Colour Vision Testing
On this page
Direct answer
Pseudoisochromatic plates hide their numbers in a mist of equal-luminance dots, which is why Ishihara's book screens red-green defects so well — and why it deliberately screens nothing else: it is a red-green test built for congenital defects, wrong for blue-yellow loss. Classification then climbs a ladder: D-15 arrangement for significant defects, FM 100-hue for quantitative scoring, and the Nagel anomaloscope as gold standard. Congenital red-green defects are X-linked and overwhelmingly male — deuteranomaly alone affects about 5 per cent of men — while acquired defects follow Kollner's rule: retinal disease takes blue-yellow first, optic nerve disease red-green.
What you must remember
- Trichromatic basis: three cone populations — L (560 nm), M (530 nm) and S (420 nm); Young–Helmholtz trichromacy with opponent processing explains hue perception.
- Congenital taxonomy: protan (L-cone), deutan (M-cone), tritan (S-cone), each as anopia (dichromat) or anomaly (shifted trichromat); deuteranomaly is the commonest defect, about 5 per cent of males.
- Inheritance: red-green defects X-linked recessive, female carriers usually unaffected; tritan rare, autosomal dominant; congenital defects are bilateral, symmetric and lifelong.
- Acquired defects: often asymmetric and progressive; Kollner's rule — outer retinal disease (maculopathy, diabetic retinopathy) loses blue-yellow early, optic nerve and media disease lose red-green; glaucoma is the known early-tritan exception from nerve disease.
- Ishihara: full 38-plate edition; plate 1 is the demonstration number every defective eye reads (malingering check); plates 26–38 separate protan from deutan; good daylight at 50–75 cm, about 3–4 seconds per plate; failure commonly 3 or more errors on screening plates.
- Farnsworth D-15: 15 hues plus a fixed pilot cap; errors plot as lines along protan, deutan or tritan axes on the score chart — a pass/fail grading of marked defects.
- FM 100-hue: 85 caps in four boxes, total error score with diagnostic fan patterns — quantitative grading and monitoring.
- Nagel anomaloscope: gold standard — the Rayleigh match of a red-plus-green mixture to yellow classifies protan/deutan and grades the anomalous range.
- Lantern tests judge occupational signal recognition; Indian Railways medical boards grade colour perception (CP categories) for safety-critical posts such as drivers.
A failed plate at a railway recruitment exam
A 22-year-old arrives with a form: he failed Ishihara screening for a locomotive post. The job is to characterise, not overturn, the screen. Conditions are corrected first — daylight-equivalent illumination, correct distance, brisk exposure — because dim light manufactures failures. He again misses multiple screening plates, reads plate 1 correctly (literacy and effort are not the issue), and the diagnostic plates tilt protan. A D-15 arrangement follows: his cap sequence crosses the protan axis. An anomaloscope or the board's lantern test completes the picture, and the report states defect class and severity in the terms the railway rules use — grading exists so borderline cases are classified by defined tests, not examiner mood.
Contrast the second visitor: a 45-year-old diabetic newly confused by blue traffic signals. His Ishihara could pass, because his loss is early tritan from outer retinal disease, exactly as Kollner's rule predicts. Detection needs a blue-yellow capable test — D-15 with tritan scoring, FM 100-hue — and the finding triggers fundus referral. The battery must match the defect class suspected.
Where students slip
The over-learned error is treating Ishihara as a general colour test: asked how to screen an acquired blue-yellow defect, "Ishihara" is wrong, because the book is engineered for congenital red-green loss. Confusion follows in plate arithmetic — plate 1 is the demonstration number every defective eye reads, and plates 26–38 are the protan-versus-deutan diagnostic section. Candidates also swap D-15 and FM 100-hue roles: D-15 is a rapid grading of marked defects; the 100-hue is the quantitative, monitorable score. Kollner's rule gets reversed (retina blue-yellow, nerve red-green), and examiners relish the glaucoma exception — an optic nerve disease that begins with tritan thresholds. Finally, the anomaloscope is the gold standard — writing "Ishihara is the gold standard" surrenders the whole question.
Frequently asked questions
Which is the commonest congenital colour defect and its prevalence?
Deuteranomaly, an X-linked red-green defect, affecting about 5 per cent of males and roughly 0.4 per cent of females.
What is Kollner's rule for acquired defects?
Outer retinal disease (maculopathy, diabetic retinopathy) impairs blue-yellow early, while optic nerve and media disorders impair red-green — glaucoma being a noted early-tritan exception.
Which test is the gold standard for classifying defects?
The Nagel anomaloscope, using the Rayleigh equation — matching red-green mixtures to a yellow standard — to classify and grade protan and deutan defects.
What are the limitations of Ishihara plates?
They screen congenital red-green defects only, cannot assess blue-yellow loss, and need daylight-equivalent light at proper distance.
How do the D-15 and FM 100-hue differ?
D-15 is a quick 15-cap arrangement grading marked defects along protan, deutan and tritan axes; FM 100-hue is a longer quantitative test yielding a total error score.
How do Indian Railways assess colour vision for safety posts?
Through graded colour perception standards (CP categories), applying Ishihara screening and lantern-based signal recognition tests for driving and safety-critical categories.