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Fluorescent Tube Flicker

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A fluorescent lamp that is running from a mains AC supply will flicker a twice the mains frequency, this is because the power being delivered to the fluorescent tube drops to zero twice per cycle.

This means that in a country that uses a 60-cycles-per-second (60Hz) AC frequency the fluorescent tube will flicker at a rate of 120 times per second and, likewise, in countries that use a 50Hz AC frequency the tube will flicker at 100 times per second.

The same principle is also responsible for the annoying hum that can be heard from some fluorescent tubes, although it is actually the ballast and not the tube making this noise.

Both the flicker and the hum can be eliminated by use of a high-frequency electronic ballast, such as those used in compact fluorescent tubes.

The majority of people cannot visibly notice any flicker in a fluorescent tube operating at 120Hz, but a number of people have been known to complain that these lamps are responsible for causing eyestrain and headaches.

A study by Dr J. Veitch has found that this situation is improved when using a high-frequency electronic ballast (20-60kHz) instead of a magnetic ballast (100-120Hz).

Sometimes that fluorescent tube may flicker at the mains frequency itself (50-60Hz) and then the flicker is noticeable by many more people.

This usually happens near the end of the life of the tube and is due to the emission coating at one end of the tube almost running out, this means that this cathode has difficulty emitting enough electrons, resulting in uneven light output in positive and negative mains cycles.

Flickering at these frequencies is much more noticeable in the peripheral vision, much like the flicker of a television or computer monitor operating at 50Hz that you can sometimes see out of the corner of your eye, it is much harder to see the flicker when looking directly at it.