fundamentals: , and . These functions correspond to the response curves for the three types of color photoreceptors found in the human retina: long-wavelength (L), medium-wavelength (M), and short-wavelength (S) cones. The three cone fundamentals are related to the original color matching functions by the following linear transformation (specific to a 10° field): The L, M, and S primaries correspond to imaginary lights that stimulate only the L, M, and S cones respectively. These primaries are the basis for LMS color space, which has significant physiological relevance as these three photoreceptors mediate trichromatic color vision in humans. The R, G, and, B primaries as described here are real in that they represent physical lights but incomplete since some colors cannot be matched with primary intensity coefficients that are all nonnegative. The X, Y, Z and the L, M, S primaries are imaginary, since none can be represented by real lights or colorants, and complete since all colors can be defined in terms of primary intensity coefficients that are all nonnegative. Other color spaces such as sRGB and scRGB are partially defined in terms of linear transformations from CIE XYZ which have their own specific primaries. The choice of which color space to use is essentially arbitrary and depends on the utility to a specific application. The color-matching context is always three-dimensional (as seen in all the previously described color spaces) but more general color appearance models like CIECAM02 describe color in more dimensions and can be used to predict how colors appear under different viewing conditions. Humans are normally trichromats and use three (or more) primaries for color reproduction applications requiring a diverse gamuts. Some humans are monochromats or dichromats, corresponding to specific forms of color blindness in which color vision is mediated by only one or two of the types of color receptors. Participants with color blindness in color matching experiments were essential in the determination of cone fundamentals. There is one scholarly report of a functional human tetrachromat. Most other mammals are dichromats while birds and many fish are tetrachromats. Psychological primaries The opponent process was proposed by Ewald Hering in which he described the four "simple" or "primary" colors (einfache or grundfarben) as red, green, yellow and blue. To Hering, colors appeared either as these pure colors or as "psychological mixes" of two of them. Furthermore, these colors were organized in "opponent" pairs, red vs. green and