Colour Management Dictionary

Colour Management terms explained

Whether you are a 'Newbie' after some quick facts or simply need a reminder we hope you will find this dictionary useful.

© Finer Image.

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Absolute Colorimetric

One of the 4 rendering intents in the ICC Profile Specification. Absolute Colorimetric Rendering is matching the colours in the destination color space exactly to the colors of the source color space. By definition this match is relative to the source colour space white point. Where a color is outside the gamut of the destination color space it is mapped to the nearest in gamut colour.

Additive Synthesis

Additive Synthesis is the method of simulating many colours using a small number of primary colors, usually Red Green and Blue. The synthesis is said to be additive because pure Red Green and Blue light combined results in white light.

Adobe RGB

An RGB Colour Space developed by Adobe Systems. Often referred to as AdobeRGB1998 'cause this was the year they developed it :) Like sRGB it has a gamma of 2.2 but contains approximately 40% more colours (Gamut) than sRGB. Adobe RGB was intended to reflect the printable colours of common colour printers of the day.

Assigned Profile

Images don't always have an icc profile embeded in them but it is often known in what colour space they were created. In this case a profile can be assumed. The act of associating an ICC profile with an image is usually refereed to as Assigning a Profile. Often when resaving an image with an assigned profile, the profile is actually embedded into the image.

Assumed Profile

An ICC profile is not always embedded in an image but it is often known in what colour space the image was created. That being the case one can assume the profile of an image and render it's colours correctly. An example is images published on the internet where the sRGB profile is assumed. Of course if the profile is assumed incorrectly the rendering of colors in the image will be wrong.

Black Point

Black Point refers to the darkest black that a device can produce. In Photoshop "setting the black point" refers to matching the darkest black in a digital image to the darkest black of the output device.

Black Point Compensation

Black Point Compensation was developed by Adobe and ensures that the maximum black of an output device is mapped to the maximum black of an input device during a colour conversion. This can be critical when converting from a CMYK colour space to an RGB colour space and vice versa. Without Black Point Compensation an image going from CYMK to RGB might not contain any true Blacks. Going the other way an RGB to CMYK conversion could easily result in clipping of dark areas of the image.

Brightness

Brightness describes the subjective response of a person to light intensity. Brightness perception varies from person to person because it is a response to an apparent light source not a property of the actual light source.

Calibration

Calibration is the process of adjusting the variable characteristics of a device in order to achieve a specified state or set of parameters. For example Monitor Calibration which is the process of adjusting a computer display to achieve measurably accurate colour.

Candela

Candela is the (SI) unit of luminous intensity. A lit candle emits a luminous intensity of approximately 1 Candela

CIE

Commission Internationale de l'éclairage (French). Translates as International Commission on Illumination. The CIE are the international authority on Light, Illumination and Colour. The CIE's various definitions of colour spaces and standard illuminants are intrinsic to modern colour management.

CMM

A Colour Management Module is the software that performs colour transformations in colour managed software such a Photoshop. Colour transformations are needed to relate the colours of one device to another.

CMYK

CMYK (Cyan Magenta Yellow Black) is a four component model for describing colour. Each colour within the CMYK model is described as proportions of C,M,Y & K. CMYK is used largely in printing and is known as a subtractive colour model - the less of each colour printed, the closer you get to white (white paper).

Colorimeter

A Colorimeter is actually a Tristimulus Colorimeter - a device for measuring emitted light. Colorimeters are typically used to calibrate computer displays.

Colour Management

Colour Management is the name given to the science and process of maintaining accurate colour between various input and output devices. The goal is to capture or render colour in a consistent way no matter what the device.

Colour Model

Colour Models are ways of describing colour. There are many Colour Models such as RGB, CMYK, CIELAB, HSL and HSV. In order to describe a set of colours or 'colour space' you need to decide which colour model to use.

Colour Rendering Index

Colour Rendering Index is an objective measure of the ability of a light source to reproduce the colours of objects faithfully in comparison to natural daylight. High CRI lights are essential for Print Viewing and Assessment

Colour Space

A colour space is a range of colours (gamut) described in absolute terms with the use of a colour model. It is a sub-set of all possible colours. The absolute reference is usually a standardized description of colour such as CIEXYZ but can be any absolute reference such as a pantone colour swatch.

Colour Temperature

Colour Temperature is a description of the colour of light when compared to the temperature of an 'ideal black body'. Lower temperatures are redish and higher temperatures blueish.

Colour Wheel

A Colour Wheel is a way of ploting colours around a wheel. The most common presentation of a colour wheel is organized so that Complementary Colours are opposite each other on the wheel.

Complementary Colour

Complementary Colours are opposing colours. When you mix them together equally you get gray. Blue and Yellow are examples of complementary colours. [A common error is to miss-interpret complement (meaning to complete) with compliment (meaninging flattery). Complemetary colours are often called clashing colours - No compliments there!]

D-Max

D-max is the maximum density that can be achieved by a material (such a ink on paper). The higher the D-max, the less light is transmitted and the blacker the material will appear

D50

D50 is a "Standard Illuminant" as defined by the Commission internationale de l'Eclairage (CIE). It describes the Spectral Power Distribution (SPD) of natural daylight at 5003 K.

D65

D65 is a "Standard Illuminant" as defined by the Commission internationale de l'Eclairage (CIE). It describes the Spectral Power Distribution (SPD) of natural daylight at 6504 K.

Display Profile

Display Profiles (often refered to as Monitor Profiles) are ICC Profiles that describe the precise colorimetric characteristics of a computers display system (including Monitor and video card). The profiles are used by Colour Managed software to accurately render colours on screen.

Dynamic Range

When applied to digital imaging, Dynamic Range is the ratio of the whitest white to the darkest black distinguishable and recordable on a device. For a digital camera this is a measure of the contrast range the sensor can capture. For a printer this is the difference between paper white and the most dense printable black.

Embedded Profile

An embedded profile is an ICC Profile that has been saved onto the data of an image. JPEG and TIFF images are examples of image formats that allow ICC profiles to be embedded.

Gamma Curve

A gamma curve is a non-linear curve. An example would be graph where x = y2. Gamma correction curves are used in digital imaging for many reasons not least of which is the fact that the human perception of brightness on a monitor is logarithmic. Typically for an observer to perceive a doubling in brightness, the output intensity needs to be more than quadrupled.

Gamut

A Colour Gamut is a range of destinct colours. Often Gamuts are displayed as 3 dimensional plots of colour. In this way the measured gamuts of various output devices can be compared.

Gamut Warning

If a colour is outside the set of colours that a device can reproduce it is said to be "Out Of Gamut". Photoshop has a Gamut Warning feature that highlights the parts of an image that cannot be reproduced on the output device.

Kelvin

Kelvin is the SI unit of temperature even though most of us use Celcius or Fahrenheit. In digital imaging it is used to express Colour Temperature

Luminous Intensity

Luminous intensity is proportional to the power output of a light source in a given direction

Monitor Profile

A Monitor Profile is more accurately described as a Display Profile since the entire Display System including the computer graphics card is profiled. See Display Profile for further explanation.

Out Of Gamut

When a Colour defined in a Source Colour Space falls outside the range of colours defined in the Destination Colour Space, that Colour is said to be " Out Of Gamut". In practical terms this means it cannot be reproduced by the output device.

Relative Colorimetric

One of the 4 rendering intents in the ICC Profile Specification. Colors in the destination colour space are matched directly to the colors in the source colour space except that the colours in the destination colour space are adjusted relative to the white point of the destination colour space. Colours outside the gamut of the destination colour space are matched to the nearest in gamut colour.

Rendering Intent

The ICC (International Colour Consortium) specification for ICC profiles includes 4 Rendering Intents. Rendering Intents are methods for handling Out of Gamut Colours - that is colours in a Source Colour Space that are not reproducible in the Destination Colour Space. The 4 rendering intents are: Perceptual, Saturation, Relative Colorimetric and Absolute Colorimetric

RGB

RGB stands for Red Green Blue and refers to a colour model in which colours are expressed in amounts of Red Green and Blue light. RGB is known as an additive model of colour as adding Maximum Red Maximum Green and Maximum Blue light produces white.

Saturation Intent

One of the 4 rendering intents in the ICC Profile Specification. Saturation Intent tries to maintain the relative saturation of colors in the destination colour space relative to the source colour space. It gives priority to saturation over hue and tonal accuracy. Saturation Intent tends to be used for graphs and other applications where relative saturation of a colour is used to convey information. (e.g hotspots on a weather map).

sRGB

sRGB stands for StandardRGB. It is a colour space that was intended to reflect the gamut of a typical CRT monitor at the time it was developed by HP and Microsoft in 1996. In the age of LCD panels sRGB is less relevant but still the defacto standard for images on the World Wide Web.

Video Card

An electronic component commonly found in computers with the function of generating output images on a display. The output from a video card is adjusted as part of a Monitor Calibration

White Point

White Point describes the colour temperature of a 'white colour' captured or produced by a device. For example when the RGB colour control signal 255,255,255 is sent to a monitor, the 'white colour' produced will have a tint that can be expressed in terms of colour temperature.