Describing Coloured Stimuli and the Impact of Colour Vision

Describing Coloured Stimuli

• Requirement for Description: The ability to describe coloured stimuli accurately is essential for discussing colour in academic contexts and for conducting formal colour vision testing.

• Primary Systems for Description:

  • CIE Diagram: This is particularly useful for describing and classifying light sources.

  • Munsell System: This is the standard for classifying pigment samples and surface colours.

The CIE Chromaticity Diagram

• Structural Components of the CIE Diagram:

  • Axes: The diagram is plotted on a coordinate system with the $y$ axis and $x$ axis ranging from $0$ to $1.0$.

  • The Spectral Locus: The curved boundary of the diagram representing the monochromatic spectral wavelengths, measured in nanometers ($nm$). The visible spectrum shown ranges from approximately $380\,nm$ (violet) to $700\,nm$ (red).

  • Specific Wavelength Benchmarks:

    • $520\,nm$ to $540\,nm$: Green region.

    • $570\,nm$ to $590\,nm$: Yellow to Orange region.

    • $470\,nm$ to $480\,nm$: Blue region.

  • Non-Spectral Purples: The straight line along the bottom of the diagram connecting the blue/violet end to the red end. These colours do not exist as single wavelengths but are mixtures of short and long-wavelength light.

  • The Alychne: A term noted near the base of the diagram (y=0 line) representing a line of zero luminance.

• Standard Light Sources and Colour Temperatures:

  • Equal Energy Point: Located at the center of the diagram where $x \approx 0.33$ and $y \approx 0.33$, representing a theoretical "perfect white" where all wavelengths are present in equal amounts.

  • Black Body Locus: A curved line within the diagram representing the chromaticity of an ideal black body radiator as it is heated to different temperatures in Kelvin ($K$).

  • Source "A": Represents incandescent light, specifically a lamp range of $2600K$ to $3100K$.

  • Source "B": Represents sunlight at noon with a colour temperature of approximately $4870K$.

  • Source "C": Represents average daylight with a colour temperature of approximately $6700K$ to $7000K$.

  • Additional Benchmarks:

    • Sunlight at sunrise: $1800K$

    • Northwest sky: $25,000K$

    • Fluorescent sources: Indicated by specific points (e.g., WWX-3000K, CWX-4200K).

The Munsell Colour System

• Definition: A system that represents colour appearance in a three-dimensional diagram, allowing for the numerical specification of colour samples.

• The Three Dimensions of Munsell:

  1. Hue: Defined by a circle or the pages of a book. It corresponds to the dominant wavelength. The scale includes Red ($R$), Yellow-Red ($YR$), Yellow ($Y$), Green-Yellow ($GY$), Green ($G$), Blue-Green ($BG$), Blue ($B$), Purple-Blue ($PB$), Purple ($P$), and Red-Purple ($RP$).

  2. Value (Lightness): Represented on a vertical scale from $0$ (Pure Black) at the bottom to $10$ (Pure White) at the top.

  3. Chroma: Represented radially or horizontally on a scale starting from $0$ at the center. It measures the "amount of colour present" (saturation or purity).

• Specification Format: Munsell samples are always specified numerically in the order: Hue Value/Chroma.

Categorization of Colour Coding

• Connotative Colour Coding:

  • Definition: Colour is the only means of conveying the information.

  • Requirement: The code must be interpreted correctly for the task to be carried out safely.

  • Implication: Failure to distinguish the colour leads to a failure to receive the information.

• Denotative Colour Coding:

  • Definition: Colour enhances information that is already provided by other redundant features such as position, shape, or text.

  • Utility: While not strictly necessary for task completion, colour allows users who can distinguish it to perform the task more quickly and effectively.

• Real-World Example (Nutrition Labeling):

  • Labels for Calories, Sugar, Fat, Saturated Fat, and Salt use both text and colour (Red, Amber/Yellow, Green) to indicate nutritional levels.

  • Example data: $353$ Calories ($18\%$ GDA), $0.9\,g$ Sugar ($1\%$), $20.3\,g$ Fat ($29\%$), $10.8\,g$ Saturated Fat ($54\%$), and $1.1\,g$ Salt ($18\%$).

Challenges and Occupational Impacts of Colour Vision Defects

• Prevalence of Difficulties in Everyday Tasks (Study by Steward and Cole, 1989):

• Occupations Involving Color Use:

  • Safety Critical: Transport, Armed Forces, Police.

  • Aesthetic/Industry: Fine art, Interior design, Paints, Textiles and dyes.

  • Scientific/Medical: Botany/Horticulture, Geology, Chemistry, Histopathology, General Practitioner ($GP$), Optometrist, Pharmacist.

  • Administrative/Technical: Bank employees, Electrical work (resistors/cables), Pipelines, Filing systems, Computer software.

  • General Safety: Fire extinguishers, Safety notices.

• Career Limitations: Individuals with colour vision defects are only excluded from occupations in limited cases where connotative colour coding is safety-critical and no colleague is available to assist. Most cases require only professional advice and minor adaptation.

Electrical Standards and Colour Coding

• Wiring Changes (UK/Europe):

  • Phase: Formerly Red; Now Brown.

  • Neutral: Formerly Black; Now Blue.

  • Earth: Formerly Green; Now Green/Yellow.

• Resistors: Use a color-coded band system to indicate resistance value, multiplier, tolerance, and temperature coefficient.

Illumination and Metamerism

• The Impact of Illumination: Colour appearance is not fixed; it is a product of the light source and the object's reflectance.

  • Daylight conditions (Illuminant C): Provides a spectral energy distribution that mimics natural light.

  • Bulb light conditions (Incandescent): Shifting the energy incident on the cornea toward longer wavelengths (reds/yellows).

• Metamerism:

  • Definition: A condition where two different physical stimuli (different spectral power distributions) result in the same perceived colour under specific lighting conditions.

  • Observed Scenarios:

    • Sample 1 (Orange pigment only) vs. Sample 2 (Combination of red and yellow pigments).

    • Under one light source, they may appear identical (Metamerism exists).

    • Under a different light source, they may appear distinct (No metamerism exists).

  • Influence Factors: Colour judgments are fundamentally affected by either the illumination quality or individual colour vision defects.

Commercial and Practical Examples

• Anecdote (Protanopia): An image illustrates the potential aesthetic mismatch when a protanope (lacking long-wavelength/red sensitive cones) selects paint for a house without guidance.

• Farrow & Ball Paint Examples: The use of descriptive, non-standard names for pigments such as "Snow White" (No. W1), "Skimmed Milk White" (No. W7), "Ash Grey" (No. W9), "Broccoli Brown" (No. W108), and "Ultra Marine Blue" (No. W29).

• RAL Classic System: A standardized color collection (e.g., RAL Yellow hues) containing specific codes like RAL 1000 through RAL 1037 to ensure industrial consistency.