The invention refers to an arrangement and a method for determining a longitudinal position of a stopper for sealing a compartment of a translucent medicament container for a liquid medicament (M). The arrangement comprises a circular light source and a photo sensitive sensor, wherein either the light source or the sensor is laterally arrangeable next to the medicament container extending over at least part of the length of the medicament container, and wherein the respective other of the light source and the sensor is arrangeable in a circular manner around a head of the medicament container. The light source is arranged to emit light into the medicament container. The light is scattered by the medicament or medicament container and detected by the sensor. The sensor is connected to a processor unit for detecting the position of the stopper).