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optical crown

British  

noun

  1. an optical glass of low dispersion and relatively low refractive index. It is used in the construction of lenses

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012