Chromaticity is a property that characterizes the color quality of an object seen by the human eye and gives a measure of how well an object is seen by the human eye. It is independent of the brightness and so is different from luminance where brightness is indicated. It can be determined by the spectral content of light. Light sources with the same chromaticity indices will emit identically colored light.

While pointing out the color of an object, each person may describe it slightly differently which is due to the subjective nature of human color perception that depends on various factors like eye fatigue, age, lighting, background effects, etc. This necessitates the need for a way to objectively measure the color quality. It can be done by describing a color space where each color has a location with a unique set of three independent parameters.

Color spaces

Color space is a complete scheme in which a mathematical model or space is used to map all colors based on human color perception.

CIE 1931 XYZ color space is one of the earliest defined and most important color spaces, which is widely used for scientific purposes. This color space was developed by Commission Internationale de l’Eclairage(CIE), an authority on the subject of light. It also serves as a basis for the development of various other color spaces such as the CIE xyY color space.

In the CIE 1931 XYZ color space, a color is defined by a set of three coordinates known as tristimulus values (X, Y, Z). Each of these three values expresses the strength of a primary color. Mixing these primary colors can produce all perceived colors. 

The luminance-chromaticity color space is another genre of color space in which a color is defined by specifying three coordinates; one coordinate specifies luminance or the brightness of the light while the other two describe chromaticity. In this approach, chromaticity can be defined in many ways. One such way is to choose hue and saturation as chromaticity coordinates. Saturation is also known as colorfulness, chroma, or excitation purity. These two values represent the type of color and how pure the color is.

The CIE xyY color space is a luminance-chromaticity color space, where x and y represent chromaticity while Y represents luminance. The chromaticity coordinates are obtained by normalizing the tristimulus values (X, Y, Z) of the CIE XYZ color space as follows:

CIE xy chromaticity diagram

The chromaticity diagram is a horseshoe-shaped graph plotted based on the chromaticity coordinates (x and y) of the CIE xyY color space as shown in the figure below. Any chromaticity can be located on this diagram. The colors at the boundary represent spectral monochromatic light except in the bottom part. The rest of the colors, including colors at the bottom boundary, are produced by a combination of various spectral components, i.e., there is no single wavelength that represents it in the entire visible spectrum. Hence, these colors are termed nonspectral. For example, the color along the bottom boundary of the color gamut is called nonspectral purple and can be produced by various ratios of blue and red colored lights. 

The color resulting from the mixing of two arbitrary colors in the diagram will be positioned on a straight line joining their respective coordinates, depending on their quantities. If their quantities are of equal strength, then the resulting color would be at the center of the straight line.

The point with x=1/3 and y=1/3 is the white point on the chromaticity diagram and a small region surrounding that point represents the white color. This can also point to the fact that to develop a white light source, Phosphors that emit different colors must be mixed at a certain ratio which results in a color coordinate very close to the white point. Along the boundary of the color gamut, the hue value changes while the saturation value increases outwards from the white point.

Knowledge of chromaticity helps to design and ensure the color specifications in various industries like paints, textiles, lighting & display systems, etc.