Equipment

Although the human eye can control color, there is a need for instruments in order to provide objective color measuring and evaluation as well as help in matching colors.

There are two basic methods for measuring surfaces' color:

• The first is to imitate the analysis made by the eye in terms of responses to three stimuli. This technique, known as "tristimulus colorimetry", sets out to measure X, Y, and Z directly.
• The second method is to determine reflectance (R) for each wavelength band across the range of the spectrum to which the eye is sensitive, and then to calculate the visual responses by summing products of R and the standard values for distribution of the sensitivity of the three-color responses (2. J, and Z).

The tristimulus method has theoretical advantages where the materials to be measured are fluorescent, but there are serious practical problems in assuming that a tristimulus colorimeter exactly matches human vision, that is, in eliminating color blindness from the instrument.

Two commonly used types of color measurement equipments are a colorimeter and a spectrophotometer.

Colorimeters

A tristimulus colorimeter has three main components:

• a source of illumination (usually a lamp functioning at a constant voltage);
• a combination of filters used to modify the energy distribution of the incident / reflected light;
• a photoelectric detector that converts the reflected light into an electrical output.

Each color has a fingerprint reflectance pattern in the spectrum. The colorimeter measures color through three wide-band filters corresponding to the spectral sensitivity curves. Measurements made on a tristimulus colorimeter are comparative, the instrument being standardized on glass or ceramic standards. To achieve the most accurate measurements it is necessary to use calibrated standards of similar colors to the measured materials. This "hitching post" technique enables reasonably accurate tristimulus values to be obtained even when the colorimeter is demonstrably colorblind.

Tristimulus colorimeters are most useful for quick comparison of near-matching colors. They are not very accurate. Large differences are evident between the various instrument manufacturers. However, colorimeters are less expensive than spectrophotometers.

Spectrophotometers

To get a precise measurement of color, it is advisable to use a spectrophotometer. A spectrophotometer measures the reflectance for each wavelength, and allows to calculate tristimulus values.

The advantage over tristimulus colorimetry is that adequate information is obtained to calculate color values for any illuminant and that metamerism is automatically detected.
The negative is that high quality spectrophotometers are very expensive and measurements take longer (although this disadvantage has been greatly reduced by instrument development).

In a spectrophotometer, the light is usually split into a spectrum by a prism or a diffraction grating before each wavelength band is selected for measurement. Instruments have also been developed in which narrow bands are selected by interference filters. The spectral resolution of the instrument depends on the narrowness of the bands used for each successive measurement. In theory, a spectrophotometer could be set up to compare reflected light directly with incident light, but it is more usual to calibrate it against an opal glass standard itself previously calibrated by an internationally reknown laboratory. Checks must also be made on the optical zero (e.g. by measurements with a black light trap) because dust or other problems can give rise to stray light in an instrument giving then false readings.

Today's spectrophotometers contain monochromators and photodiodes that measure the reflectance curve of a product's color every 10 nm or less. The analysis generates typically 30 or more data-points, with which a precise color composition can be calculated.