Synthetic-organic chemists have developed to a high state the art of prediction of changes in absorption spectra with changes in molecular architecture. But they are not able to visualize the colors resulting from specific changes. The colorimetrist is frequently asked what spectra constitute ideal goals at which to shoot in a particular field of colors. Previously (see reference 7) the author had calculated the specifications of the colors caused by a V-shaped absorption band moving through the visible spectrum. In order to obtain a more detailed picture of the effects of changes in absorption spectra on the resulting colors, particularly in limiting cases, 100 selected ordinate calculations have now been made of the colors produced by idealized (rectangular) bands, of various widths, when moved across the spectrum.
The points representing the ideal colors in the 3-dimensional color solid lie on the same surface as do points found by MacAdam, but the new ones are of much more direct interest to chemists, for they relate to the two main and direct variables of the spectra, which are the chemists’ current tools in precise studies of “color” and chemical structure.
The spectra are of two types with ideal cut-off (ordinates all zero or unity), for bands of various widths and wavelength centers. Munsell re-notations of all the colors have also been computed, and they have been compared in saturation with the excellent TCCA silk colors.
As previously pointed out, no true greens can be made with a single absorption band; and no magentas with a single transmission band. Saturation of dyes increases with width of absorption or narrowness of transmission bands; but a “law of diminishing returns” operates, there being a point beyond which it is unprofitable to go. A 5-mμ shift in the yellow has more effect, e.g., on the color than a 50-mμ, shift in the blue-green. In two spectral-transmission regions, it makes little difference whether very wide or very narrow bands are used; in others the reverse is true. Saturation maxima were found at 3 absorption wave-length positions, and for transmission at 400–450 mμ. Many previous correlations of spectra and “color” are invalidated because of the “crowding” found of some hues into narrow wave-length regions, and great spread in others. In practice, yellows approach close to to the ideal maximum possible saturation, greens are very far away indeed; the spectral conditions for better greens are outlined.
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