spectrophotometry conducted on colored samples, or more generally, descriptions and measurements undertaken to describe or characterize color, such as in the color of a paint chip or fiber.
Visible light (colored light) makes up a small portion of the electromagnetic spectrum bordered on one side by ultraviolet radiation (uv) and on the other by infrared radiation (ir). White light can be broken into component colors as seen in rainbows, and these component colors can be separated from each other using a prism or a similar dispersive device. The colors of visible light range from blue to red and are often identified by the acronym ROY G. BIV, which stands for red orange yellow green blue indigo violet, the familiar colors of a rainbow. Colorimetry relies on the principle of complements, which states that the wavelengths of visible light being absorbed by a sample will be the complementary colors of the sample itself. Roughly speaking, a sample of a dark blue food coloring appears as that color to the eye because the red and yellow components of visible light are absorbed by the sample, removing them before they can be seen. The color that is absorbed is the complement of the color seen by the eye. A color wheel device used by artists shows what colors are complementary, and colorimetric techniques exploit this relationship to measure the concentration of colored components in a liquid sample by relating the color and intensity of absorbed light to the concentration. In the food coloring example, a more concentrated solution will appear as a very dark blue and absorb more red and yellow light than diluted samples. Accordingly, the amount of light absorbed can be related to concentration. Because the eye can be used as a detector, colorimetry was one of the first forms of spectrophotometry developed. In modern instruments, instrumental detectors replace the eye with devices that convert light to electrical current that can be displayed or further manipulated.