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 non-transmissive part located on an optical path between a first one of the one or more sources of light.本發明揭露一種用於附接到注射裝置的輔助裝置,包括覆蓋一套筒的劑量視窗,在該劑量視窗上標示著劑量值。該輔助裝置包含一主體;一排列,用於支撐與該注射裝置呈預定位置關係之輔助裝置的主體;一透明保護視窗,放置在當使用時與該注射筆之劑量視窗對準之主體的表面上;一感應器排列,在該主體中被支撐並且具有在該保護視窗上被引導的感應器;一照明排列,包含一或更多光源,該一或更多光源的每一個均在該保護視窗上被引導;以及一非透射部份,放置在該一或更多光源之第一個之間的光路徑上。13‧‧‧劑量視窗19‧‧‧套筒25‧‧‧照相機25A‧‧‧透鏡29C‧‧‧發光二極體29D‧‧‧發光二極體80‧‧‧保護視窗85‧‧‧斑痕86‧‧‧斑痕87‧‧‧光學路徑88‧‧‧光學路徑
