Porro Prism

A Porro prism is an isosceles right-angle optical prism crafted from glass, with its end face aligned to the right-angle vertex. In operation, light enters through the prism’s largest rectangular face, undergoes two total internal reflections on the inclined surfaces, and exits through the original incident plane. This normal-incidence input/output ensures the Porro prism introduces no chromatic dispersion, preserving image clarity. Notably, the prism flips the image 180° while emitting light along the original incident direction; two total internal reflections preserve the image’s handedness, maintaining consistent orientation.

Porro prisms are commonly deployed in pairs as a double Porro prism system, with the second prism rotated 90° relative to the first. This configuration redirects incident light to a parallel path, rotates the image 180°, and retains its original handedness. Double Porro prism systems excel at image orientation adjustment in compact optical telescopes—most notably binoculars—where they enable image erecting and extended optical path folding. This design effectively shortens the objective-to-eyepiece distance, enabling compact, high-performance binocular construction. In practical double Porro prism assemblies, the two prisms are often bonded together, with excess material trimmed to minimize weight and optimize compactness for portable optical devices.