A roof prism is created by machining a 90° "roof" shaped double reflective surface (resembling a roof) on the hypotenuse of a traditional right-angle prism. This allows the light beam to undergo two total internal reflections, turning 90° while also performing a mirror flip, thereby achieving an erect image output without increasing the optical path length. It combines the dual functions of "optical path folding" and "left-right reversal," making it a key component in modern binoculars, scopes, periscopes, and miniature camera viewfinders.

1. Optical Principle
   ? Optical Path: Incident light → first reflective surface (first roof face) → second reflective surface (second roof face) → emergent light. The total optical path length after two total internal reflections is the same as that of a 90° right-angle prism, but with an additional mirror symmetry operation.
   ? Phase Compensation: A 180° phase difference exists at the junction of the roof surfaces, requiring a phase compensation coating (Phase Coating) to prevent polarization cancellation that would reduce imaging contrast.
   ? Emergent Angle: 90°±1′; the emergent image is oriented the same as the incident image, with correct top, bottom, left, and right alignment, eliminating the need for an additional image-erecting system.

2. Key Specifications
   ? Roof Angle Accuracy: 90°±2″; increased error may cause double images.
   ? Surface Quality: 10-5 scratch-dig, surface figure λ/4@632.8 nm; roof edge straightness ≤ 1 μm.
   ? Coatings: Broadband anti-reflection coating on entrance/exit surfaces for 400-700 nm (R<0.5 %); silver + protective paint or high-reflection dielectric coating on roof surfaces (R>98 %).