The color of an object is determined by illuminant (light source), object, and observer (detector, eyes).
illuminant
The above are the common illuminants and their spectra.
standard observer, color matching function
A standard observer is developed by the CIE to describe an average human’s response to wavelengths of light (as shown in the above Figure). It is worthy to remember that the standard observer is angle dependent; typically the angle is 2 or 10 degree.
Refelction
Reflection is not only the property of the object to be measured, but also related to the optical geometry. There are three primary types of light path, as shown in the following figure:
(1) traditional 0°/45° (or 45°/0°)
(2) sphere (or diffuse/8°)
(3) multi-angle (MA) spectrophotometers (more angle options than (1))
SCE and SCI are spectral component excluded and included, respectively. It is measured with the light trap either open or close in the above Figure.
color, tristimulus value
Color can be described by:
(1) Hue. Or simply hue is how we perceive an object’s color
(2) Chroma. It describes the vividness or dullness of a color or how close to pure hue
(3) Value. It is the luminous intensity of a color; — i.e., its degree of lightness.
Connection can be built based on the following equation, between color tristimulus and the spectra of illuminant, reflection, and color matching function.
X Y Z are the tristimulus; they are defined by CIE as shown in the above Equation; where S is illuminant spectrum, x y z are color matching functions; R is reflection spectrum. However X Y Z are NOT directly correlated to hue, chroma, and value. x y z Chromaticity Coordinates are built based on XYZ as the following equation. And the xy diagram is plot. The perimeter is the hue. Saturation increases from center to perimeter (getting purer or more saturated from center to the edge)
However, x y 2D diagram is not a uniformly diagram; the L* a* b* color space is more commonly used in industry. It is calculated as:
It can be plotted as in the above Figure. L*, a* and b* describes brightness, red/green, and yellow/blue, respectively. L* is for the lightness from black (0) to white (100); the scaling and limits of the a* and b* axes will depend on the specific implementation, often run in the range of ±100 or −128 to +127 (signed 8-bit integer).
There are other color spaces in use as well, such as L*C*h* color space, Hunter lab color space, Munsell color space, and L*u*v* color space, etc.
One standard of L*a*b* measurement is based on ASTM E 1164, Standard Practice for Obtaining Spectrometric Data for Object-Color Evaluation.