What is the difference between a Shielded and Unshielded Proximity sensor?

Flush and non-flush mounting

A Shielded Proximity sensor has the face exposed and can be mounted flush. An Unshielded Proximity sensor has the face and the sides exposed as well, allowing for detection of objects from the front and the side. Being Unshielded also allows for a longer sensing distance but they can not be flush mounted.