A cap designed for friction-fit with the Reed LM-81LX light meter to protect the translucent optical dome, and to provide a consistent 'zero' reference. Designed to thread the manufacturer provided strap through.Printed at 1.01x scale to get a good friction fit, adjust accordingly for your printer tolerances. Loosen the 4 screws holding the case to release the strap, thread the strap through the cap, then re-attach the strap to the retaining pin on the case and secure the screws again.