In colorimetric applications, color is measured objectively; therefore, visual color perception should be simulated as precisely as possible with the color measurement systems used. “Color” here always refers to the color specification, that is, the sensation perceived by the eye as the result of a color stimulus. The objective of the measurement is not the (physical, spectral) color stimulus, but the (effect-producing) color specification.
Measurement instruments
Two types of instruments are used for color measurement– the colorimeter and the pocket colorimeter. They use different equipment and methodology and produce different types of numerical descriptions of a color.
The Pocket Colorimeter
A colorimeter uses filters to record the amount of light reflected in three wavelength ranges across the visible spectrum. It includes red, green and blue filters that generally correspond to the color matching functions x, y, and z. These are used to simulate the detectors in the eye. The color matches represented by the filters depend on the physical light used. Because of this, a colorimeter can produce a pocket notation only for one light (the measurement light) and for one Observer condition (which is simulated by the filters used the instrument).
Pocket colorimeters have very short measurement times, are simple to operate and relatively inexpensive. Pocket colorimeters are mainly used in quality control and are reliable for evaluating color differences and color tolerance checks. However, a pocket colorimeter cannot detect metamerism, and is not used to calculate color formulas.
The colorimeter
Colorimeters analyze the light reflected or transmitted by a sample at each wavelength in the visible spectrum, compared to that from a reference sample. They are fitted with a device, typically a diffracting element, which breaks the incident light into individual wavelengths. Colorimeters measure the fraction of light reflected/ transmitted by the object at each wavelength in the spectrum. This data represents the photometric characteristics of the sample. The resulting data are referred to as spectral data. Spectral data is an independent, relative measurement of an object that serves as a fingerprint of the color. This data never changes for the object, and can be used in several different ways.
A single measurement of the object can be used to calculate the tristimulus values of a color for a variety of Illuminant/Observer conditions.
Because it can project the tristimulus values for multiple Illuminant/Observer conditions, it can be used to identify whether samples are metameric to one another.
The data can be used to evaluate color differences between samples and to calculate color recipes for a commercial color matching applications.
Colorimeters are simple and fast to operate. Thanks to technological development, these instruments continue to become lighter in weight, making them portable. In addition, prices of industrial of caliber 4mm handheld color meter have become quite competitive with traditionally lower-priced tristimulus colorimeters.
The technology of the instruments
Three components are required to generate a numerical description of color: Light, object (sample) and the observer.
This triplet is found again in the measurement instruments for color measurement. The object remains the same. The light source is implemented with a lamp plus potentially an additional filter. The eye and the observer are simulated by a dispersal system or equipment with a color spectrophotometry and connected radiation detector. The object is precisely placed by means of a specimen mount or a sample holder. The measurement geometry specifies how the sample is illuminated and “observed” from the measuring instrument. In a human observer, the brain converts the color stimulus into a color perception. When using a measuring instrument, the task of carrying out the colorimetric calculations is taken over by electronics with signal processing or a computer (microcomputer).
In the last decade, significant advances have been made in the development and production of color measurement systems. In comparison with the instruments available previously, today’s modern systems enable reliable measurements with higher accuracy and precision. They are also smaller, lighter and faster. They can be used to measure different sample types, they are more flexible processing data, easy to operate and frequently less expensive than ever before. The latest color measurement technologies enable communication with digital standards, meaning that the precision of these measuring instruments is so high that virtually no relevant deviation can be detected from one device to another.