Nikkor lenses necessitate the best optical glass, that meets strict specifications.
Nikon's glass is made by Hikari Glass, established in Hikari, Japan, in 1975. They conceit themselves on having the skilled craftsmanship required to make optical standard glass.
There are only a few companies around the world that generate optical glass, Hikari Glass say that they have so much technical ability and knowhow that other companies simply can't vie. The plant designs, assembles and repairs all the equipment used to produce optical glass themselves, enabling them to improve equipment in order to consummate the specifications Nikon demands.
Nikon is proud to be one of very few manufacturers to carry out every element of manufacture, from initial development, straightaway through to completion of the final product itself.
You can find out more about Hikari plant, and how they produce Nikkor lens microscope spectacles to such exacting standards on the Nikkor website.