Breakthrough Photography introduced a line of high-grade filters in 2015, dubbed the “X3” line. See the in-depth description of the technology used to manufacture these filters.
Due to trademark issues X3 became “X4”.
The coatings on the BreakThrough Photography neutral density filters are outstanding, and the neutrality is far superior to other brands for the more dense filters (6 and 10 stop in particular are vastly better).
The latest versions have even better coatings than the X3 version I reviewed last year: