Lens Import Unknown Materials
When importing a lens prescription from a Zemax file, you might see some materials replaced with a dummy one called "unknown" that has a refractive index of 1. This happens when the referenced material(s) in the file can't be found in FRED's equivalent glass catalogs.
it is good practice to add all glass catalogs pertinent to the prescription to same directory as the Zemax file before importing. This will allow FRED to read the Zemax catalog and use its definition of the materials (FRED does not search the user's disk in an attempt to locate these catalogs).