EN 169 Personal eye protection - Filters for welding and related applications - Transmittance requirements and recommended use 

This standard specifies the scale numbers and transmittance requirements for filters intended to protect operators performing work involving welding, braze-welding, air-arc gouging, and plasma jet cutting. It also includes requirements for welding filters with dual scale numbers. The other applicable requirements for these types of filters are detailed in EN 166. The requirements for fitting frames and mountings are described in Annex EN 175. 

The numbering table for filters and the reference for marking oculars and frames are available in EN 166.