Human color and contrast perception differ from person to person, not only in terms of vision deficiency, but also among color-normal observers. However, this variability is not taken into
account in current display technologies, and it is assumed that a single average standard observer can represent the entire population.
Historically, the objective of colorimetry was to integrate properties of the human color vision into the measurement of visible light to define visual equivalents of colored stimuli. More recently, its objective has been to provide procedures that enable quantifying color matches and differences. In that sense colorimetry is based on the assumption that everyone’s color response can be quantified with the CIE standard observer functions, which predict the average viewer’s response to the spectral content of light. However, individual observers may have slightly different response functions, which may cause disagreement about which colors match and which do not. For colors with smoothly varying (broad) spectra, the disagreement is generally small, but for colors mixed using a few narrow-band spectral peaks, differences can be as large as 10 CIELAB units. The human visual system (HVS) works over a remarkably wide range of illumination thanks to two classes of photoreceptors, rods and cones. In daylight, vision relies mainly on three types of cones photoreceptors, and is referred as photopic vision. In low light conditions, only rods are active, and this is referred as scotopic vision. In between photopic and scotopic conditions, both cones and rods operate simultaneously, and this is referred as mesopic vision.