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Color Vision

Optics & Refraction

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1. Physiology of Colour Vision

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  • Clinical Analogy: Think of colour vision as a three-channel RGB camera: the retina has three types of 'sensor' (S, M, L cones), each absorbing different wavelengths. The brain then computes colour by comparing the ratio of signals - just as a camera's chip produces every hue from just red, green and blue channels. A defect in one channel doesn't erase colour entirely; it collapses the space from 3D to 2D (or 1D), causing specific confusion axes.

Photoreceptor Basis

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  • The human retina contains ~6 million cones concentrated in the fovea (peak density ~150,000/mm²).
  • Three cone subtypes are defined by their peak spectral sensitivity:
Cone TypeGene LocusPeak λ (nm)PhotopigmentOpsin Class
S (Short / Blue)Chromosome 7~420–440CyanolabeOPN1SW
M (Medium / Green)X-chromosome~530–535ChlorolabeOPN1MW
L (Long / Red)X-chromosome~560–565ErythrolabeOPN1LW
Exam pearl
  • Medium-wavelength-sensitive (M) and long-wavelength-sensitive (L) cone genes are tandemly arranged on the X chromosome; their homology predisposes to recombination causing red-green defects.
  • X-linked recessive inheritance explains the high prevalence of red-green defects in males.
  • Short-wavelength-sensitive (S) cone gene on chromosome 7 is autosomal, so blue-yellow defects are rarer and equally distributed by sex.

Opponent-Colour Theory & Trichromacy

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Young-Helmholtz trichromatic theory (peripheral level): Any colour can be matched by mixing three primary colours in appropriate proportions. Three cone types provide three independent signals. Hering's opponent-colour theory (post-receptoral level): Signals are re-coded into three opponent channels:

  • Red–Green channel: long-wavelength cone signal − medium-wavelength cone signal (parvocellular pathway)
  • Blue–Yellow channel: short-wavelength cone signal − (long-wavelength cone signal + medium-wavelength cone signal) (koniocellular pathway)
  • Luminance channel: long-wavelength cone signal + medium-wavelength cone signal (achromatic; magnocellular pathway)

Both theories are correct at different levels - trichromacy at the cone level, opponent coding at post-receptoral levels including retinal circuitry and the lateral geniculate nucleus. This stage theory (Hurvich & Jameson) is the modern synthesis.

CIE Colour Space & Colour Confusion Lines

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The Commission Internationale de l’Éclairage (CIE) 1931 chromaticity diagram maps all visible colours on an x-y plane. Each dichromatic colour deficiency defines a set of confusion lines that converge at a point called the copunctal point:

DeficiencyCopunctal PointConfusion Axis
• Protanopia• Red end of spectrum• Red-Green (red axis)
• Deuteranopia• Off to the left of the chromaticity diagram• Red-Green (green axis)
• Tritanopia• Violet end of spectrum• Blue-Yellow axis

2. Classification of Colour Vision Defects

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Congenital Defects

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  • Congenital defects are usually stationary, bilateral, symmetric, and non-progressive.
  • They affect the red-green axis (X-linked) far more commonly than the blue-yellow axis (autosomal).
TypeMechanismPrevalence (M/F)SeverityResidual Vision
Protanomaly• Anomalous L cone (shifted toward M)• 1% / 0.01%• Mild• Reduced red sensitivity
Protanopia• L cone absent• 1% / 0.01%• Severe• Dichromat - red-green confusion
Deuteranomaly• Anomalous M cone (shifted toward L)• 5% / 0.4%• Mild• Most common colour vision defect (CVD)
Deuteranopia• M cone absent• 1.5% / 0.01%• Severe• Dichromat - red-green confusion
Tritanomaly• Anomalous S cone• Rare (both)• Mild• Blue-yellow confusion
Tritanopia• S cone absent• 1:12,500• Severe• Dichromat - blue-yellow
Achromatopsia (rod)• All cones non-functional• <1:30,000• Complete• Achromat • photophobia • nystagmus
Blue Cone Monochromacy• L & M cones absent • S intact• Rare X-linked• Severe• Low visual acuity (VA) • nystagmus • myopia
Exam pearl
  • Protanopes also have a reduced luminance response to red (brightness deficit)
  • - clinically important: red traffic lights appear dimmer.
  • This is absent in deuteranopes.
  • This is a key differentiating viva point.

Acquired Defects

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Exam pearl
  • Deuteranomaly is the MOST COMMON colour vision defect (5% of males) - NOT protanopia.
  • Confusing 'anomaly' (trichromat, shifted) with 'anopia' (dichromat, absent) is a classic multiple-choice question (MCQ) error.
  • Acquired defects can involve the red-green and/or blue-yellow axes.
  • In optic neuritis, mixed defects are common; blue-yellow defects may predominate acutely, whereas red-green defects may be more common at 6 months.
Köllner's Rule AxisAssociated DiseaseMechanism
Type I (Red-Green)• Macular / photoreceptor disease (e.g., cone dystrophy) • Central serous chorioretinopathy: blue-yellow colour vision defects may persist despite normal visual acuityL/M cone damage
Type II (Red-Green, severe)• Optic nerve disease (glaucoma, anterior ischaemic optic neuropathy) • Optic neuritis: both blue-yellow and red-green colour vision defects can occurParvocellular loss
Type III (Blue-Yellow)• Glaucoma (early) • Optic neuritis: both blue-yellow and red-green colour vision defects can occur • Diabetic maculopathyIn diabetic retinopathy, selective S-cone loss may contribute to tritan-like colour vision loss.
  • Köllner's Rule
  • Outer retinal/photoreceptor → blue-yellow.
  • Optic nerve → red-green.
  • Important exceptions
  • Glaucoma (optic nerve but blue-yellow defect early).
  • Dominant optic atrophy (commonly blue-yellow defect, although red-green, mixed, or nonspecific colour defects can also occur).
  • Chloroquine maculopathy (blue-yellow tritan defect early; red-green-axis abnormalities may occur in more advanced retinopathy).
Oral-exam tip
  • In optic neuritis, colour desaturation (especially red) is a common sign and may be disproportionate to visual-acuity loss.
  • Colour-vision abnormalities may persist during recovery, including after visual acuity (VA) returns to 6/6.
  • Use red desaturation test as a sensitive marker of optic nerve dysfunction.
DiseaseTypical AxisNotes
Glaucoma (early)Blue-Yellow• Short-wavelength-sensitive cone pathway deficits can occur early.
Optic NeuritisMixed Red-Green and Blue-Yellow; one axis may predominate.• Use Farnsworth-Munsell 100-hue testing to monitor recovery.
Dominant Optic AtrophyVariable; Blue-Yellow and Red-Green.• Dominant optic atrophy may show tritanopia. • Leber hereditary optic neuropathy is predominantly Red-Green (protan), often with Blue-Yellow loss.
Leber hereditary optic neuropathyPredominantly Red-Green (protan), often with Blue-Yellow loss.• As optic nerve involvement progresses.
Central Serous ChorioretinopathyBlue-Yellow• Blue-yellow colour discrimination may be impaired in central serous chorioretinopathy.
Chloroquine / HydroxychloroquineRed-Green (bull's eye)• Parafoveal cone damage.
Diabetic MaculopathyBlue-Yellow• Blue-yellow or tritan-like colour-vision deficits can occur and may correlate with diabetic retinopathy or macular-oedema severity.
Age-related Macular DegenerationVariable; Red-Green and Blue-Yellow defects may occur, with Blue-Yellow loss often greater.• Retinal pigment epithelium dysfunction.

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Keep reading Color Vision

8 more sections are included with the Clinical plan.

  • Colour Vision Examination Tests
  • Acquired Colour Vision Disorders - Clinical Correlations
  • Congenital Colour Vision Defects - Deeper Dive
  • Colour Vision in Special Clinical Contexts
  • Clinical Approach: Systematic Testing Strategy
  • Evolving Concepts & Emerging Therapies

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