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Colorimetry:

How to Measure Color Differences

Konica Minolta Photo Imaging U.S.A., Inc.Instrument Systems Div.

Colorimetry, the science ofcolor measurement, is widely employed in com- merce, industry and the lab- oratory to express color in numerical terms and to mea- sure color differences be- tween specimens. Applica- tions include paints, inks, plastics, textiles and apparel, food and beverages, phar- maceuticals and cosmetics, displays, and other parts and products that reflect or trans- mit color.

)Z–X ( )––( )Y ( Analysis, Test & Measurement )X– 400 500 600 700wavelength (nm) 2.01.51.00.5( Figure 1. Spectral sensitivity corresponding to the human eye (color-matchingfunctions of the 1931 Standard Observer).

How to measure color difference - 18425 T

he use and importance of col-orimetry has grown in unison with the increase of globalmanufacturing and processing. Whenplastic automotive trim produced on one continent, for example, must match a painted metal finish applied on another, an objective and precisedescription of color becomes an ab-solute necessity.Unfortunately, human color per-ception varies widely and is affected by illumination, sample size, sur-rounding color and the angle of ob-servation. Colorimetric instruments provide a set of standardized condi- tions that help assure consistency and repeatability.While the term colorimetry oftenis used in a general sense to mean color measurement, it differs from spectrophotometry, a related but dis- tinct method of color measurement. provides high accuracy and is gener-ally used in research and color for-mulation applications. Colorimeters are generally used in production and quality control applications.

Anatomy of a colorimeter

A colorimeter consists of a lightsource, fixed-geometry viewing op- tics, three photocells matched to an internationally established Standard Observer, and an on-board processoror cable connection to a proces-sor/display unit or computer. In operation, the sensor lens of thecolorimeter typically is placed directly over the area of the specimen to bemeasured — for example, a piece ofdyed cloth or a colored plastic chip. For liquid or colored film specimens, where transmitted light must be measured, the specimen is placed ina transmittance compartment of the In colorimetry, the quantification of color is based on the three- component theory of color vision,which states that the human eye pos-sesses receptors for three primarycolors (red, green and blue), and thatall colors are seen as mixtures of these primaries. In colorimetry, these com-ponents are referred to as X-Y-Z coordinates. Colorimeters, based on this theory of color perception, em- ploy three photocells as receptors to see color in much the same way asthe human eye. Spectrophotometry, on the otherhand, uses many more sensors (40 or more in some spectrophotometers) to separate a beam of reflected ortransmitted light into its componentwavelengths. It measures the spec- tral reflectance of an object at each wavelength on the visible spec- trum continuum. Spectrophotometry Reprinted from the 2005 edition of The Photonics Handbook
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