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Dave Rants & Raves [Issue 2, October 2009]                                                                         Previous Issues     
  

Electrodeless Fluorescent Lamps
By: Dave Johnson  
Compact fluorescent lamps are quite popular these days.  They can offer more light for less power when compared to standard incandescent lamps. When made by quality companies, they can also outlast standard lamps.  But, many of the lamps coming out of China are poorly made.  Very few of the lamps I have purchased lately have lasted more than two years, even when the package says that the lamp would last 5 years. Iím sure many of you have had similar experiences.
The weakest part of a standard hot cathode fluorescent lamp is the cathode itself. Over time the hot cathode electrode weakens and will fail, killing lamp operation.  Frequent on/off cycling of the lamp will further weaken the cathode. Cold cathode lamps, which donít have hot cathodes and are often found in laptop computer screens, have much longer lives.  I have one laptop that is over 10 years old and its screen is still just as bright now as when it was new.  But, sputtering action inside those lamps eventually will cause enough of the metal from the cathode electrodes to be deposited inside the lamp, obscuring the light. 
Compact Fluorescent Lamps  Fluorescent Lamp Cathode Cold Cathode Fluorescent Lamps
But, what if you could do away with the metal electrodes completely?  Then, the lamp should last much longer.  The only thing that will eventually fail in such a lamp would be the control electronics and perhaps over time, the phosphors inside the lamp will fade.  A 10 year life might be possible.  My definition of the end of life is when the light output drops by 50% of the original intensity.
Iím now beginning to see some of these new ďelectrodelessĒ lamps being promoted.  The lamp uses much of the same mixture of gases, phosphors and a tiny bit of mercury as in the conventional lamp, but they have no metal electrodes at all.  To excite the gasses inside, a magnetic induction method is used.  This is basically a toroid shape transformer with the glass tube of the lamp forming a single turn secondary.  A high frequency magnetic field produced in the center of the coil induces enough current to flow in the lamp to cause it to glow brightly.  There is not much data yet on the overall efficiency or brightness of these devices but I expect them to be close to that of a conventional hot cathode lamp.  In applications where lamp life if more important than brightness, I expect these to outshine most any other type of lamp, including LED lamps.
Electrodeless Fluorescent Lamp  

October 2009      Issue 2

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