Colour Glossary

[A] [B] [C] [D] [E] [F] [G] [H] [I] [J] [K] [L] [M] [N] [O] [P] [Q] [R] [S] [T] [U] [V] [W] [X] [Y] [Z]

A

Appearance

The display of the visual characteristics of objects through attributes such as size, shape, color, texture, gloss, transparency, opacity, etc.

Artificial Daylight

Terminology applied to lightsources available in lightbooths and luminaires which reproduce the color and spectral distribution of daylight.

B

Brightener, Optical

Fluorescent dye or pigment which absorbs UV radiation and re-emits light of a violet or bluish hue. Used to increase the luminance factor. (also called Brightening Agent)

C

CIELab

This is the most commonly used color space worldwide. It was first recommended by the CIE in 1976. In this coordinate system L* is a measure of the lightness of a sample, and ranges from 0 (black) to 100 (white).The quantities a* and b* (called opponent-type coordinates), and define the degree of: redness (positive a*) or greenness (negative a*) yellowness (positive b*) or blueness (negative b*) These coordinates (a* and b*) approach zero for neutral colors (white, grays, and blacks). The higher the values for a* and b*, the more saturated a color is.

Examples:

1. Pale gray (nearly white): L* = 83.70 a* = - 0.50 b* = 0.50

2. Medium gray: L* = 59.60 a* = 0.00 b* = 0.50

3. Brilliant red: L* = 43.70 a* = 37.1 b* = 18.7

4. Brilliant yellow: L* = 83.30 a* = 1.90 b* = 77.0

5. Green: L* = 56.80 a* = - 30.0 b* = 15.4

6. Deep blue: L* = 29.30 a* = 8.00 b* = -17.9

CIELCh

This color space is often referred to simply as LCh. The system is the same as the CIELab color space, except that it describes the location of a color in space by use of polar coordinates rather than rectangular coordinates.L* is a measure of the lightness of a sample, ranging from 0 (black) to 100 (white). C* is a measure of chroma (saturation), and represents distance from the neutral axis. h is a measure of hue and is represented as an angle ranging from 0° to 360°. Angles that range from: 0° to 90° are reds, oranges, and yellows; 90° to 180° are yellows, yellow-greens, and greens; 180° to 270° are greens cyans(blue-greens) and blues. From 270° to 360° are blues, purples, magentas, and return again to reds. An h value that is equal to 360° is reported as 0°.

Examples:

1. Pale gray (nearly white): L* = 83.70 C* = 0.71 h = 315°

2. Medium gray: L* = 59.60 C* = 0.50 h = 270°

3. Brilliant red: L* = 43.70 C* = 41.55 h = 26.8°

4. Brilliant yellow: L* = 83.30 C* = 77.02 h = 88.6°

5. Green: L* = 56.80 C* = 33.72 h = 152.7°

6. Deep blue: L* = 29.30 C* = 19.61 h = 294.1°

CMC

CMC stands for Colour Measurement Committee (of The Society of Dyers and Colourists). This "color space" is actually not a color space, but rather a color difference equation based on CIELCh color space. CMC (l:c) applies an elliptical tolerance whose size and shape depends on the location of the standard in LCh space. Samples with a DE(CMC) less than 1.0 fall within the ellipse, and samples with a D E(CMC) greater than 1.0 fall outside the ellipse. The equation is designed such that a DE(CMC) equal to 1.0 represents a just-noticeable-difference from the standard. The (l:c) notation refers to the correction factors for lightness and chroma. The CMC color difference equation is designed so that you can adjust the tolerance ellipse axes to put more or less emphasis on lightness and chroma. In general, a CMC (l:c) ratio of:

1. (1:1) is appropriate for determining perceptible color difference, while

2. (2:1) is appropriate for determining acceptable color difference.

Color space

Color space is an arrangement of colors in some orderly fashion. For color measurement, the most useful color spaces are ones that are calculated based on tristimulus values. Some color spaces are physical arrangements of colors based on visual assessment (for example, the Munsell system). Other color spaces are mathematical representations based on instrumental color measurement.

Color space values are sometimes easier to understand when they are graphed. For CIELab and CIELCh color spaces, each point is plotted on a two-dimensional graph with a* on the x axis and b* on the y axis. L* is represented as a bar of the same scale next to the two-dimensional graph. (This also applies when CMC is chosen, because the CMC color difference formula is based on CIELCh). Color Plot for FMC-2 represents the standard in the center. Each trial is plotted relative to the standard, based on the delta RG, delta YB and delta LD values for the trial versus standard.

Contrast Ratio

This is a measure of opacity (sometimes called hiding power). Basically, it is a measure of how opaque a substance is. It is the ratio of Y tristimulus for the sample measured over a black substrate divided by Y tristimulus for the sample measured over a white substrate, expressed as a percentage:

A contrast ratio of 100 (%) means the substance is totally opaque (Y is the same no matter what the substrate is, black or white). In theory, a perfectly clear sample would have a contrast ratio of 0 (%); the lowest practical value is about 1 (%).

D

E

The Eye

The normal human eye contains three types of color-sensitive receptors. Each is sensitive to a particular portion of the visible spectrum. When the eye is exposed to a particular color, each sensor type responds in proportion to the amount of light to which it is sensitive. The brain receives three color signals from the eye¾one for each type of sensor. The brain then decides which color the eye is looking at by comparing the amounts of response from each sensor type. The eye and brain together constitute the human visual system. The signal from the eye is just a signal; it becomes meaningful only after it has been processed in the brain.

F

FMC-2 is not really a color space, but rather a color difference equation developed by Friele, MacAdam and Chickering (hence the abbreviation F-M-C). It was developed from test results based on just-noticeable-difference. Traditionally, when this color space is selected, Optiview displays tristimulus values (XYZ) for the absolute values, intermediate delta values, and DE (total color difference). XYZxy. These values are simply tristimulus values and chromaticity coordinates and represent the absolute values displayed when FMC-2 is selected. DRG and DYB: the delta values between two colors. DL is an abbreviation for "Delta Light/Dark" and represents change in lightness/darkness. DRG is an abbreviation for "Delta Red/Green" and represents a change in redness/greenness. DYB is an abbreviation for "Delta Yellow/Blue" and represents change in yellowness/blueness.

DE: Delta E (DE) represents total color difference. The FMC-2 equations are designed so that a DE of 1.0 represents a just-noticeable-difference, regardless of the color in question.

Examples:

1. Pale gray: X = 62.051 Y = 63.491 Z = 74.413

2. Medium gray: X = 27.148 Y = 27.682 Z = 32.353

3. Brilliant red: X = 20.025 Y = 13.632 Z = 8.832

4. Brilliant yellow: X = 62.345 Y = 62.730 Z = 12.356

5. Green: X = 17.933 Y = 24.718 Z = 19.734

G

Gloss

Term used to describe the relative amount and nature of mirror-like (specular) reflection. Flat is virtually free of sheen and is usually less than 15 gloss units on an 85 degree glossmeter; Eggshell is usually 20-35 gloss units on a 60 degree glossmeter; Semi-gloss is usually 35-70 gloss units on a 60 degree glossmeter; and a high gloss surface is greater than 70 gloss units.

H

HAZE

The Color-Eye® 7000 is capable of measuring percent haze (in transmission) as specified by ASTM method D 1003. Haze is defined as the percentage of light that is scattered so that its direction deviates more than a specified angle from the direction of the incident beam. If a transmission sample exhibits haze, it causes objects viewed through it to appear lower in contrast and out of focus ("hazy").

I

J

K

L

LAB

This color space was developed by Richard S. Hunter in 1942 for use with three-filter colorimeters. The L, a, and b notations represent the same color characteristics as in the CIELab system. For any given color, CIELab values and Hunter Lab are similar. However, the CIELab equations are the current CIE recommendation, and are in more common use. The Lab color space is generally used only by manufacturers who need to correlate with historical data stored in Hunter Lab values.

Light source

A Light source is defined as a physical origin of light, such as a tungsten lamp. An illuminant is a numerical representation of a source. The set of numbers used in this numerical representation describes how much light of each wavelength the source contains. Illuminants have been created to represent most commonly available sources.

M

Metamer one of a pair of colours which matches the other under one lightsource, observer or angle but does not match if one of the conditions change.

Munsell is a three-dimensional concept of color. The three dimensions or attributes of color are hue, value, and chroma. Hue is that quality by which we distinguish one color family from another, as red from yellow or blue from green. The circular band represents the hues. Value is the quality by which we distinguish a light color from a dark color. The upright center axis is the value scale. Chroma is the strength of a color. It is the quality by which we distinguish a weak color, as a dull green from strong color such as a vivid green. The paths that radiate outward from the value scale represent chroma. This color space is a simple transformation of tristimulus values that allow chromaticity (hue and chroma) to be represented on a two-dimensional graph. The third dimension is tristimulus Y (luminosity), which represents how bright the color is.

N

Nanometer is the unit of length used for describing the wavelength of light. The visual wavelengths of light are between 400 and 700 nanometers.

O

Opacity is a term used to describe the degree to which a material obscures a substrate. It is the opposite of transparency.

P

Photochromism is a phenomenon (colour change) exibited by some materials when exposed to light.

Q

R

S

Spectral data (reflectance or transmittance) specifies the spectral reflectance or transmittance characteristics of a sample. This is measured on a spectrophotometer, and shows how much light (as a percentage of incident light) of each wavelength is reflected or transmitted by the sample.

Spectral Match

A pair of colours with no perceptible difference when observed under different conditions. Macbeth's ProPalette colour matching software uses spectral matching as apposed to Tristimulus matching.

T

Tristimulus values are numbers that represent how the human visual system responds to a given colour. These values are the input for all subsequent colour calculations (CIELab, CIELCh, Lab, FMC-2 , CMC, and most indices). Understanding the human visual system is, therefore, important for understanding tristimulus values.

Tristimulus Matching

Colour matching using the tristimulus values (X,Y,Z) instead of the actual spectral data. Tristimulus matching is less accurate than spectral matching

U

V

W

X

Y

Z

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