A supplemental device configured for attachment to an injection device, the supplemental device comprising: a camera configured to generate camera output indicative of a scene viewable by the camera, and to be located on an optical path with a surface of an injection device attached to the supplemental device in use； a light source having an emission spectrum and configured to illuminate the surface with light within the emission spectrum in use； and a filter configured to be located on the optical path between the camera and the surface in use, the filter being substantially opaque to light across the entire emission spectrum of the light source and substantially transparent to light of at least one frequency outside the emission spectrum of the light source.
