UV-IR imaging of medium-pressure mercury arc and electrode

This closeup shot of a Philips HO 450W was taken through a U360 filter which transmits primarily shortwave radiation between 300 and 400 nm, peaking at 360 nm, and some deep red and infrared radiation between 700 and 830 nm to a much lesser extent. The aspect of the resulting image depends primarily on the dominant spectral emission of various parts of the lamp. Mercury's I-line cluster around 365 nm emitted by the hot arc results in the excitation of red and blue pixels, while the predominantly long-wave emission from the incandescent electrode caused green and red pixels to be excited. The greenish background comes from parasitic reflections in the infrared caused by the lens's (and filter's) imperfect AR coatings.