Explanation: A tungsten filament requires a higher operating temperature to achieve sufficient thermionic emission compared to a thoriated tungsten filament. This is due to the difference in their work functions. Thermionic emission is the process where electrons escape from a heated metal surface. The work function is the minimum energy required for an electron to escape the surface. Pure tungsten has a relatively high work function, meaning it needs to be heated to very high temperatures (typically white-hot) to emit electrons effectively. When a thin layer of thorium is deposited on tungsten, it significantly lowers the effective work function of the filament. This "thoriated" surface can emit the same number of electrons at a much lower temperature (typically bright-orange) than pure tungsten. Therefore, a pure tungsten filament must operate at a higher temperature to achieve comparable electron emission.