Colour vision guidance material GM

(a) Applicants shall be assessed as unfit, where they cannot demonstrate their ability to readily perceive the colour that are necessary for the safe exercise of the privileges of the licence.

(b) Examination and assessment

(1) Applicants shall be subjected to the Ishihara test for the initial issue of a medical certificate. Applicants who pass that test may be assessed as fit.

(2) For a class 1 medical certificate:

(i) Applicants who do not pass the Ishihara test shall be referred to the medical assessor of the licensing authority and shall undergo further colour perception testing to establish whether they are colour safe.

(ii) Applicants shall be normal trichromats or shall be colour safe.

(iii) Applicants who fail further colour perception testing shall be assessed as unfit.

(3) For a class 2 medical certificate:

(i) Applicants who do not pass the Ishihara test shall undergo further colour perception testing to establish whether they are colour safe.

(ii) Applicants who do not have satisfactory perception of colours shall be limited to exercising the privileges of the applicable licence in daytime only.

UK Alternative AMC to MED B.075 Colour vision (Class 1 and 2)

1. At revalidation colour vision should be tested on clinical indication.
2. The Ishihara test (24 plate version) is considered passed if the first 15 plates, presented in a random order, are identified without error.
3. Those failing the Ishihara test should be examined by:
   1. Anomaloscopy (Nagel or equivalent). This test is considered passed if the colour match shows normal trichromacy, i.e. a matching midpoint of 38-42 scale units and the matching range is 4 scale units or less; or by
   2. Colour Assessment and Diagnosis (CAD) Test. This is considered passed if the threshold is less than 6 SU for deutan deficiency, or less than 12 SU for protan deficiency. A threshold greater than 2SU for tritan deficiency indicates an acquired cause which should be investigated.

The Ishihara test is conducted using the first 15 plates of the 24 plate book presented to the applicant in a random order. Otherwise the test is conducted as per the manufacturer’s instructions: test distance 75cm with the plane of the plates at right angles to the line of vision under daylight or daylight simulated light (usually colour temperature around 6500K) allowing 3 seconds per plate for response. Ishihara plates should be updated periodically or if showing any signs of fading.

The Alternative Means of Compliance submitted by the UK CAA can be accessed below this table.
The UK CAA does not accept lantern testing as evidence of being colour safe.

Anomaloscopy (Nagel or equivalent) may be considered provided the full protocol used for testing is enclosed with the result. This test is only considered passed if the colour match shows normal trichromacy, i.e. a matching midpoint of 38-42 scale units and the matching range is 4 scale units or less. Tests that have not been performed in the UK must have been conducted by an Aeromedical Centre in another Competent Authority. Applicants failing the Anomaloscope test may undergo the CAD test.

All applicants in the UK for advanced colour vision testing should be tested using the CAD test conducted under CAA protocols (available on request).

The CAD test will only pass as colour safe, those individuals who perform as well as individuals with colour vision in the normal range on the most difficult aviation colour vision tasks. See CAA papers:

• CAA Paper 2006/04 Part 1 Minimum Colour Vision Requirements for Flight Crew: The Use of Colour Signals and the Assessment of Colour Vision Requirements in Aviation
• CAA Paper 2006/04 Part 2 Minimum Colour Vision requirements for Professional Flight Crew: Task Analysis
• CAA Paper 2009/04 Minimum Colour Vision Requirements for Professional Flight Crew: Recommendations for new colour vision standards.

For further additional reading, see CAP 1429 Analysis of European colour vision certification requirements for air traffic control officers.

There is a wide diversity of colour testing methods employed and standards used for the assessment of flight crew minimum colour vision requirements throughout the world, including amongst European States.

Ishihara (IH) tests

Colour vision requirements and assessment of 'colour safety' based on Ishihara (IH) tests have the following problems:

1. Inconsistent application of the manufacturers' instructions / CIE protocols for the conduct of the tests by the test operator/institution. 
   
   
2. Variation in the lighting conditions used to view the test plates (illuminant spectral power distribution and illuminance level).
   
   
3. Use of different test plate editions that are by no means identical, and the current availability of very inexpensive 'Ishihara' test plates sets on the web that may not be genuine.
   
   
4. The possible use of other cues such as the recognition of vertically and/or horizontally arranged dot patterns, or the learning of the order of plates when the presentation sequence is not randomised.
   
   
5. A large proportion of normal trichromats fail the IH plates (various editions) when the protocol requires zero errors for a Pass.  
   
   
6. A large proportion of applicants with congenital colour deficiency (some with severe loss of RG colour vision) that pass with 3 or fewer errors on the 38 plates edition. There is little or no correlation between the applicant's severity of colour vision loss and the number of failed IH test plates. 
   
   
7. When more than three errors are allowed as a pass, some applicants with congenital colour deficiency that pass can have severe loss of colour vision. For example, having a pass standard (e.g. for LAPL) that requires fewer plates to be correctly identified (LAPL 9 of the 15 plates) allows applicants with severe colour deficiency to pass. 

Lantern Tests

Colour vision requirements and assessment of 'colour safety' based on lantern tests have the following problems.

1. Inconsistent application of the manufacturers' instructions for the conduct of the tests by the test operator/institution.
   
   
2. Maintenance and calibration is usually not carried out. Old lanterns are difficult to service and many types are no longer manufactured. 
   
   
3. Applicants can learn the order of the lights presented and use other cues to correctly name the lights, particularly if the starting point and order of presentation are not varied.
   
   
4. The variability in outcome on repeated lantern test protocols is high which results in many false positives and negatives.
   
   
5. Lanterns do not diagnose or quantify either the type or the severity of colour vision loss.
   
   
6. A significant proportion of deutan subjects (in particular) pass lantern tests based on red, green and white lights without guaranteeing minimum colour deficiency.
    
7. Different organisations/states performing the tests and interpreting the results have different definitions of what constitutes a pass.
   
   
8. Many lanterns were not specifically designed for aviation purposes so the colour of the lights used and the intensity do not necessarily represent a proper representation of the coloured signals/ lights used in aviation.

Anomaloscope Tests

Colour vision requirements and assessment of 'colour safety' based on anomaloscope tests (i.e., dichromatic, RG colour matching tests) have the following problems.

1. Inconsistent application of the manufacturers' instructions for the conduct of the tests by the test operator/institution.
   
   
2. Calibration and proper maintenance cannot be demonstrated and 'normal' match parameters are usually needed when the light source is replaced, etc.
   
   
3. There can be substantial differences in testing between anomaloscope type and models,  such as the use of white, interstimulus adapting fields.
   
   
4. Although anomaloscopes (which employ a dichromatic Rayleigh match) distinguish between the type of RG colour deficiency (e.g., protan- vs deutan-like deficiency) the severity of colour vision loss and whether the applicant is 'colour safe' cannot be demonstrated.
   
   
5. Different organisations/states performing the tests and interpreting the results have different definitions of what constitutes a pass. This particularly relates to interpretation of the matching midpoint and the size of the matching range. Applicants with a 'normal' matching mid-point as tested might have a large range, and those with a very abnormal midpoint might have a small matching range, often well within the mean matching range measured in normal trichromats.
   
   
6. Some subjects exhibit 'extreme' anomalous matches that spread over the midpoint measured in normal trichromats. These subjects cannot therefore be diagnosed as either deutan- or protan-like.
   
   
7. A small proportion of subjects exhibit normal Rayleigh matches, but demonstrate significant loss of RG chromatic sensitivity in other tests. The opposite is also the case when subjects with heavily abnormal anomaloscope midpoints exhibit completely normal RG chromatic sensitivity.
   
   
8. Anomaloscopes were designed for clinical diagnostic reasons and not specifically designed for use in aviation to determine whether an individual is colour safe of not. They can determine whether subjects are normal trichromats with a normal matching mid-point and  normal matching range.

The Colour Assessment and Diagnosis (CAD) Test

The Colour Assessment and Diagnosis (CAD) Test provides an accurate and reproducible assessment of an applicant's class of colour vision and severity of RG and YB colour vison loss. The latter can be used to set Pass / Fail limits that do not discriminate against applicants with mild to moderate RG colour deficiency who have been shown to carry out the safety-critical, colour related tasks as well as normal trichromats.  

The CAD test cannot be learnt and there are no cues the applicant could use to pass it. The results reflect only the RG and the YB sensitivity of the eye.  The results are expressed in Standard Normal CAD units (i.e., RG = 1.0 and YB = 1.0) which represent the median RG and YB colour signal strengths for young, healthy normal trichromats. A threshold of 6 units means that the applicant requires 6 times greater colour signal strength than the standard CAD observer. 

Upper limits that describe the binocular and the monocular performance of normal trichromats as a function of age (~ 8 to 85 yrs of age) are incorporated in the test. These are used to screen reliably for normal trichromatic colour vision and also make it possible to detect the presence of retinal or / and systemic diseases that affect vision.  The CAD test can also detect acquired deficiencies, even when acquired loss is present in applicants with congenital RG colour deficiency.

Reference Material

1. CAA Paper 2006/04 Part 1: Minimum Colour Vision Requirements for Flight Crew: The Use of Colour Signals and the Assessment of Colour Vision Requirements in Aviation.
   
   
2. CAA Paper 2006/04 Part 2: Minimum Colour Vision requirements for Professional Flight Crew: Task Analysis
   
   
3. CAA Paper 2009/04: Minimum Colour Vision Requirements for Professional Flight Crew. Recommendations for new colour vision standard 
   
   
4. Barbur JL, Rodriguez-Carmona M, Harlow JA, Mancuso K, Neitz J, Neitz M. A study of unusual Rayleigh matches in deutan deficiency. Vis Neurosci. 2008 May-Jun;25(3):507-16.
   
   
5. Squire TJ, Rodriguez-Carmona M, Evans Adb, Barbur Jl. Color Vision Tests For Aviation: Comparison Of The Anomaloscope And Three Lantern Types. . Aviat Space Environ Med 2005; 76:421-9.
   
   
6. Detailed Interpretation Of The Nagel Anomaloscope.
   
   
7. Cole BL, Vingrys AJ. Who fails lantern tests? Doc Ophthalmol. 1983 May 1;55(3):157-75.
   
   
8. Watson DB. Lack of international uniformity in assessing color vision deficiency in professional pilots. Aviat Space Environ Med. 2014 Feb;85(2):148-59.
   
   
9. Barbur JL, Rodriguez-Carmona M. 'Color vision changes in normal aging'. In: A.J. E, M.D. F, A. F, eds. Handbook of Color Psychology. 1. Cambridge, UK: Cambridge University Press; 2015. p. 180-96.
   
   
10.   Barbur, JL and Rodriguez-Carmona, M. Colour vision requirements in visually demanding occupations Dr Medical Bulleting. 2017,1-27.
    
    
11.   Hovis, JK. Repeatability of the Holmes-Wright Type A Lantern Color Vision Test. Aviat. Space Environ. Med 2008; 79 1028-33.