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Colorfulness
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Colorfulness, chroma and saturation are attributes of perceived color relating to chromatic intensity. As defined formally by the International Commission on Illumination (CIE) they respectively describe three different aspects of chromatic intensity, but the terms are often used loosely and interchangeably in contexts where these aspects are not clearly distinguished. The precise meanings of the terms vary by what other functions they are dependent on.
- Colorfulness is the "attribute of a visual perception according to which the perceived color of an area appears to be more or less chromatic (Any color that is absent of white, grey, or black)[clarification needed]".[1][2] The colorfulness evoked by an object depends not only on its spectral reflectance but also on the strength of the illumination, and increases with the latter unless the brightness is very high (Hunt effect).
- Chroma is the "colorfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears white or highly transmitting".[3][2] As a result, chroma is mostly only dependent on the spectral properties, and as such is seen to describe the object color.[4] It is how different from a grey of the same lightness such an object color appears to be.[5]
- Saturation is the "colorfulness of an area judged in proportion to its brightness",[6][2] which in effect is the perceived freedom from whitishness of the light coming from the area. An object with a given spectral reflectance exhibits approximately constant saturation for all levels of illumination, unless the brightness is very high.[7]
As colorfulness, chroma, and saturation are defined as attributes of perception, they can not be physically measured as such, but they can be quantified in relation to psychometric scales intended to be perceptually even—for example, the chroma scales of the Munsell system. While the chroma and lightness of an object are its colorfulness and brightness judged in proportion to the same thing ("the brightness of a similarly illuminated area that appears white or highly transmitting"), the saturation of the light coming from that object is in effect the chroma of the object judged in proportion to its lightness. On a Munsell hue page, lines of uniform saturation thus tend to radiate from near the black point, while lines of uniform chroma are vertical.[7]
Chroma
[edit]The naïve definition of saturation does not specify its response function. In the CIE XYZ and RGB color spaces, the saturation is defined in terms of additive color mixing, and has the property of being proportional to any scaling centered at white or the white point illuminant. However, both color spaces are non-linear in terms of psychovisually perceived color differences. It is also possible — and sometimes desirable — to define a saturation-like quantity that is linearized in term of the psychovisual perception.
In the CIE 1976 LAB and LUV color spaces, the unnormalized chroma is the radial component of the cylindrical coordinate CIE LCh (lightness, chroma, hue) representation of the LAB and LUV color spaces, also denoted as CIE LCh(ab) or CIE LCh for short, and CIE LCh(uv). The transformation of to is given by: and analogously for CIE LCh(uv).
The chroma in the CIE LCh(ab) and CIE LCh(uv) coordinates has the advantage of being more psychovisually linear, yet they are non-linear in terms of linear component color mixing. And therefore, chroma in CIE 1976 Lab and LUV color spaces is very much different from the traditional sense of "saturation".
In color appearance models
[edit]Another, psychovisually even more accurate, but also more complex method to obtain or specify the saturation is to use a color appearance model like CIECAM02. Here, the chroma color appearance parameter might (depending on the color appearance model) be intertwined with e.g. the physical brightness of the illumination or the characteristics of the emitting/reflecting surface, which is more sensible psychovisually.
The CIECAM02 chroma for example, is computed from a lightness in addition to a naively evaluated color magnitude In addition, a colorfulness parameter exists alongside the chroma It is defined as where is dependent on the viewing condition.[8]
Saturation
[edit]The saturation of a color is determined by a combination of light intensity and how much it is distributed across the spectrum of different wavelengths. The purest (most saturated) color is achieved by using just one wavelength at a high intensity, such as in laser light. If the intensity drops, then as a result the saturation drops. To desaturate a color of given intensity in a subtractive system (such as watercolor), one can add white, black, gray, or the hue's complement.
Various correlates of saturation follow.
CIELUV and CIELAB
[edit]In CIELUV, saturation is equal to the chroma normalized by the lightness: where is the chromaticity of the white point, and chroma is defined below.[9]
By analogy, in CIELAB this would yield:
The CIE has not formally recommended this equation since CIELAB has no chromaticity diagram, and this definition therefore lacks direct connection with older concepts of saturation.[10] Nevertheless, this equation provides a reasonable predictor of saturation, and demonstrates that adjusting the lightness in CIELAB while holding (a*, b*) fixed does affect the saturation.
But the following verbal definition of Manfred Richter and the corresponding formula proposed by Eva Lübbe are in agreement with the human perception of saturation: Saturation is the proportion of pure chromatic color in the total color sensation.[11] where is the saturation, the lightness and is the chroma of the color.
CIECAM02
[edit]In CIECAM02, saturation equals the square root of the colorfulness divided by the brightness:
This definition is inspired by experimental work done with the intention of remedying CIECAM97s's poor performance.[8][12] is proportional to the chroma thus the CIECAM02 definition bears some similarity to the CIELUV definition.[8]
HSL and HSV
[edit]Saturation is also one of three coordinates in the HSL and HSV color spaces. However, in the HSL color space saturation exists independently of lightness. That is, both a very light color and a very dark color can be heavily saturated in HSL; whereas in the previous definitions—as well as in the HSV color space—colors approaching white all feature low saturation.
Excitation purity
[edit]
The excitation purity (purity for short) of a stimulus is the difference from the illuminant's white point to the furthest point on the chromaticity diagram with the same dominant wavelength; using the CIE 1931 color space:[13] where is the chromaticity of the white point and is the point on the perimeter whose line segment to the white point contains the chromaticity of the stimulus. Different color spaces, such as CIELAB or CIELUV may be used, and will yield different results.
References
[edit]- ^ "colourfulness | eilv". eilv. Archived from the original on August 6, 2017. Retrieved December 20, 2017.
- ^ a b c Fairchild, Mark (2013). Color Appearance Models. John Wiley & Sons., page 87.
- ^ "CIE e-ILV 17-139". Archived from the original on April 10, 2017.
- ^ "CIE e-ILV 17-831". Archived from the original on April 10, 2017.
- ^ "The Dimensions of Colour". www.huevaluechroma.com. Archived from the original on March 30, 2017. Retrieved April 10, 2017.
- ^ "CIE e-ILV 17-1136". Archived from the original on April 10, 2017.
- ^ a b "The Dimensions of Colour". www.huevaluechroma.com. Archived from the original on March 30, 2017. Retrieved April 10, 2017.
- ^ a b c Moroney, Nathan; Fairchild, Mark D.; Hunt, Robert W.G.; Li, Changjun; Luo, M. Ronnier; Newman, Todd (November 12, 2002). IS&T/SID Tenth Color Imaging Conference (PDF). The CIECAM02 Color Appearance Model. Scottsdale, Arizona: The Society for Imaging Science and Technology. ISBN 0-89208-241-0. Archived from the original (PDF) on October 2, 2011.
- ^ Schanda, János (2007). Colorimetry: Understanding the CIE System. Wiley Interscience. ISBN 978-0-470-04904-4. Archived from the original on January 17, 2017., page 88.
- ^ Hunt, Robert William Gainer (1993). Leslie D. Stroebel, Richard D. Zakia (ed.). The Focal Encyclopedia of Photography. Focal Press. p. 124. ISBN 0-240-51417-3.
- ^ Lübbe, Eva (2010). Colours in the Mind - Colour Systems in Reality- A formula for colour saturation. [Book on Demand]. ISBN 978-3-7881-4057-1.
- ^ Juan, Lu-Yin G.; Luo, Ming R. (June 2002). Robert Chung; Allan Rodrigues (eds.). Magnitude estimation for scaling saturation. 9th Congress of the International Colour Association. Proceedings of SPIE. Vol. 4421. pp. 575–578. doi:10.1117/12.464511.
- ^ Stroebel, Leslie D.; Zakia, Richard D. (1993). The Focal Encyclopedia of Photography (3E ed.). Focal Press. p. 121. ISBN 0-240-51417-3.
excitation purity.
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Colorfulness
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Colorfulness is an attribute of visual perception according to which the perceived color of an area appears to be more or less chromatic.[1] In color science, it quantifies the absolute intensity or vividness of a color stimulus, independent of surrounding or reference conditions, and is one of six key attributes of perceived color defined by the International Commission on Illumination (CIE): hue, brightness, lightness, colorfulness, saturation, and chroma.[1] For a color stimulus of given chromaticity, colorfulness typically increases with luminance, except at very high brightness levels where it may plateau.[1]
Distinct from related attributes, colorfulness differs from chroma, which is the colorfulness of an area judged relative to the brightness of a similarly illuminated area that appears gray, white, or highly transmitting, making chroma more constant across illuminance levels for related colors.[2] Similarly, saturation measures colorfulness in proportion to the area's own brightness, emphasizing the perceived purity or freedom from achromatic dilution within the stimulus itself.[3] These distinctions are formalized in the CIE International Lighting Vocabulary (ILV), which underpins color appearance models like CIECAM02 and CIECAM16, where colorfulness is often denoted as a perceptual correlate (e.g., M) to predict how colors appear under varying viewing conditions such as adaptation luminance and background.[1][2][3]
In practical applications, colorfulness plays a critical role in fields like digital imaging, display technology, and textile design, where accurate reproduction requires modeling its dependence on absolute luminance to avoid perceptual distortions in rendered scenes.[1] For instance, under dim illumination, colors may exhibit reduced colorfulness compared to brighter conditions, influencing judgments of vibrancy in environments from art conservation to virtual reality.[1]
Fundamentals
Definition
Colorfulness is the attribute of a visual perception according to which the perceived color of an area appears to be more or less chromatic.[1] This perceptual attribute describes the intensity of the chromatic component in a visual sensation, which depends on the absolute luminance of the stimulus.[4] The term "colorfulness" was proposed by R. W. G. Hunt in 1977 to denote this distinct aspect of color appearance, distinguishing it from physical properties of light and earlier terms like saturation.[4] It was subsequently formalized in the International Commission on Illumination (CIE) vocabulary as a key psychophysical attribute of color perception.[1] For example, a vivid red apple appears more colorful than a muted grayish red under the same lighting conditions, even though both share the same hue and lightness. Colorfulness is perceived relative to the viewer's state of chromatic adaptation, which influences how chromatic the color seems in a given viewing context.[4] It is related but distinct from chroma, which quantifies colorfulness for object colors relative to a reference white under specified viewing conditions.[1]Perceptual Aspects
The perception of colorfulness in the human visual system begins at the retinal level, where three types of cone photoreceptors—long-wavelength-sensitive (L) cones peaking around 564 nm, medium-wavelength-sensitive (M) cones peaking around 534 nm, and short-wavelength-sensitive (S) cones peaking around 420 nm—respond to different portions of the visible spectrum. These cones generate signals based on the intensity of light they absorb, and colorfulness emerges from the magnitude of differences in their activation patterns, particularly the strength of the opponent signals in the chromatic channels (L-M for red-green and S-(L+M) for blue-yellow). This differential stimulation allows the brain to interpret the absolute chromatic content of a stimulus, distinguishing it from achromatic luminance signals processed primarily by rod cells in low-light conditions.[5] Viewing conditions significantly modulate perceived colorfulness, with higher luminance levels amplifying the attribute even when the stimulus's chromaticity remains unchanged. This phenomenon, known as the Hunt effect, results from the visual system's nonlinear scaling of chromatic responses with overall light intensity, making colors appear more vivid under brighter illumination. Chromatic adaptation further influences this perception by adjusting cone sensitivities to the ambient spectral distribution, enhancing the relative colorfulness of stimuli that deviate from the adapting field; for instance, in dim environments with neutral adaptation, a brightly lit chromatic source like a neon sign can appear exceptionally vivid due to the contrast with the low-chromatic surround.[6] Unlike relative attributes such as saturation, which scale colorfulness against a reference white or the stimulus's own achromatic component, colorfulness is inherently absolute and dependent on the stimulus's overall excitation level, making it sensitive to absolute photometric conditions rather than proportional purity. This scale-dependence underscores colorfulness as a holistic measure of chromatic strength, varying predictably with environmental luminance to support adaptive object recognition.[6]Related Attributes
Chroma
Chroma refers to the colorfulness of an area relative to the brightness of a similarly illuminated area that appears white, and it is primarily applied to the perceived colors of objects or surfaces rather than light sources. This attribute quantifies the intensity of chromatic deviation from a neutral color of the same lightness, providing a measure of how vivid or strong an object color appears under specified viewing conditions. In color science, chroma is essential for describing surface colors, such as those in paints, fabrics, or printed materials, where it captures the perceptual purity independent of absolute brightness levels.[7] A key distinction exists between chroma and colorfulness: while colorfulness pertains to the absolute chromatic intensity of light stimuli or overall visual appearance, chroma is specifically relative and suited to object-mode perception, such as the hue strength in a pigmented coating viewed under daylight. This separation allows for more precise modeling of how colors are judged in real-world contexts, like assessing the vibrancy of a wall paint compared to a reference white surface. In practice, chroma helps in applications requiring consistent object color reproduction, for instance, measuring pigment chroma in the printing industry to match batches and maintain uniformity across productions.[8][9] The Munsell color system exemplifies chroma's perceptual scaling, where it ranges from 0 for achromatic neutrals to 16 or higher for highly vivid colors, with steps designed to appear equally spaced to the human eye. This scale reflects the limited strength of pigments, as stronger materials can extend beyond typical maxima, aiding artists and designers in specifying intense hues like a bright red with high chroma. Historically, the CIE formalized the term chroma in its 1976 recommendations for uniform color spaces, establishing it as a distinct perceptual attribute to differentiate from saturation and enable better color specification.[10]Saturation
Saturation refers to the colorfulness of a color relative to its own brightness, representing the perceived intensity of the chromatic component normalized by the overall luminance.[3] Equivalently, it can be understood as the proportion of chromatic to achromatic components in the sensory response to a stimulus of a given hue. This attribute emphasizes the purity of the hue, independent of absolute brightness levels, allowing for consistent evaluation across varying illumination conditions. A key aspect of saturation is that it diminishes as the grayness of a color increases while maintaining the same hue and lightness; for instance, desaturating a vivid red by mixing in gray results in a muted tone with reduced perceptual strength.[11] A pure spectral color, such as a monochromatic wavelength of light, exhibits maximum saturation due to its complete absence of achromatic dilution.[12] In contrast, adding white to this spectral color progressively reduces saturation, yielding softer pastel shades that appear less intense.[13] In human color perception, saturation is not solely determined by the stimulus itself but is also influenced by the luminance of the surrounding field, as explained by opponent-process theory, which posits that chromatic signals are processed in opposition to achromatic ones, modulating perceived purity based on contextual contrast. This contextual effect arises because higher surround luminance can enhance the relative prominence of chromatic channels, altering saturation judgments even for fixed stimuli.[14] Unlike colorfulness, which measures absolute chromatic strength, saturation is a normalized attribute that facilitates comparisons between colors of differing brightness, making it particularly valuable in perceptual studies and color reproduction.[1] Saturation can be briefly contrasted with chroma, a related but non-relative measure that assesses color intensity for object colors in proportion to an equally bright white.[15]Excitation Purity
Excitation purity, denoted as , is a colorimetric measure that quantifies the degree to which a color stimulus approaches a pure spectral color in terms of its chromaticity. It is calculated as the ratio of two collinear distances on the CIE 1931 xy chromaticity diagram: the distance from the achromatic (white) point N to the color point C, divided by the distance from N to the point D on the spectrum locus (or purple boundary for non-spectral hues) along the same line through C.[16] Mathematically, this is expressed as: where are the chromaticity coordinates of the color C, are those of the white point N (e.g., illuminant E at (1/3, 1/3)), and are those of point D; the formula using the coordinate yielding the larger numerator is preferred for numerical stability.[16] This metric is dimensionless and ranges from 0 for achromatic stimuli (pure white, where C coincides with N) to 1 for spectral colors (where C lies on the spectrum locus).[16] It proves particularly valuable for evaluating the chromatic properties of light sources, as it directly reflects their spectral composition relative to ideal monochromatic emissions.[16] The concept of excitation purity was introduced by the International Commission on Illumination (CIE) in 1931 as part of the foundational CIE XYZ color space and chromaticity diagram, providing a straightforward way to assess the "purity" or saturation-like quality of color stimuli without requiring perceptual scaling.[17] For practical applications, excitation purity highlights differences in spectral bandwidth among sources. Laser light, with its narrow emission approximating a single wavelength, achieves values close to 1, representing near-ideal spectral purity. In contrast, broadband sources like light-emitting diodes (LEDs) exhibit lower values, typically in the range of 0.3 to 0.7 for common colored LEDs, due to their wider spectral output; for example, high-quality red LEDs can reach up to 0.95 under optimal conditions.[18] As a physical metric derived from chromaticity, excitation purity approximates perceptual attributes like saturation but remains tied to the diagram's geometry rather than human vision models.[16]Color Models and Measurements
Uniform Color Spaces
Uniform color spaces, such as CIELUV and CIELAB, provide a framework for quantifying chroma through metrics that approximate perceptual uniformity, allowing equal numerical steps to correspond to equal perceived differences in color attributes.[19] These spaces transform tristimulus values into coordinates where deviations from the neutral axis represent chroma, a relative measure of colorfulness, facilitating precise measurements independent of device-specific representations. Colorfulness, being an absolute attribute, is more directly addressed in appearance models. In the CIELUV space, adopted by the CIE in 1976, chroma is measured by the , where and are transformed coordinates derived from the uniform chromaticity scale (UCS) to enhance perceptual uniformity. Similarly, the CIELAB space uses chroma , which quantifies the deviation from the neutral axis in the opponent-color dimensions a^* (red-green) and b^* (yellow-blue).[19] These metrics build on conceptual chroma by embedding it within a three-dimensional structure that separates lightness from chromatic attributes. A core objective of these spaces is to achieve equal perceptual steps across lightness, chroma, and hue, enabling the color difference formula (or analogous for CIELUV) to incorporate chroma differences alongside other attributes for overall perceived variation.[19] In CIELAB, for instance, a of 1 unit approximates a just-noticeable change in chroma, particularly for low-chroma colors near the neutral axis.[20] These uniform spaces find practical application in industries requiring accurate color matching, such as textiles, where CIELAB metrics help ensure consistent color reproduction during quality control.[21]Appearance Models
Appearance models in color science extend beyond uniform color spaces by incorporating dynamic perceptual factors such as luminance adaptation, viewing surround, and background influences to predict colorfulness under realistic conditions. These models transform device-independent tristimulus values, like CIE XYZ, into perceptual attributes that account for the human visual system's adaptation mechanisms, providing a more accurate representation of how colorfulness is perceived in context. Unlike static uniform spaces, appearance models emphasize psychophysical realism, making them essential for applications requiring perceptual fidelity, such as cross-media color reproduction.[22] The CIECAM02 model, recommended by the International Commission on Illumination (CIE) in 2002, exemplifies this approach by deriving colorfulness from adapted cone responses. It begins with chromatic adaptation using the CAT02 transform to obtain post-adaptation cone signals (R_c, G_c, B_c), which are then nonlinearly compressed to R'_a, G'_a, B'_a, incorporating the luminance adaptation factor F_L that scales with adapting luminance L_A. Opponent chromatic signals a and b are computed from these: a = (R'_a/100 - 12 * G'_a/100 / 11 + 1/11 * B'_a/100) * 50000 / 13 * N_c * N_cb * (f / F_L)^{0.8}, and similarly for b, where N_c and N_cb account for surround and background induction, respectively. The temporary magnitude t is derived from the hue angle h and eccentricity e_t based on a and b, leading to chroma C = t^{0.9} \sqrt{J/100} (1.64 - 0.29^n)^{0.73}, where J is lightness and n = Y_b / Y_w (background relative to white). Colorfulness M is then M = C \cdot F_L^{0.25}, capturing the absolute chromatic intensity relative to a neutral stimulus under the given adaptation state. This formulation integrates surround effects (via parameters F, c, N_c for average, dim, or dark conditions) and background relative luminance, enabling predictions that align with perceptual phenomena like the Hunt effect.[22] CIECAM02 represents an update to the earlier CIECAM97s model, incorporating a linear chromatic adaptation transform, revised nonlinear response functions, and simplified perceptual correlates to improve accuracy and computational efficiency. It is widely adopted in high-fidelity imaging and color management systems for tasks like soft-proofing and gamut mapping, where contextual color appearance must be preserved across displays and prints. For instance, CIECAM02 predicts greater colorfulness for the same chromatic stimulus in a bright viewing booth (high L_A, larger F_L) compared to a dim room (low L_A, smaller F_L), reflecting enhanced chromatic response under higher luminance adaptation. Uniform color spaces like CIELAB serve as precursors by providing baseline uniformity, but appearance models like CIECAM02 advance this by dynamically modeling adaptation and context.[22] In 2022, the CIE updated its recommendations with CIECAM16 (CIE 248:2022), refining the framework to better handle high dynamic range (HDR) content and modern imaging workflows while maintaining compatibility with CIECAM02 structures. CIECAM16 improves predictions for extreme luminance levels and viewing conditions, enhancing colorfulness estimation in HDR scenarios by optimizing parameters for broader adaptation ranges and reducing artifacts in uniform color spaces derived from it, such as CAM16-UCS.Digital Color Spaces
In digital graphics and design, color spaces like HSV (Hue, Saturation, Value) and HSL (Hue, Saturation, Lightness) provide practical approximations of saturation through their saturation components, enabling intuitive manipulation of perceived color vividness in software and rendering pipelines.[23] These models transform RGB values into cylindrical coordinates where saturation quantifies the departure from achromatic colors, roughly aligning with a relative measure of colorfulness by emphasizing chromatic intensity relative to lightness or value.[23] The HSV saturation is defined as when the maximum is nonzero, otherwise , where are normalized to [0,1]; this measures color purity as the relative difference from the brightest channel, approximating perceptual saturation but introducing distortions at low saturation levels where small changes yield disproportionate perceptual shifts.[23] In contrast, HSL saturation normalizes for lightness and is computed as , providing a more hue-uniform scaling that reduces some distortions in mid-tones and offers perceptual consistency across color families. While HSV chroma better captures vividness in high-value scenarios, HSL's lightness-adjusted approach makes it preferable for balanced hue editing. For instance, in Adobe Photoshop, HSL adjustments allow users to selectively increase saturation for specific hues, directly enhancing perceived vividness by amplifying chromatic content without altering overall brightness, as seen in the Saturation slider which intensifies colors toward full purity. In web design, CSS supports HSL via thehsl() function, where developers tweak saturation percentages for intuitive color editing, such as boosting vividness in user interfaces by adjusting the second parameter independently of hue or lightness.[24]
However, neither HSV nor HSL achieves perceptual uniformity, as equal numerical steps in saturation do not correspond to equal perceived colorfulness differences, leading to inconsistencies in low-saturation and extreme lightness regions. Despite this, modern applications like AR/VR rendering leverage HSL/HSV saturation for depth cues, such as desaturating virtual objects to simulate distance and enhance colorfulness gradients in mixed realities.