PORRO PRISM, also known as the PRO prism, is a refractive prism used in optical instruments to modify the orientation of images. Paul's prism is an isosceles right angle prism made of glass blocks, with the end plane facing the right angle. In use, the light enters from the largest rectangular surface in the prism, undergoes two total reflections on the inclined plane, and then penetrates the original incident plane to emit. Because light only enters and exits in a normal state, the prism does not experience dispersion. But the image passing through the Paul prism will be flipped 180 ° and will move in the direction it originally entered, which means the direction of travel has also changed by 180 °. But because the image undergoes two reflections, the chirality remains unchanged. Paul prism is most commonly used in pairs with a combination of double Paul prisms, with the second prism rotated 90 ° relative to the first. The net effect of a prism system that allows light to pass through two triangular prisms arranged in this way is that the incident light is changed in a parallel direction, the image is rotated 180 °, and the chirality remains unchanged.