in which participants observing a foveal 2° bipartite field with a dark surround. Half of the field is illuminated with a monochromatic test stimulus (ranging from 380 nm to 780 nm) and the other half is the matching stimulus illuminated with three coincident monochromatic primary lights: 700 nm for red (R), 546.1 nm for green (G), and 435.8 nm for blue (B). These primaries correspond to CIE RGB color space. The intensities of the primary lights could be adjusted by the participant observer until the matching stimulus matched the test stimulus, as predicted by Grassman's laws of additive mixing. Different standard observers from other color matching experiments have been derived since 1931. The variations in experiments include choices of primary lights, field of view, number of participants etc. but the presentation below is representative of those results. Matching was performed across many participants in incremental steps along the range of test stimulus wavelengths (380 nm to 780 nm) to ultimately yield the color matching functions: , and that represent the relative intensities of red, green, and blue light to match each wavelength (). These functions imply that units of the test stimulus with any spectral power distribution, , can be matched by , , and units of each primary where: Each integral term in the above equation is known as a tristimulus value and measures amounts in the adopted units. No set of real primary lights can match another monochromatic light under additive mixing so at least one of the color matching functions is negative for each wavelength. A negative tristimulus value corresponds to that primary being added to the test stimulus instead of the matching stimulus to achieve a match. The negative tristimulus values made certain types of calculations difficult, so the CIE put forth new color matching functions , , and defined by the following linear transformation: These new color matching functions correspond to imaginary primary lights X, Y, and Z (CIE XYZ color space). All colors can be matched by finding the amounts , , and analogously to , , and as defined in . The functions , , and based on the specifications that they should be nonnegative for all wavelengths, be equal to photometric luminance, and that for an equienergy (i.e., a uniform spectral power distribution) test stimulus. Derivations use the color matching functions, along with data from other experiments, to ultimately yield the cone