Formatt-Hitech 100x100mm (4x4") Resin ProStop IRND 6 (6 Stops) Hitech filters are made from the highest quality 1.5mm optical resin. The filter is also available in 85mm, 150mm and 165mm sizes. The 100mm size is useable with Hitech 100mm holder systems or any 100mm (4-Inch) holder.