Porro Prism is a kind of reflection prisms, which is designed to modify the orientation of image.
By twice internal reflection, light incoming or image from the largest face will exit through the same surface. In other words, the image will rotate by 180°but not inverted. And porro prisms are often used as a pair named double Porro prism. This kind of prisms is often used in the field of optical telescope and binocular for modifying the orientation of image.