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Blue coating
Glass absorption Simple lens Complex lens
Magenta coating
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700
500 600
Wavelength in m/^
Fig. 1. Transmittance losses from glass absorption and coated surface reflection.
color, generally yellowish, is caused by the ultraviolet absorption band of glass. Modern experiments have proven that in most optical glasses the visual color is a function of impurities present in the raw materials. At some future date, production quantities of all the common optical glasses may be available free of color. Until then we must learn how to use existing glasses. We have made a fairly extensive study of all the commonly used glasses from many sources. Two important findings that have come from this study are that there is a considerable difference between manufacturers and that the sources supplying color-free glasses appear to be able to maintain their quality. These findings have enabled us to put a certain degree of control over this first source of color in a lens.
It is generally recognized that the antireflecting coatings impart a degree of color to the transmitted light. If we observe the light reflected from different thicknesses of these coating films we see that the thinner films are yellowish or amber, the thicker ones bluish, with magenta
films lying in between. The yellow reflection means that less blue light is reflected than red or green which in turn can be interpreted as meaning that more blue light is transmitted. Likewise the magenta coating means more green light is transmitted and the blue coating means more yellow light is transmitted. Since coatings do transmit light selectively we have here the second contributing source of color in the lens. An advantage can obviously be gained if we can get these two factors to cancel one another. Figure 1 is a plot of the two color-contributing factors in terms of wavelength and transmittance density. It can be seen that the curve shapes are essentially different so that complete cancellation cannot be hoped for, but a good approximation is possible. Simple lenses having little glass absorption will need less color compensation from the coatings than the more complex lenses.
The complete color specification for a lens is given by its spectrophotometric curve as shown in Fig. 2. However, a single quantity would be convenient to use in expressing the varying degrees of
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September 1952 Journal of the SMPTE Vol. 59