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Early studies of color
The full dispersion curve of a colorless crown glass. At the left end, at low energies, there are features derived from the absorption of infrared energy, which produces excited-lattice vibrations, originating in the molecular framework derived from the bonding between atoms. At the right end, at high energies, there are features derived from the un-pairing and excitation of previously paired electrons on individual atoms, leading to absorptions in the ultraviolet.
The detailed understanding of the science of color began in 1666, when Newton first used the word "spectrum" for the array of colors produced by a glass prism. He recognized that the colors comprising white light are "refracted" (bent) by different amounts and he also understood that there is no "colored" light, the color being in the eye of the beholder. There is merely a range of energies -- or the proportional frequencies and the inverse wavelengths. Newton assigned seven colors to the spectrum by analogy with the musical scale.
Importance of electrons
It might seem remarkable that so many distinct causes of color should apply to that small band of electromagnetic radiation to which the eye is sensitive, a band less than one "octave" wide in an electromagnetic spectrum of more than 80 "octaves." So much happens in this narrow band because this is the region in which the interaction of radiation with electrons first becomes important. Radiation at lower energies induces a relatively small motion of atoms and molecules, which we sense as heat, if at all. Radiation at higher energies has a destructive effect since it can ionize atoms, that is, completely remove one or more electrons, and can permanently damage molecules. Only in the narrow optical region, just that region to which the human eye is sensitive, is the energy of light well attuned to the electronic structure of matter with its wide diversity of colorful interactions.
Electrons are involved in essentially all of our 15 mechanisms and we "see" electrons in a real sense whenever we perceive color.
We now know that the normal dispersion curve leading to Newton's spectrum, with the refractive index increasing with increasing energy (decreasing wavelength), is only the small central region of the full dispersion curve as shown for a colorless glass (above right).
The infrared absorptions of crown glass can shift to occur in the visible region with light, strongly bonded atoms, as in Mechanism 3 (Vibrations and rotations). Similarly, the ultraviolet absorptions of crown glass can also occur in the visible region in a wide variety of situations, leading to color of most other Mechanisms (except those optical in origin: refraction, scattering, interference, diffraction).
Color Sphere according to Philipp Otto Runge (1810). Being a painter, mixing of yellow and blue pigments to obtain green color was so familiar to him that he had refused to recognize green as a pure color. His sphere represents basically mere mixing of pigments.
Color atlases
How can we understand and compare colors? How do we name them?
The multitude of colors can be depicted by ascribing a single point in an abstract space to each color and placing similar colors close to each other. Such space can then be mapped in a series of single maps which then constitute a color atlas. Every possible color can be described by three numerical attributes: hue, saturation and brightness (HSB). The first such atlas was developed by the American painter Albert H. Munsell (1858-1918). Another type of color atlases is based on the original work of Isaac Newton (1643-1727) on additive mixing of colors. The Standard Table of the International Commission on Lighting (Commission Internationale de l'Éclairage, CIE) is one of this type of atlases. The third type of color systems is based on the perceptional relationship amongst the colors. We perceive red, green and blue as pure colors containing no other hues whereas orange, for example, seems to contain yellow and red. Moreover red/green and blue/yellow are opposites in the sense that no two colors of the pair are simultaneously contained in one hue. There is no yellowish blue or greenish red. The Swedish "Natural Color System (NCS)" is based on these observational facts.
The necessity of ordering colors in space was demonstrated for the first time by the color pyramid of Johann Heinrich Lambert (1772). The color samples were prepared by mixing colored waxes in integral proportions.
Rotating top with clipped-on colored discs. This was the first color system based on scientific principles authored by James Clerk Maxwell around 1850.
How can the color systems based on physical characteristics of light on one side and the perception based systems on the other side be related to each other? Visible light can be described as electromagnetic radiation of a wavelength between 380 and 740 nanometers. Every kind of observed light can be characterized by its spectral distribution, i.e. intensity of light as function of the wavelength. Describing the color of light in this way is much more complex than using the three attributes described above. Dividing the whole interval of visible wavelengths into slices 10 nanometers wide would only reduce the number of necessary attributes to 35. Thus it follows that for each perceived color there must exist many physically different spectra giving the same color impression. Such spectra are called metamer.
Color wheel according to Ewald Hering. The four prime colors red, green, blue and yellow are positioned according to their polarity as poles of two perpendicular axes. Intermediate colors are formed by additive mixing from the prime colors. The modern sweedish Natural Color System (NCS) is based on this color wheel.
Color wheel according to Michel-Eugène Chevreul. Adjacent colors should all show the same difference to each other.
Additional factor of the quality of illumination comes into play in describing metamerism in the case of colored surfaces or pigments. The color of non radiating surfaces can be characterized by a reflectance spectra: intensity of reflected light as function of the wavelength. The reflected light perceived by the observer is the product of the illumination and the reflectance. It is thus possible that two pigments reflect metamer lights when illuminated by one light source but reflect different spectra when illuminated by a different light source. Two colored textiles may look the same color when illuminated by incandescent bulb but look different under a fluorescent tube.
Detail from The Standard Color Table DIN 6164 (DIN = German Industrial Norm).
Sample page from the color atlas of Wilhelm Ostwald. Black is increasing from top to bottom while hue is increasing from left to right. This is the predecessor of the DIN Color Table.
A "natural" color system based on the colorful hues of maples leaves.
Detail from the color system in Munsell Book of Colors. The color space defined by Albert H. Munsell is defined by three coordinates: brightness increasing from top to bottom, hue and saturation. The latter correspond to the polar coordinates (angle and radius) in the horizontal plane.
