A supplemental device (2) for attachment to an injection device (1) including a dosage window (13)covering a sleeve (19) on which dose values are marked is disclosed. The supplemental device (2) comprises a main body (20) an arrangement for supporting the main body of the supplemental device in a predetermined positional relationship with the injection device (1) a transparent protection window (80) located at a surface of the main body (20) that is aligned with the dosage window (13) of the injection device (1) when in use a sensor arrangement (25) supported in the main body (20) and having a sensor directed at the protection window (80) an illumination arrangement (29a d) comprising one or more sources of light each of the one or more sources of light being directed at the protection window (80) and a non transmissive part (85 86) located on an optical path between a first one of the one or more sources of light.
