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: