In one aspect the present disclosure relates to a monitoring device for attachment to an injection device, the monitoring device comprising:a body (51) having an inside surface (52) for engagement with an outside surface (19) of the injection device (1) and having an outside surface (53) opposite to the inside surface (52),an light transmission area (60) extending from the inside surface (52) to the outside surface (53), the light transmission area (60) having an inner end (62) adjacent to the inside surface and having an outer end (63) adjacent to the outside surface (52), wherein the light transmission area (60) provides transmission of light from the inner end (62) to the outer end (63),an image acquisition system (70) arranged inside or on the body (51), andan optical coupling (80) arranged in or extending into the light transmission area (60), wherein the optical coupling (80) is optically coupled to the image acquisition system (70) in a light-transmitting way.
