Sodium Vapour Lamp

Sodium Vapour Lamp Principal :

Electrons are emitted from the cathode by subjecting the surface of the cathode to very high electric stress so that electrons are forcibly ejected from orbits of atom against nuclear centrifugal force

sodium vapour lamp

Construction and Working

Diagram of sodium vapour lamp

  • It consists of inner tube of U shape fitted with neon gas and globules of sodium. For good performance 300°C temperature is to be maintained.
  • To protect discharge tube it is contained in evacuated double walled glass jacket. This prevents heat loss.
  • At starting, electric discharge is to be established through neon gas till sodium gas vapourized, so at the start .or 10-15 min. discharge through lamp will be pink and then as sodium vaporizes, it will turn to yellow.
  • For this voltage of about 410 V is required while normal voltage is about 165 V. Supply is given through a leak transformer which has high leakage reactance.
  • Ás soon as discharge through the tube starts, heavy voltage drop takes place in the transformer so that required operating voltage is automatically impressed across cathodes.
  • P.F. of these lamps is 0.3 lagging and therefore condenser can be used for P.F. connection. Run up time is 7 to 10 minutes.

Advantages of sodium Vapour Lamp and Disadvantages :


  1. Given Yellow light.
  2. It is Suitable for street light.
  3. More light output for the same energy than other sources.
  4. It Can be disposed easily.
  5. Effective in rainy and foggy season.
  6. Low operating temperature.
  7. Good for tunnel lighting and avoids stroboscopic effect.
  8. Luminous efficiency 200 lumen /watt or more.
  9. More life 12, 000 To 20, 000 burning hours.
  10. Effect of ambient temperature is negligible.


  1. it requires additional transformer.
  2. Starting time is required.
  3. Poor power factor.
  4. Cost is more.
  5. Size is bulky.

Measures :

It Care should be taken in handling these lamps, for if inner tube is broken and sodium comes in contact with moisture, fire will result.

Life :

Normal average life of sodium vapour lamps is 2500 hrs.

Applications :

Due to yellow light its use is limited to street lighting, flood lighting of docks, quarries, railway yard where color rendering is not required.

Sizes available : 45 W, 60 W, 85 W, 140 W.

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