In the cleaning process, fully automated cleaning by a machine and manual cleaning by skilled craftsmen is performed (the photograph and animation show manual cleaning).
Here, “cleaning” denotes not only removing “stains” simply but also removing unnecessary materials on the surface of glass as a lens filter.
This is because “what is an unnecessary material” for glass having a complicated structure depends on its usage.
Cleaning of an optical lens filter is a process of a high difficulty level because higher quality is required than a common glass product and coating on the surface is necessary after cleaning.
Through multiple cleaning processes while paying full attention, a glass material is completed.