The numbers, 1 and 3, in the names Cx1 and Cx3 refer to the lens filtering category that is standard in the lens industry. It indicates the amount of light that is transmitted or filtered through the lens. Cx1 lenses are Category 1 filters (44-80% transmission) for indoor, lower light conditions and Cx3 lenses are Category 3 filters (9-18% transmission), typical for sunglasses and bright sun.
