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How does a discharge UV lamp works

  1. May 28, 2013 #1
    I think i have looked at like 20-30 web pages, and i cant find a simple step-by-step explanation of the operational principle.

    I know, that when a rare gas like Helium, relaxes from an excited state two characteristic emission lines occur. But how do you excite the gas in the first place?

    I know you can excite it, by shooting some electrons through the gas, but we have no free electron in the system, since the electric field should not be strong enough to ionize the helium atom by itself.
  2. jcsd
  3. May 28, 2013 #2
    There are free electrons in the system. The electrons get accelerated by the electric field. Once the electrons are a high enough kinetic energy, the atoms absorb a discrete quantity from them in an inelastic collision, becoming excited. The unstable, excited atoms quickly fall back to a lower state, emitting a UV photon in the process. This UV photon interacts with the coating of the tube in a similar manner, transferring energy to excited/ground states, emitting visible light.

    I read your post again, and I am not sure what you mean. You have no free electrons in a system you are doing? I am not sure how that would work in this case. All gas discharge lamps I know of have free electrons flowing through an ionized gas.
    Last edited: May 28, 2013
  4. May 28, 2013 #3
    The (electrons in the) atoms can also be excited through inter-atom collsions.
  5. May 28, 2013 #4

    okay so for a normal gas discharge lamp, we have free electrons you say. Are these made, by heating a filament or similar and then accelerated?
  6. May 29, 2013 #5
    It depends on if it is a hot cathode or a cold cathode. Hot cathode lamps use thermionic emission to release the electrons from the electrodes to maintain conduction in the gas. Cold cathodes need an initial (much higher than hot cathode) voltage to ionize the gas and make it conductive.

    Atmospheric gas is typically non-conductive until this "breakdown voltage" is applied. The voltage necessary is reduced in a vacuum and with certain gases, which is why these lamps have the gases they do.
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