Definition of porro prism in English:

porro prism

(also Porro prism)

Pronunciation /ˈpôrō ˌprizəm/ /ˈpɔroʊ ˌprɪzəm/


  • 1A reflecting prism in which the light is reflected by two 45° surfaces and returned parallel to the incoming beam.

    Compare with roof prism

    ‘While not as sexy as a roof prism, the porro prism is a dependable unit made for the outdoors.’
    • ‘Two porro prism binocular models are new, the 8x42 mm and the 10x42 mm.’
    • ‘Modern roof prism designs are capable of performance just as good as a comparable porro prism design, due to current technology such as phase coating and improved reflective backing materials on the prisms.’
    • ‘Bushnell's latest couples a waterproof 8x36 porro prism center-focus binocular with a laser rangefinder.’
    • ‘In binoculars, it has added porro prisms to its line.’
    1. 1.1A pair of binoculars using two Porro prisms at right angles, resulting in a conventional instrument with objective lenses that are further apart than the eyepieces.


Named after the Italian scientist Ignazio Porro (1801–75).