What is Sodium Vapor Lamp : Working & Its Applications

A lamp in which an electric discharge takes place throughout a metallic vapor is known as a vapor lamp. These lamps are available in two types like sodium vapor lamp and mercury vapor lamp which emit different colors of lights like thick blue and yellow color respectively. These types of lamps are very efficient & provide light with high intensity which is applicable in open areas and also for illuminating large distances.

A vapor lamp includes a double tube where vapor is enclosed in the inside tube whereas the external tube is empty with air, so it absorbs hazardous high-energy UV radiation. This article discusses an overview of sodium vapor lamps and it’s working with applications.

What is a Sodium Vapor Lamp?

A gas-discharge lamp that uses sodium (Na) in an exciting condition to generate light at a 589 nm of characteristic wavelength is known as a sodium vapor lamp. These lamps were commercially designed first sodium lamps by Philips in the year 1932, Holland.

The sodium vapour lamp working principle is, that it mainly works through vaporized sodium metal by creating an electric arc. Other gases & materials will help in activating the lamp and also controlling its color. These lamps are mostly used in street lights and industrial purposes. The sodium vapor lamp symbol is shown below.

Sodium Vapour Lamps
Sodium Vapour Lamps

Sodium Vapour Lamp Circuit

The sodium vapor lamp circuit diagram is shown below. Here, the type of lamp used in this circuit is a low-power sodium vapour lamp (LPSVL). The construction of this lamp is similar to a mercury vapour lamp. In this lamp, two electrodes like E1 & E2 are mounted within a G1 glass tube including sodium with a small quantity of argon or neon gases. This G1 glass tube is placed in a G2 evacuated external tube.

Sodium Vapor Lamp Circuit Diagram
Sodium Vapor Lamp Circuit Diagram

Sodium vapour is very active chemically, so the glass lamp is designed very strongly to oppose the hot sodium vapour action. Here, efficiency & best-operating conditions can be obtained by maintaining the exact temperature within the discharge tube about 300 degrees centigrade, so that we can reduce heat loss from the tube.


In the above circuit, capacitor (C), choke (L) & a small step-down transformer ‘T’ is connected in series with the lamp. The choke (L) is an inductor that is connected in series with a low-power sodium vapor lamp. It functions like overload protection for stabilizing the discharge. The transformer used in the above circuit is a leaking transformer for reducing the voltage supply & stabilizing the arc. All these capacitors, choke & transformer is connected in series with vapour lamp for heating the cathode electrode ‘E1’.

Lamp Working

Once the sodium vapour lamp is turned ON, it generates a discharge within the neon gas, which produces sufficient heat to evaporate the Na (sodium). A small transformer is used to heat a cathode filament within the discharge tube whereas the anode terminal is connected directly to the 230V AC supply throughout a choke. Because of both the transformer and choke, the power factor (P.F) of the above circuit will turn very low, so a capacitor is included in the circuit for rectification purposes.

When the vapour lamp is not operated, Na (sodium) is deposited on the sidewalls of the tube in solid condition. Once the vapour lamp is connected to the power supply, neon gas is discharged & generates red-orange color light. The heat produced by the emission of neon gas changes into solid sodium vapor. After fifteen minutes, it starts generating complete yellow light.

These sodium vapor lamps are available in 50 Watt, 100 Watt, 200 Watt, 500 Watt. This lamp applies simply to AC supply.

Sodium Vapour Lamp Types

There are two types of sodium vapour lamps available in the market like LPS vapor lamp (Low-Pressure Sodium Vapor Lamp) and HPS vapor Lamp (High-Pressure Sodium Vapor Lamp).

Types of Sodium Vapour Lamp
Types of Sodium Vapour Lamp

Low-Pressure Sodium Vapor Lamp

Low-pressure sodium vapour lamps (LPSV) were first invented by Arthur H. Compton at Westinghouse in the year 1920. The first developed sodium lamp is an LPS lamp or low-pressure sodium lamp. This lamp is identified through its signature color that is monochromatic yellow. It is frequently used in Europe because it did not demand in other markets because of its poor color rendering index (CRI).

This type of sodium lamp is a very efficient lamp around the world due to its several uses. The current obtained by this lamp make a light at the most responsive color to the human eye. On the contrary, an incandescent lamp will form light at all frequencies which range from IR to UV. The low-pressure sodium lamp is also known as a SOX lamp.

The LPS vapour lamp is mostly used for outdoor lighting in Europe. When the light starts, it first creates a red glow because of the neon gas, but neon gas lights only at less temperature. Once the temperature is increased, the sodium starts vaporizing & turns into a pure monochromatic yellow color light.

High-Pressure Sodium Vapor Lamp

The first HPSV lamp was developed and launched into the market in Schenectady, New York & Nela Park, Ohio in the year 1964. The HPSV lamp is the most frequently used in street lighting. An improvement of the LPS lamp is the HPS lamp but it has less efficiency as compared to LPS. The HPS lamp includes a narrow arc tube that is supported through a frame within a bulb.

The arc tube is designed with aluminum oxide ceramic because it opposes the acidic effects of alkalis like Na (sodium). The arc tube includes high pressure for higher efficiency. In this tube, xenon, sodium, and mercury are usually used.

The most common method to activate the lamp is through a pulse start. There is an ignitor within the ballast which transmits a high voltage signal throughout the arc tube. This signal will activate an arc with the xenon gas. So the lamp will turn sky blue like xenon lights.

After that, the arc heats the mercury (Hg) then its vapor provides a bluish color to the lamp. The lamp gets heat & the sodium is the final material to evaporate. The vapor of sodium hits an arc above 240 C.
The sodium (Na) is mixed with different impurities to form white light. The mercury helps in adding blue light to the clean yellow color of the sodium.

Differences between Sodium Vapour Lamp and Mercury Vapour Lamp

The difference between sodium vapour lamp and mercury vapour lamp include the following.

Sodium Vapour Lamp Mercury Vapour Lamp
Sodium Vapour Lamp was invented in the year 1920, by by Arthur H. Compton at Westinghouse Mercury vapour lamp was invented by American engineer namely Peter Cooper Hewitt in the year 1901 & improved in 1903.
Sodium vapour lamp efficiency is about 100 lumens/watt. Its efficiency is about 35 to 65 lumens/watt.
Sodium vapour is a gas discharge lamp. Mercury vapour is also a gas discharge lamp.
This lamp uses sodium in an energized condition to emit light. This lamp uses an electric arc with vaporized mercury to emit light
The light in this lamp is from the process of an atomic emission. The light in this lamp is from the process of fluorescence emission.
It is used in street lighting & other illumination. It is used to provide lighting for large areas like streets, sports arenas, gyms, stores, or banks.
Its life span is about 18,000 hours. Its life span is about 24,000 hours.
Sodium vapour lamp wavelength is 589 nm Mercury vapour lamp wavelength is 254 nm


The advantages of sodium vapor lamps include the following.

  • Its efficiency is high.
  • Longer life.
  • It can be easily disposed of.
  • The operating temperature is low.
  • Energy-efficient.
  • Effective in foggy & rainy reasons.
  • Cool source.
  • Heat is fairly low.
  • It ranges from fewer grades to high grades.
  • Its color temperature is warm always.
  • These are ideal for different applications.


The disadvantages of sodium vapor lamps include the following.

  • Color temperature
  • It needs ballast
  • It needs controlling elements for controlling glare.
  • It is not applicable in color identification areas.
  • Sodium element is dangerous because it can catch fire in contact with air.
  • It needs an extra transformer
  • The power factor is poor.
  • For indoor lighting, the yellow color is not suitable.
  • For sufficient light output, it needs long tubes.
  • To provide a complete output, it needs 5 to 10 minutes.


The applications of sodium vapor lamps include the following.

  • These lamps are broadly used for airports, goods yards, lighting of roads, etc
  • Sometimes, these lamps are used for advertisement purposes.
  • The most commonly used ratings of lamps are 250 watts and above.
  • HPS lamps are extensively used to provide lighting in industries
  • These lamps are used for outdoor area lighting like roadways, security areas, parking lots, etc.

Why does sodium vapour emit yellow light?

Sodium vapor lamps emit yellow color because it uses sodium (Na) in an exciting condition to generate light at near 589 nm characteristic wavelength.

When sodium Vapour lamp burns light is emitted?

When sodium vapour lamp burns then yellow color light emits.

Why capacitor is used in sodium Vapour lamps?

The Metalized polypropylene film capacitor is used to compensate for the inductive idle current which is drawn by the lamp.

Which gas is used in the sodium Vapour lamp?

Neon gas is used in sodium vapour lamps because it is inactive.

Thus, this is all about an overview of a sodium vapor lamp, circuit diagram, types, working with advantages, disadvantages & its applications. This lamp is the most efficiently used lamp in different applications in the world. These types of lamps have up to 190 lumens of efficiency for each watt as compared to an incandescent lamp. Here is a question for you, what is a mercury vapour lamp?