A mercury-vapor lamp is a type of gas discharge lamp that produces light through the excitation of mercury vapor. It consists of a glass bulb filled with low-pressure mercury vapor and an inert gas, such as argon. When an electrical current passes through the bulb, it ionizes the gases, resulting in a discharge that emits ultraviolet (UV) light. This UV light then excites the mercury vapor, causing it to emit visible light primarily in the blue and green spectrum.
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A mercury-vapor lamp excites Mercury atoms in an electrical discharge and collects the light emitted as they relax. Its output contains characteristic spectral lines; the lamp's pressure and construction affect their widths and the balance between lines and background.
A spectroscope separates those lines, making the lamp useful for wavelength references and optical excitation. Some emitted ultraviolet radiation can excite a phosphor, which converts it into a broader visible output. The observed spectrum then depends on the discharge, envelope transmission and any conversion coating; it is not simply the isolated-atom spectrum at every operating condition.