Neutral Density Filters UQG manufactures two types of neutral density filters. Absorptive glass nd filters and reflective metallic nd filters for the visible/NIR and UV transmitting. Absorptive Neutral Density Filters Absorptive Schott glass nd filters are used to control the intensity of light and are stackable without resulting in multiple reflections. The nd glass absorbs the light within the filter. Each glass type is polished to required thickness and measured to produce the exact optical density and transmission. Custom or standard optical density’s and transmission can be produced to a particular wavelength.