The beauty of the golden sky at sunset, the splendor of peacock feathers, and the glorious spectacle of fireworks are displays of changing color. Our visual sense is greatly enriched by our perception and appreciation of colors. Although our color perception seems to be direct and effortless, it is a very interesting subject of immense complexity, as are other aspects of our visual perception. In the last 70 years, we have made a lot of progress in understanding the physics, chemistry, optics, physiology, psychophysics, anatomy, neural science, and molecular biology of human color vision, but we are still very far from being able to describe exactly how it works. Therefore, practical use of color requires certain empirical rules. These rules, which are by no means perfect, are based on many years of experimentation and observation, and they form the empirical foundation of colorimetry, the science of measuring color.

The basic measurement of a color stimulus is its spectral power distribution as a function of wavelength (or frequency). The spectral power distribution of a reflecting surface is the product of the spectral power distribution of the illumination and the spectral reflectance factor of the surface. Although the same spectral power distribution may produce different color sensations, depending on its surroundings, background, illumination, and viewing geometry, all physical specifications of color stimuli start from their spectral power distributions. The link between the objective physics and the subjective perception is provided by photometry and colorimetry. These two fields of scientific study attempt to quantify the capacity of light stimuli to produce color sensation.