Fluorescent Lighting

Fluorescence occurs when an electron in an atomic orbital absorbs energy from some source (like an interaction with a photon, or a collision with another atom) is promoted to a higher energy level and then, on 'falling back', releases some of that energy in the form of visible light.The missing energy is usually converted to thermal energy (infra red transitions), making the tube slightly hot.


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A fluorescent light is

  • a type of gas discharge tube,
  • it is made of glass and is narrow, with two electrical connections on each of the metal caps that seal the ends of the tube.
  • it contains an inert gas (such as argon, neon, or krypton) and mercury vapour.
  • The gases inside the tube have a pressure of about 0.003 atmospheres.
  • The mixture of mercury and gas is not conductive when the tube is off.

 

How it works:

  • A high voltage discharge is needed to start the flow of current. The initial high voltage burst is necessary to produce electrons with enough kinetic eergy to ionise the mercury atoms - the atoms in the gas need to be sparse enough (low pressure) so that KE can be built up to a level that will allow ionisation of the mercury atoms.
  • The high speed electrons cause the mercury atoms to ionize. After this has taken place, the voltage can lowered - as the initial ionisations produce more of the same. The voltage required ranges from 100 volts for tubes under 30 watts and 100 to 175 volts for tubes of 30 watts or more.
  • A pair of electrode filaments are located at the metal ends of the tube. They produce electrons that ionise the mercury atoms. They do this by thermionic emission. The filaments remain hot when the tube is lit producing a continuous electrical discharge.
  • The flow of electrons through the gases excites the electrons in the mercury atoms, which then emit ultraviolet (UV) radiation.
  • There is a good reason why the lamp contains only a small amount of mercury, which must be vaporized to support the lamp current and generate light. At low temperatures the mercury is in the form of dispersed liquid droplets. As the lamp warms, more of the mercury is in vapour form - therefore more can be ionised by the electrons passing through it BUT at higher temperatures, self-absorption in the vapour reduces the yield of UV and visible light - so you cannot have too much mercury in the vapour form otherwise it will not produce light. Since mercury condenses at the coolest spot in the lamp, careful design is required to maintain that spot at the optimum temperature, around 40 °C.
  • The inside of the tube is coated with a phosphorous material that emits visible light when excited with UV and the tube gives off light. By carefully selecting the fluorescent powders (called "phosphors"), the manufacturer of the light can tailor the light's coloration. Phosphors in CF bulbs are a mix to produce excellent color rendering and warmth similar to incandescent bulbs. Other phosphor mixtures are warm white, cool white, deluxe warm white, and deluxe cool white.

Advantages over the filament light bulb

  • Fluorescent bulbs are four to six times more efficient than incandescent bulbs.
  • The lamp allows brightly lit workplaces to remain at a cool temperature due to its greatly increased efficiency.

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More detailed background reading

A fluorescent tube (or bulb) consists of a partially evacuated glass tube filled with mercury vapour. Only a small fraction of the gas within the bulb is mercury vapour; argon gas atoms outnumber the mercury atoms by about 300 to 1. Both kinds of atoms combined are only at a total of about 0.3% of atmospheric pressure.

Fluorescent tubes and electroluminescent panels typically require 200 to 600 V for starting and running illumination, so a voltage of about 500 V is applied across the tube. This causes a small percentage of the mercury atoms to ionize (release electrons). These free electrons then accelerate towards the positive electrode and collide with the mercury atoms on the way. If an electron's energy is high enough, it can strip an electron off the target atom and create an additional free electron. This collection of free electrons and residual mercury ions classifies the argon-mercury combination as a plasma. If the impact electron is of lower energy, excitation of electrons in the orbitals around the mercury atom can occur. The mercury gives emission lines at 254 nm and 367 nm (in the ultraviolet region of the electromagnetic spectrum). See emission spectrum.

The light from fluorescent light bulbs looks white in most cases, and that white colour is a combination (as it is with sunlight) of all of the colours of the visible spectrum.

The inside of the fluorescent tube, or bulb, is coated with a powder such as Sr5(PO4)3F mixed with small amounts of rare earth elements. This powder (commonly called a phosphor, although it may not have any phosphorus in it) gives off the white light we see through a process called fluorescence, which is the basis of the name fluorescent light bulb.The UV photons emitted by the excitation of mercury orbital electrons are absorbed by the coating, resulting in the emission of light in the visible portion of the electromagnetic spectrum.

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