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Message: <blockquote data-quote="henry link" data-source="post: 3353247" data-attributes="member: 6806">Thanks for the effort, Lee.You can find out more than you want to know about the low dispersion glass types in the Schott catalogue here:<a href="http://www.schott.com/advanced_optics/english/download/schott-fluoro-phosphate-glasses-may-2014.pdf" target="_blank">http://www.schott.com/advanced_optics/english/download/schott-fluoro-phosphate-glasses-may-2014.pdf</a>I don't think anything new has been developed since N-FK58 XLD in 2014. Only one or possibly two types in the catalogue are reasonable candidates for the original marketing term "FL". FK51A and PK52A were the only truly extra low dispersion types in the catalogue when the original Victory FL appeared in 2004 and only FK51A is a fluoro-crown, the stuff that is sometimes called Fluoride glass. N-FK58 XLD, with an Abbe# above 90, falls into a category of glass types that is sometimes called UD (ultra low dispersion) to distinguish them from ordinary ED (extra low dispersion). That's why I wondered whether "Ultra-FL" might be marketing code for FK58.Certainly makes sense for HT glass types to be used in the SF where they're appropriate. Henry</blockquote>