Difference between Sodium Vapor Lamp and Mercury Lamp

May 22, 2013 April 13, 2021

Difference	Sodium Vapour Lamp	Mercury Lamp
Light Source	Works by electric discharge (passage of electricity through sodium vapours at high/low pressure)	Works through the combined effect of electric discharge through mercury vapours and fluorescence from phosphors (luminescent materials).
Process of Lightning	Filaments of the lamp sputter fast moving electrons, which hit the sodium atoms (vapour) causing the valence electrons of the sodium atoms to excite to higher energy levels and the electrons thus excited relax by emitting the characteristic monochromatic bright yellow light (589nm).	The mechanism in mercury vapour lamp is more involved and sequential. The sputtered electrons from the filaments, after having been accelerated by high voltage, hit the mercury atoms. Here also, the excited electrons of mercury atoms relax by emitting characteristic but ultraviolet (254nm, invisible) light.
		The photons of this ultraviolet light fall on the fluorescent layer on the inner walls of the tube and excite the molecular bonds of the fluorescent material to various electronic and vibrational energy states. Hence, the light from the mercury vapour lamp is white.

Applications	Although sodium vapour lamps produce much higher light output (about 90 lumens/watt) they cannot be used in lighting applications where colour-rendering property is very crucial. This is because most of the light emitted from a sodium vapour lamp is concentrated in the yellow part of the visible spectrum (around 580-590 nm).	Mercury vapour lamp is quite suitable for lighting applications. This is because, the mercury vapour lamp can feed almost the entire visible region (380-780 nm) of the human visual system