I'm thinking of installing a motion detector inside an existing recessed fixture over my front door in order to illuminate the stairs and door lock at night. Home Depot sells a small sensor that can be installed between the socket and bulb but nobody has been able to tell me if the frosted glass cover will block the sensor.
IR (infra red) motion sensors are generally housed behind frosted plastic housings. Camera lenses also let infra red light pass through. Assuming yours is an infra red motion detector, frosted plastic or glass will not be a problem. The frosting is there to block your view of the lens and to "smear" the background contrast projected onto the IR sensor such that the IR blob's motion is better detected.