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.