APPENDIX C:
BACKGROUND ON COLOR
Color measurement1 is most often performed using the Commission Internationale de l 'Eclairage system, with values frequently given in terms of “x”, “y”, and “Y.” Here, x and y are the coordinates on the chromaticity diagram or “color triangle,” and Y is the intensity. Several other systems are used; interconversion is relatively simple and automatic in modern, computerized color-measuring instruments.
The Commission Internationale de l'Eclairage has also specified a number of standard illuminates, in particular “A,” corresponding to a tungsten filament bulb with a color temperature of 2854K, “C” for noon sunlight, and “D65” for daylight with a color temperature of 6500K. A last type of illumination, not standardized but important for currency, is the reddish candle or low-level incandescent tungsten filament light so often found in dimly lit surroundings.
Nine of the fifteen physical and chemical causes2 that can cause a non-white distribution of light from non-color light have been used or could be used in currency:
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Transition metal compounds as in pigments such as chrome green, where the colors are caused by light absorption at restricted energies, producing the excitation of unpaired electrons.
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Organic compounds as in dyes and pigments such as in the dye indigo, where the colors derive from absorptions leading to excited electrons in extended conjugated organic molecules having alternating single and double bonds.
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Charge transfer compounds as in the pigment Prussian blue, where electrons absorb energy in moving from one atom, ion, or group to another.
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Metals as in reflective pigments such as those based on aluminum flakes, where light is totally absorbed into an electron band at the metal surface, but essentially all of it is immediately re-emitted in the metallic reflection.
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Pure semiconductors as in some pigments such as cadmium yellow, where selected absorption of light is limited by an energy gap in the structure of the electron band, in which gap electrons cannot exist.
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Refractive index differences as in transparent and translucent materials and in watermarks, where the refractive index depends on the physical nature of the substance, including its chemical composition, structure, and the strength of the bonding.
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1 |
Color measurement and related topics are well covered in Billmeyer and Saltzman, 1981, and in Hunt, 1987. |
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2 |
Details of the 15 causes of color are given in Nassau, 1983. |
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Scattering as in opaque regions and in watermarks where opacity and scattering depend on the nature of the material and the size of the particles as well as on the refractive index of the medium surrounding them.
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Interference as in optically variable pigments holograms, where the waves of light scattered by structured thin films or different parts of an object interfere with each other and either cancel or reinforce to produce patterns of colors from white light.
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Diffraction as in diffraction gratings, where a similar result is produced by regular two- or three-dimensional geometrical arrays or structures that scatter and break up reflected or transmitted light into spectral arrays of color.
REFERENCES
Billmeyer, F. W., Jr., and M. Saltzman. 1981. Principles of Color Technology, 2nd ed.New York: John Wiley and Sons.
Hunt, R. W. G.. 1987. Measuring Color. New York, New York: John Wiley and Sons.
Nassau, K.. 1983. The Physics and Chemistry of Color. New York: John Wiley and Sons.
