The main materials of prisms are BAK4 and BK7. These two types of glass have different refractive indices. BAK4 has a high refractive index. For the Paul prism made of BAK4 material, it can fully reflect 100% of the incident light 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.