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Optical Materials 225

Figure 10.10 Spectral transmission curves for several optical glass filters.

as well as a number of establishments which make commercial colored
glass (as opposed to “optical” glass, which is more carefully controlled).
The transmission characteristics of glass filters vary from melt to
melt for any given type. If a filter application requires that the trans-
mission be accurately controlled, it is frequently necessary to adjust
the finished thickness of the filter to compensate for these variations.
The red filters are probably the most variable; since they are sensitive to
heat, some red glasses cannot be re-pressed into blanks. Spectral
transmission data for filters is usually given for a specific thickness
and includes the losses due to Fresnel surface reflections. To determine the
transmission for thicknesses other than the nominal value, the trans-
mittance, that is, the “internal” transmission of the piece without the
reflection losses, must be determined. In most cases, it is sufficient to
divide the transmission by Eq. 10.4 to get the transmittance. Then
Eq. 10.2 or 10.3 can be used to determine the transmittance of the new
thickness. This transmittance times the T of Eq. 10.4 will then give
the total transmission for the filter to a reasonable accuracy.
This process is greatly simplified by the use of a log-log plot of the
transmittance. The Schott catalog of filter glass makes use of this type
of scale. A transparent overlay makes it possible to evaluate instantly
the effect of a thickness change. A study of Fig. 10.11 will indicate the
utility of this type of a transmittance plot; the same filter is shown in