Pentagonal prism is the beam Angle (90 °), one of the steering gear.It has two purposes: one is that no matter how much the first incident Angle on the side is the emergent light incident light steering Angle (90 °);The other is that, unlike a rectangular prism, the resulting image has neither rotation nor specular reflection.Pentaprism is commonly used in camera viewfinders, image observation systems or measuring instruments.