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0.20
0.10
400 500 600 700
Wavelength in m/i
Fig. 2. Spectral transmittance of coated lenses.
color. It was decided that since we are dealing with more or less similar curves the difference in characteristics at two points of the spectrum would suffice to specify the color. From the standpoint of measurement accuracy, wavelengths should be chosen to give as large a density difference as possible. On the other hand the limits of sensitivity of color films are a consideration. As a compromise 400 and 700 imt were chosen. Since transmittance densities are additive we shall speak of the color contribution in terms of the transmittance density difference at 400 and 700 mju. That is, Z)4oo— Z>7oo equals the color contribution.
Having defined this quantity it will be convenient to evaluate the two sources of color in terms of this quantity. To this end we have defined a term known as "color index" for a description of the color of a piece of glass.
If t is the glass thickness in millimeters,
Color Index =
~ Aoo
The glasses most free of color have values from 0 to 0.0005. The worst glasses for color have indices of 0.0300. When using color index as a manufacturing tolerance it has been convenient to multiply by a factor of 104 to give integral values 0-300.
To compute the glass contribution we have merely to multiply the lens thicknesses in millimeters by the color index of the glass from which the lenses were made. The use of densities instead of per cent transmittance permits adding the values from each lens element. This gives us a simple method of determining quantitatively the effect of the glass absorption, and now we turn to the coating problem.
The color arising from the film coating is a function of three variables — the index of the glass, the number of coated surfaces, and the thickness of the coated film. The first two are fixed in the design of a lens so that we have only the last variable left for control. The trans
Philip T. Scharf: Transmission Color in Lenses
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