Glossary of Colour Terms

This Colour Glossary is a reference page covering descriptions of key colour terms. To learn more about Designer’s colour management tools and workflows, see the Colour Management section.

Common Colour Space Examples

A colour space is a complete model that defines how colours are represented across different devices or systems. It maps the entire range of colours (the gamut) that can be produced within a given environment.

Rec. 709

Rec. 709 is the standard camera encoding colour space for HDTV with a gamut identical to sRGB. However, Rec. 709 differs slightly in its encoding transfer function, and doesn’t actually specify an inverse transfer function for display.

For broadcast encoding, it is defined in 8-bit colour depth (values between 0 and 255) where black is level 16, and white is level 235. These are often referred to as “video levels”.

In the case of 10-bit colour depth which is common for post production, full range levels are between 0 and 1023, but the final output is mapped to broadcast standard 8-bit 16-235 when creating common deliverables.

Rec. 709 is by far the most common working and delivery colour space for most video projects. If you’re creating video for broadcast delivery, or that will be consumed online, then Rec. 709 is most likely what you need to work and monitor in. The Rec. 709 gamut is supported by all common display technologies across many devices. Most computer video players know how to deal with Rec. 709 encoded video, and can display it correctly on an sRGB computer display.

sRGB

sRGB is a display referred colour space originally created for CRT computer monitors, but has become standardised for graphics and print. It is almost identical to the Rec. 709 video colour space. It’s based on the same primaries and has the same gamut as Rec. 709 but specifies a different transfer function and gamma profile.

sRGB is still the standard for computer imaging, most consumer to mid level photo cameras and home printers. For professional printing and pre-press purposes Adobe RGB is often used which has an extended gamut that can be reproduced with professional CMYK printing. sRGB doesn’t have anything to do with video in the context of broadcast standards, except to know that for the most part Rec. 709 video will look fine on a sRGB computer display. There might be a slight difference in brightness due to the difference in transfer function.

DCI P3

DCI-P3 is a wide gamut video colour space introduced by SMPTE for digital cinema projection. It is designed to closely match the full gamut of colour motion picture film.

It is generally not a consumer standard and is mostly used for content destined for digital theatrical projection. However, Apple have adopted P3 colour across many device displays, and the ability to capture photo and video in the P3 colour space since iOS10.

Most professional reference monitors are able to display the full DCI P3 gamut.

You will often see a white point specified along with the colour space, such as P3 D55, P3 D61 or P3 D65. The D number indicates the target white colour temperature given in degrees Kelvin. D55 is 5500K, D61 is 6100K, D65 is 6500K, and the DCI standard white point is 6300K.

P3 is commonly expressed with both a D65 and D63 (DCI) white point. P3-D65 is increasingly common for ICVFX pipelines.

Rec. 2020

Rec. 2020 defines the colour specifications for UHD HDR. Its colour gamut covers a large percentage of the full CIE XYZ colour space. The standard defines 10-bit or 12-bit colour depth. A few display technologies are fully Rec. 2020 compliant but as yet, it is not a common video colour space to be working in for the average video creator.

HDR finishing is becoming more common for commercial high-end delivery, but not something the home freelance DP/colourist or video enthusiast will be equipped to undertake for some time to come. The average consumer HDR television does not meet the requirements as a reference display for post production.

A wide gamut colour space encompassing approximately 75% of all visible colours. Intended for implementation in future UHDTV standards and HDR Cinema. Seen as a future end-to-end colour space standard for digital cinema and consumer applications alike.

White Point, Gamma Offset, and Chroma Bit Depth

White Point, Gamma Offset, and Bit Depth are distinct from colour spaces, though they are closely related concepts in colour management.

White Point

White Point refers to the colour temperature of the “white” in an image or display. It defines the reference white in a colour space, influencing how all other colours are perceived.

On a piece of film, the measured density in the area of least opacity.

In a digital image, the numerical value that corresponds to the brightest area that will be represented when the image is eventually viewed in its final form.

In Virtual Production, white point registers as a calibration value used for determining the “colour temperature” of an image. Just as with cameras and film, this value is measured in degrees kelvin.

D65 (6500K) – Used in Rec 709 and most other colour spaces. It is often used in sRGB.

D65 (6500K) is a standard white point used in a wide variety of digital and print applications. It represents a colour temperature of approximately 6500 Kelvin and is intended to mimic natural daylight at midday, which is considered a neutral white. The “D” in D65 stands for “Daylight,” and 65 refers to 6500K.

Common uses of D65 (6500K) include sRGB and Rec. 709 colour spaces, consumer displays and monitors, photography annd video including video streaming such as YouTube. D65 is also used in colour management systems for graphic design and publishing to ensure that the colours on the screen are consistent with those in printed materials when viewed under natural daylight.

For broadcast television and some forms of digital video post-production, the D65 white point is used for consistency across the vast range of screens that viewers may use, including TV sets, laptops, and mobile devices. When mastering content for home video, D65 is often the white point of choice to ensure that the content looks correct on consumer displays.

DCI White (6300K) – Used specifically in DCI-P3 target colour gamut.

DCI White (6300K) is the white point standard used in Digital Cinema Initiatives (DCI), which is a specification designed for the digital cinema industry. The white point of 6300K is slightly warmer than the commonly used D65 (6500K).

DCI White (6300K) is used for digital cinema projection, colour graing in film, and the calibration of Cinema Projectors.

DCI White (6300K) may suit the darkened theater environment, where human perception of colour is different from brighter, ambient-lit environments (like a living room or office). In dark settings, a slightly warmer white (like 6300K) tends to appear more neutral to the human eye than a cooler white like D65 (6500K).

By using DCI White, the digital cinema standard ensures that films are projected with the colour balance and contrast that filmmakers intended, maintaining a cinematic look.

Gamma Offset

In order to most effectively compress and deliver digital video, it is useful to use a gamma offset to limit the number of colours addressed by the compression engine. This is a vital detail to find out from:

2.2 gamma – Most common gamma offset, used in Rec709 and sRGB colour profiles.
2.35 gamma – The technical EBU standard for Rec709 profile, but not as widely used as 2.2 or 2.4 gamma.
2.4 gamma – Used by many colourists when grading Rec709 projects to compensate for the higher contrast of modern displays.
2.6 gamma – The standard gamma for P3, P7, and Rec2020 profiles.

Chroma Bit-Depth

Bit depth is the measure of resolution within a pixel. That is to say, how many discrete colours can a pixel address. In video we measure this a function of bits per channel (bpc). The greater the bit-depth, the more colours are available to record. Typically, video is encoded with at least 8 bits per channel. This gives us a total of 2^8 = 256 discrete values for each colour primary. When taken together, this gives us a total of 16,777,216 addressable RGB values. Expanding from this:

8-bit (256 colours per channel) = millions of colours.
10-bit (1024 colours per channel) = billions of colours.
12-bit (4096 colours per channel) = trillions of colours.

When an image tries to encode colour data that has finer detail than the bit depth of the image, we perceive banding. This artifact is most evident in images featuring pronounced gradients like sunsets. Digital cinema supports full 12-bit colour, which effectively eliminates any perception of banding and allows for explicitly wider colour gamuts like P3

Legal/Full Range

Legal range, also known as Video or SMPTE range, video is a technique used in broadcast to limit the information addressed by an encoder on the luminance channel. If we think of our luminance channel having values from 0-100, legal range squeezes that video to an effective 7.5-92.5. The purpose of this is due to the way some digital signals are distributed.

When encoding video, codecs tend to eliminate detail in the extremities or at the very least introduce artifacting. By delivering content in legal range and expanding it during playback or exhibition, these effects. Two examples of this range include:

Legal 709 is 16 to 235 in 8-bit RGB values from black to white. Full range 709 is 0 to 255. And yes, it will affect how the footage looks.
Legal 709 is the most common colour space for broadcast and streaming.

Glossary

Absolute white

In theory, a material that perfectly reflects all light energy at every visible wavelength. In practice, a solid white with known spectral reflectance data that is used as the reference white for all measurements of absolute reflectance. When calibrating a spectrophotometer, often a white ceramic plaque is measured and used as the absolute white reference.

Absorb/absorption

Dissipation of the energy of electromagnetic waves into other forms (e.g., heat) as a result of its interaction with matter; a decrease in directional transmittance of incident radiation, resulting in a modification or conversion of the absorbed energy.

ACES

The Academy Color Encoding System (ACES) is a global standard for interchanging digital image files, managing color workflows and creating masters for delivery and archiving. See ACES Central for further information about ACES in general, and ACES for an overview of how ACES is implemented in Designer.

Appearance

A visual perception through which an object is seen to have attributes such as size, shape, colour, texture, glossiness, transparency, opacity, etc.

Artificial daylight

Term loosely applied to light sources, frequently equipped with filters, that try to reproduce the colour and spectral distribution of daylight. A more specific definition of the light source is preferred.

Attribute

Distinguishing characteristic of a sensation, perception or mode of appearance. Colours are often described by their attributes of hue, chroma (or saturation) and lightness.

Black

In theory, the complete absorption of incident light; the absence of any reflection. In practice, any colour that is close to this ideal in a relative viewing situation i.e., a colour of very low saturation and very low luminance.

Black level

The lowest level of brightness that can be displayed on a given screen.

Brightness

The dimension of colour that refers to an achromatic scale, ranging from black to white. Also called lightness, luminous reflectance or transmittance (q.v.). Because of confusion with saturation, the use of this term should be discouraged.

C*

Abbreviation for chromaticity.

Chroma/Chromaticity

The intensity or saturation level of a particular hue, defined as the distance of departure of a chromatic colour from the neutral (grey) colour with the same lightness value. In an additive colour-mixing environment, imagine mixing a neutral grey and a vivid red with the same value. Starting with the neutral grey, add small amounts of red until the vivid red colour is achieved. The resulting scale obtained would represent increasing chroma. The scale begins at zero for neutral colours, but has no arbitrary end. Munsell originally established 10 as the highest chroma for a vermilion pigment and related other pigments to it. Other pigments with higher chroma were noted, but the original scale remained. The chroma scale for normal reflecting materials may extend as high as 20, and for fluorescent materials it may be as high as 30.

Chromatic

Perceived as having a hue not white, grey or black.

Chromaticity coordinates (CIE)

The ratios of each of the three tristimulus values X, Y and Z in relation to the sum of the three designated as x, y and z respectively. They are sometimes referred to as the trichromatic coefficients. When written without subscripts, they are assumed to have been calculated for illuminant C and the 2Â° (1931) standard observer unless specified otherwise. If they have been obtained for other illuminants or observers, a subscript describing the observer or illuminant should be used. For example, x10 and y10 are chromaticity coordinates for the 10Â° observer and illuminant C.

Chromaticity diagram (CIE)

A two-dimensional graph of the chromaticity coordinates (x as the abscissa and y as the ordinate), which shows the spectrum locus (chromaticity coordinates of monochromatic light, 380 nm -770 nm). It has many useful properties for comparing colours of both luminous and non-luminous materials.

CIE (Commission Internationale de l’Eclairage)

The International Commission on Illumination, the first global organization for the standardization of colour and light measurement.

CIE 1976 Lab* Colour Space

A uniform colour space introduced in 1976 using the Adams-Nickerson cube root formula, designed for precise measurement of small colour differences.

CIE 1976 Luv* Colour Space

A uniform colour space also adopted in 1976, suited for additive colour mixing, particularly in applications like colour television.

CIE Chromaticity Coordinates

Numerical values that represent colour in a two-dimensional plane, independent of brightness, as defined by the CIE system.

CIE Chromaticity Diagram

A graphical tool that maps chromaticity coordinates, used to compare and analyze colours in the CIE system.

CIE Daylight Illuminants

Standard light sources defined by the CIE to simulate natural daylight for accurate colour assessment and comparison.

CIE Luminosity Function (y)

A curve describing the average human eye’s sensitivity to different wavelengths of light, crucial for calculating perceived brightness.

CIE Standard Illuminants

Reference light sources established by the CIE for consistent colour measurement across various lighting conditions.

CIE Standard Observer

A model developed by the CIE to represent the average human visual response to light, used in colourimetry for accuracy in colour perception.

CIELAB (or CIE Lab*, CIE Lab)

The CIELAB (or CIE Lab*, sometimes just Lab) colour space is a perceptually uniform colour space defined by the International Commission on Illumination (CIE). It was designed to approximate human vision and is used in various industries for accurate colour representation, colour management, and colour difference measurement. The values L*, a*, and b* are plotted in a Cartesian coordinate system to represent colours in a three-dimensional space.

Components of CIELAB (L*, a*, b*):

The CIELAB colour space is plotted in a three-dimensional Cartesian coordinate system:

L* (Lightness):

The L* value represents lightness, ranging from 0 (black) to 100 (white). It measures the brightness of the colour, independent of its hue or chroma. L* essentially corresponds to how light or dark a colour appears.

a* (Green–Red Axis):

The a* value represents the colour’s position between green and red. The a* axis runs horizontally, representing the green-red axis. Positive values along the a* axis indicate red, and negative values indicate green. This axis does not measure lightness, only the chromatic information related to these colours.

b* (Blue–Yellow Axis):

The b* axis runs perpendicularly to the a* axis, representing the blue-yellow axis. Positive values on the b* axis indicate yellow, and negative values indicate blue. Like a*, this axis captures chromatic information but along the blue-yellow dimension. .

CMC (Colour Measurement Committee of the Society of Dyes and Colourists of Great Britain)

Organisation that developed and published in 1988 a more logical, ellipse-based equation based on LCh colour space for computing rE (see delta E*) values as an alternative to the rectangular coordinates of the CIELAB colour space.

Colour

One aspect of appearance; a stimulus based on visual response to light, consisting of the three dimensions of hue, saturation and lightness.

Colour attribute

A three- dimensional characteristic of the appearance of an object. One dimension usually defines the lightness, the other two together define the chromaticity.

Colour difference

The magnitude and character of the difference between two colours under specified conditions.

Colour-matching functions

Relative amounts of three additive primaries required to match each wavelength of light. The term is generally used to refer to the CIE standard observer colour- matching functions.

Colour measurement

Physical measurement by visual or instrumental means of light radiated, transmitted or reflected by a specimen under specified condition and transformed into numeric colourimetric terms describing its numeric colour attributes.

Colour model

A colour-measurement scale or system that numerically specifies the perceived attributes of colour. Used in computer graphics applications and by colour measurement instruments.

Colour order systems

Systems used to describe an orderly three- dimensional arrangement of colours. Three bases can be used for ordering colours: 1) an appearance basis (i.e., a psychological basis) in terms of hue, saturation and lightness; an example is the Munsell System; 2) an orderly additive colour mixture basis (i.e., a psychophysical basis); examples are the CIE System and the Ostwald System; 3) an orderly subtractive colour mixture basis; an example is the Plochere Colour System based on an orderly mixture of inks.

Colour specification

Tristimulus values, chromaticity coordinates and luminance value, or other colour- scale values, used to designate a colour numerically in a specified colour system.

Colour temperature

A measurement of the colour of light radiated by a black body while it is being heated. This measurement is expressed in terms of absolute scale, or degrees Kelvin. Lower Kelvin temperatures such as 2400K are red; higher temperatures such as 9300K are blue. Neutral temperature is white, at 6504K.

Colour wheel

The visible spectrum’s continuum of colours arranged in a circle, where complementary colours such as red and green are located directly across from each other.

Colourants

Materials used to create colours dyes, pigments, toners, waxes, phosphors.

Colourimeter

An optical measurement instrument that responds to colour in a manner similar to the human eye by filtering reflected light into its dominant regions of red, green and blue.

Colourimetric

Of, or relating to, values giving the amounts of three coloured lights or receptors red, green and blue.

Colourist

A person skilled in the art of colour matching (colourant formulation) and knowledgeable concerning the behavior of colourants in a particular material; a tinter (q.v.) (in the American usage) or a shader. The word “colourist” is of European origin.

Complements

Two colours that create neutral grey when combined. On a colour wheel, complements are directly opposite from each other: blue/yellow, red/green and so on.

Contrast

The level of variation of a measured quantity such as lightness between two areas in an image. This is expressed as a number computed by a specified formula.

Daylight illuminants (CIE)

Series of illuminant spectral power distribution curves based on measurements of natural daylight and recommended by the CIE in 1965. Values are defined for the wavelength region 300 to 830 nm. They are described in terms of the correlated colour temperature. The most important is D65 because of the closeness of its correlated colour temperature to that of illuminant C, 6774K. D75 bluer than D65 and D55 yellower than D65 are also used.

Display-referred

A colour space which is referenced against the space in which images are displayed on screens or outputs after being processed.

Dynamic range

An instrument’s range of measurable values, from the lowest amount it can detect to the highest amount it can handle.

Electromagnetic spectrum

The massive band of electromagnetic waves that pass through the air in different sizes, as measured by wavelength. Different wavelengths have different properties, but most are invisible and some completely undetectable to human beings. Only wavelengths that are between 380 and 780 nm are visible, producing light. Waves outside the visible spectrum include gamma rays, x-rays, microwaves and radio waves.

Emissive object

An object that emits light. Emission is usually caused by a chemical reaction, such as the burning gasses of the sun or the heated filament of a light bulb.

Exposure

The overall brightness of an image. This can be controlled when capturing in a camera by adjusting shutter speed and aperture values, or while rendering or compositing by applying an adjustment to the overall luminance of the image.

Fluorescent lamp

A glass tube filled with mercury gas and coated on its inner surface with phosphors. When the gas is charged with an electrical current, radiation is produced. This, in turn, energises the phosphors, causing them to glow.

Gloss

Attribute due to surface reflected light, at specular angle to illumination, responsible for the degree to which reflected highlights or images of objects may be seen as superimposed on a surface. An additional parameter to consider when determining a colour standard, along with hue, value, chroma, the texture of a material and whether the material has metallic or pearlescent qualities. Gloss is an additional tolerance that may be specified in the Munsell Colour Tolerance Set.

The general rule for evaluating the gloss of a colour sample is the higher the gloss unit, the darker the colour sample will appear. Conversely, the lower the gloss unit, the lighter a sample will appear. Gloss can be measured in gloss units, which are expressed using the angle of measurement and the gloss value (e.g. 60Âº gloss = 29.8). A 60Âº geometry is recommended by the American Society for Testing and Materials (ASTM) D523 standard for the general evaluation of gloss

Greyscale

An achromatic scale ranging from black through a series of successively lighter greys to white. Such a series may be made up of steps that appear to be equally distant from one another (such as the Munsell Value Scale), or it may be arranged according to some other criteria such as a geometric progression based on lightness. Such scales may be used to describe the relative amount of difference between two similar colours.

Hue

Attribute of a visual sensation according to which an area appears to be similar to one, or to proportions of two of the perceived colours, red, yellow, green and blue. Munsell defined five principal hues (red, yellow, green, blue and purple) and five intermediate hues (yellow-red, green-yellow, blue-green, purple-blue and red- purple. These 10 hues (represented by their corresponding initials R, YR, Y, GY, G, BG, B, PB, P and RP) are equally spaced around a circle divided into 100 equal visual steps, with the zero point located at the beginning of the red sector.

Adjacent colours in this circle may be mixed to obtain continuous variation from one hue to another. Colours defined around the hue circle are known as chromatic colours. 2) The attribute of colour by means of which a colour is perceived to be red, yellow, green, blue, purple, etc. White, black and grey possess no hue.

Illuminant

Mathematical description of the relative spectral power distribution of a real or imaginary source i.e., the relative energy emitted by a source at each wavelength in its emission spectrum that can in illuminating objects affect their perceived colours. Often used synonymously with light source or a lamp, although such usage is not recommended.

Illuminant A (CIE)

Incandescent illumination, yellow-orange in colour, with a correlated colour temperature of 2856K. It is defined in the wavelength range of 380 nm to 830 nm.

Illuminant C (CIE)

Tungsten illumination that simulates average daylight, bluish in colour, with a correlated colour temperature of 6774K.

Illuminants D (CIE)

Daylight lluminants, defined from 300 nm to 830 nm (the UV portion 300 nm to 380 nm being necessary to correctly describe colours that contain fluorescent dyes or pigments). They are designated as D, with a subscript to describe the correlated colour temperature; D65 is the most commonly used, having a correlated colour temperature of 6504K, close to that of illuminant C. They are based on actual measurements of the spectral distribution of daylight.

Integrating sphere

A sphere manufactured or coated with a highly reflective material that diffuses light within it.

Kelvin (K)

Unit of measurement for colour temperature. The Kelvin scale starts from absolute zero, which is -273° Celsius.

Light

Electromagnetic radiation of which a human observer is aware through the visual sensations that arise from the stimulation of the retina of the eye. This portion of the spectrum includes wavelengths from about 380 nm to 780 nm. Thus, to speak of ultraviolet light is incorrect because the human observer cannot see radiant energy in the ultraviolet region.
Adjective meaning high reflectance, transmittance or level of illumination as contrasted to dark, or low level of intensity.

Light source

An object that emits light or radiant energy to which the human eye is sensitive. The emission of a light source can be described by the relative amount of energy emitted at each wavelength in the visible spectrum, thus defining the source as an illuminant. The emission also may be described in terms of its correlated colour temperature.

Linear

A linear colour space is one where the numeric values of a pixel are directly proportional to the physical brightness represented by the pixel. Compositing and rendering operations which are representative of the behaviour of physical light should generally be carried out in a linear working space.

Luminous efficiency (V(l)) function (CIE)

A plot of the relative magnitude of the visual response as a function of wavelength from about 380 nm to 780 nm, adopted by CIE in 1924.

Metamerism

A phenomenon exhibited by a pair of colours that match under one or more sets of illuminants (be they real or calculated), but not under all illuminants.

Munsell Colour System

The colour identification of a specimen by its Munsell hue, value and chroma as visually estimated by comparison with the Munsell Book of Colour.

Nanometer (nm)

Unit of length equal to 10-9 metre (a.k.a. one billionth of a metre, or a milli-micron).

Observer

The human viewer who receives a stimulus and experiences a sensation from it. In vision, the stimulus is a visual one and the sensation is an appearance.

OCIO

OpenColorIO (OCIO) is an open colour management solution for authoring, sharing and applying colour spaces and transforms across different applications. See OCIO for an overview of how OCIO is implemented in Designer.

Primaries

Colour primaries define the range of possible colours which can be represented within a given colour space. They can be considered independently of brightness, to define the available chromaticity values of a space. Common colour primaries include Rec. 709, Rec. 202 and P3 D60.

Radiant energy

A form of energy consisting of the electromagnetic spectrum, which travels at 299,792 kilometers/second (186,206 miles/second) through a vacuum, and more slowly in denser media (air, water, glass, etc.). The nature of radiant energy is described by its wavelength or frequency, although it also behaves as distinct quanta (corpuscular theory). The various types of energy may be transformed into other forms of energy (electrical, chemical, mechanical, atomic, thermal, radiant), but the energy itself cannot be destroyed.

Reflectance

The ratio of the intensity of reflected radiant flux to that of incident flux. In popular usage, it is considered the ratio of the intensity of reflected radiant energy to that reflected from a defined reference standard.

Reflectance, specular

See specular reflectance.

Reflectance, total

See total reflectance.

Saturation

The attribute of colour perception that expresses the amount of departure from a grey of the same lightness. All greys have zero saturation (ASTM). See chroma/ chromaticity.

Scattering

Diffusion or redirection of radiant energy encountering particles of different refractive index. Scattering occurs at any such interface, at the surface, or inside a medium containing particles.

Scene-referred

A colour space which is referenced against the space in which images were originally captured or rendered.

Spectral power distribution curve

Intensity of radiant energy as a function of wavelength, generally given in relative power terms.

Spectrophotometer

Photometric device that measures spectral transmittance, spectral reflectance or relative spectral emittance.

Spectrophotometric curve

A curve measured on a spectrophotometer; a graph with relative reflectance or transmittance (or absorption) as the ordinate, plotted with wavelength or frequency as the abscissa.

Spectrum

Spatial arrangement of components of radiant energy in order of their wavelengths, wave number or frequency.

Specular gloss

Ratio of reflectance from a surface in the mirror or specular direction to the incident flux. It is sometimes measured at 60° relative to a perfect mirror.

Specular reflectance

Reflectance of a beam of radiant energy at an angle equal but opposite to the incident angle; the mirror-like reflectance. The magnitude of the specular reflectance on glossy materials depends on the angle and the difference in refractive indices between two media at a surface. The magnitude may be calculated from Fresnel’s Law.

Specular reflectance excluded (SCE)

Measurement of reflectance made in such a way that the specular reflectance is excluded from the measurement; diffuse reflectance. The exclusion may be accomplished by black absorbers or light traps at the specular angle when the incident angle is not perpendicular, or in directional measurements by measuring at an angle different from the specular angle.

Specular reflectance included (SCI)

Measurement of the total reflectance from a surface, including the diffuse and specular reflectances.

Standard

A reference against which instrumental measurements are made.

Standard illuminants (CIE)

Known spectral data established by the CIE for different types of light sources. When using tristimulus data to describe a colour, the illuminant must also be defined. These standard illuminants are used in place of actual measurements of the light source.

Standard observer (CIE)

A hypothetical ideal observer having either the tristimulus colour-mixture data recommended in 1931 by the CIE for a 2º viewing angle or for a larger angle of 10º as adopted in 1964.
The spectral response characteristics of the average observer defined by the CIE. Two such sets of data are defined, the 1931 data for the 2º visual field and the 1964 data for the 10º visual field. By custom, the assumption is made that if the observer is not specified, the tristimulus data has been calculated for the 1931, or 2º field observer. The use of the 1964 data should be specified.

Subtractive primary colours

Cyan, magenta and yellow. Theoretically, all three subtractive primaries on white paper are 100% combined, black. When combined with different intensities, a scale is made up of different colours. When combining two primary colours at 100%, an additive primary colour, red, green, or blue, is equivalent to:

100% cyan + 100% magenta = blue
100% cyan + 100% yellow = green
100% magenta + 100% yellow = red

Tint

verb: To mix white pigment with absorbing (generally chromatic) colourants.
noun: The colour produced by mixing white pigment with absorbing (generally chromatic) colourants. The resulting mixture is lighter and less saturated than the colour without the white added.

Total reflectance

Ratio to the incident flux of the radiant flux reflected at all angles from the surface, thus including both diffuse and specular reflectances.

Transfer function

In colour science, a transfer function generally refers to the function which maps the numeric brightness value of a pixel to its physical brightness (e.g. in nits) when displayed. Transfer functions are generally applied to store a greater range of brightness values in ranges where the human eye is more perceptive to brightness changes. Common transfer functions include linear, gamma curves and the sRGB curve.

Transparent

Describes a material that transmits light without diffusion or scattering.

Tristimulus

Of, or consisting of, three stimuli; generally used to describe components of additive mixture required to evoke a particular colour sensation. Tristimulus colourimeter

An instrument that measures colour in terms of tristimulus values for a standard illuminant.

Tristimulus values (CIE)

Percentages of the components in a three-colour additive mixture necessary to match a colour; in the CIE system, they are designated as X, Y and Z. The illuminant and standard observer colour-matching functions used must be designated; if they are not, the assumption is made that the values are for the 1931 observer (2Âº field) and illuminant C. The values obtained depend on the method of integration used, the relationship of the nature of the sample and the instrument design used to measure the reflectance or transmittance. Tristimulus values are not, therefore, absolute values characteristic of a sample, but relative values dependent on the method used to obtain them. Approximations of CIE tristimulus values may be obtained from measurements made on a tristimulus colourimeter that gives measurements generally normalised to 100. These must then be normalised to equivalent CIE values. The filter measurements should be properly designated as R, G and B instead of X, Y and Z.

Value

Indicates the degree of lightness or darkness of a colour in relation to a neutral grey scale. The scale of value (or V, in the Munsell system of colour notation) ranges from 0 for pure black to 10 for pure white. The value scale is neutral or without hue.

Working space

The colour space in which rendering and compositing applications are carried out. Generally input sources will be transformed into the working space before processing, and outputs will be transformed from the working space to the output colour space.

X

One of the three CIE tristimulus values; the red primary.
Spectral colour-matching functions of the CIE standard observer used for calculating the X tristimulus value.
One of the CIE chromaticity coordinates calculated as the fraction of the sum of the three tristimulus values attributable to the X value.

Y

One of the three CIE tristimulus values, equal to the luminous reflectance or transmittance; the green primary.
Spectral colour-matching function of the CIE standard observer used for calculating Y tristimulus value.
One of the CIE chromaticity coordinates calculated as the fraction of the sum of the three tristimulus values, attributable to the Y value.

Z

One of the three CIE tristimulus values; the blue primary.
Spectral colour-matching function of the CIE standard observer used for calculating the Z tristimulus value.
One of the CIE chromaticity coordinates calculated as the fraction of the sum of the three tristimulus values attributable to the Z primary.