A roof prism (or Dach prism) is a specialized optical prism designed to invert an image in one axis while folding an optical path, all within an extremely compact form factor. Its name derives from the distinct “roof-like” structure (two surfaces meeting at a 90° angle) on one of its sides, which is responsible for the critical image-erecting function. This design is fundamental to creating slim, high-performance optical instruments like binoculars and rangefinders.

The primary purpose of a roof prism is to produce a fully corrected, erect, and right-reading imagewhile keeping the instrument’s physical barrel straight and narrow. It achieves this through a combination of reflections:

1. Inversion and Reflection: Like a Porro prism system, it inverts the image using multiple internal reflections.

2. The Roof Section: The defining feature is the roof (two surfaces meeting at a precise 90° edge). This roof introduces an additional reflection that flips the image in the second axis, correcting the left-right orientation that other prisms may reverse. This is often accompanied by a phase-shift correction.

• The Amici Prism: The most common type of roof prism is the Amici prism, which is essentially a right-angle prism with a roof section built onto its hypotenuse face. It both deviates the line of sight by 90° and erects the image.

• Materials: Fabricated from high-quality optical glass (e.g., N-BK7, BaK4) to ensure excellent light transmission and minimal internal scattering.

• Extreme Precision: The 90° roof edge must be manufactured to exceptional accuracy (often within arc-second tolerances). Any error in this angle causes double images or loss of resolution, a flaw known as “chromatic blurring.”

• Phase Correction Coatings: A critical advanced feature. The two reflections on the roof surfaces can introduce a phase difference between light waves, reducing image contrast and resolution. To combat this, specialized dielectric phase-correction coatings are applied to the roof surfaces to realign the light waves, ensuring a sharp, high-contrast image. This is a key differentiator between low- and high-quality roof prism instruments.

• Compact Binoculars and Monoculars: This is the predominant application. Roof prisms allow for straight-through, lightweight, and streamlined optical tubes, making them ideal for modern, portable designs.

• Rangefinders and Telescopic Sights: Used where a compact form factor and a clear, correctly oriented image are paramount.

• Complex Optical Systems: Incorporated as an erecting subsystem in various scientific, medical, and industrial imaging devices.