The roof prism deviates or deflects the image through an angle of 90 degrees. Basically the roof prism is a right-angle prism whose hypotenuse has been replaced by a 90-degree total internal reflection roof. Cleanliness and quality of the total internal reflection roof surfaces is important. The 90-degree deviation applies only to rays which are precisely normal to the entrance and exit faces. All other rays are deflected by varying amounts in exactly the same manner of an ordinary right-angle prism. The roof prism finds applications in situations that demand both right-angle deflection and image erection such as in some instrument viewfinders.