A supplemental device for attachment to an injection device including a dosage window covering a sleeve on which dose values are marked is disclosed. The supplemental device comprises a main body an arrangement for supporting the main body of the supplemental device in a predetermined positional relationship with the injection device a transparent protection window located at a surface of the main body that is aligned with the dosage window of the injection pen when in use a sensor arrangement supported in the main body and having a sensor directed at the protection window an illumination arrangement comprising one or more sources of light, each of the one or more sources of light being directed at the protection window and a dispersing part, which may be diffusing or scattering, located on an optical path between a first one of the one or more sources of light.
