[′pȯ·rō ‚priz·əm i′rek·tiŋ ‚sis·təm] (optics) A compound erecting system, designed by M. Porro, in which there are four reflections to completely erect the image; two Porro prisms are employed; the line of sight is bent through 360°, is displaced, but is not deviated; used in prism binoculars and some telescope systems.