The three additive primary colours are red, green, and blue; this means that, by additively mixing the colours red, green, and blue in varying amounts, almost all other colours can be produced, and, when the three primaries are added together in equal amounts, white is produced.
Additive mixing can be demonstrated physically by using three slide projectors fitted with filters so that one projector shines a beam of saturated red light onto a white screen, another a beam of saturated blue light, and the third a beam of saturated green light. Additive mixing occurs where the beams overlap (and thus are added together), as shown in the figure (left). Where red and green beams overlap, yellow is produced. If more red light is added or if the intensity of the green light is decreased, the light mixture becomes orange. Similarly, if there is more green light than red light, a yellow-green is produced. The RGB colour model, one of the three main colour models, is an additive model used in digital devices and light-based media to create a gamut of colours from just red, green, and blue.
Subtractive colour mixing involves the absorption and selective transmission or reflection of light. It occurs when colorants (such as pigments or dyes) are mixed or when several coloured filters are inserted into a single beam of white light. For example, if a projector is fitted with a deep red filter, the filter will transmit red light and absorb other colours. If the projector is fitted with a strong green filter, red light will be absorbed and only green light transmitted. If, therefore, the projector is fitted with both red and green filters, all colours will be absorbed and no light transmitted, resulting in black. Similarly, a yellow pigment absorbs blue and violet light while reflecting yellow, green, and red light (the green and red additively combining to produce more yellow). Blue pigment absorbs primarily yellow, orange, and red light. If the yellow and blue pigments are mixed, green will be produced since it is the only spectral component that is not strongly absorbed by either pigment.
Because additive processes have the greatest gamut when the primaries are red, green, and blue, it is reasonable to expect that the greatest gamut in subtractive processes will be achieved when the primaries are, respectively, red-absorbing, green-absorbing, and blue-absorbing. The colour of an image that absorbs red light while transmitting all other radiations is blue-green, often called cyan. An image that absorbs only green light transmits both blue light and red light, and its colour is magenta. The blue-absorbing image transmits only green light and red light, and its colour is yellow.
No concepts in the field of colour have traditionally been more confused than those just discussed. This confusion can be traced to two prevalent misnomers: the subtractive primary cyan, which is properly a blue-green, is commonly called blue; and the subtractive primary magenta is commonly called red. In these terms, the subtractive primaries become red, yellow, and blue; and those whose experience is confined for the most part to subtractive mixtures have good cause to wonder why the physicist insists on regarding red, green, and blue as the primary colours. The confusion is at once resolved when it is realized that red, green, and blue are selected as additive primaries because they provide the greatest colour gamut in mixtures. For the same reason, the subtractive primaries are, respectively, red-absorbing (cyan), green-absorbing (magenta), and blue-absorbing (yellow).
Calculating chromaticity and luminance is a scientific method of determining a colour, but, for the rapid visual determination of the colour of objects, a colour atlas such as the Munsell Book of Color is often used. In this system colours are matched to printed colour chips from a three-dimensional colour solid whose parameters are hue, value (corresponding to reflectance), and chroma (corresponding to purity, or saturation). These three parameters are illustrated schematically in the figure. The central vertical axis provides a 10-step value scale extending from black at the bottom to white at the top. There are 100 hues divided into 10 groups around the vertical axis; each group has a colour name and consists of 10 subdivisions assigned a number from 1 to 10. The chroma scale starts at 0 at the vertical axis and extends radially outward from 10 to 18 steps depending on hue and value. The red apple discussed earlier would be designated 10RP 4/10 in the Munsell system, indicating a specific reddish purple hue 10RP, a value of 4, and a chroma of 10. Interpolated values are used to give more precise designations, so the emerald-green pigment can be specified as 5.0G 6.7/11.2.
A system that is useful when such precision is not required is the ISCC-NBS (Inter-Society Color Council–National Bureau of Standards) Centroid Color Charts. It has 267 numbered colour designations and uses descriptive terms such as very pale purple, light yellowish brown, and grayish blue; the red apple is 258 (moderate purplish red) in this system, and the emerald-green pigment is 139 (vivid green). Other colour atlases include the Ostwald colour system, based on mixtures of white, black, and a high chroma colour; the Maerz and Paul dictionary of colour; the Plochere colour system; and the Ridgway colour standards.
Gas excitation involves the emission of light by a chemical element present as a gas or vapour. When a gas such as neon or a vaporized element such as sodium or mercury is excited electrically, the electrical energy raises the atoms into high energy states, from which they decay back to ground state with the emission of photons. This leads to the red light seen in neon tubes and the yellow and blue light seen in sodium and mercury vapour lamps, respectively. The same yellow sodium light is emitted when sodium atoms are thermally excited by being heated in a gas flame. In addition to being produced electrically or by chemical reactions, gas excitations can also result from interaction with energetic particles, as in auroras, where energetic particles emitted in solar storms excite gases high in the Earth’s atmosphere to produce various colour effects.
All molecules have some vibration or rotation energy as a result of chemical bonding, but the energy involved is too low to interact directly with visible light. The frequency of vibration can be increased, however, by strengthening the chemical bonding involving very light atoms. For example, the bond between hydrogen and oxygen is stronger in liquid water and solid ice than in an isolated H2O molecule. The corresponding increase in vibration frequencies allows some absorption at the red end of the spectrum and produces the pale blue colour characteristic of pure water and ice when seen in bulk.
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