What are the core characteristics of VIS neutral density filters?
Spectral Neutrality: ideally, ND filters have uniform transmission for all wavelengths in the visible range (actual products may have small deviations, especially at marginal wavelengths such as deep red or deep blue).
Attenuation range: common optical densities (OD) are 0.1-4.0 (transmittance from 80% to 0.01%), higher ODs require multi-layer combinations or special materials.
Surface quality: optical grade surface (no scratches, bubbles)

What materials are commonly used for VIS neutral density filters?
Germany Schott: NG series (e.g. NG4, NG9): borosilicate glass doped with iron, nickel and other metal ions, excellent spectral neutrality, high temperature resistance.
Hoya ND series.
Chinese materials: AB series materials.
Russian materials: HC series.