In a fluorescent lamp the ultraviolet radiation, efficiently produced by an electric discharge in a mixture of low pressure mercury vapor and low pressure inert gas such as argon, krypton or neon, stimulates the phosphor material coated on the inside of the lamp tube to emit visible light. Therefore phosphors could be called converters for changing invisible ultraviolet light into visible light.
When a fluorescent lamp is switched on, first a heating current passes through the cathodes in order to increase the temperature of the cathodes and an arc discharge (a current flow through a gas) is produced by applying a high voltage between the cathodes. Once a discharge is produced, the temperature of the cathodes can be maintained by the collision of electrons without the need for any other cathode heating, and in addition the mercury vapor pressure increases due to vaporization by the discharge, and the current increases. Fluorescent lamps are designed to incorporate ballast and therefore provide continuous discharge in a balanced state at an optimum value.
The fast electrons emitted by the cathode collide with mercury atoms and a transfer of energy occurs between the two. As a result of this transfer of energy the mercury atom emits an ultra-violet radiation with a wavelength of 253.7nm. Another energy transfer occurs when this ultra violet radiation collides with the phosphor and visible light is radiated as a result.