The Optical Wavelength Labs U2.5-4 2.5mm Universal Adapter Cap connects to 2.5mm connectors such as ST, SC, and FC. The cap is designed so that once the ferrule is completely inserted, the cone of acceptance from a fiber connector falls completely onto the photodetector, regardless of how the connector may turn, twist, or wiggle in the port. Because of this, users can be assured that the optical power reading will always be accurate.