The SIGMA WR CERAMIC PROTECTOR from this advanced material is a highly reliable filter with outstanding protection. Glass-ceramic is an extremely resistant, crystallized glass, which serves as the basis for clear-glass ceramics and is widely used in space and other industries. This advanced new material combines greater hardness than chemically toughened glass and greater flexibility than sapphire crystal glass, and has the high light transmittance required of optical devices. These properties make clear glass ceramics the ideal material for lens protection filters