Sensor device for attachment to a drug delivery device

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 comprising:

• a light guide having a first surface, a second surface opposed to the first surface, at least one side surface extending between the first and the second surfaces and a light out-coupling structure configured to cause diffuse light to be emitted from the first surface when light is input into the light guide;
• at least one light source configured to in-couple light into the light guide;
• and
• an optical sensor arranged to receive light reflected from a surface of the drug delivery device,
• wherein the sensor device further comprises a reflective backing layer adjacent the second surface configured to reflect any light which escapes the light guide through the second surface back into the light guide.