Low Pressure Mercury Fluorescent Lamps or Tube Lights: How a Fluorescent Lamp Works

Low Pressure Mercury Fluorescent Lamps or Tube Lights: How a Fluorescent Lamp Works
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Tube Light

Low pressure mercury fluorescent lamps are commonly called tube lights. They are available in lengths from the very small at 150 mm to up to 2400 mm. The power ratings are from 4 watt to 125 watts. The average life of tube lights is about 5000 hours and also they are very cheap, hence they are very popular. In these types of lights the inside of the tubes are coated with a coating of fluorescent phosphor which emits visible light when the arc establishes and the gas inside ionizes. They give colder light than incandescent lamps and give a whiter light. In air conditioned spaces where the heat load is to be kept at a minimum there tube lights and CFL’s are used.

Nowadays CFL’s have become very popular and give stiff competition to tube lights as they can be fitted in the fittings for the incandescent bulbs and no new modifications are required. However the CFL cause nausea in some people due to its light. Also in installations like merchant ships CFL cannot be used as they create interference with the communication equipment.

The variations of the coating of the phosphor material produce different colors known as warm white, warm white de luxe, daylight, and white.

Construction of Tube Light

The tube light has a very simple construction. It consists of a glass tube sealed at its ends by metal caps through which electrodes pass. The tube is filled with a low pressure mixture of argon gas and mercury. The insides of the tubes are coated with fluorescent phosphor coating. The electrodes at the ends of the glass tube are connected to the bi-pin cap.

Operating Principle

tube light construction

The tube light is started by striking an arc between the electrodes using high voltage. This is done with the help of a glow type starter. The starter has a bimetallic strip that allows an arc to form and the starter glows. The glowing produces heat which bends the strip and make contact. Now the glowing stops and the current is applied to the electrodes which glow at the ends.

After some time the starter contacts cool down and they separate and the circuit opens. When the circuit opens it interrupts the choke circuit which produces the voltage surge that lights the tube.

Two or three times this happens before the tube if sufficiently warm to establish the arc. After the arc has established the starter can be removed and the tube light will still keep working. It will not restart without the starter though.

The mercury gets depleted over the time ending the life of the tube. When the tube blackens at the ends it is a sign that its life is finished.

tube light circuit


  • Tube Light: Wikipedia Commons
  • Practical Marine Electrical Engineering by Dennis T. Hall