There are mainly bak4 and BK7 prism materials. The refractive index of the two glasses is different. The refractive index of bak4 is high. For the Paul prism made of bak4 material, the incident light can be 100% fully reflected without considering absorption, while BK7 can only reflect about 83% of the incident light. Therefore, bak4 has high optical efficiency, but its price is also very high.