The heat energy is converted in the form of light in the filament. The filament is heated due to electrical energy supplied to the bulb is radiated in the form of heat. Fraction of the heat is converted into visible light because the waves of heat is not visible. So, the luminous

flux is less as compared to the radiant flux.

In case of mercury lamp, the amount of heat converted into visible light is more. So, the for this lamp the luminous efficiency is more.