A supplemental device for attachment to an injection device, the supplemental device comprising: an imaging arrangement configured to capture an image of one or more numbers present on a moveable component of the injection device； and a processor arrangement configured to run a first algorithm to: receive data representing an image； determine the position of at least one number in the image by determining the boundaries of each visible number； select one of the visible numbers to be identified； perform a pattern matching process to identify the value of the selected number； and determine a dose dialled into the injection device using the determined position of the selected one of the at least one visible number in the image.一種用於附接至一注射裝置之輔助裝置，此輔助裝置包含：一取像配置，組態成用於擷取一出現在一注射裝置之可移動組件上之一個或多個數之影像；以及一處理器配置，組態成用於執行一第一演算法，用於：接收表示一影像之資料；藉由決定每個可見數之邊界來決定影像中之至少一數之位置；選擇待識別之可見數之其中一個；執行一圖案匹配過程以確認選擇的數之數值；以及藉由使用影像中之至少一可見數之選擇的其中一個之確定位置，以決定一撥入注射裝置中之劑量。
