Glass data in ZEMAX is managed within catalogs, which are ASCII text files with the extension .agf. The glass catalog contains refractive, thermal, transmission and other data on each glass inside the catalog. Many glass catalogs are supplied with ZEMAX (schott.agf, ohara.agf, infrared.agf etc).

Important: you should never edit or change the glass catalogs supplied with ZEMAX. When you update ZEMAX at its next release, all files included with the installation (including glass catalogs) are overwritten with the versions supplied with that release. For this reason, when you want to add your own glass, the first step is to create your own catalog. Then add as many glasses as you want to your catalog. Your private catalogs will not be affected by updates to ZEMAX.

Also Important: never delete a glass from a catalog once it has been used. If you delete a glass from the catalog, any file that used the now-deleted glass will fail to trace, and will be useless. The correct approach is to mark the glass as 'Obsolete', and to use the comment field to explain why the glass should not be used. Both of these are discussed later in this article. The System Check utility will detect and warn the user if an obsolete glass is used, but the file will still ray-trace correctly. 

Chapter 18 of the User's Guide (entitled Using Glass Catalogs) should be read in conjunction with this article.