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The light in Sodium Vapour lamp is from an atomic emission process whereas in Mercury Vapour Lamp it is, finally, from fluorescence emission. The mechanism of light emission in a sodium vapour lamp is simple and straight-forward. The 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 emiting 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 atomsrelax by emitting characteristic but ultravilot(254nm,invisible) light. The photons of this ultravilot 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. The basic difference between the two is, the former works by electric discharge (passage of electricity through sodium vapours at high/low pressure) while the latter works through the combined effect of electric discharge through mercury vapours and fluorescence from phosphors (luminescent materials). 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) On the other hand, a 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. Conventional fluorescent lamps can also be called as low-pressure mercury discharge lamps. In this system, when electric discharge strikes mercury vapours held at low pressure ( a few mm of mercury) it produces a lot of ultravilot radiation dominantly at 254 nm inside the column of the discharge tube. This UV radiation when impinging on the white coating made of fluorescent materials coated inside the discharge column of the tube will generate white light(called daylight). Light output from a fluorescent lamp is moderate (60 lumens/watt) while the colour-rendering index is high. posat by Chinmoy kanjilal and Tushar dhara,research scientists,SMIT,Sikkim,India.
Type of Lamp
Lum / Watt
Color RenderingIndex
Typical Application
Life (Hours)
Range
Avg.
Incandescent
8-18
14
Excellent
Homes, restaurants, general lighting, emergency lighting
1000
Fluorescent Lamps
46-60
50
Good w.r.t. coating
Offices, shops, hospitals, homes
5000
Compact fluorescent lamps (CFL)
40-70
60
Very good
Hotels, shops, homes, offices
8000-10000
High pressure mercury (HPMV)
44-57
50
Fair
General lighting in factories, garages, car parking, flood lighting
5000
Halogen lamps
18-24
20
Excellent
Display, flood lighting, stadium exhibition grounds, construction areas
2000-4000
High pressure sodium (HPSV) SON
67-121
90
Fair
General lighting in factories, ware houses, street lighting
6000-12000
Low pressure sodium (LPSV) SOX
101-175
150
Poor
Roadways, tunnels, canals, street lighting
6000-12000
The best way of distinguishing between the two types of lamps is by looking at the lamp's light output. A sodium vapour lamp's output is in the red end of the light spectrum. A mercury vapour lamp in in the blue end of the light spectrum.
With both types of lamps, they do not give a good accurate true colour rendition of the object that they shine on as compared to daylight.
Sodium vapor lamps usually emit a warm yellow or orange light, compared to the white light of other types of streetlights. You can also look for a yellowish glow surrounding the light fixture.
They both are 75 watts so they use the same amount of electricity.
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