Abstract: A retrofittable apparatus for illuminating the face of a lock comprised of an annular outer housing with an aperture which receives the lock cylinder such that when the cylinder's face is within the outer housing a gap is formed between a housing front wall and the cylinder front face. An annular body having an aperture accepts the lock cylinder such that a shoulder of the body engages a lock cylinder flange. A lamp illuminates the face of the lock cylinder through the gap when a switch is closed. In the preferred embodiment the outer housing forms part of the electrical path from the battery supply to the lamp.