Neutral Density (or ND) filters produce a grey neutral effect and are used to reduce light in cinematography, enabling more control over exposure and depth of field without affecting color or contrast. ND's are made in different densities according to the level of light reduction from 1/3 to as much as 4 stops of light.
Formatt Hitech HD ND filters are made of Schott Super White Glass. They are a standard dye sublimation filter which effects the visible spectrum only. These are great for use with digital sensors up to 4 stops (ND1.2) or on film cameras in any situation.
The HD ND do not filter for near-infrared light. Firecrest Ultra IRND
filters are also available for camera sensors that require near IR filtration.