Fig. 11 Chromatic valence functions for the CIE standard observer. Observers view a monochromatic test light to which is added one of the four unique hues (blue, green, yellow, and red). The task of the observer is to adjust the intensity of the unique hue to make the test light appear either neutral between blue and yellow or neutral between red and green. At each test wavelength, a positive red value means the observer added green to neutralize the color. A negative green value means red was added. Similarly, a positive yellow means blue was added and a negative blue value means yellow was added. The curves show the relative amounts of red, green, blue, and yellow perceived in each wavelength of the spectrum. They also show that no wavelength is perceived to simultaneously contain both red and green, or both blue and yellow. For example, a 600-nm light, which appears “orange,” required the observer had to add green light to cancel the red component of the stimulus and blue light to cancel its yellow component. (From Wyszecki G, Stiles WS: Color science. Concepts and methods, quantitative data and formulae. New York: John Wiley and Sons, 1982. Copyright © 1982 Wiley-Liss, Inc., A Wiley Company. Reproduced with permission of John Wiley and Sons, Inc.)