HSE Guide to Colour Vision Examination

Colour vision assessment is essential for safety-critical roles (e.g., train drivers) to prevent accidents and for quality control (e.g., textiles, paper, paints) to avoid costly errors.
Employers should refer to Guidance Note MS7 Colour vision examination: A guide for occupational health providers for detailed clinical strategies.

Inherited red-green defects affect approximately $8\%$ of the male population ( $1 \text{ in } 12$ ) and are rarely found in women (approximately $1 \text{ in } 200$ ).
Acquired defects are common in those over age $45$ ( $1 \text{ in } 12$ ) and increase to $1 \text{ in } 4$ for those over $75$ .
Causes of acquired defects include eye diseases (glaucoma, cataracts), general diseases (diabetes, multiple sclerosis), industrial chemicals (solvents), and certain medications or substance abuse.

Cones: Central retina cells responsible for daylight vision and colour discrimination; normal vision requires three functional cone types (red, green, and blue sensitivity).
Rods: Peripheral retina cells that function in low light to provide night vision.
Dichromatism: A major defect where one cone type is absent or faulty.
Anomalous trichromatism: A minor or moderate defect where a cone type has different spectral sensitivity.

Connotative Coding: Information is transferred solely via colour (e.g., electrical wiring, railway signals). This should be modified to denotative coding where possible.
Denotative Coding: Colour serves as a redundant cue alongside shape, position, text, or sequence (e.g., road traffic signals, gas cylinders).
Colour-deficient workers often require higher illumination levels and may work more slowly on tasks involving search and sorting.

Pre-employment assessment is typical, followed by annual testing for safety-critical roles or workers over $40$ years of age.
A formal policy should specify task lists, acceptable deficiency degrees ( $if any$ ), and referral procedures for borderline or acquired cases.
Testing must be conducted by trained examiners (occupational nurses or optometrists) under specified lighting conditions.

Ishihara plate test: A screening tool specifically for red-green defects; it cannot detect blue-yellow (tritanopia) defects.
City University test: A diagnostic tool used to grade the severity and type the defect (red, green, or blue deficiency).
Lantern tests: Standardized clinical tests specifically for roles involving coloured lights, such as maritime, aviation, and rail transport.
Trade tests: Non-standardized practical assessments based on specific workplace tasks used to determine fitness for work in borderline cases.