Colorimetry is the science that deals with quantifying and measuring color. It helps to express color numerically which makes it easy to compare. Chromaticity gives a qualitative measure of the colors as seen by the human eye whereas colorimetry gives a numerical measure. The same color may be expressed differently by each person depending on their age, illumination, sample size, background effects, observing angle, eye fatigue, etc. This subjective nature points to the necessity of developing color standards that help to describe color objectively and precisely. These standards ensure consistency and repeatability.

The development of various theories like the principles of additive & subtractive color mixing, the opponent color theory, and the trichromatic theory of human color vision, in the 19th & 20th centuries, laid the foundation for modern colorimetry. The Commission Internationale de l’Eclairage (CIE) established one of the earliest systems to describe color based on these developments in 1931.

In colorimetry, color quantification is developed based on the theory of human tri-color vision. The human eye has special receptors with a peak response at each of the three primary colors (say, green, blue, and red). All other colors are viewed as a combination of these primary colors. Similarly, in colorimetry, three color coordinates, for example, XYZ coordinates, are used to represent the primaries known as XYZ tristimulus values. Their combinations at a specific ratio can represent any color. 

Color spaces

For easier visualization and understanding of color, various mathematical and graphical models were developed known as color spaces. CIE XYZ color space, CIE Yxy color space, L*a*b color space, and L*C*h color space are some of them. 

CIE XYZ color space and CIE Yxy color space

In CIE XYZ color space, a color is expressed by tristimulus values (X, Y, and Z), each representing the strength of a primary color. CIE Yxy color space is a cousin color space of CIE XYZ color space. The x and y values are obtained by normalizing the tristimulus values. They represent chromaticity while Y represents luminance.

L*a*b color space and L*C*h color space

In L*a*b color space, L represents lightness or luminance, while a and b indicate the color direction. i.e., +a* is the red direction, -a* is the green direction, +b* is the yellow direction, and -b* is the blue direction. The L*C*h color space follows the cylindrical coordinate approach for the same color space of L*a*b. L represents lightness, C denotes chroma and h is the hue angle of the color.

Colorimeter

A colorimeter is a device developed to measure color. Colorimeter based on tri-color theory uses three photocells with red, green, and blue filters as receptors.

The light entering the colorimeter is separated into red, green, and blue wavelengths using filters. Then the intensity of each of these wavelengths is analyzed and a set of tristimulus values are computed to obtain color coordinates. These values can be further processed to obtain color coordinates of the same color in any color space. 

Spectrometer

Spectrometers can also measure color. Instead of color filters, they use gratings, which can get into finer details of each wavelength present in the light. So, they can more accurately analyze color and pinpoint the location of the color in the color space. 

Colorimeters are used for simpler applications like inspection, comparing color samples, etc. while a spectrometer is used for LED manufacturing, R&D work, academics, etc. where more accurate measurements are required. 

The colorimetric analysis is essential in plastics, paints, inks, textiles, food, beverage, cosmetics, displays, pharmaceuticals, and footwear industries. It is helpful in the production, inspection, and quality control of the products.