Retroreflecting Hollow Roof Prism Mirrors consists of two right angle prisms and a rectangular substrate made of H-K9L glass. The inclined surface of the right angle prism is coated with a protective layer of metal film, and it has high reflectivity over a wide spectral range. The angle between the inclined surfaces of the two prisms is 90 degrees. When the incident light is perpendicular to the bottom surface, 180 ° reflection can be achieved. Retroreflecting Hollow Roof Prism Mirrors are used to reflect light that enters the prism slope from the outside. The difference from mirrors is that the reflected light is parallel to the incident light, avoiding the influence of beam interference. It is more accurate than manually adjusting two flat mirrors to achieve 180 ° reflection.