Clinical and Occupational Colour Vision Analysis

CIE Diagram: Standard for classifying light sources. Plotted on $x$ and $y$ axes ( $0$ to $1.0$ ). Features include the Spectral Locus ( $380\,nm$ to $700\,nm$ ), Non-Spectral Purples line, and the Alychne (zero luminance line).
Standard Illuminants:
- Equal Energy Point (E): $x \approx 0.33$ , $y \approx 0.33$ .
- Source A: Incandescent ( $2600\,K-3100\,K$ ).
- Source B: Noon sunlight ( $4870\,K$ ).
- Source C: Average daylight ( $6700\,K-7000\,K$ ).
Munsell System: 3D system for pigment samples defined by Hue (dominant wavelength), Value (lightness, $0-10$ ), and Chroma (saturation).
Metamerism: Occurs when two different spectral stimuli appear identical under specific lighting.

Normal Trichromacy: Based on three cone types with specific peak sensitivities:
- S-cones (Short-wavelength): Approx. $435\,nm$ .
- M-cones (Medium-wavelength): Approx. $535\,nm$ .
- L-cones (Long-wavelength): Approx. $570\,nm$ .
Colour Perception: Result of the pattern of stimulation across these three receptors.

Monochromat (Achromat): Only one or zero cone photopigments; no hue discrimination.
Dichromat: Two cone photopigments; severely impaired hue discrimination. Sub-types include Protanope (L-cone absent), Deuteranope (M-cone absent), and Tritanope (S-cone absent).
Anomalous Trichromat: Three cone types present but one is abnormal. Includes Protanomalous, Deuteranomalous, and Tritanomalous.
Incidence: Red-Green (R-G) defects affect up to $8\%$ of males and $0.5\%$ of females. Tritan defects are rare ( $1:15,000$ to $1:50,000$ ).

Congenital: Present at birth, stationary, binocular, normal visual acuity, and usually X-linked (R-G type).
Acquired: Late onset, progressive, often monocular, abnormal visual acuity/fields, affects sexes equally, and usually Blue-Yellow (Tritan) type.
Occupational Risk: Neurotoxic chemicals and organic solvents can cause acquired loss; monitored via baseline and interval testing.

Anomaloscope: The only tool for a positive diagnosis of congenital defects using a bipartite field and the Rayleigh Match ( $R + G = Y$ ).
- Protan Match: Requires more red mixture.
- Deutan Match: Requires more green mixture.
- Protanopes: Distinctive feature includes the "darkening" of the red end of the spectrum.
Ishihara Test: Screening for R-G defects using pseudo-isochromatic plates. Relies on Confusion Lines on the CIE diagram.
- Plates include Vanishing, Hidden Digit, and Classification types.
- Requires lighting at $6500\,K$ (Illuminant C), $67\,cm$ distance, and $4\,s$ viewing time.
Hue Arrangement Tests:
- Farnsworth-Munsell 100-Hue: Comprehensive test using $85$ caps to assess hue discrimination and locate an "axis of confusion."
- Farnsworth D-15: Binary pass/fail test for moderate/severe defects used in industrial settings.
City University Test: Hue-matching test checking for both R-G and Tritan defects.
Lantern Tests: Vocational, strictly pass/fail instruments (e.g., Holmes-Wright, Giles-Archer) simulating safety signals for aviation and maritime use.
CAD (Colour Assessment and Diagnosis) Test: Computerized, 4-AFC method that detects R-G and B-Y defects without luminance clues.

Colour Coding:
- Connotative: Colour is the only information source (safety-critical).
- Denotative: Colour enhances redundant features (position, shape).
Safety Critical Roles: Visual performance in transport (PAPI lights in aviation), armed forces, and electrical work.
UK Wiring Standards: Phase (Brown), Neutral (Blue), Earth (Green/Yellow).
Road Safety: CCVD is linked to a higher risk of rear-end collisions due to difficulty detecting red brake lights.
Professional Reports: Optometrists must use exact prescribed tests and adhere to specific pass/fail cut-offs (e.g., firefighters fail if $> 2$ plates missed on Ishihara).