W3: Colour Vision I

Role of colour vision (CV) in daily functioning
Review basic CV principles from a clinical perspective
Understand processes underlying abnormal CV and theoretical constructs in the design of clinical CV tests
Become familiar with the various CV tests
Recognise and differentiate specific CV defect patterns
Understand occupational & safety implications of abnormal CV
Clinical recording and reporting of results

Colour allows interpretation of information and signals
Connotative colour codes – colours convey specific meanings
- Signals: lighting, signage, electronics
- Examples: Red (stop/danger), Yellow (warning/caution), Green (go/safety)
- Redundancy: information may also be conveyed by text, shape, or relative position
Denotative colour codes – colours have no inherent meaning; used to signify or organize

Colour is used in education
Colour used across many industries and occupations
- Medicine and medical imaging
- Mapping
- Meteorology, etc.

Colour is a subjective visual sensation
Psychological concepts: color described by three variables – Hue, Saturation, and Luminosity (brightness)
Psychophysical concepts: spectral sensitivity curves, wavelength discrimination, saturation discrimination, colour mixture
Physiological aspects of colour (refer to Visual Science 4 CV lecture notes)

Visual system is sensitive to visible light: $380\le \lambda \le 780\ \mathrm{nm}$
Normal CV: $\sim 150\text{, }$ different hues; with all luminance and saturation variations, potential CV sensations number in the tens of thousands
Retina contains 3 cone photoreceptors: $S,\ M,\ L$
Post-receptoral colour pathways: Red/Green and Blue/Yellow channels; Luminance channel
- Channels: Red/Green, Blue/Yellow, and Luminance (often denoted as $R/G\,, B/Y\,, \text{Luminance}$ )

Colour processing is explained by the Young–Helmholtz theory (trichromacy)
Retina cone types and peak sensitivities:
- Short-wavelength cones (S) – Cyanolabe; peak sensitivity $\lambda_{max} \approx 420\ \mathrm{nm}$
- Medium-wavelength cones (M) – Chlorolabe; peak sensitivity $\lambda_{max} \approx 530\ \mathrm{nm}$
- Long-wavelength cones (L) – Erythrolabe; peak sensitivity $\lambda_{max} \approx 560\ \mathrm{nm}$

Individual variations akin to a fingerprint
Typical ratio: $\text{L:M:S} \approx 10:5:1$
Approximately $2\times$ more L than M cones
S-cones are rare (≈ $7\%$ of cones)
Variation with eccentricity
No S-cones in central $100\ \mu\mathrm{m}$
Central vision can simulate a tritanopic CV defect if CV is measured in that region

Colour attributes model:
1) Hue – the distinctive name of the colour
2) Value (brightness) – represented by the perpendicular axis of the colour circle, from black to white
3) Saturation/Chroma – the percentage of “pure” hue in a colour

Hue – associated with the dominant wavelength
- e.g., red, yellow, blue
Saturation – amount of grey content; spectral colours are fully saturated
- White/black/grey – fully desaturated
Value/Chroma (brightness) – related to perceived radiant energy; analogous to a dimmer switch

With three primary spectral wavelengths a match can be found for any reference wavelength
- Primary colours commonly cited:
  - Red (650 nm), green (540 nm) and blue (460 nm)
- CIE 1931 - standardised international system
  - Red (700 nm), green (546.1 nm) and blue (435.8 nm)
Metameric match: different spectral distributions that are perceived as identical colours.
Different spectral power distributions can be perceived as identical colours (metamers)
Colour obeys algebraic laws, including additive, subtractive, and scalar properties; Grassmann’s laws of metamers describe these relations

Conceptually, a 3D space with coordinates (x, y, z) satisfying $x+y+z=1$
1931 CIE (x,y) chromaticity diagram
Spectral colours lie along the spectral locus curve where colours are fully saturated
Colour temperature described via the black body locus
- Candle flame around $1900\ \mathrm{K}$ appears more yellow
- Incandescent around $2800\ \mathrm{K}$
- LEDs around $4000\ \mathrm{K}$
- Direct sunlight around $5000\ \mathrm{K}$

Relative luminous efficiency, $V(\lambda)$ , describes visual sensitivity across wavelengths
Under photopic conditions, a monochromatic stimulus at $\lambda=555\ \mathrm{nm}$ is perceived as brighter than other monochromatic stimuli of equal energy
Scotopic conditions V’(lambda)

Wavelength difference that induces a noticeable hue difference
We can discriminate different hues with small wavelength changes in the range of approximately $2\ \text{to } 6\ \mathrm{nm}$
More wavelength difference is required at the ends of the visible spectrum

Just noticeable difference from white can be produced by a proportional mixture of a monochromatic light and white light
Yellow, approximately $570\ \mathrm{nm}$ , is the most desaturated
- The ratio of yellow to white must be high for the colour to appear different from white
Blue and red are more saturated; a small amount of these colours can make the mixture appear different from white

Bezold–Brücke (Bezold-Brucke) effect: hue changes with luminance
- At high luminance, hues below $500\ \mathrm{nm}$ appear bluer; hues above $500\ \mathrm{nm}$ appear yellower
Invariant hues: examples include $570\ \mathrm{nm},\; 508\ \mathrm{nm},\; 476\ \mathrm{nm}$
Contrast effects on colour perception:
- Simultaneous contrast (background effects)
- Successive contrast (over time effects)
Stiles-Crawford effects: perceived colour and brightness depend on the part of the pupil through which light enters

Normal physiological variations:
- Ocular media
  - Age-related lens changes (cataract) increase absorption of short wavelengths, reducing blue-end sensitivity
- Macular pigment (MP) – dietary pigments lutein/zeaxanthin accumulate in the RNFL
  - MPs absorb short-wavelength light before photoreceptor stimulation, altering blue sensitivity
Retinal eccentricity
- Colour vision deteriorates toward the periphery
- CV is relatively poor outside the central 2° of the visual field
- Therefore, most CV tests are designed for foveal viewing