Roof Prism A roof prism is a reflective optical prism to make two cross beams be parallel with each other. It contains a section where two faces meet at a 90 angle. These two 90 faces resemble the roof of a building, giving this prism type its name. Reflection from the two 90�° faces returns an image that is flipped laterally across the axis where the faces meet. In some cases, passing through the prism, the image is both deflected right-to-left and top-to-bottom. The hypotenuse of the prism utilizes total internal reflection (TIR) to reflect the image through the prism. Polarization states may become rotated during reflection. As a rich experienced manufacturer, substrate material in Photonchina can be chosen from all optical glasses (N-BK7, for instance), Fused Silica, Si, ZnSe, ZnS, CaF2, MgF2 etc.