Glossary of Light Color, Chromaticity, and Photometry

This reference is designed for professionals, engineers, and scholars in lighting, optical engineering, and color science. Each entry offers a rigorous definition, theory, measurement methodology, and practical aviation applications, referencing ICAO, CIE, and other authoritative sources.

A

Absorption (of Light)

Absorption is the process by which a material captures incident electromagnetic radiation (including visible light), converting its energy—usually into heat, sometimes into photochemical energy. The degree of absorption depends on the material’s properties, the incident wavelength, and angle.

Application:
Absorption affects the spectral composition of light in lighting, photometry, and colorimetry. For example, in aircraft cabin lighting, materials with low visible absorption are selected for efficiency, while coatings may target selective absorption for glare or UV reduction.

Measurement:
Quantified by the absorption coefficient; measured using spectrophotometers or integrating spheres. ICAO standards (Annex 14) require surface materials for runway lighting to be tested for absorption to ensure visibility and color fidelity.

Example:
Neutral density filters and sunglasses employ controlled absorption to attenuate light. In photometry, absorption corrections are made for accurate luminous flux or chromaticity measurements.

B

Blackbody (Planckian Radiator)

A blackbody is an idealized object that absorbs all incident radiation and re-emits energy according to Planck’s law. Its emitted spectrum is determined solely by temperature, with a peak wavelength shifting inversely with temperature per Wien’s displacement law.

Application:
Blackbody radiation underpins color temperature definitions. ICAO specifies blackbody chromaticities for aerodrome lighting, ensuring global consistency. The Planckian locus in chromaticity diagrams references correlated color temperature (CCT).

Measurement:
Blackbody sources (e.g., tungsten lamps) calibrate photometric instruments. Their spectral power distribution is compared to real sources to determine CCT.

Example:
CIE Standard Illuminant A (2856 K) is a blackbody-based reference for calibration and color rendering. Cockpit displays use blackbody references for consistent white balance.

Brightness

Brightness is the subjective attribute describing how luminous an area appears to a human observer, regardless of its physical luminance. Influenced by light intensity, observer adaptation, background, and contrast, brightness is a perceptual phenomenon, not a directly measurable physical value.

Application:
Luminance (cd/m²) is the physical correlate, but human perception may diverge due to factors like the Purkinje effect or glare. ICAO specifies minimum luminance and uniformity for airfield lighting to ensure perceived brightness for pilot safety.

Measurement:
Luminance meters or imaging photometers provide objective measurements. Psychophysical tests relate perceived brightness to luminance in research.

Example:
Runway edge lights must meet minimum luminance for visibility, accounting for atmospheric attenuation. Cockpit lighting brightness is adjusted to avoid glare and preserve night vision.

C

Candela (cd)

The candela (cd) is the SI base unit of luminous intensity—the luminous power emitted by a source in a specific direction per unit solid angle (steradian), weighted by the V(λ) luminosity function for human photopic vision.

Application:
Candela quantifies the directional output of signal and navigation lights. ICAO sets minimum/maximum candela values for aviation lighting to ensure both visibility and avoidance of glare.

Measurement:
Measured with goniophotometers or photometric benches, aligning a sensor at specific angles and quantifying the flux per solid angle.

Example:
Runway centerline lights must emit at least 200 cd in the primary beam, per ICAO Annex 14. Aircraft anti-collision lights are rated in candelas for visibility from specified angles.

Chromaticity

Chromaticity specifies a color’s quality independent of luminance—defining hue and saturation. Represented by coordinate pairs (x, y) in CIE 1931 or (u’, v’) in CIE 1976 diagrams.

Application:
Fundamental for color specification in lighting, displays, and material science. Aviation standards define chromaticity regions for signal lights, ensuring colors are distinguishable for safety.

Measurement:
Derived from tristimulus values (X, Y, Z) using CIE color matching functions. Instruments like spectroradiometers measure spectral power distributions to calculate chromaticity.

Example:
ICAO defines green chromaticity boundaries for taxiway lights. LED manufacturing requires tight chromaticity control to prevent visible color variation.

Chromaticity Coordinates

Numeric values defining a color’s position within a chromaticity diagram—(x, y) in CIE 1931, (u’, v’) in CIE 1976 UCS.

Application:
Used to precisely communicate and specify color tolerances in manufacturing and regulation. ICAO defines allowable coordinates for aviation lighting compliance.

Calculation:
From tristimulus values:

• x = X/(X+Y+Z)
• y = Y/(X+Y+Z)
• u’ = 4X/(X+15Y+3Z)
• v’ = 9Y/(X+15Y+3Z)

Example:
LED modules for aircraft interiors are tested for (x, y) coordinates to ensure color uniformity.

Chromaticity Diagram

A 2D graphical representation of all perceivable chromaticities. The CIE 1931 (x, y) and CIE 1976 (u’, v’) diagrams are most common. The spectral locus forms the boundary of pure spectral colors.

Application:
Used in lighting and display design, calibration, and regulation. ICAO and CIE standards define color regions for compliance.

Visualization:
The CIE 1931 diagram forms a horseshoe shape; the Planckian locus traces color temperature progression.

Example:
Certifying new LED runway lights involves plotting their chromaticity to verify compliance with ICAO color boundaries.

CIE (Commission Internationale de l’Éclairage)

The CIE is the international authority for light and color measurement standards. It establishes systems and nomenclature for photometry, colorimetry, and radiometry, forming the basis for ICAO, ISO, and IES standards.

Key Contributions:

• CIE 1931 Standard Observer and color matching functions
• Standard illuminants (A, D65, etc.)
• Chromaticity, color temperature, color rendering definitions
• Measurement protocols for photometric units

Example:
ICAO’s color specifications for runway lighting are based on CIE standards, ensuring global consistency and safety.

CIE 1931 Standard Observer

A mathematical model of average human eye color response under photopic (daylight) conditions, based on the color matching functions (x̄(λ), ȳ(λ), z̄(λ)) for a 2° field of view (foveal region).

Application:
Reference for all colorimetric calculations—XYZ values, chromaticity coordinates, and color spaces. ICAO mandates the CIE 1931 observer for aviation lighting certification.

Measurement:
Colorimetric instruments use these functions to derive XYZ values from spectral measurements.

Example:
A spectroradiometer measures the SPD of navigation lights, applying CIE 1931 functions to compute color coordinates for regulatory compliance.

Color Coordinate

A numeric value (or set) locating a color stimulus within a color space or chromaticity diagram—(x, y) in CIE 1931, (u’, v’) in CIE 1976, (L*, a*, b*) in CIE Lab*.

Application:
Used for color specification, manufacturing, lighting design, and compliance. Aviation lights are defined by specific color coordinates in ICAO documents.

Example:
Taxiway lights are batch-tested for (x, y) coordinates to confirm emission of the specified green.

Color Matching Functions

Standardized spectral sensitivity curves (x̄(λ), ȳ(λ), z̄(λ)) representing the CIE Standard Observer’s response to monochromatic light. They define how much of each primary is needed to match a given wavelength.

Application:
Form the mathematical basis for transforming SPDs into XYZ tristimulus values, used in all colorimetric computations.

Measurement:
Applied as weighting factors to measured SPDs between 380–780 nm.

Example:
Aircraft warning lights’ SPDs are multiplied by color matching functions to verify chromaticity compliance.

Colorimetry

The science of quantifying and describing human color perception using standardized systems and numerical descriptors. It includes color spaces, measurement protocols, and mathematical transformations linking physical light to perceived color.

Application:
Foundation for specification and quality control of color in lighting and displays. ICAO and CIE standards use colorimetric methods to define acceptable parameters for aviation lighting.

Measurement:
Involves measuring SPD, applying color matching functions, and calculating tristimulus values, chromaticity, and correlated color temperature.

Example:
LED navigation lights are evaluated with colorimetric analysis to ensure ICAO compliance.

Color Rendering (Color Rendering Index, CRI)

Color rendering describes how faithfully a light source reproduces the colors of objects compared to a reference. The Color Rendering Index (CRI, Ra) is a CIE metric (0–100) quantifying this property.

Application:
Critical for environments where color accuracy matters—aircraft cabins, control rooms, and signage. ICAO and IES recommend minimum CRI for interior/exterior lighting for safety and visual clarity.

Measurement:
Assessed by illuminating standard color samples, measuring color differences from a reference, and averaging the first eight test colors.

Example:
Passenger aircraft cabin lighting is specified with a CRI above 80 for comfort and accurate safety sign color perception.

Color Space

A mathematical system defining a set of colors as coordinate combinations. Common spaces: CIE XYZ (device-independent), sRGB (displays), CIE Lab* (perceptually uniform).

Application:
Facilitates consistent color communication across devices and media. Aviation lighting, displays, and materials reference color spaces for technical specification and compliance.

Example:
Aircraft cockpit displays are calibrated in sRGB to ensure correct symbol and warning color appearance.

Color Temperature

The temperature in kelvin (K) of a blackbody radiator that emits light of a color most similar to that of the light source. Describes the hue of white light, from warm (low K) to cool (high K).

Application:
Specifies white light source hue in lighting design. ICAO references color temperature to distinguish classes of white airfield lighting.

Measurement:
Determined by matching the source’s chromaticity to the Planckian locus in a chromaticity diagram.

Example:
Runway edge lights may be specified at 4000–6500 K for neutral to cool white, maximizing visibility.

Correlated Color Temperature (CCT)

CCT is the temperature of the blackbody whose chromaticity most closely matches that of a non-blackbody source (like LEDs), expressed in kelvin (K). Used to describe the hue of white light when the source does not emit perfect blackbody radiation.

Application:
CCT specifies the “whiteness” of LEDs, fluorescent, or other engineered lights. In aviation, CCT ensures consistent signal and runway lighting appearance, even with modern non-blackbody sources.

Measurement:
The source’s chromaticity is plotted on a diagram, and CCT is determined by the nearest point on the Planckian locus.

Example:
LED runway lights are specified for CCT to ensure their white matches existing incandescent systems.

_This glossary will continue to expand as lighting technology, measurement science, and standards evolve.and standards evolve.