Color and Appearance Instruments

Color and appearance instruments are used to measure the properties of paints and coatings including color, gloss, haze and transparency.  Appearance is the manifestation of the nature of objects and materials through visual attributes such as size, shape, chroma, color, texture, glossiness, haze, transparency, opacity, hue, luster, orange peel, translucency, etc.

Color and appearance instruments generally fall into one of four categories, colorimeters, densitometers, spectral cameras, and spectrophotometers.  Colorimeters measure color using three or four filters that match human color receptors.  Colorimeters can show L, a, b or L*, a*, b* numbers but can only measure in one light source.  Densitometers measure the density of ink films using one or more filters.  Densitometers do not give complete color information, but are useful for specification and control of printed colors.  Spectral cameras provide measurements with full spectral and spatial information.  Spectrophotometers operate on the principle of reflected light.  Spectrophotometers measure individual wavelengths and then calculate L, a, b or L*, a*, b* values from this information.  These color and appearance instruments can measure in all standard illuminants.

To accomplish their readings, color and appearance instruments many use any of a number of measurement scales.  These include:

Hunter L, a, b: a color standard that was finalized in 1958.  L=lightness, a=green and red and b=blue and yellow.

CIELAB: an international color standard adopted in 1976.  CIE is a tricolor system that is based on the fact that any color can be matched by a suitable mix of the 3 primary colors.

CIELCH: a color standard developed from CIELAB.

cE*CMC: the abbreviation stands for delta E Colour Measurement Committee of Dyers and Colourists.  Built upon CIECH's attention to what direction a color is changing.  Adopted as British Standard BS6923: 1988.

XYZ: the XYZ space allows colors to be expressed as a mixture of the three tristimulus values X, Y, and Z. The term tristimulus comes from the fact that color perception results from the retina of the eye responding to three types of stimuli. After experimentation, the CIE set up a hypothetical set of primaries, XYZ, that correspond to the way the eye's retina behaves.

Yxy: Yxy space expresses the XYZ values in terms of x and y chromaticity coordinates, somewhat analogous to the hue and saturation coordinates of HSV space.

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