Retroreflectors reflect an image or beam back 180° toward its original direction. Prisms achieve this either through total internal reflections (TIR) or specular reflections, depending on whether the reflective faces are coated. "Hollow" retroreflector mirrors employ first-surface specular reflections in order to eliminate dispersion, chromatic aberrations, and material absorption inherent in prisms.
Thorlabs offers a variety of free-space optics, including retroreflector prisms and mirrors (a.k.a. corner cubes), right-angle (a.k.a. porro) prisms, and hollow roof mirrors, and Dove prisms. Fiber optic retroreflector patch cables reflect light input through the connector backward through the fiber and can be used to create a fiber interferometer or to build a low-power fiber laser.