A system comprising an injection device and a supplementary device for removable attachment to the injection device, the injection device comprising: a moveable number sleeve having numbers printed thereon; and a transparent dose window through which the numbers printed on the moveable number sleeve are visible; and the supplementary device comprising: a first optical sensor and a second optical sensor each configured to capture an image of a portion of the transparent dose window of the injection device from different respective angles wherein the first optical sensor is orientated and focused to capture an image of only an upper half of the dose window and the second optical sensor is orientated and focused to capture an image of only a lower half of the dose window; a plurality of point light sources (600, 610); and a processor arrangement configured to control operation of the first optical sensor and the second optical sensor and the plurality of point light sources (600, 610) and to receive image data from each of the optical sensors, wherein the processor arrangement is configured to: activate the plurality of point light sources (600, 610) sequentially; control the first optical sensor to capture the image of the upper half of the dose window under a first illumination condition; control the second optical sensor to capture the image of the lower half of the dose window under a second illumination condition; combine the image of the upper half of the dose window captured by the first optical sensor and the image of the lower half of the dose window captured by the second optical sensor into a final image; and perform an OCR process on the final image to identify the numbers visible in the final image. a moveable number sleeve having numbers printed thereon; and a transparent dose window through which the numbers printed on the moveable number sleeve are visible; and a first optical sensor and a second optical sensor each configured to capture an image of a portion of the transparent dose window of the injection device from different respective angles wherein the first optical sensor is orientated and focused to capture an image of only an upper half of the dose window and the second optical sensor is orientated and focused to capture an image of only a lower half of the dose window; a plurality of point light sources (600, 610); and a processor arrangement configured to control operation of the first optical sensor and the second optical sensor and the plurality of point light sources (600, 610) and to receive image data from each of the optical sensors, wherein the processor arrangement is configured to: activate the plurality of point light sources (600, 610) sequentially; control the first optical sensor to capture the image of the upper half of the dose window under a first illumination condition; control the second optical sensor to capture the image of the lower half of the dose window under a second illumination condition; combine the image of the upper half of the dose window captured by the first optical sensor and the image of the lower half of the dose window captured by the second optical sensor into a final image; and perform an OCR process on the final image to identify the numbers visible in the final image. activate the plurality of point light sources (600, 610) sequentially; control the first optical sensor to capture the image of the upper half of the dose window under a first illumination condition; control the second optical sensor to capture the image of the lower half of the dose window under a second illumination condition; combine the image of the upper half of the dose window captured by the first optical sensor and the image of the lower half of the dose window captured by the second optical sensor into a final image; and perform an OCR process on the final image to identify the numbers visible in the final image.