes confuses red and blue-green, deuteranopes green and red-purple. In protanomaly and deuteranomaly, separation difficulties arise with the same hues, although only those of low severity and some are almost as pronounces as the dichromacies: extreme anomalous trichromacy.

 Borderline cases between normal trichromacy and anomalous trichromacy are pigment amblyopia and colour asthenopia. The former confuse pigment colours, e.g. those on pseudo-isochromatic charts but pass other colour vision tests. Colour asthenopia is essentially an increased “fatigue” to spectral lights. These and other borderline cases are usually considered a normal in practice.

 The term colour asthenopia describes a colour weakness which can be demonstrated in individuals who are not able to give a perfectly correct score when confronted with a set of pseudo-isochromatic plates, although they are able to match within the normal range on the Nagel Anomaloscope. However, this group often shows a widespread range of anomaloscopic acceptances. If it is a matter of the definition specified in the primary screening method as to weather this group should be classified as normal of defective. The knowledge of the practical performance of colour asthenopes in aviation, however, is obscure. Thus, many States simply define a colour-normal as a subject who can obtain a correct or a certain score with an authorized set of pseudo-isochromatic plates. [ICAO Manual of Civil Medicine]

 The rare congenital tritan deficiencies cause confusion between violet and yellow colours. 

Congenital defects are unaltered with age and cannot, contrary to what is sometimes claimed, be treated in any way. Tinted filters, e.g. the so-called X-chrom lens, make possible a better discrimination of some confusion colours but do not improve colour perception. Applicants passing a colour test by the use of such a device are not “colour safe”.

 Acquired colour vision deficiencies arise from diseases in the eye or the visual pathways. An ocular disorder most often gives rise to a yellow-blue defect. It is generally combined with other visual disturbances like reduced visual acuity or visual field defects and the ocular damage is thus overt. Of greater interest is the red-green deficiency caused by an optic nerve lesion. Such a problem invariably accompanies an optic neuritis and may result in difficulties in identifying colour signals although the visual acuity is normal.

 With increasing age and density of the yellow lens pigment, a slight degree of tritanomaly follows.

Practical considerations

 In aviation, colour is used in signal, instruments, signs and print. The coloured object can be self-luminous (lamp + filter, LED or colour phosphor) or can be produced with pigment colours. In the latter case, the colour appearance depends on the character of the illumination. In some cases, a luminous contrast to the background is also present, and the identification of the colour is of assistance but not necessary to read the information. In other cases, e.g. navigation light, the hue is the only clue to the correct identification. With the technical evolution, some colour signals have lost much of their significance because the message they convey has been taken over by other instruments. At the same time, a number of new colour applications have been introduced. The most recent is the colour display which presents data in a number of different hues and saturations steps. Luminous contrast is not always present and it seems most possible that the displays can give rise to practical difficulties for colour defectives. The evolution is very rapid, and the colour characteristics of the displays largely unknown. These instruments have created a serious problem which, until more knowledge is attained, necessitates a much less liberal attitude to colour vision abnormality. 

As regards to the use of colour in civil aviation, some information is used only at night ant other only in more advance aviation. It is not self-evident that normal trichromacy is a necessity in all situations. Bye setting standards for the chromaticity of various colours, an attempt has been made to make their identification easier for air personnel with a colour deficiency.

 The mere qualitative diagnosis of the colour vision deficiency is not sufficient, because the colour discrimination varies considerably between individuals with the same type of defect.

 A practical colour vision test certainly has the highest validity but only for the conditions present at the test. It has been the practice of some countries to waive applicants with simple deuteranomaly who readily pass lantern tests. In some cases, a practical test with a signal gun has been decisive. Even individuals with rather outspoken defects may pass this test which does not signify whether the applicant normally perceives other less conspicuous signals. 

In order to assess the fitness of an applicant with a colour vision deficiency with regard to a possible waiver, it is necessary to have at hand the results of a battery of colour vision tests. As many different aspects of colour vision as possible should be examined.

Tests of colour vision

 Colour vision tests are produced to identify individuals with colour vision deficiencies, to classify them, and to screen those with a mild defect from those with more severe defect.

 The most readily available tests for screening are the pseudo-isochromatic plates. Most of them are made only for detection of red-green deficiencies; some series have plates which enable a classification and graduation of severity. The different series perform the screening task more or less well; among well-known series are those of Ishihara, Dvorine, Stilling-Velhagen, and Boström-Kugelberg. These tests effectively separate normal from colour defectives.

There are, however, subjects who fail only a few plates and in these cases a definite diagnosis is only possible with the aid of an anomaloscope.

 There is a weak correlation between the number of failed charts and the severity of the defect, dichromats usually fail more plates than do anomalous trichromats. The classification of protans and deutans is not always possible with the charts. The American Optical Hardy-Rittler plates are especially designed for qualitative and quantitative diagnosis. These tasks are better fulfilled with this series than with any other plates. Unfortunately, this test, which is also excellent for testing acquired defects, is no longer available.

 Testing with pseudo-isochromatic plates should be performed according to the instructions given by each test. It is important that the quality of the illumination is correct: either northern daylight or an artificial daylight source should be used. Ordinary incandescent lamps or fluorescent tubes make these tests easier to pass, especially to deuteranomals. The daylight source should give an illumination equivalent to the standard illuminants ‘C’ or ‘D’ of CIE (Commission Internationale de l’Eclairage). The plates should be shown at right angles to the visual axis of the applicant, at the correct distance and for the time specified in the test. The applicant should not wear tinted glasses. The number of failed plates serves to classify the subject as normal, defective, or ‘doubtful’ according to the specifications of the test.

 In the assortment tests, the subject is asked to arrange a number of coloured chips or to separate them into coloured or neutrally tinted. Of these tests, the Farnsworth Panel D-15 effectively parts subjects with minor defects from those with more severe defects. The test is easily performed and evaluated and, when failed, gives a qualitative diagnosis. It may be used as a valuable adjunct to other tests.

 The exact qualitative and quantitative diagnosis is give by the anomaloscope. Looking into this instrument, the subject compares two juxtaposed fields and judges when they appear identical. Red-green deficiencies are studied with the ‘Rayleigh match’ where one field is yellow and the other ad additive mixture of red and green. The examination demands a thorough knowledge of colour vision physiology and large experience. Most widely known and used is the Nagel anomaloscope, but equally efficient other apparatuses have recently been put on the market (e.g. Heidelberg Anomaloscope from Oculus). All dichromates should be rejected as they are colour unsafe. When examining an anomalous trichromate with Nagel anomaloscope, different scale readings are used to express the result. The colour matching range is defined as the difference between the maximum and the minimum scale reading accepted by the examinee as identical to the test colour. If the colour matching range exceeds four scale units, the applicant must be considered colour unsafe. The relation between the mean scale reading for colour identify and the standard scale reading is expressed by an anomaly quotient. This quotient has diagnostic value, but provides no guidance in assessing colour safety. The anomaly quotient per se is thus irrelevant in the assessment of an applicant’s fitness for flying.

The information provided herein has been gathered from:

-JAA Manual of Aviation Medicine

-ICAO General Provisions for licensing