The NiSi 77mm Nano Coating Graduated Neutral Density Filter GND16 1.2 Nano coated optical gradient filter is made from environmental optical glass (H-K9L). The glass is annealing environmental protection optical glass H-K9L. Double sides optical grinding and polishing to make sure the optical high parallelism and smooth which ensure the high sharpness of image. Neutral density filter coated on double sides via 200C high temperature coating. The spectrum curves and straight and flat. It's soft transition from visible light to infrared light. There is anti-electrostatic coating can resit dust and also can endure wipe off organic solvent. AR and water-proof coating on double sides make the filter won't fade after several years. A graduated neutral-density filter, also known as a graduated ND filter, split neutral-density filter, or just a graduated filter, is an optical filter that has a variable light transmission. Typically half of the filter is of neutral density which transitions, either abruptly or gradually, into the other half which is clear. It is used to bring an overly-bright part of a scene into the dynamic range of film or sensor. For example, it can be used to darken a bright sky so that both the sky and subject can be properly exposed. ND filters can come in a variety of shapes and sizes and densities and can be used in all types of photographic applications from still photography, motion photography and scientific applications.