Heat Strengthened Glass or HS Glass is a Semi Tempered Glass. The process is similar to that of tempered glass, and differs only in the cooling method. HS is cooled less rapidly than tempered glass. HS Glass is two times as stronger than annealed glass. However it has lesser optical defects & bending them tempered glass. It is therefore used in applications like windows where you get better optics than tempered glass & better strength them normal glass. It is also used for making laminated glass.