Why Does the Tungsten Filament of a Tungsten Iodine Lamp Can Emit Light?

The tungsten filament of a tungsten iodine lamp is a conductor and can radiate visible light. A tungsten iodine lamp is a new type of electric light source, the main components are the internal tungsten filament and the external glass shell. Under the action of electric current, the spiral tungsten filament continuously gathers heat, making the temperature of the filament reach over 2000℃. Microscopically, the tungsten atom's electrons outside the nucleus are excited by heat to a high-energy state, to maintain stability and fall back to a low-energy state, the energy difference in the form of photons released, macroscopically visible luminescence phenomenon.

Tungsten iodine lamps have a tungsten filament that utilizes the principle of electrothermal luminescence to achieve the lighting function. The melting point of tungsten is 3410℃, which is more than the normal luminous temperature of the filament (more than 2000℃). The same type such as copper wire and iron wire is also a conductor, but the melting point is lower than 2000℃, which can't support the normal work under this temperature. At the same time, the tungsten filament resistance is large, the relative resistance of the wire is negligible, and the tungsten filament glows first when the wire is not glowing, which ensures that the tungsten filament will not be oxidized when it heats up.

In fact, all substances can emit light under certain conditions. According to the third law of thermodynamics, substances cannot reach absolute zero. Therefore, at temperatures above absolute zero, other substances also radiate light, generally invisible infrared light at room temperature. As the temperature rises, molecules move and rotate and electrons are excited, emitting visible and ultraviolet light. At very high temperatures, the nuclei of atoms are excited, emitting radiation of shorter wavelengths such as X-rays.

As the core material of tungsten iodine lamps, tungsten filament has higher requirements on light efficiency and lifetime with the implementation of energy efficiency directive for halogen lamps. On the other hand, to ensure the normal cycle of tungsten halogen, the high-temperature deformation resistance of tungsten filament and the content of impurities have also become important indicators for assessment.

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