  We demonstrate and verify the in-situ addition of a collecting lens for
electroluminescence experiments to an existing scanning tunneling microscope.
We fabricate a simple clip-on lens that we reversibly attach at the sample
plate via regular sample transfer tools to collimate the light emitted from a
plasmonic tunneling junction to the viewport ordinarily used for optical
access. The proximity of the lens to the tunneling junction allows us to
exploit the full numerical aperture which helps us achieve good collection
efficiencies, demonstrating the quick turnaround of converting an existing
setup with optical access into a practical scanning luminescence microscope. We
verify the function of the clip-on lens by measuring the bias dependent plasmon
of Au, Ag, and spatial luminescence maps.
