The following are the requirements for the medical certification of aircrew, including guidance material issued by the UK CAA Medical
Department in relation to colour vision.
2) Applicants who fail to pass in the
Ishihara test shall undergo further colour perception testing to establish
whether they are colour safe.
(c) Those failing the Ishihara test should be examined
(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 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
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
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:
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.
Colour vision requirements and assessment of 'colour safety' based on Ishihara (IH) tests have the following problems:
Colour vision requirements and assessment of 'colour safety' based on lantern tests have the following problems.
Colour vision requirements and assessment of 'colour safety' based on anomaloscope tests (i.e., dichromatic, RG colour matching tests) have the following problems.
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.
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