The eye has three types of receptors (cones) on the retina. These differ in their spectral sensitivity. One type reacts with special sensitivity to red-orange (x), the second type to green (y) and the third type to blue (z-). This allows “standard spectral functions” to be assigned to the eye. In order to vividly represent the individual colors, the red and green color components x and y are illustrated in a coordinate system. This illustration is independent of the color’s brightness ( Brightness Meter ) and shows all possible body colors.
The CIE, or Commission Internationale de l’Eclairage (translated as the International Commission on Illumination), is the body responsible for international recommendations for photometry and colorimetry. In the CIE 1931 standardized color order systems by specifying the light source (or illuminants), the observer and the methodology used to derive values for describing color.
The CIE Color Systems utilize three coordinates to locate a color in a color space. These color spaces include:
To obtain these values, we must understand how they are calculated. Our eyes need three things to see color: a light source, an object and an observer/processor. The same must be true for instruments to see color. Color measurement instruments ( Colorimeter ) receive color the same way our eyes do – by gathering and filtering the wavelengths of light reflected from an object. The instrument perceives the reflected light wavelengths as numeric values. These values are recorded as points across the visible spectrum and are called spectral data. Spectral data is represented as a spectral curve. This curve is the
color’s fingerprint. Once we obtain a color’s reflectance curve, we can apply mathematics to
map the color onto a color space. To do this, we take the reflectance curve and multiply the data by a CIE standard illuminant. The illuminant is a graphical representation of the light source under which the samples are viewed. Each light source has a power distribution that affects how we see color. Examples of different illuminants are A – incandescent, D65 – daylight and F2 – fluorescent. We multiply the result of this calculation by the CIE standard observer. The CIE commissioned work in 1931 and 1964 to derive the concept of a standard observer, which is based on the average human response to wavelengths of light. In short, the standard observer represents how an average person sees color across the visible spectrum. Once these values are calculated, we convert the data into the tristimulus values of XYZ. These values can now identify a color numerically.
A spectrophotometer measures spectral data – the amount of light energy reflected from an object at several intervals along the visible spectrum. The spectral data is shown as a spectral curve.
Tristimulus values, unfortunately, have limited use as color specifications because they correlate poorly with visual attributes. While Y relates to value (lightness), X and Z do not correlate to hue and chroma.
As a result, when the 1931 CIE standard observer was established, the commission recommended using the chromaticity coordinates xyz. These coordinates are used to form the chromaticity diagram. The notation Yxy specifies colors by identifying value (Y) and the color as viewed in the chromaticity diagram (x,y).
Hueis represented at all points around the perimeter of the chromaticity diagram. Chroma, or saturation, is represented by a movement from the central white (neutral) area out toward the diagram’s perimeter, where 100% saturation equals pure hue.
To overcome the limitations of chromaticity diagrams like Yxy, the CIE recommended two alternate, uniform color scales: CIE 1976 (L*a*b*) or CIELAB, and CIELCH (L*C*h°). These color scales are based on the opponent-colors theory of color vision, which says that two colors cannot be both green and red at the same time, nor blue and yellow at the same time. As a result, single values can be used to describe the red/green and the yellow/blue attributes.