It is easier if you insert one flange of the 55mm ring into the adapter hole, and line the opposite flange up with the wider part of the hole labeled "OPEN". Then rotate the ring slightly to lock it onto the beam splitter. (See Picture 1)A beam splitter or beamsplitter is an optical device that splits a beam of light into a transmitted and a reflected beam. It is a crucial part of many optical experimental and measurement systems, such as interferometers, also finding widespread application in fibre optic telecommunications. Indoor options encompass locations like the community's central computer room, building's weak current well, or floor wiring box.