4.10 Color terms⧉
This part has leaned on a precise vocabulary, and the words are easy to run together: radiance and luminance measure different things, luminance and luma live in different domains, lightness and brightness are different percepts, chroma and saturation are not interchangeable. This chapter gathers the terms in one place, grouped by what they measure, with the confusions named explicitly at the end.
4.10.1 Radiometry: physical light, no observer⧉
Radiometry. The physical measurement of light as electromagnetic power, with no reference to a human observer.
Radiance. Power per unit projected area per unit solid angle, in W·sr⁻¹·m⁻². The fundamental quantity a camera measures, and the one preserved along a ray through empty space; sensor- and observer-independent. Written $L$ (linear).
Irradiance. Power per unit area arriving on a surface, in W·m⁻². What the sensor plane actually receives; it is tied to scene radiance through the lens by the aperture (f-number) and the projection geometry. Written $E$.
Spectral radiance. Radiance per wavelength interval, $L(\lambda)$, with units such as W·sr⁻¹·m⁻²·nm⁻¹. A spectral power distribution (SPD) describes power or another specified radiometric quantity versus wavelength; state which quantity and normalization. A spectrum alone does not specify the spatial, angular, temporal, or polarization distribution of light.
4.10.2 Photometry: physical light weighted by the eye⧉
Photometry. Radiometry weighted by the human luminosity function $V(\lambda)$, the daytime spectral sensitivity of vision. It defines standard observer-weighted physical quantities, not a complete prediction of perceived brightness. Other photometric regimes use other specified weightings.
Luminous flux. Total light power weighted by $V(\lambda)$, in lumens.
Luminance. The photometric counterpart of radiance: radiance weighted by $V(\lambda)$, in cd·m⁻². A linear weighted integral of spectral radiance; it is not proportional to total photon count independently of spectrum. It correlates with brightness but is not itself a percept. Written $Y$ in XYZ (or $L$ where context is clear). Physical color mixing can operate on full linear RGB or spectra; luminance alone discards chromatic information.
Illuminance. The photometric counterpart of irradiance, the luminous flux arriving per unit area, in lux.
The chronic confusion, luminance $Y$ vs luma $Y'$: $Y$ is linear, photometric and device-independent; $Y'$ is computed from specified nonlinear $R'G'B'$ and depends on the encoding and coefficients; it is generally not the encoding of $Y$. Same letter, different domain, different meaning (L4.5).
4.10.3 Colorimetry: color via the standard observer⧉
Colorimetry. The measurement of color, made repeatable by fixing a standard observer whose sensitivities stand in for a human's.
Color-matching functions (CMFs). $\bar x(\lambda),\ \bar y(\lambda),\ \bar z(\lambda)$: the CIE 1931 standard observer's three sensitivity curves, measured empirically from color-matching experiments. They are the mechanism by which a spectrum is projected onto three numbers.
Tristimulus values. $X, Y, Z$: the three numbers from integrating the SPD against the CMFs over wavelength with a specified normalization. They are linear in the stimulus; $Y$ is relative luminance or absolute luminance according to the scale.
Metamerism. Two different spectra that integrate to the same XYZ for a given observer, and match under the specified viewing conditions. Camera metamerism can differ from standard-observer metamerism. Surface matches can change with illumination, whether or not the illuminant is spiky.
Chromaticity. Tristimulus coordinates with overall scale removed: $x = X/(X{+}Y{+}Z)$, $y = Y/(X{+}Y{+}Z)$. A projective quantity, reduced from three dimensions to two, so it names a color's direction but not its amount and cannot be turned back into XYZ without $Y$ supplied separately.
Chromaticity diagram. The 2-D plot of $x,y$ (or the more uniform $u',v'$) chromaticities, the horseshoe. Three-primary additive gamuts project to triangles and the blackbody illuminants as the Planckian locus.
Spectral locus. The curved boundary of the horseshoe, the chromaticities of monochromatic (single-wavelength) lights. For ordinary broadband illumination, a bounded surface reflectance approaching a spectral endpoint generally has a low maximum luminance.
Line of purples. The straight edge closing the horseshoe between the red and violet ends of the spectral locus. Its colors are real but have no single-wavelength equivalent; they are mixtures of the two spectral extremes.
4.10.4 Color appearance: perceptual, observer-dependent⧉
Hue. The perceptual attribute we name with a color word, red, yellow, green, blue, which is not fully described by a dominant wavelength, especially for purples. One of the three basic dimensions of color appearance.
Lightness. Brightness judged relative to a similarly illuminated reference white. CIELAB $L^*$ is a model correlate, ordinarily 0–100 for black through reference white, with possible extended values. It is nonlinear in relative luminance.
Brightness. The attribute by which an area appears to emit, transmit, or reflect more or less light. It depends on stimulus and context. It is not a radiometric measurement, and self-luminous stimuli can also have lightness relative to an adopted white.
Colorfulness, chroma, saturation. Three distinct answers to "how colorful." Colorfulness is the absolute perceived amount of color, and it rises with the light (the Hunt effect). Chroma $C^*$ is colorfulness judged relative to a white reference (the $C^*$ of CIELAB). Saturation is colorfulness relative to the object's own brightness, these relative attributes depend on adaptation and viewing conditions and need not remain constant across illumination changes.
The chronic confusion, chroma vs saturation vs $C_b,C_r$: perceptual chroma $C^*$ (in CIELAB, $\sqrt{a^{*2}+b^{*2}}$) is not the $C_b,C_r$ channels of Y'CbCr, though both are loosely called "chroma." $C_b,C_r$ are encoding quantities, linear combinations of gamma-encoded values; $C^*$ is a perceptual magnitude.
4.10.5 Color spaces and encoding⧉
Primary. A basis vector of the 3-D color space; three primaries span a gamut. A primary need not be a physically producible color: XYZ’s three axes are imaginary primaries, chosen for mathematical convenience.
Gamut. The set of colors a given set of primaries and technology can reach. For three independent additive primaries, its chromaticity projection is a triangle and a bounded drive cube maps to a three-dimensional gamut. Subtractive and other devices require their own characterization.
White point (color-space sense). The chromaticity that equal channel values $(1,1,1)$ map to, which pins the normalization of the primaries. A mathematical parameter of a color-space definition (see the disambiguation table below for its other senses).
Color space. A specified coordinate system for color. An RGB encoding needs primaries, a reference white, and a transfer function; range and image state are also needed for processing. Other spaces have different definitions, and appearance models require viewing conditions.
Linear RGB. RGB in linear light, no transfer function applied, scene- or display-referred depending on context. The domain in which light adds and scales, so blends, blurs and resizes belong here. Unmarked, $R,G,B$.
Encoded RGB, $R'G'B'$. RGB after a transfer function (gamma, PQ, log) has been applied, the nonlinear form that camera JPEGs, display signals and most files carry. The prime marks a specified nonlinear encoding here; a log encoding is marked $R^{\ell}$.
Luma $Y'$. A weighted sum of the gamma-encoded $R'G'B'$, an approximate brightness carried by Y'CbCr and JPEG. Nonlinear, in the gamma domain, and not luminance.
Transfer function. The nonlinear map between linear light and encoded signal values, in either direction. OETF (opto-electronic transfer function) takes scene light to encoded signal, the camera side; EOTF (electro-optical transfer function) takes encoded signal to display light, the display side; OOTF (opto-optical transfer function) is the end-to-end scene-light-to-display-light rendering, the composition of the two.
Gamma. Loosely, a power-law transfer function with exponent $\gamma$. Historically the CRT's inherent $\approx V^{2.2}$ light output and the camera's $\gamma \approx 0.45$ compensation for it; more loosely, any display transfer curve. The prime $R'$ marks gamma-encoded quantities.
Scene-referred. Values interpreted relative to a scene, before output rendering. They may be linear or nonlinearly encoded, including camera log. “Scene-linear” states both properties.
Display-referred. Values interpreted as a rendered image for an output condition. They can be linear or nonlinear. A typical sRGB photograph is display-referred, though RGB primaries alone do not determine image state.
4.10.6 Chronic confusions, named⧉
Nine pairs this part keeps apart, and the distinction in each:
| term | vs | the distinction |
|---|---|---|
| luminance $Y$ | luma $Y'$ | linear photometric quantity vs weighted nonlinear components |
| lightness $L^*$ | luminance $Y$ | perceptual correlate vs linear photometric quantity with a stated scale |
| chroma $C^*$ | $C_b,C_r$ | perceptual magnitude in CIELAB vs encoding difference signals |
| saturation | chroma $C^*$ | relative to the object's brightness vs relative to white |
| chromaticity $x,y$ | color $XYZ$ | projective 2-D (luminance discarded) vs full 3-D |
| white point (space) | white balance (camera) | a mathematical normalization vs illuminant compensation |
| OETF | EOTF | camera encoding vs display decoding |
| colorfulness | chroma $C^*$ | absolute percept vs white-relative percept |
| primaries | phosphors / LEDs | abstract basis vectors vs physical emitters |
4.10.7 One word, several meanings⧉
A few terms carry different meanings in different chapters and are worth disambiguating on sight:
White point has four senses: a color-space normalization parameter, a display's calibration target, a viewer's adaptation state, and a camera's auto-white-balance target. They are designed to agree in a calibrated pipeline but are conceptually independent.
Color temperature is the temperature (in kelvin) of the blackbody whose glow matches a light's color, so it is only exactly meaningful on the Planckian locus; it is used loosely for any illuminant's chromaticity, and as a camera white-balance control even for non-blackbody sources. Correlated color temperature specifies a nearby Planckian-locus temperature; it does not specify the source spectrum or its off-locus distance.
Neutral means achromatic under specified conditions. Equal RGB components represent the defined RGB white chromaticity; equal XYZ represents equal-energy white, not every neutral. Equal LMS is a normalization-dependent convention, not a general test of neutrality.
Linear can mean proportional to physical light (radiometrically linear), a linear-algebra operation (a matrix, no nonlinearity), or a 1-D sensor layout (a linear array). It always needs its context.