The Abney effect describes the perceived hue shift that occurs when white light is added to a monochromatic light source.
The addition of white light will cause a desaturation of the monochromatic source, as perceived by the human eye. However, a less intuitive effect of the white light addition that is perceived by the human eye is the change in the dominant wavelength that dictates which color is observed, or change in hue. This simultaneous hue shift phenomenon cannot be measured by experimental instruments; it is only discernible in the eye.
This variance of hue as a result of the addition of white light was first described by the English chemist and physicist Sir William de Wiveleslie Abney in 1909, although the date is commonly reported as 1910. A white light source is created by the combination of red light, blue light, and green light. Sir Abney demonstrated that the cause of the apparent change in hue was the red light and green light that comprise the white light, and the blue light component of white light had no contribution to the Abney effect.