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Colorimetry

Colorimetry is the science of defining and measuring colors in a way that will correlate to how an "average" person will see the color. Precisely defining and measuring colors is needed in chemistry, color printing, paint manufacturing, textile manufacturing and advertising. For example, you would not want to buy two colors of the same paint and discover that they are noticeably different.

This science has to be quite accurate, since people are said to be able to recognize over 16 million different colors. That means, as in the case of painting your house, the two cans of paint better match exactly or you are likely to notice the difference.

Defining and measuring colors is difficult since each and every person sees color a little differently. To this end, researchers have tested thousands of people to determine how each perceives color, and combined their results to create a "standard observer" or the average person's sensitivity to various lights and colors.

Even though we each perceive colors slightly differently, the International Commission on Illumination or CIE (abbreviation for its French name, Commission internationale de l'éclairage) has devised a standardized technique for defining and measuring colors using the data for a standard observer.

But even with all our modern technology, it is difficult to measure colors exactly due to the human element (slightly different perceptions) and the fact that the light source can cause a perceived change in color due to color temperature. Another difficulty relates to metamerism which is how two colors can appear to match exactly even though they may be significantly different when viewed with special equipments, since they are comprised of different spectral power distributions. In other words, because of the light source and metamerism, two colors that are not the same may look the same and two colors that are exactly the same may look different under different lighting conditions.

The Instruments

There are four main classes of instruments used in colorimetry, the colorimeter, spectrophotometer, densitometer, and spectroradiometer.

Colorimeter

A colorimeter makes a rough estimate of the color by taking three wideband readings. The readings are taken along the visible spectrum. The size of the filter is extremely important. The wavelength of light which it transmits must equal that which is absorbed by the substance.

Spectrophotometer

A spectrophotometer is used for critical color matching. It takes 31 readings along the visible spectrum. The plot produced using this device is often referred to as the DNA of the color. The 31 values are reduced mathematically to 3 tristimulus values which are converted to coordinates in a desired color space.