Color Appearance Models (Third Edition) by Mark D. Fairchild Page 57-59: Basic and Advanced Colorimetry – The Triangle of Color
Color exists due to the interaction of light sources, objects, and the human visual system.
The first requirement is a source of visible electromagnetic energy necessary to initiate the sensory process of vision. This energy is then modulated by the physical and chemical properties of an object. The modulated energy is then imaged by the eye, detected by photoreceptors, and processed by the neural mechanisms of the human visual system to produce our perceptions of color.
Light sources are quantified through their spectral power distribution and standardized as illuminants. (Standard Illuminants)
Material objects are specified by the geometric and spectral distribution of the energy they reflect or transmit.
The human visual system is quantified through its color matching properties that represent the first stage response (photopigment absorption) in the system.
Since the reflectance feature of objects and color perception of eyes do not change during short time, a reliable standardized light source becomes necessary.