Colorimetry is the science of color measurement and description. Since color is ultimately a human response, color measurement should begin with human observation. The photodetectors in the human retina consist of rods and cones. The alternative is to measure color by means of measuring the human response to patches of color. Metamerism is the key feature of human vision that allows the construction of color reproduction devices, including the color figures in this book and anything reproduced on printers, televisions, and monitors. Chromaticity diagrams provide insight into additive color mixtures. However, they should be used with care. A more uniform chromaticity diagram was developed to at least in part address the second of these problems. Color opponent spaces are characterized by a channel representing an achromatic channel, as well as two channels encoding color opponency. The human visual system accommodates these changes in the environment through a process called adaptation.