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.
