Disclosed are an apparatus and a method for monitoring a glass plate polishing state. A light detection device for detecting light is provided, wherein the light detection device includes the optical element as described above and an element for detecting light condensed by the optical element. Sensors detect the level of grinding force and motor torque required to grind a workpiece into a desired shape. The join request manager receives a request to receive a multicast traffic flow, the request being received from one of the user devices within one of the user systems, and denies the request if a system metric is above a threshold. The system enables parties to be organized into groups based upon member profiles.