Abstract: Retrodirective optical prisms are tested for light reflection and efficiency by generating two substantially co-axial beams of light, a signal beam and a reference beam. The beams are passed through an optical beam splitter arranged at a predetermined angle to the axis of the beams, thereby separating them. The separated signal beam is passed into the prism to be tested, reflected back along its initial path, and back through the beam splitter. The intensity of the reflected beam is measured and compared with the intensity of the reference beam. It thus gives an indication of the light reflective efficiency of the prism.