A thru-beam sensor is positioned such that the beam passes through the crease line of the envelope when the envelope is parked in the insertion area. As the envelope enters the insertion area, the through beam sensor is set to current level A. The sensor is used to accurately position the envelope based on the passing of the lead edge. Once the envelope is in position, the current in the thru-beam emitter pair is raised until the envelope is no longer obstructing the line of sight of the sensor. From this state, the insert or plurality of inserts that are entering the accumulator can be detected, as the presence of the inserts will inhibit light transmission between the emitter and receiver. Using the sensor system proposed here, the inserts trailing edge can be detected at the crease line of the envelope.