A sensor device configured to be attached to a drug delivery device and configured to illuminate the drug delivery device when attached, the sensor device having an OLED having a transparent first electrode, a transparent second electrode and a central layer disposed between the first and second electrodes, the central layer comprising at least one organic layer, the at least one organic layer configured to emit light through the transparent first electrode, and an optical sensor arranged to receive light reflected from a surface of the drug delivery device, wherein the central layer of the OLED has a region without the at least one organic layer and wherein the optical sensor is arranged, when the sensor device is attached to the drug delivery device, to view a predetermined area of the surface of the drug delivery device through the region without the at least one organic layer.