Arrangement for determining a longitudinal position of a stopper

An arrangement for determining a longitudinal position of a stopper for sealing a compartment of a translucent medicament container for a liquid medicament (M), the arrangement comprising at least one light source and at least one photo sensitive sensor, wherein either the at least one light source or the at least one sensor is laterally arrangeable next to the medicament container extending over at least part of the length of the medicament container, wherein the respective other of the at least one light source and the at least one sensor is arrangeable in a circular manner around a head of the medicament container, wherein the light source is arranged to emit light into the medicament (M) container and wherein the medicament container and/or the medicament is arranged to scatter the light so as to allow the sensor to detect it, wherein the at least one sensor is connectable to a processor unit for processing sensor data.