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Colour Vision Assessment Clinical Guide

Comprehensive clinical protocol for colour vision testing, interpretation, and management decisions in optometric practice.

Last updated: March 2026

Purpose: Colour vision testing is a fundamental component of comprehensive eye examination, assessing the eye's ability to discriminate and perceive colours. Approximately 8% of males and 0.5% of females have colour vision deficiency (CVD), with inherited red-green deficiency being most common. Colour vision assessment screens for congenital and acquired anomalies, identifies functional limitations, and informs occupational suitability decisions.

Scope and Application Note: Colour vision deficiency can affect quality of life, educational choices, occupational opportunities, and safety. Early detection in children enables educational accommodation and career guidance. In adults, colour vision changes may indicate neurological or retinal pathology requiring further investigation. Standardized, systematic colour vision testing provides objective documentation of colour discrimination abilities and guides clinical management.

Clinical Guide Overview

1. Clinical Importance

Diagnostic Applications

  • Congenital CVD screening: Identifies inherited red-green and blue-yellow colour deficiencies, enabling early patient counselling and occupational guidance
  • Acquired CVD detection: New or progressive colour vision loss may indicate macular disease (AMD, diabetic maculopathy), optic neuropathy, CNS disease, or medication toxicity
  • Disease monitoring: Tracks colour vision changes in diabetic retinopathy, multiple sclerosis, syphilis, and other systemic conditions
  • Medication effects: Monitors for colour vision changes from ethambutol, tamoxifen, chloroquine, and other medications
  • Toxic exposure assessment: Screens for occupational exposures (carbon monoxide, solvents, heavy metals) affecting colour vision

Occupational and Legal Applications

  • Professional licensing: Colour vision requirements for pilots, commercial drivers, electrical workers, military personnel, and other safety-sensitive positions
  • Workplace accommodation: Identifies need for workplace modifications or alternative job assignments
  • Disability determination: Documents colour vision deficiency for insurance, disability, and educational support eligibility
  • Travel and mobility: Some countries restrict colour-blind individuals from certain professions or activities
  • Legal documentation: Standardized testing records provide objective, defensible documentation for employment and regulatory decisions

Patient Quality of Life

  • Education: Early detection permits academic accommodations and realistic career planning
  • Career guidance: Counsels patients regarding colour-vision-dependent occupations
  • Daily function: Affects ability to match clothing, interpret visual signals, and perform colour-dependent tasks
  • Assistive device selection: Colour vision status informs selection of eyeglass tints, contact lens options, or colour-enhancement devices
  • Psychological impact: Early counselling reduces frustration and supports mental health

Clinical Foundations: Colour Vision Physiology

Trichromatic Vision

Normal colour vision (trichromacy) depends on three types of cone photoreceptors in the retina, each sensitive to different wavelengths of light:

  • L-cones (Long wavelength): Red-sensitive, peak wavelength ~564 nm
  • M-cones (Medium wavelength): Green-sensitive, peak wavelength ~533 nm
  • S-cones (Short wavelength): Blue-sensitive, peak wavelength ~420 nm

The brain integrates signals from all three cone types to perceive the full spectrum of visible colour. Approximately 6-7 million cones in the retina provide the neural basis for colour discrimination.

Colour Vision Pathways

Colour information is processed through multiple visual pathways from retina to cortex:

  • Retinal cone circuits: Opponent colour processing in horizontal and bipolar cells creates red-green and blue-yellow opponent channels
  • Magnocellular pathway: Primarily luminance (brightness) processing
  • Parvocellular pathway: Primarily colour processing through lateral geniculate nucleus
  • V4 cortex: Colour-selective neurons in visual cortex V4 process and interpret colour information

Clinical Foundations: Types of Colour Vision Deficiency

Red-Green Colour Blindness (90% of CVD)

Protanopia (Red-blind):

Complete absence of L-cones (long wavelength). Rare. Affects ~1 in 30,000 males. Appears to see red as dark brown, yellow as orange, green as white. Variable severity.

Protanomaly (Red-weak):

Abnormal L-cone function. Milder than protanopia. Affects ~1 in 100 males. Can distinguish red tones but with reduced saturation and intensity.

Deuteranopia (Green-blind):

Complete absence of M-cones (medium wavelength). Rarer than protanopia. Sees red as yellow, green as white. Similar functional impact to protanopia.

Deuteranomaly (Green-weak):

Abnormal M-cone function. Most common form of CVD, affecting ~1 in 100 males and ~0.4% of females. Milder colour discrimination defect, often undiagnosed in childhood.

Blue-Yellow Colour Blindness (Rare)

Tritanopia (Blue-blind):

Very rare. Autosomal inheritance (not X-linked). Equal prevalence in males and females (~1 in 10,000). Absent S-cones. Bright blue appears white, yellow appears pink.

Tritanomaly (Blue-weak):

Extremely rare. Abnormal S-cone function. Similar visual effects to tritanopia but milder.

Achromatopsia (Complete colour blindness):

Rare (~1 in 30,000). Rod monochromatism—absence of functional cones. Sees only grayscale, photophobic, poor visual acuity (6/60 or worse). Usually identified in infancy.

Acquired Colour Vision Deficiency

Causes:

  • Optic neuropathy: Demyelinating disease, glaucoma, ischemic optic neuropathy (typically blue-yellow defect)
  • Macular pathology: Age-related macular degeneration, diabetic maculopathy, central serous chorioretinopathy
  • Medications: Ethambutol (TB treatment), tamoxifen, chloroquine, hydroxychloroquine, phenothiazines
  • Systemic disease: Diabetic retinopathy, syphilis, Lyme disease, multiple sclerosis
  • Occupational/environmental: Carbon monoxide toxicity, solvent exposure, lead exposure

Genetics Reminder

X-linked inheritance: Red-green CVD is X-linked recessive. Males with one mutant allele are affected. Females need two mutant alleles to be fully colour blind (rare), but heterozygous carriers may show mild colour discrimination defects. Females inheriting from affected father and carrier mother have 50% chance of full expression. Males inheriting from affected mother have 100% chance of expressing CVD.

2. Equipment and Tools

Screening Tests

Ishihara Colour Test (Most Common):
  • Standard: 38-plate test (diagnostic and screening)
  • Quick version: 24-plate test (faster screening)
  • Concise version: 14-plate test (ultra-compact)
  • Contains pseudo-isochromatic plates with colored dots
  • Classifies red-green colour blindness into protanopia, deuteranopia, and anomalies
  • Sensitive and specific for congenital red-green CVD
  • Limited ability to detect acquired CVD or blue-yellow defects

Quantitative Tests (Diagnostic)

Farnsworth D-15 (Dichotomous-15):
  • Gold standard for acquired colour vision deficiency assessment
  • 15 coloured caps with reference cap
  • Patient arranges caps in order of hue progression
  • Detects all forms of colour blindness, including blue-yellow defects
  • Error patterns reveal type of colour defect
  • More sensitive to mild colour defects than Ishihara
  • Takes 3-5 minutes per eye
Farnsworth-Munsell 100 Hue Test:
  • 100 coloured caps arranged in hue order
  • More sensitive quantitative measure of colour discrimination
  • Takes 10-15 minutes (more time-consuming)
  • Used primarily in research and detailed diagnostics
  • Can quantify severity of colour defect
City University Colour Vision Test:
  • Modern pseudo-isochromatic plate test
  • 4-plate screening test or 10-plate detailed test
  • Specifically designed to detect acquisition of colour defects
  • More sensitive than Ishihara for acquired CVD
  • Particularly good for detecting blue-yellow defects

Objective Tests

Anomaloscope (Reference Standard):
  • Gold standard for quantifying colour vision defects
  • Patient matches reference colour by adjusting wavelength and intensity
  • Provides objective measurement of colour discrimination
  • Expensive and time-consuming (research instrument)
  • Rarely used in clinical practice
Chromatic Discrimination Tests (Advanced):
  • Computer-based colour vision assessment systems
  • Precise wavelength and saturation control
  • Research and specialized clinics

Environmental Requirements

  • Illumination: Standardized lighting is critical. Use Munsell C illuminant (6500K daylight) or cool-white fluorescent (approximately D65). Poor lighting invalidates results.
  • Room conditions: Minimal glare, shadows, and visual distractions. Neutral background wall behind testing materials.
  • Test plate care: Keep plates/caps clean and protected from UV light (fading changes colour properties). Replace plates annually if frequently used.
  • Distance and viewing angle: Maintain standardized distance (typically 30-50 cm for plates, arm's length for D-15). Ensure perpendicular viewing to prevent colour distortion.

Best Practice: Test Selection Algorithm

Screening (All patients): Ishihara 38-plate test or City University 4-plate test.If abnormal screening: Farnsworth D-15 test to confirm, quantify severity, and determine type. If acquired CVD suspected: Use City University test or FM100.If baseline documentation needed: Farnsworth-Munsell 100 Hue test for numerical score for future comparison.

3. Patient Preparation

Pre-Test Requirements

  1. Refractive correction: Ensure patient wears current best refraction (glasses or contacts)
  2. Cataract assessment: Significant cataracts can affect colour discrimination. Note and consider in interpretation.
  3. Media clarity: Ensure adequate visual media for accurate colour perception
  4. Visual acuity check: Patient should have adequate VA for test. If VA worse than 6/24, colour testing may be difficult to interpret.
  5. Pupil dilation: Generally not required for colour testing. Avoid testing shortly after mydriasis if possible (altered light adaptation).
  6. Eye fatigue: Test colour vision early in exam, before patient fatigue affects performance

Environmental Setup

  1. Standardized illumination: Use colour-correct lighting (D65 or Munsell C standard). Avoid tungsten (yellow) or poor fluorescent lighting.
  2. Test distance: Follow test manufacturer specifications. Typically 30-50 cm (12-20 inches)
  3. Viewing angle: Present test materials perpendicular to line of sight. Angle alters colour perception.
  4. Background: Neutral, non-reflective background wall or tester's shirt
  5. Time allowance: Allocate 3-5 minutes for Ishihara screening, 5-10 minutes for D-15

Patient Education

  • Explain test purpose: "This test checks how your eyes see colours. It's quick and easy."
  • Instructions clarity: Give clear, specific instructions for each test before starting
  • Normalize: Explain that colour vision variations are common and not dangerous to eyes
  • Honesty encouragement: Ask patient to report what they see, not guess. Guessing invalidates result.
  • Monocular testing: Explain need to test each eye separately for occupational testing

4. Ishihara Colour Test Protocol

Step-by-Step Procedure

  1. Position plate correctly:

    Hold Ishihara plate at distance of 30-50 cm (about 12-20 inches), perpendicular to line of sight. Ensure standardized lighting without glare on plate.

  2. Monocular testing:

    Test right eye first, then left eye. Occlude opposite eye completely. For occupational testing, always use monocular testing.

  3. Time limit:

    Typically 3-5 seconds per plate for identification. If patient hasn't answered in that time, record as incorrect and move to next plate. Avoid encouraging second-guessing.

  4. Record responses verbatim:

    Write exactly what patient says (number, symbol, or "nothing"). Do NOT correct or coach patient to "try again." If patient states uncertainty, record that uncertainty.

  5. Scoring:

    Compare responses to test key. Typically, 17 or more correct answers (of 38 plates) indicates normal colour vision. 15-16 correct = suspect colour deficiency. 0-14 correct = colour blindness (specify type).

  6. Record result:

    Document as "Ishihara: Normal" or "Ishihara: Protanopia," "Deuteranopia," "Protanomaly," "Deuteranomaly" per interpretation key.

Plate Types and Interpretation

  • Plates 1-2 (Demonstration): Seen by all patients regardless of colour status. Used to ensure patient understands task. All patients should see numbers.
  • Plates 3-14 (Normal/anomaly plates): Differentiate normmals from those with colour defects
  • Plates 15-17 (Confusion plates): Protanopia vs deuteranopia patterns (protans read differently than deutans)
  • Plates 18-21 (Transformation plates): Used in some versions to grade severity
  • Plates 22-38 (Advanced plates): Detailed analysis of type and severity. Protans typically miss certain plates; deutans miss different plates.

Ishihara Advantages

  • ✓ Quick screening (3-5 minutes)
  • ✓ Good for red-green colour blindness detection
  • ✓ Portable and inexpensive
  • ✓ Well-established standardization
  • ✓ Easily reproducible

5. Farnsworth D-15 Test Protocol

Step-by-Step Procedure

  1. Setup and lighting:

    Place D-15 test box on table with standardized lighting perpendicular to viewing surface. Ensure no glare. Patient seated comfortably with eye level aligned to caps.

  2. Explain task:

    "These caps are arranged in random order. I want you to place them in order by hue - arrange them from the reference cap as if they flow in a continuous rainbow. Match colours that look most similar to each other."

  3. Anchor cap placement:

    Place the reference cap (fixed middle cap) in its slot. This serves as starting point. Patient arranges other 15 caps adjacent to reference in hue order.

  4. Allow patient to manipulate:

    Patient is allowed to pick up, move, compare, and reorganize caps until satisfied with arrangement. No time limit, but typically takes 3-5 minutes.

  5. Record arrangement:

    Write down exact order of cap numbers as patient arranged them. This is critical for error pattern analysis.

  6. Repeat for second eye:

    Reset caps in random order and repeat for left eye. For occupational assessment, record both eyes separately.

  7. Calculate error score:

    Count number of incorrect adjacent placements (transpositions). Total error score is sum of all transposition errors. Normal score: 0-15 errors. Score over 50 indicates colour vision defect.

Error Pattern Analysis

Plotting results: Plot cap positions on error graph to visualize colour discrimination profile.

  • Random errors (no pattern): Scattered plot indicates normal colour vision or non-colour vision related explanation for errors (patient confused, low motivation)
  • Protan pattern: Characteristic crossover errors in red-green region (axis typically 65-90 degrees on error plot)
  • Deutan pattern: Characteristic crossover errors at different axis (typically 105-125 degrees)
  • Tritan pattern: Errors in blue-yellow region (axis 155-180 degrees, indicates blue-yellow defect or acquired CVD)
  • Achromatic pattern: Errors across all hues (suggests complete colour blindness or low acuity)

Farnsworth-Munsell 100 Hue Test

Protocol: Identical to D-15 but uses 100 caps divided into four segments. Patient arranges all caps in complete hue spectrum.

Advantages: More sensitive to mild colour defects, provides quantified error score, error pattern very specific for identifying type of CVD.

Disadvantages: Time-consuming (10-15 minutes), patient fatigue, requires careful lighting and setup. Space-intensive.

Clinical use: Primarily research and academic settings. When detailed colour vision quantification needed for baseline documentation or monitoring acquired CVD progression.

Farnsworth Limitations

  • • Patient must be able to manipulate caps and understand spatial ordering
  • • Cognitive impairment or tremor can affect results
  • • Age-related colour discrimination decline can confound interpretation
  • • Caps fade with UV exposure and age—must replace regularly
  • • Lighting standardization critical; results invalid under poor lighting

6. City University Colour Vision Test Protocol

Overview and Purpose

The City University Colour Vision Test is a modern pseudo-isochromatic plate test specifically designed to identify acquired colour vision deficiency. It's a hybrid screening tool that detects red-green, blue-yellow, and acquired colour anomalies with good sensitivity compared to traditional Ishihara.

Particularly valuable for monitoring patients on colour-affecting medications (ethambutol, tamoxifen) or those with systemic/neurological conditions affecting colour vision. Available as 4-plate screening version or 10-plate detailed test.

Procedure (4-Plate Screening)

  1. Present plates at 30-50 cm distance in standardized lighting
  2. 3-5 second time limit per plate
  3. Patient identifies symbols or numbers on plate
  4. Record all responses verbatim
  5. Interpret per test manual scoring criteria

Scoring: Correct responses all 4 plates generally indicates normal colour vision. 1-2 errors suggest possible colour defect requiring confirmation with D-15 or additional testing.

Advantages Over Ishihara

  • Better sensitivity for blue-yellow colour defects (important for acquired CVD)
  • Specifically designed to detect acquired CVD progression
  • More consistent plate luminance and saturation
  • Fewer false positives/false negatives than Ishihara
  • Modern design based on contemporary colour science

7. Other Colour Vision Tests

Nagel's Anomaloscope

  • Reference standard for colour vision assessment in research settings
  • Objective measurement: Patient matches monochromatic test field against mixture of reference wavelengths
  • Provides precise quantification of colour anomalies
  • Clinical application: Limited due to cost, complexity, and time required
  • Primarily used in research institutions, ophthalmology departments, occupational health settings

Hardy-Rand-Rittler (HRR) Test

  • Pseudo-isochromatic plate test designed to detect tritan (blue-yellow) defects better than Ishihara
  • 16 plates total with geometric shapes in background and foreground
  • Particularly good for pediatric colour detection and acquired CVD
  • Similar protocol to Ishihara but specific pattern interpretation for tritan detection

D-100 (Farnsworth-Munsell 100 Hue Continuation)

  • Extended version of FM-100 with specific error pattern analysis software
  • Digitized scoring provides error coordinates for precise pattern identification
  • Allows trend analysis over time for monitoring acquired CVD progression
  • Used in specialized occupational and medical monitoring applications

Lanthony D-15 Desaturated (D-15d)

  • Modified D-15 with pastel (desaturated) colours
  • More sensitive to mild colour vision defects than standard D-15
  • Excellent for detecting early-stage acquired CVD and occupational exposures
  • Recommended when baseline colour vision monitoring needed

Digital and Vision Apps

  • Browser-based tests: Color Vision Testing Made Easy (CVT), Enchroma colour blindness test
  • Smartphone apps: ColorBlindr, Ishihara Test app, CVT mobile
  • Limitations: Lighting not standardized, monitor colour calibration variable, not validated for medical decision-making
  • Use case: Patient self-awareness and educational screening only, not clinical testing

8. Interpretation of Results

Classification Framework

Normal Colour Vision (Trichromacy):

Ishihara 17+ correct, D-15 error score <15, all normal hue discrimination. Full colour perception across spectrum. No colour discrimination limitations.

Congenital Red-Green Colour Blindness:

Characterized error patterns on Ishihara or D-15. Protanopia/protanomaly (protan) shows specific plate misidentifications. Deuteranopia/deuteranomaly (deutan) shows different pattern. Early-life onset, stable over lifetime. X-linked inheritance pattern.

Acquired Colour Vision Deficiency:

New colour defect not present on prior testing. Often blue-yellow pattern on D-15. Associated with recent systemic disease, medication change, occupational exposure, or retinal/optic nerve pathology. Requires investigation for underlying cause.

Incomplete Penetrance / Heterozygous Carriers:

Some female carriers (heterozygous for X-linked defects) show mild colour discrimination defects on detailed testing despite normal screening. Subtle deficiency most apparent on desaturated tests (D-15d).

Severity Grading

Severity LevelIshiharaD-15 ScoreClinical Significance
Normal17-38 correct0-15No colour vision restrictions
Suspect/Mild15-16 correct16-30Confirm with detailed testing
Moderate10-14 correct31-50Significant colour discrimination loss
Severe/Complete0-9 correct>50Marked functional limitation, occupational restrictions

Pattern Recognition for Type Identification

D-15 Error Plot Interpretation:

  • Random scatter: Normal colour vision or non-colour-related factors (patient confusion, low motivation, tremor)
  • Linear axis 65-90°: Protan (protanopia/protanomaly) - red-weak or red-blind pattern
  • Linear axis 105-125°: Deutan (deuteranopia/deuteranomaly) - green-weak or green-blind pattern
  • Linear axis 155-180°: Tritan (tritanopia/tritanomaly) - blue-yellow defect or acquired CVD
  • Diffuse errors across all hues: Achromatic pattern - complete colour blindness, very low acuity, or cognitive/attention issues

9. Treatment and Management Protocols for Optometry Practice

Congenital Colour Blindness Management

1. Patient Counselling and Education
  • Explain genetic basis (X-linked inheritance for red-green)
  • Discuss non-progressive nature and no threat to eye health
  • Address practical daily challenges (clothing matching, traffic lights, colour coding)
  • Provide empathetic support; many patients unaware of deficiency until adolescence
  • Emphasize normal life expectancy and visual function in most tasks
2. Occupational Guidance
  • Discuss colour vision requirements for careers of interest (pilot, electrician, police, military, driver)
  • Provide career counselling resources and occupational health referrals
  • Document colour vision status for employment/licensing purposes
  • Explore alternative careers where colour vision not essential
  • Connect with occupational therapist for workplace accommodation strategies
3. Assistive Devices and Adaptations
  • Colour-filtering glasses: Enchroma, ChromaGen glasses enhance colour discrimination by filtering specific wavelengths. Modest improvement for some patients (protans benefit more than deutans). Variable effectiveness; trial period recommended.
  • Electronic colour identifiers: Smartphone apps, wearable devices that identify and audibly report colours (CColor app, others)
  • Workplace modifications: Label colour-coded items with text/symbols, use redundant coding systems (shape + colour), high-contrast design
  • Screen readers and colour management: Computer settings, accessibility software to customize colour schemes
4. Educational Support
  • Early identification in children enables school accommodations
  • Communicate with teachers regarding colour-dependent assignments (maps, graphics)
  • Provide documentation for disability/accommodation services at schools/universities
  • Recommend books/resources on colour blindness for patient education
5. Routine Follow-up
  • Congenital CVD is stable. No need for frequent colour testing unless occupational changes
  • Screen for acquired CVD at comprehensive exams (new colour defect would be pathological)

Acquired Colour Blindness Management

1. Diagnosis and Underlying Cause Investigation
  • New colour vision loss is abnormal and warrants investigation
  • Confirm with repeat testing (Farnsworth D-15 recommended) to exclude false positive
  • Take detailed systemic history: Recent illness, medication changes, occupational exposures, trauma
  • Examine for retinal/optic nerve pathology (macular disease, optic disc appearance, visual fields)
  • If no ocular cause identified, refer to internist/neurologist for systemic investigation
2. Medication Review
  • Ethambutol (TB treatment): Most common medication-induced CVD. Check dose and duration. Baseline colour testing recommended before starting. Monthly monitoring if on therapy. Discontinue if colour vision loss detected; usually reversible if caught early.
  • Tamoxifen (breast cancer): Can cause yellow-brown vision. Document status. Ophthalmology referral if significant symptoms.
  • Chloroquine/hydroxychloroquine: Rare at therapeutic doses but monitor if prolonged use. Retinal toxicity more common concern than colour.
  • Others: Phenothiazines, amiodarone, sildenafil. Document and educate patient.
  • Action: Collaborate with prescribing physician. Document colour vision status for legal protection. Weigh risks/benefits of continuing medication versus colour vision loss.
3. Disease-Specific Management
  • Diabetic retinopathy: Optimize glycemic control, refer to endocrinologist. Monitor colour vision as indicator of macular involvement. Intensive monitoring if diabetic macular edema present.
  • AMD: Refer to ophthalmology for advanced imaging (OCT, angiography). Colour vision loss may precede visible macular changes on examination.
  • Optic neuropathy: Refer for imaging and systemic workup (MRI brain for demyelination). Blue-yellow colour loss common pattern.
  • Glaucoma: Colour vision can decline due to optic nerve damage. Use as additional marker of disease progression alongside visual fields, OCT.
4. Monitoring Protocol for Acquired CVD
  • Establish baseline colour vision on first detection (D-15 or FM-100)
  • For ethambutol exposure: Monthly colour testing during therapy
  • For systemic disease: Repeat colour testing at 3-6 month intervals or per disease management plan
  • Document serial results to track progression
  • Communicate findings with primary care physician and specialists

Occupational Health Testing

Safety-Sensitive Professions Requiring Colour Vision:
  • Pilots: FAA standard - typically normal or mild deficiency acceptable depending on aircraft type
  • Commercial drivers: Varies by jurisdiction; some require normal, others accept mild
  • Electricians/Utility workers: Wire colour coding critical. Normal/near-normal required
  • Military: Strict colour vision requirements for combat pilots and special operations
  • Police/Emergency services: Varies by jurisdiction
  • Railroad/Maritime: Signal interpretation essential. Colour vision standards vary by country
Testing Recommendations for Occupational Assessment:
  • Use standardized, well-validated tests (Ishihara 38-plate + Farnsworth D-15 minimum)
  • Test monocularly if job-specific requirements address individual eye colour vision
  • Ensure standardized lighting (critical for legal defensibility)
  • Document lighting conditions, distance, test type, and complete findings
  • Provide written report suitable for employer/licensing authority
  • Understand jurisdiction-specific standards for that profession

Optometrist's Role in Treatment

While optometrists typically cannot treat the underlying disease causing acquired colour vision loss, your role is critical: Early detection and referral for serious pathology, baseline documentation for monitoring progression, patient counselling regarding functional implications, occupational suitability assessment, and communication with other healthcare providers. Establish relationships with ophthalmologists, internists, and specialists for appropriate referrals. Document thoroughly for legal defensibility, especially in occupational health context.

10. Special Populations

Pediatric Colour Vision Assessment

  • Age of testing: Screen all children by age 4 (kindergarten entry). Earlier if family history positive.
  • Test selection: Ishihara 24-plate preferred (simpler for children). Alternative: pseudoisochromatic test with animal/car symbols if letters difficult.
  • Environmental support: Parent or guardian should accompany child. Explain test in age-appropriate manner.
  • Early identification benefits: School accommodations, realistic career discussion, family counselling (especially affected siblings, females for genetic counselling).
  • Limitations: Valid testing requires cooperation; some 4-5 year-olds unable to reliably verbalize responses. Retest at age 6-7 if initial results uncertain.
  • Acquired CVD warning: Consider investigation if colour defect noted after age 5 (unusual for congenital to be newly detected past that age).

Elderly Patients

  • Age-related colour discrimination decline: Normal aging affects colour perception. Yellowing of lens reduces blue colour perception. Slightly reduced saturation perception normal.
  • Interpretation caution: Mild colour defects on screening may reflect age-related changes rather than pathology. Repeat testing and clinical correlation necessary.
  • Acquired CVD concern: New or progressive colour loss warrants investigation for AMD, optic neuropathy, or systemic disease (diabetes, B12 deficiency, neurological disease).
  • Medication review: Elderly patients often on multiple medications. Check history for colour-affecting drugs.
  • Test modifications: Ensure adequate lighting, extra time allowed, consider large-print or high-contrast test plates if visual acuity reduced.

Low Vision Patients

  • Visual acuity threshold: If VA worse than 6/24 (20/80), colour testing reliability questionable. May be unable to discriminate small differences or read test properly.
  • Test modifications: Use larger print tests or pseudoisochromatic plates with larger symbols. Increase distance to chart if necessary. Farnsworth D-15 may be more feasible than small Ishihara plates.
  • Functional assessment: More important to assess functional colour discrimination for daily tasks (reading labels, matching clothes) than formal test results.
  • Occupational limitation: A colour visual acuity loss significant enough to need large-print tests likely precludes colour-vision-dependent occupations regardless of test results.

Patients with Neurological Conditions

  • Multiple sclerosis: Optic neuritis (demyelination) causes blue-yellow colour loss. Colour vision can be early indicator of MS exacerbation or new lesion. If patient reports colour change, refer promptly.
  • Lyme disease: Acrodin-positive B6 deficiency from borreliosis can cause colour (especially blue-yellow) visual loss.
  • HIV/AIDS: Opportunistic infections (CMV retinitis), medications can cause colour vision changes. Monitor closely.
  • Dementia/cognitive decline: Unreliable colour testing responses. Simplified pseudoisochromatic test may be only feasible option. Document limitations in medical record.

Female Carriers of Red-Green CVD

  • Inheritance pattern: X-linked recessive. Heterozygous female carriers typically have normal colour vision on Ishihara screening.
  • Subtle defects: Some female carriers show mild defects on sensitive tests (desaturated D-15, FM-100), particularly if skewed X inactivation pattern.
  • Genetic counselling: If female carrier status confirmed, discuss reproductive implications: 50% sons will have CVD if inherited from affected mother; daughters will be carriers.
  • Occupational implications: Most female carriers pass routine screening and occupy colour-vision-dependent jobs. Only those with mild confirmed defects need occupational limitation.

11. Clinical Pearls and Best Practices

Pearl #1: Lighting Changes Everything

Poor lighting invalidates colour vision testing results. Use properly calibrated D65 (6500K daylight) illumination or standard Munsell C lighting. Tungsten or yellow fluorescent lights shift colour perception. Invest in proper colour-corrected lighting; it's essential for valid testing.

Pearl #2: Congenital vs. Acquired Clues

Ask the history: "Have you always had trouble with colours, or is this new?" Lifelong colour deficiency = congenital. New colour loss = acquired (investigate). Blue-yellow pattern on D-15 = acquired until proven otherwise. Red-green pattern can be either; age of onset is key discriminator.

Pearl #3: Screen Before Referring

Always do basic colour screening (Ishihara) before referring for occupational colour vision testing. Knowing whether screening is normal vs. abnormal helps guide referral decisions and may save patients money on occupational health testing fees.

Pearl #4: Reproducibility is Professional

Test procedures should be identical every time. Standardize: test distance, time allowed, lighting, testing room appearance. This protects you medico-legally and provides reproducible baseline for monitoring suspected acquired colour deficiency.

Pearl #5: Ishihara Plates Fade With Age

UV light fades pseudoisochromatic plate colours. As plates age, saturation decreases and colour can shift, affecting test validity. Replace annually if heavy use (20+ exams/week). Keep plates in original case and covered when not in use to preserve colour properties.

Pearl #6: Ethambutol Monitoring is Critical

TB patients on ethambutol: baseline colour vision BEFORE starting medication, then monthly monitoring during therapy. If colour vision loss detected, inform prescribing physician about potential drug-induced toxicity. Reversible if caught early, but irreversible if delayed. Document everything carefully.

Pearl #7: Protan vs. Deutan Classification Matters

Protan (red) and deutan (green) colour blindness are different for occupational assessment. Some professions accept one type but not the other. The D-15 error pattern tells you the type. Know the difference for your local profession-specific requirements.

Pearl #8: D-15 Detects What Ishihara Misses

If Ishihara borderline or patient reports acquired defects, use Farnsworth D-15. Much better at detecting blue-yellow and acquired defects. Error plot patterns are diagnostic. Takes 5 minutes but provides clarity that Ishihara cannot.

Pearl #9: Refractive Correction is Essential

Uncorrected refractive error impairs colour discrimination testing through lens blur and saturation reduction. Always perform colour testing with best refractive correction in place. Refract first if patient unsure of habitual correction. Document correction status in record.

Pearl #10: Documentation Protects You Legally

Colour vision determination affects employment, licensing, and disability status. Document: date/time of testing, lighting conditions, test used, exact patient responses, monocular results, and precise findings. Be specific: "Ishihara: 12/38 correct, protanopia pattern." This protects you legally and provides defensible documentation.

Golden Rule of Colour Vision Testing

"Standardization, reproducibility, and documentation." Colour vision testing appears simple but is surprisingly sensitive to environmental and procedural variables. Excellent colour vision assessment reflects attention to detail, proper technique, and commitment to quality. Your patients and your professional reputation depend on getting colour vision testing right, especially for occupational and clinical decision-making.

Quick Reference Protocol

  1. Prepare testing conditions: Verify standardized illumination, calibrated test materials, and appropriate distance before testing.
  2. Confirm correction and baseline status: Test with best refractive correction and document congenital/acquired history context.
  3. Run screening first: Start with Ishihara plates using standardized timing, plate distance, and response recording.
  4. Quantify and classify: Use Farnsworth D-15 when screening suggests deficiency to classify axis and severity patterns.
  5. Refine with City University test: Apply targeted plate-based assessment for additional discrimination and confirmation.
  6. Use adjunctive tests when indicated: Apply other colour vision tests where clinical, occupational, or diagnostic indications require added detail.
  7. Interpret in full clinical context: Correlate test outcomes with symptoms, ocular findings, and risk factors for acquired pathology.
  8. Document and counsel clearly: Record exact responses and communicate occupational relevance, accommodations, and follow-up plan.

Documentation and Communication

Essential Clinical Documentation

  • Record test name, edition, testing distance, illumination conditions, and correction status at time of testing.
  • Document monocular and binocular outcomes where applicable, including exact plate or arrangement responses.
  • State classification and pattern clearly (e.g., red-green axis, suspected protan/deutan/tritan profile).
  • Include interpretation context, including congenital vs acquired concern and any related ocular/systemic risk factors.
  • Capture occupational relevance and referral/follow-up recommendations when colour-vision-critical tasks are involved.

Patient and Family Communication

  • Explain results in clear language, including what deficiency pattern means functionally in daily life.
  • Discuss educational and occupational implications early, especially in children and career-planning settings.
  • Review practical adaptation strategies for colour-dependent environments and safety tasks.
  • Clarify when findings suggest possible acquired disease and why further investigation or referral is needed.
  • Provide clear expectations for monitoring intervals and repeat testing where progression risk exists.

References

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