Understanding the CAD Test: A Guide to Colour Vision Assessment

The Colour Assessment and Diagnosis (CAD) test is a modern computer-based colour vision test developed at City University London, adopted internationally for occupational colour vision assessment (aviation, maritime, military, some medical roles). Unlike the older Ishihara plates (colour dot patterns showing hidden numbers) that only give pass/fail results, CAD: (1) Quantifies the SEVERITY of colour deficiency (mild, moderate, severe) using ‘standard normal units’ (SN units); (2) Distinguishes between protan (red weakness), deutan (green weakness) and tritan (blue weakness) types; (3) Uses moving coloured stimuli on a grey background — harder to guess; (4) Provides pass/fail against occupation-specific thresholds; (5) Is standardised and reproducible across centres. Aviation authorities (DGCA in India, ICAO globally) accept CAD test for pilot medical assessment when Ishihara fails — allows some mildly colour-deficient candidates to still qualify as commercial pilots. Availability is limited — the CAD test is offered at specialised aviation medical centres and some tertiary eye hospitals, not general ophthalmology practices; DGCA-approved aviation medical examiners are the reliable source list.

Colour vision deficiency (often called 'colour blindness') has a strong genetic pattern. Red-green colour deficiency affects roughly 5-8% of men (it's X-linked, inherited from the mother) and only 0.4-0.5% of women (who need both X chromosomes affected). Blue-yellow (tritan) deficiency is very rare and usually acquired later — from diabetes, glaucoma, or optic nerve disease. Signs suggesting a colour vision issue: difficulty distinguishing red/green traffic signals, trouble matching clothing colours, difficulty seeing changes in food browning during cooking, confusion about red/green marks on maps, or a family history of colour blindness on the mother's side. Testing options: Ishihara plates are the most common screening test, widely available at optometrists. Farnsworth D-15 arrangement test is used at some ophthalmology centres. Anomaloscope testing is the gold standard for genetic colour blindness diagnosis, available at university hospitals. CAD is used for occupational assessment as described above. Testing is best done in an optometry clinic under proper daylight illumination. Once diagnosed, genetic deficiency is permanent — no cure exists — but EnChroma-type filter glasses can enhance colour perception for some types, useful for hobbies but they don't help pass professional colour tests.

Occupations with colour vision requirements in India: (1) Commercial pilot (DGCA) — historically strict; CAD test allows some mild colour deficiency to qualify; military aviation stricter; (2) Indian Air Force pilot — stricter than commercial; typically excludes any colour deficiency; (3) Indian Railways loco pilot, guard, station master — signal identification critical; strict standards; (4) Merchant Navy — international standards; CAD test increasingly used; (5) Indian Armed Forces (Army, Navy, Air Force) — various roles have different requirements; (6) Fire service, some police roles; (7) Electrician (wire colour coding safety); (8) Certain medical specialties — histopathology, dermatology (colour-based diagnosis); most medical roles accessible; (9) Some art/design/photography roles — self-selection typical; (10) Chemistry lab work (colour reactions). Testing before career commitment is prudent — a child with confirmed colour deficiency can plan alternate careers rather than discovering barriers years into training. School-age colour vision screening should be routine at first eye exam; parents with family history should test children by age 5-6. No treatment available for genetic colour blindness — accept and adapt is the approach. Acquired colour deficiency (from diabetes, glaucoma, medications, MS, optic neuritis) may improve if underlying cause treated — needs ophthalmologist workup.