Alright help an old fogie out here.  When you use the term "Rectify" regarding lamps, what are you talking about? 

I always use this term in the situation where a fluorescent lamp has one dead electrode and one good electrode. When the lamp is lit, it conducts electriciy only in one direction (from good electrode to bad electrode) instead of both directions with the mains AC frequency. This means that a normal fluorescent (two good electrodes) will flicker with a frequency of 120 Hz on typical North American mains. A rectifying tube will flicker with 60Hz frequency since there is no light emitted on three quarters of the cycle and the light is created on only one quarter of the cycle.

Here are the sequences : Normal tube is on for 1/4 cycle on electrode A and then off for 1/4 cycle. Then on again on electrode B for 1/4 cycle and finally off again for the remainder 1/4 of the cycle and then repeat. Rectifying tube is on for 1/4 cycle on good electrode, then off for 1/4 cycle. Then off for 1/4 cycle because bad electrode doesn't emit and finally off again for the remainder 1/4 cycle.

Usually, bad electrode is on heavily blackened end of the tube.

Rectification occurs only in magnetically ballasted lamps.

The term rectify comes from the fact that the tube only conducts in one direction so as to try to rectify AC (ie correct it) so it would be theoretically DC. However, the DC producing part is not true for fluorescent lamps.

In some cases, since the tube conducts only in one direction, a phenomenon known as mercury migration occurs. This is when the mercury in the tube is drawn to one end, making it bright on one end and darker on the other.

Hope this helps.

Discharge lamp have current going from 1 electrode to the other. On AC the current direction is changing - 100/120 times/sec on magnetic or few 10000's times on electronic ballast

It can happen that 1 electrode in the lamp is bad (usually EOL of the electrode itself - the most common failure mode of fluorescents, sometimes failure of the ballast in RS ballasts). A bad electrode will not work when it is negative so there will be no current, but it will work normally when it is positive

As the current is changing directions, the bad electrode will not allow the current to go in 1 direction - so instead of the current going either way, it will go in 1 direction 1/2 of the time, and not go at all in the other 1/2 of the time - which can be seen as harsh flickering (50/60 Hz) on magnetic ballast lamps

The resulting current is essentially DC as it is going in 1 direction only (actually pulsed DC since it is interrupted at a constant frequency), so the lamp is kinda converting the AC to DC same as a purpose built rectifier does



When the electrode is negative, its purpose is to emit electrons into the discharge. There is Thermionic emission (which is low wear on the electrode) and Cold cathode (which wears the electrode) emission modes

For thermionic emission (release of electrons by the heat of the material) to happen the electrode should be hot enough (typically incandescent), and have a coating of material that gives out electrons at this temperature. The material is called emitter and it is a coating on the tungsten electrode. An electrode is EOL when all of the emitter coating is worn up, and if the ballast attempts to keep the lamp going, it is now the tungsten (that the electrode is made of) to provide the electrons. Tungsten does not provide thermionic emission at this temperature - so discharge directly off tungsten is cold cathode even if the electrode is incandescent

If the 2 requirements for thermionic emission are not satisfied but the voltage is high enough to get the electrons flowing anyway, cold cathode discharge will take place

Instant start fluorescent and most HID ballasts are starting the lamp in cold cathode mode (due to no preheating but just starting with high voltage), then the electrodes heat up from the heat of the discharge and it changes to thermionic emission. The high voltage also allows them to keep running AC through a lamp with bad electrode (in this case in cold cathode mode on that electrode), as the voltage is high enough to make it work. This might go on untill the electrode wears out so much that the sealing of the lamp around it is compromised and the lamp lose vacuum

Most other fluorescent ballast heat the electrodes to start the lamp directly in hot cathode mode, so minimize the wear of the lamp when starting. Their voltage is lower, and not enough to force a bad electrode to work in cold cathode mode. This is when the lamp won't light up when the bad electrode is negative and will rectify

Cold cathode fluorescents (often found in LCDs and scanners etc as well as most indicator / sign gas discharge lamps) are working all the time with cold cathodes and not only when starting