The UV filter, also known as the ultraviolet filter, is referred to as Ultra Violet. It is usually colorless and transparent, but some appear purple or purplish-red when viewed from certain angles due to the addition of an anti-reflective coating. Many people purchase UV filters to protect the delicate lens coating, but this is actually just a secondary function. The main function of a UV filter is to absorb ultraviolet light with wavelengths below 400 nanometers, while it does not filter other visible or invisible light. The reason it can filter ultraviolet light is that the lens contains lead, making UV filters heavier compared to other lenses of the same size and thickness.