Use of rare earths in cinema arc carbons

For the production of white light, a temperature of 1500°C is required. The incandescent carbon volatilizes above 3600°C and hence reasonable brightness can be obtained from the arc produced from carbon electrodes. 

The path of least resistance for the electric current is from negative carbon to the positive core. Hence most of the electrons forming the high current density in the arc stream travel from the positive core. At this point the energy is high such that the core vaporizes faster than the shell, thus forming a cup on the face of the positive carbon, which is the main source of light. 

Rare earth compounds in the form of oxides and fluorides are mixed with carbon powder and introduced into the core of the positive arc carbon electrode. The amounts of rare earth compounds used and the size of the core are dependent on the operational requirements. 

The demand for a brighter light source with a specific spectral range has been on the increase due to a continuing need for the projection of colour movies on wide screens. Rare earth compounds have thus become essential in the cinema industry. 

---