A type of prism used in some binoculars and scopes. A Porro prism is a right geometric prism used to change the orientation of an optical image. In binoculars, two Porro prisms are combined (a.k.a double Porro prism), with the second prism offset by 90 degrees. This configuration re-orients an image that would otherwise be inverted by the optics. Porro prism binoculars tend to be larger and bulkier than roof prism binoculars, but may also provide better light transmission for a brighter image.