Fluorescent lamps are also called mercury lamps or fluorescent lamps. The inner wall of cathode fluorescent lamp has phosphor powder, and the tube is filled with 0.8 Pa mercury vapor and inert gas, and the tube end has two electrodes respectively, which are communicated with a sealed spiral tungsten wire coated with thorium oxide and a pair of diagonal corners in cathode fluorescent lamp. When the filament is electrified and heated to 850 ~ 900℃, thorium oxide begins to emit electrons, and the electrons get high speed under the action of electric field, hitting mercury atoms and ionizing. When mercury atoms are ionized, they emit ultraviolet rays with a wavelength of 253.7nm, and the fluorescent powder on the wall of the excitation tube emits visible light. Some ultraviolet rays may radiate through the lamp tube. Relatively speaking, fluorescent lamps emit more ultraviolet rays than incandescent lamps. It is not suitable as an artificial light source for archives.