A supplemental device comprising: a connector configured to attach the supplemental device to an injection device in use a sensor configured to generate sensor output dependent on an intensity of light on the sensor and having a field of view directed at a surface area of the injection device in use, the surface area being external to the supplemental device a light source configured to generate illumination which in use can be directed out of the supplemental device onto the surface area a shield configured to limit the field of view of the sensor such that the field of view is entirely directed at the surface area in use and a processor configured to use the sensor output generated when illumination from the light source is reflected from the surface area to determine a property of the injection device.
