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Glossary of Terms

Absolute Reflectance Value

Reflectance value relative to the perfectly reflecting and perfectly diffusing surface, which is assigned a value of 1.0.


Process by which light or other electromagnetic radiation is converted into heat or other radiation when incident on or passing through material.


Conformity of a measured result to an accepted reference value or scale.

Achromatic Color

A neutral color, such as white, gray or black, that has no hue. Also termed nonchromatic.

Angle of Incidence

The angle between the axis of an impinging light beam and the perpendicular to the specimen surface.

Angle of View

The angle between the axis of observation and the perpendicular to the specimen surface.


Distinguishing characteristic of a sensation, perception or mode of appearance. Distinction is made between chromatic and geometric appearance attributes.


The attribute of visual sensation by which an observer is made aware of differences in luminance.


The graphical or mathematical relationship of a desired property to an instrument's output.


Perceived as having a hue - not white, gray or black.

Chromatic Attributes

Those attributes associated with the spectral distribution of light, hue and saturation.


That part of a color specification which does not involve luminance.

CIE 1976 L*a*b* Color Space

A uniform-color space utilizing an Adams-Nickerson cube root formula, suggested in 1976 for adoption by the CIE in 1976 for use in measurement of small color differences.

CIE Chromaticity Coordinates (Trichromatic Coefficients)

The ratios of each of the tristimulus values of a color to the sum of the tristimulus values. In the CIE system they are designated by x, y and z.

CIE Luminosity Function

A plot of the relative magnitude of the visual response as a function of wavelength from about 380 nm to 770 nm, adopted by CIE in 1924.

CIE Standard Observer

A hypothetical observer having the tristimulus color-mixture data recommended in 1931 by the CIE for a 2° field of vision (x, y, z or x2, y2, z2). A supplementary observer for a larger 10° field (x10, y10, z10) was adopted in 1964.

CIE Tristimulus Values

The amounts of the three reference or matching stimuli required to give a match, with the color stimulus considered, in a given trichromatic system.

CIE, Commission Internationale de l'Eclairage

In English, the International Commission on Illumination; the main international organization concerned with color and color measurement.


The characteristic of a transparent material whereby distinct images may be observed through it.


Color difference based on the CIELAB color scale which can automatically generate tolerances for perceptible or acceptable differences.

Color Attribute

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

Color Constancy

Substantial invariance of object-color perceptions in the presence of changes in illumination or viewing conditions.

Color Difference

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

Color Measurement Scale

A system of specifying numerically the perceived attributes of color.

Color Perceived

The visual sensation produced by light of different wavelengths throughout the visible region of the spectrum. By such perception an observer may distinguish differences between two objects of the same size, shape, and structure.

Color Specifications

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


Instrument which senses tristimulus values and converts them to chromaticity components of color.

Contrast Ratio

A measure of opacity; the ratio of the luminous reflectance of a specimen backed with black material of specified reflectance to the reflectance of the same specimen backed with white material of specified reflectance.

Delta Descriptor

A comparison of a sample to a standard in words. It is based on color difference data. When the selected color scale is L, a, b or L*a*b*, the delta descriptors are lighter/darker, redder/greener, or yellower/bluer. When the selected color scale is LCh, the descriptors are lighter/darker, less saturated/more saturated and less chromatic/more chromatic.

Diffuse Reflection

Process by which incident light is redirected over a range of angles from the surface on which it is incident.

Diffuse Transmission

Process by which incident light, while being transmitted through an object, is redirected or scattered over a range of angles.


The scattering of light within or at the surface of a nearly clear specimen, responsible for cloudy appearance seen in transmission.

Hitch Standards

Standards close in color to the specimens being measured. These are the same as transfer standards.


The attribute of color perception by means of which an object is judged to be red, yellow, green, blue or purple.

Hunter L,a,b Scale

A uniform color scale devised by Hunter in 1958 for use in a color difference meter. It is based on Hering's opponent-colors theory of vision.


A table of spectral distribution as close as possible to that of the natural light source, usually daylight, to be duplicated.

Instrument Standards

Secondary standards which are only used with a particular instrument for maintaining the calibration of the instrument.


Electromagnetic radiation in the spectral range (approximately 380 nm to 780 nm) detectable by the normal human eye.

Light Source

That element in an instrument or in the visual observing situation that furnishes radiant energy in the form of light.


Perception by which white objects are distinguished from gray objects and light objects from dark color objects.


The phenomenon whereby colors of specimens match when illuminated by light of one spectral composition despite differences in spectral reflectance of the specimens, and that consequently may not match in light of some other spectral composition.

Metamerism Index

A measure of the degree of metamerism. A general index of metamerism is derived from two metameric spectral distribution curves without regard to the illuminant. A special index of metamerism is specified to two different illuminants.

Munsell Color System

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

Nanometer (nm)

Unit of length equal to 10-9 of a meter.

Object Color

The aspect of the appearance of an object dependent upon the spectral composition of the incident light, the spectral reflectance or transmittance of the object, and the spectral response of an observer.

Observing Conditions

The geometric and spectral conditions of illuminating and viewing a specimen for visual or instrumental evaluation. In visual observations, the conditions include the surroundings and the state of adaptation of the observer.


The degree to which a sheet or film obscures a pattern beneath it.

Opponent-Colors System

A color system based on Hering's opponent-colors theory which states that there are six independent color dimensions which are perceived by three opponent-color systems: black-white, red-green and yellow-blue.


The combination of different sensations and the utilization of past experience in recognizing the objects from which the stimulation comes.

Perfect Diffuse Reflector

An ideal uniform diffuser with zero absorbance and zero transmittance.

Perfect Diffuse Transmitter

An ideal uniform diffuser with zero absorbance and zero reflectance.


A device for isolating narrow portions of the spectrum by dispersing light into its component wavelengths.


The degree of agreement of repeated measurements of the same property.

Primary Light

Any one of three lights in terms of which a color is specified by giving the amounts required to duplicate it by additive combination.

Primary Standard

A standard whose calibration is determined by the measurement of parameters usually different from the parameter for which it will be used as a standard.


The ratio of reflected to incident radiation.


Process by which incident light leaves a surface or medium from the side on which it is incident.


The bending of light rays as they pass from one medium into another having a different index of refraction.

Regular Transmittance

Process by which incident light is transmitted through an object in a rectilinear, straight-through manner, without diffusion.


The degree to which a single instrument gives the same reading on the same specimen.


The agreement attainable between measurements performed by different instruments in different laboratories.


To set the top of the standardization scale only.


The attribute of color perception that expresses the degree of departure from the gray of the same lightness.


The process by which light passing through granular, fibrous or rough surface matter is redirected throughout a range of angles.

Secondary Standards

All standards other than primary standards.

Shade Sorting

The grouping together of similarly-colored materials so that the materials within each group may be used together in a finished product.


Visible spectrum-sensing full-scanning or abridged spectrophotometer with either an integral microprocessor or a personal computer programmed to perform tristimulus integrations, normally with a broad bandpass of 10 nm to 20 nm.


An instrument used for measuring the transmittance and/or reflectance of specimens as a function of wavelength.


Spatial arrangement of electromagnetic energy in order of wavelength. For visible radiation, the spectrum is a band of color produced by breaking white light into its component colors.


Having the qualities of a speculum or mirror; having a smooth reflecting surface.

Specular Reflectance

Process by which incident light is redirected at the specular angle, as from a mirror, without diffusion.


A reference against which instrumental measurements are made.


Process by which a given method, procedure or protocol is made to conform to prescribed conditions. Standardization can only follow calibration.


Limits that determine how far a sample can deviate from a standard. Tolerances can be set for any color scale or index parameter.

Total Reflection

Diffuse plus specular reflection.

Total Transmission

Diffuse plus regular transmission.

Transfer Standards

Standards close to the color of the specimens being measured. These are the same as hitch standards.


The property of a material by which a major portion of the transmitted light undergoes scattering.


Process by which incident light is transmitted through an object.


The property of a material by which a negligible portion of the transmitted light undergoes scattering.

Uniform Color Scale

A color scale or color solid in which the differences between points correspond to the perceptual visual differences between the colors represented by these points.


The distance, measured along the line of propagation, between two points that are in phase or on adjacent waves. Wavelength distribution determines the color of light. Wavelengths of visible light range from about 380 nm to about 780 nm.