This article is also available in Japanese.

Optimization of glasses is handled somewhat differently than other data. Optimizing the glass choice directly is a difficult and unpredictable process because there does not exist a continuum of glasses on the glass map.

For example, a thickness can be easily adjusted from 10 to 10.1 mm. But it is not possible to perturb say F2 glass to something just slightly more dispersive. Glasses exist only with discrete properties.

The traditional method for handling glass choice is to use some kind of model glass. The model glass method is to idealize the glass dispersion using a few numerical parameters, and then optimize these parameters while constraining either the parameter values or the computed index values to be similar to available glasses. In the visible region the method can be simplified further by using Conrady d-D methods, in which only a few rays of different wavelengths are traced, scaled and their differences driven to zero by the optimizer.

If a glass is made into a "model" glass described by the index, Abbe, and partial dispersion deviation, then the parameters to the model may be made variable and optimized just like any other numerical parameters. However, the model glass method has one serious drawback. After a good solution is found using model glasses, a conversion must be made back from the model glass to a real glass. The design must then be reoptimized using the new glass selection.

Unfortunately, for many systems the newly optimized design will perform worse than the model glass design. Even more frustrating is that the optimal design using real glasses may have a different form than that found using the model glass.

Glass substitution is the most effective method for choosing glasses. The glass substitution method is to directly alter the glass types, and then reoptimize to see if the new glasses yield a better solution. ZEMAX automates this procedure by allowing glasses to have a "substitute" status associated with them. If a glass is marked as a substitute glass, then the global optimization algorithms (both Hammer and Global Search) automatically perform iterative substitution of similar glass types during optimization. This allows ZEMAX to optimize not only the numerical prescription values such as radius and thickness, but also allows direct optimization of real glass selection, without resorting to idealizing the glass dispersion. This is the most direct and powerful way to choose glasses in any wavelength region.