terminology

Common Color Terms

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

Accuracy:  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.

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

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

Calibration:  The graphical or mathematical relationship of a desired property to an instrument’s output.

Chromatic:  Perceived as having a hue – not white, gray, or black.

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

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

CIE, Commission Internationale de l’Eclairage:  In English, the International Commission on Illumination; the main international organization concerned with color and color measurement.

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 to 770 nm, adopted by CIE in 1924.

CIE 1976 L*a*b* Color Space:  A uniform-color space utilizing an Adams-Nickerson cube root formula, suggested in 1974 for adoption by the CIE in 1976 for use in measurement of small color differences.

CIE Standard Observer:  A hypothetical observer having the tristimulus color-mixture data recommended in 1931 by the CIE for a 2° field of vision ( ).   A supplementary observer for a larger 10° field ( ) 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.

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

CMC:  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.

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

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.

Contrast Ratio Opacity:  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.

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

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

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

Hygrochromic or Hydrochromic: Descriptor for optically active samples that change color with moisture.

Hunter L,a,b Scale:  A uniform color measurement scale devised by Richard S. Hunter in 1958 for use in a color difference meter.  It is based on Hering’s opponent-colors theory of vision.

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

Instrument Geometry: As referenced in CIE Publication 15.2:2004 Colorimetry and ASTM E179 Guide for Selection of Geometric Conditions for Measurement of Reflection and Transmission Properties of Materials, the geometry of an instrument defines the relative position of light source, sample plane and detector. Effectively this is a condition of the instrument design and conformance to specification.

There are two broad categories of instrument geometries – directional and diffuse.

  •  The 45°:0° directional geometry provides reflectance measurements which best correlate to visual changes in the appearance of the sample including changes in pigment absorption and surface roughness. 
  • The diffuse d:8° (sphere) geometry provides reflectance measurements in the specular included mode that tend to negate surface roughness and primarily quantify changes in colorant absorption characteristics.Diffuse d:8° (sphere) geometry typically can also measure the transmitted color of transparent samples.

Instrument Mode: For an instrument of a defined geometry, a mode is a configuration of the optical path of the sensor that defines all configurable optical elements such as:

  • Sample measurement position at the reflectance port in RSIN specular included or RSEX specular excluded modes, or at the TTRAN or RTRAN port positions
  • Area of sample view
  • Port diameter
  • UV Filter position

Standardization is dependent on instrument geometry and mode conditions.

Instrument Standards:  Secondary instrument transfer standards which are used for maintaining the calibration of the instrument. To be effective, instrument standards must be stable, durable, uniform and cleanable. Typically, glass, ceramic or porcelain-on-steel tiles or commercial opal materials are used for instrument transfer standards.

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

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

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

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 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.

Opacity or Contrast Ratio Opacity:  The degree to which a sheet or film obscures a pattern beneath it. Typically, this assumes that the sheet is near-opaque. Typically Contrast Ratio Opacity is measured as a brightness ratio  for a translucent sheet over a white and black background. An opaque material will have 100% Opacity; a near-opaque sheet is typically above 80%, and often above 90%.

An alternative definition for ‘opacity” used for a translucent sheets measured in transmission, is the ability of an object to prevent the transmission of light. This is the reciprocal of transmittance factor.  If the translucent sample exhibits more opacity, there is less light transmitted through the sample.

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.

Perception:  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.

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

Reflectance:  The ratio of reflected to incident radiation.

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

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

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

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

Saybolt Color: As referenced in section 3.2.1 of ASTM D156 “Saybolt color, n—an empirical definition of the color of a clear petroleum liquid based on a scale of −16 (darkest) to +30 (lightest).”

Spectrum:  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.

Specular:  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.

Standard:  A reference against which instrumental measurements are made.

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

Thermochromic: Descriptor for optically active samples that change color with heat.

Tolerance:  Limits that determine how far a sample can deviate from a standard.  Tolerances can be set for each color difference and index difference.

Total Reflection:  Diffuse plus specular reflection.

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

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

Transmission Haze quantified as Haze%: Haze in transmission is the forward scattering of light by an object resulting in a reduction in the contrast of objects seen through it.  Light that is scattered upon passing through a film or sheet of a material can produce a hazy field when objects are viewed through the material. Haze% quantifies the degree of scattering of light passing through the sample with a “perfect clear” having a Haze% of 0.0. If the sample is a solid, the reference for a perfect clear would be air. if the medium is a transparent liquid, the reference for a perfect clear would be a transmission cell of define path length, filled with a clear colorless solvent such as distilled water. As the scattering increases and the medium becomes less clear, Haze% will increase in value.

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.

Visible Light Transmission (VLT) quantifies the total amount of light passing through a transparent sample as measured with a CIE Y Total or Y Transmission (Y) value.

Wavelength:  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.  Officially the CIE range of visible light in the electromagnetic spectrum is from 360 to 780 nanometers. However, as there is very little human sensitivity to light at the ends of the visible spectrum, the CIE accepts a truncated range of 400 to 700 nanometers.

Y Total or Y Transmission (Y) serves as a companion index to CIE L*, a*, b* values to differentiate small differences in overall Trasnsmission among the neutral or chromatic samples. The Y value is equivalent to Visible Light Transmission (VLT) quantifies the total amount of light passing through a transparent sample, with the top-of-scale being 100. If the sample is a solid, a reference for a perfect clear would be air. if the sample is a transparent liquid, the reference for a perfect clear would be a transmission cell of defined path length, filled with a clear colorless solvent such as distilled water. As the transparent sample increases in colorfulness or solids content, Y Transmission will drop.

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References

Berns, Roy S., Billmeyer and Saltzman’s Principles of Color Technology, New York: John Wiley & Sons, 2000.

Evans, Ralph M., An Introduction to Color, New York: Wiley, 1948.

Evans, Ralph M, The Perception of Color, New York: Wiley, 1974.

Francis, F. J. and Clydesdale, F. M., Food Colorimetry: Theory and Application, Westport: AVI Publishing Co., Inc., 1975.

Hardy, Arthur C., Handbook of Colorimetry, Cambridge: The Technology Press, 1936.

Hunter, Richard S. and Harold, Richard W., The Measurement of Appearance, New York: Wiley, 1987.

Judd, Deane B. and Wyszecki, Gunter, Color in Business, Science and Industry, New York: Wiley, 1975.

Kelly, Kenneth L. and Judd, Deane B., The ISCC-NBS Method of Designating Colors and a Dictionary of Color Names, NBS Circular 533, U.S. Government Printing Office, 1955.

MacAdam, David L., Sources of Color Science, Cambridge: The MIT Press, 1970.

Mackinney, Gordon and Little, Angela C., Color of Foods, Westport: AVI Publishing Co., 1962.

Munsell, A. H., A Color Notation, Baltimore, MD, 1936-1963.

Optical Society of America, Committee on Colorimetry, The Science of Color, New York: Thomas Y. Crowell, Co., 1953. Reprinted by the Optical Society of America, 1963.

Wright, W. D., The Measurement of Color, New York: D. Van Nostra Co., 1964.

Wyszecki, Gunter and Stiles, W. S., Color Science: Concepts and Methods, Quantitative Data and Formulae, New York: John Wiley & Sons, 1982.