Abstract: Angle measuring apparatus including a disc having a circular track of marks on one surface concentric with the disc, such disc being rotated about its axis. A mark sensor located adjacent the disc surface detects each mark and generates a signal as it passes thereby. A pair of angle-input means are located adjacent the disc for defining the angle to be measured. Reference means rotate with the disc and interact with each input-means to generate a reference signal each time the reference means passes thereby. Control circuitry counts the number of mark signals occurring between selected reference signals and from such count, the defined angle can be accurately computed.

Abstract: A sealed prism assembly including a prism having a cylindrical body region which terminates at one of its ends with an annular flange. Encasing the body region and fitting closely thereabout is a cylindrical cup terminating at a circular end which contacts the flange. A silicone rubber sealant distributed between the prism body region and the cup forms an expandable-contractable air-tight seal therebetween. The prism and cup are encased in a canister in an axially compressed condition.

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.