Testing fluorescent light tube. Researcher check- ing a fluorescent light tube. Fluorescent lights are much more efficient than filament light bulbs as less of the radiation is in the infrared. The electric current passing through the low pressure mercury vapour in the glass tube is causing the mercury to emit ultraviolet radiation. This is converted into visible light by the phosphors on the inside of the tube. Phosphors, substances that emit light without being heated, are chosen so that the whole light spectrum is emitted, produc- ing white light. Photographed at Siemens' Osram Sylvania Laboratory, Beverly, Massachusetts, USA.