K9 (BK7) is a common borosilicate glass with a transmission range of 350nm--2100nm. It has high homogeneity and low content of bubbles and impurities, and can be directly used as a transmission optical material. Because of its stable chemical properties and high hardness, it can be used to make various optical elements from visible light to near infrared light, such as plane window, lens, prism and so on. These components are widely used in optoelectronics, microwave technology and other fields.