Fluorescent Lamp and Working Principle of Fluorescent Lamp

What is Fluorescent Lamp?

A fluorescent lamp is a low-pressure mercury vapour lamp with a phosphor coating inside its glass tube. An electric current through mercury vapour produces mainly ultraviolet radiation. The phosphor absorbs this radiation and emits visible light.A fluorescent system generally uses less electrical power than incandescent lamps for the same light output. Actual efficacy depends on the lamp, ballast, temperature and operating conditions, so the often quoted range of 50 to 100 lumens per watt is not a universal rating for all incandescent lamps replacements. Fluorescent tubes contain mercury. Avoid breakage, follow local recycling rules and use official cleanup guidance if a tube breaks.

How does a Fluorescent Lamp work?

The diagram shows a traditional preheat fluorescent-lamp circuit with a magnetic ballast and glow starter. Modern rapid-start and electronic-ballast systems use different starting circuits.
In this circuit, the ballast, two lamp filaments and starter form the starting path. After ignition, the ballast limits lamp current because the gas discharge has negative differential resistance.

• When the supply is switched on, the unlit tube does not yet conduct. Most of the supply voltage appears across the lamp-and-starter branch.
• The voltage establishes a glow discharge in the starter bulb. This heats its bimetal contacts.
• The heated bimetal closes the starter contacts. Current then flows through the ballast, both lamp filaments and the closed starter, preheating the oxide-coated electrodes. Closing the contacts also extinguishes the starter’s glow discharge.
• As the bimetal cools, its contacts open and interrupt the preheat current. The changing current creates an inductive voltage drop and transient across the inductor ballast. Together with the supply voltage and hot electrodes, this helps strike the lamp discharge.
  
• The starting voltage ionizes the low-pressure fill gas and mercury vapour between the lamp electrodes. The exact fill and starting voltage depend on the lamp design.
• Once the discharge is established, current flows through the tube. The ballast limits that current; without it, the falling effective resistance of the arc could cause destructive current. The starter remains across the lamp but its effective resistance is high while it is off.
• Excited mercury atoms emit ultraviolet radiation, dominated by the 253.7 nm resonance line in a low-pressure discharge. The phosphor converts part of this energy to visible light.
• After the lamp starts, its operating voltage is normally too low to re-ignite the glow starter, so the starter stays open.

Physics Behind a Fluorescent Lamp

A sufficient voltage produces an electric field through the tube. In a hot-cathode lamp, current preheats oxide-coated filaments and promotes electron emission. Electrons accelerate between collisions with the fill gas and mercury atoms, sustaining a low-pressure discharge. On AC, each electrode alternates between cathode and anode.

The basic discharge process always follows three steps:

1. Electrons leave an electrode and accelerate in the applied electric field.
2. Collisions transfer some electron energy to the gas and excite mercury atoms.
3. Excited atoms emit radiation as they return to lower energy states, and the phosphor converts much of the ultraviolet output to visible light.

A low-pressure mercury discharge emits several spectral lines, with strong ultraviolet output near 253.7 nm. Lamp output depends on mercury vapour pressure, which in turn depends on the coldest tube-wall temperature. The original 105 to 115^oF range is therefore a useful approximate condition for some lamps, not a setting for every design.
Voltage also falls near each electrode, and electrode heating consumes part of the input power. Tube dimensions, gas fill, phosphor, electrode design and ballast operation all affect lamp efficacy and life.
Hot-cathode lamps use heated oxide-coated electrodes to supply electrons at moderate voltage. Cold-cathode fluorescent lamps use different electrodes and higher starting and operating voltages. Values such as 11 kV, 100 to 200 V or a 10 V cathode fall are design-dependent and should not be applied without the lamp and ballast specifications.

History & Invention of Fluorescent Lamp

• In 1852, George Gabriel Stokes described fluorescence and showed that some materials convert ultraviolet radiation to visible light. Later discharge-lamp experiments combined this effect with mercury-vapour ultraviolet output. Practical development required suitable phosphors, electrodes, gas fills and current-control equipment.
• By the 1920s, patents described many features of the modern tube. A low-pressure mercury vapor discharge could produce strong ultraviolet output near 253.7 nm.
  Phosphors made it possible to convert this ultraviolet radiation into useful visible light. The quoted 60% figure refers to a particular conversion stage or experiment, not the wall-plug efficiency of every complete lamp system.
• In the early 1930s, William Enfield at General Electric learned of European work on phosphor-coated discharge tubes and formed a development team. GE and Westinghouse demonstrated fluorescent lamps to the US Navy in 1936. Claims about one 1935 prototype and a 60% efficiency need a primary source before they are treated as settled facts.
• The public saw commercial fluorescent lamps at the New York World’s Fair and Golden Gate International Exposition in 1939. Different phosphor mixtures produced white and coloured lamps. Exact first-market wattages and lengths varied by manufacturer and should be checked against a dated catalogue.
• Four-foot 40 W T12 lamps later became common in offices, schools and factories. Phosphor blends provided different correlated colour temperatures, including nominal 3500 K white and 6500 K daylight products. Colour temperature describes appearance, not an exact reproduction of daylight.
• By the 1940s, the four-foot T12 format was widely available, and later designs changed tube diameter, phosphor, gas fill and ballast operation. Argon remains one possible fill gas, but other mixtures are also used. A 425 mA current and 100 to 105 V voltage drop describe certain lamp-and-ballast combinations, not all fluorescent lamps.