Which term is defined as a ratio on a chromaticity diagram represented as a/(a+b)?

The concept of Excitation Purity is crucial in color science and specifically refers to the purity of a color in relation to a specific reference white point. It is calculated using the formula a/(a+b), where 'a' denotes the intensity of the color in question and 'b' represents the intensity of the complementary color. This ratio provides a measure of how "pure" a hue is within the context of the chromaticity diagram.

The chromaticity diagram itself plots colors based on their chromaticity coordinates, allowing for a visual representation of color relationships without considering brightness or luminance. Within this framework, Excitation Purity indicates how much of the perceived color can be attributed to the specific hue rather than being mixed with other colors, which contributes to a clearer understanding of color quality and saturation.

Other terms like Color Saturation, Chromaticity, and Luminance have specific definitions that do not encompass the ratio of a/(a+b) as directly as Excitation Purity does. Color Saturation refers to the vividness or dullness of a color, Chromaticity encompasses the hue and saturation of a color excluding brightness, and Luminance relates to the intensity of light emitted or reflected by a surface. Thus, among these