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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.

Glass substitution is a powerful optimization feature, unique to ZEMAX. The glass substitution method uses real glasses from the glass catalog to replace the glasses in the Lens Data Editor during optimization.
 

To see the glass substitution in action, we will optimize the glasses in the sample Doublet.zmx file located in {ZEMAXroot}/Samples/Sequential/Objectives/Doublet.zmx.

Open the sample file and set the substitute status on the two glasses in the Lens Data Editor, by opening the solve dialog box (double or left click on the glass parameter) and setting the solve type to “Substitute”. 

The letter “S” will appear next to the glasses, indicating that theses glasses have the substitute status.

Build the RMS wavefront default merit function using the default merit function tool in the merit function window.

Click Ok and the Merit Function Editor will show:

Since each time the glass is substituted during optimization the merit function value makes a discontinuous jump, glass substitution requires the use of global optimization algorithms such as Global Search or Hammer, instead of the damped least squared method (local optimization). 

The glasses selected for substitution are chosen from the current glass catalogs for the system, listed under System...General...Glass Catalog, which in this case is the Schott catalog. 

Open the OPD fan plot to note the performance of the system before optimization. The OPD scale is 0.5 waves.

Perform the Hammer optimization by clicking Tools...Optimization...Hammer Optimization in the main ZEMAX menu. Check the “Auto Update” option to see the changes in the Lens Data Editor and the OPD fan plot during optimization.

Click “Hammer” to optimize

You will see the glasses being substituted during optimization and see the changes in the OPD fan window.

Stop the optimization after a minute or so. The glasses in the Lens Data Editor might be as following:

Set the OPD fan scale to 0.5 for easy comparison to the pre-optimization performance.  It is obvious that the performance has greatly improved. 

All glasses listed under System...General...Glass Catalog will be considered for substitution. If you want to limit the substitution to subset of catalogs for a specific glass, you can type the name of the catalog(s) in the solve window. For example, if you have both Hoya and Schott listed in the General window:

but you want to use only Hoya glasses for surface #1, all you need to do is to type “Hoya” in the solve window of that glass.

Now only Hoya glasses will be chosen.

You can further restrict the substitution within the catalogs by using the glass substitution template, under Tools...Optimization...Glass Substitution Template. Make sure to check the “Use Glass Substitution Template” box; if you do not, then all glasses in the catalog will be considered for substitution, as in the previous exercise.   

The substitution template uses data in the glass catalog such as “relative cost” and “status” to filter the glasses. For example, it if often advantageous to check the “Preferred” option,since Preferred glasses are usually frequently melted glasses, and more likely to be available upon demand.

To see the data or status for a particular glass, open the glass catalog window by clicking Tools...Catalog...Glass Catalog in the main ZEMAX menu.

The glass catalog gives the glass status, which can be Standard, Preferred, Special, Obsolete or Melt. Unless you really need to choose otherwise, always select 'Preferred' glasses only, as these are the lowest cost, most chemically stable, most commonly used etc glasses. 

The glass catalog also lists relative cost (price poer unit weight compared to N-BK7), climate resistance, stain resistance, acid resistance, alkali resistance and phosphate resistance. By using the Glass Substitution Template, you can ensure that the glasses chosen meet other important criteria rather than just optical criteria.

Glass Substitution is the most powerful method available for selecting the best glasses to use in your optical design. It uses no approximations, and always uses the real dispersion data of the glasses. It also allows glasses to be chosen on the basis of their status, cost, and other properties.