Abstract: The apparatus is a laser transmitter and receiver unit with independent adjustment of the focus of each and preservation of precise parallel alignment between the two lines of sight. The transmitter laser and the receiver photodetector are each mounted in a precisely machined two part mounting assembly. The first part of both mounting assemblies includes an objective lens and is fitted against the same precise surface of a reference fixture. The second part of each mounting assembly, holding the laser or photodetector, is connected to the first part by means of precisely mated coaxial sliding cylinders, so that the laser or photodetector can be replaced on the fixture with high accuracy. Axial adjustment of the distance of the laser or photodetector from their respective objective lenses is furnished by spacer washers inserted between the two parts of the assemblies.