Sodium Vapor Lamp

A Sodium Vapour Lamp is a type of gas-discharge lamp that produces light by passing an electric current through vaporised sodium metal. It is widely used for street lighting, industrial illumination, and other outdoor applications due to its high luminous efficiency and long operational life. Sodium vapour lamps are recognised for their characteristic yellow or golden light, which provides excellent visibility under foggy or misty conditions.

Historical Background

The development of sodium vapour lighting dates back to the early twentieth century. The first practical sodium lamp was developed by Arthur H. Compton and N. B. Hannay in the 1930s, following earlier research on gas discharge and metal vapour illumination. These lamps became commercially available around 1932, initially used in Europe and later across the world.
The introduction of sodium lamps marked a major advancement in lighting technology, as they offered much higher luminous efficacy compared to incandescent lamps of that time. Over the years, two main types of sodium vapour lamps were developed — Low-Pressure Sodium (LPS) and High-Pressure Sodium (HPS) lamps — each with distinct properties and applications.

Construction and Working Principle

A sodium vapour lamp operates on the principle of gas discharge — when electric current passes through a gas or vapour at a suitable pressure, it excites the atoms of the vapour, causing them to emit light of specific wavelengths.
The basic components of a sodium vapour lamp include:

• Discharge Tube: Made of glass or ceramic, containing a small amount of sodium and an inert gas such as neon or argon to assist in ignition.
• Electrodes: Two electrodes are placed at either end of the tube to initiate and maintain the discharge.
• Outer Bulb or Enclosure: The discharge tube is enclosed within an evacuated or gas-filled outer glass envelope that provides thermal insulation and protects the inner tube from oxidation.
• Ballast: A control device connected in series with the lamp to limit the current and provide the proper operating voltage.

When the lamp is switched on, the initial discharge occurs through the inert gas, producing a dim reddish glow. As the tube warms up, the sodium metal vaporises and participates in the discharge, producing the characteristic bright yellow light due to the emission of sodium D-lines at wavelengths 589.0 nm and 589.6 nm.

Types of Sodium Vapour Lamps

There are two main categories of sodium vapour lamps, differing in pressure, colour, and efficiency.
1. Low-Pressure Sodium (LPS) Lamp:

• Operates at very low pressure (about 0.001 torr).
• Emits monochromatic yellow light of almost pure wavelength (589 nm).
• Has a luminous efficiency of up to 200 lumens per watt, making it one of the most efficient light sources.
• Produces poor colour rendering, as all objects appear in shades of yellow or grey.
• Commonly used in street lighting, tunnel illumination, and areas requiring high efficiency over colour quality.

2. High-Pressure Sodium (HPS) Lamp:

• Operates at a higher pressure (0.1–1 atm), resulting in a broader emission spectrum.
• Emits a warm, golden-white light with better colour rendering than LPS lamps.
• Has luminous efficiency ranging from 90 to 150 lumens per watt.
• Used in highways, stadiums, warehouses, and industrial facilities.
• Often requires a pulse-start ignitor to initiate the discharge at higher pressures.

Advantages

Sodium vapour lamps offer several benefits that have made them popular for large-scale and outdoor applications:

• High Efficiency: Converts a large portion of electrical energy into visible light, reducing energy consumption.
• Long Life: Typically lasts between 12,000 and 24,000 hours, reducing maintenance costs.
• High Luminous Output: Provides bright and widespread illumination suitable for large areas.
• Excellent Penetration: The yellow light cuts effectively through fog, smoke, and dust, improving visibility.
• Low Operating Cost: Economical over long periods due to low power usage and long service life.

Disadvantages

Despite their efficiency, sodium vapour lamps also have certain limitations:

• Poor Colour Rendering: Especially in LPS lamps, the light is nearly monochromatic, making it difficult to distinguish colours accurately.
• Warm-up Time: Requires several minutes to reach full brightness as sodium needs time to vaporise.
• Re-ignition Delay: After being switched off, the lamp must cool down before it can restart.
• Bulky Ballast Requirement: The lamp cannot operate directly on mains voltage and requires auxiliary control gear.
• Environmental Concerns: Disposal requires care due to sodium content and materials like mercury (in some HPS versions).

Applications

Sodium vapour lamps have been extensively used where high efficiency and reliability are more critical than colour rendering. Typical applications include:

• Street and highway lighting.
• Railway yards and ports.
• Industrial and warehouse illumination.
• Airport runways and docks.
• Foggy and coastal areas requiring strong penetration light.

While Low-Pressure Sodium lamps were once dominant in urban street lighting, High-Pressure Sodium lamps gradually replaced them for their better light quality and compact design.

Comparison with Other Lighting Technologies

With the advent of modern technologies such as Light Emitting Diodes (LEDs), sodium vapour lamps are gradually being phased out in many regions. LEDs offer superior colour rendering, instant illumination, and lower environmental impact, though at higher initial costs.