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A common question about the use of fluorescent lights, such as the long tubes and the spiral compact fluorescent lights regards the potential for headaches, particularly migraines and some types of seizures. Due to the State of California's recent proposal to mandate the use of fluorescent lights (to save energy), this question has been frequently brought up in the media lately, such as on the Neal Boortz radio show and some newspapers.
There is some truth to these claims, although they generally affect only a very small percentage of the population and there are solutions. For people affected by the flicker of fluorescent lights, the issue is very real.
Since the electricity supplied by the power company (in North America) reverses polarity 110 to 120 times per second ( 60Hz in US and perhaps other countries, and usually 50Hz in Europe.), fluorescent lights turn on and off that often, too, which can cause a flicker. Incandescent lights don't produce this flicker because the light is produced by a hot glowing wire. It doesn't matter if the electricity switches polarity, since the wire stays hot and keeps glowing steadily.
The flicker isn't even perceptible to the vast majority of people. Only a tiny percentage are affected.
The severity of the flicker depends on several factors such as:
Lamps operating on AC electric systems (alternating current) produce light flickering at a frequency of 120 Hertz (Hz, cycles per second), twice the power line frequency of 60 Hz (50 Hz in many countries outside North America). Essentially, the power is turning on and off 120 times a second (actually the voltage varies from +120 volts to -120 volts, 60 times or cycles a second and is at zero volts twice in one cycle).
It depends on the frequency of the flicker. People can see lights flashing on and off up to about 50 flashes per second (50 Hz) - they are most sensitive to time-varying illumination in the 10-25 Hz range. The actual critical flicker frequency increases as the light intensity increases up to a maximum value, after which it starts to decrease. When a light is flickering at a frequency greater than 50 or so Hertz, most people can no longer distinguish between the individual flickers. At this frequency - the critical flicker frequency or flicker fusion threshold - the flashes appear to fuse into a steady, continuous source of light. This happens because the response to the light stimulus lasts longer than the flash itself.. This implies that the problem may be greater in counters with a 50/60 cycle (like England and Europe) than in countries with 110-120 cycles (like the United States and Canada).
People cannot notice the flicker in fluorescent lights that have a flicker rate of 120 cycles per second (or 120 Hz). However, just because you can not notice doe snot mean that it can not affect you.
The light flicker may be detected by its stroboscopic effect. When objects move or rotate rapidly, they may be lit at or about the same position during each cycle or rotation. This makes objects look as if they are moving more slowly than their actual speeds - they may even appear stationary if the object is moving at the same rate as the flicker frequency (or a multiple of it). This fact is the principle behind a strobe light but it is not the desired effect in general lighting. In fact, it could be a safety hazard if someone mistakenly thought that some equipment was stationary or was moving slowly.
Although, as stated earlier, humans cannot see fluorescent lights flicker, the sensory system in some individuals can somehow detect the flicker. Ever since fluorescent lighting was introduced in workplaces, there have been complaints about
Some of these complaints have been associated with the light flicker from fluorescent lights. When compared to regular fluorescent lights with magnetic ballasts, the use of new high frequency electronic ballasts (20,000 Hz or higher) in fluorescent lights resulted in more than a 50% drop in complaints of eye strain and headaches. There tended to be fewer complaints of headaches among workers on higher floors compared to those closer to ground level; that is, workers exposed to more natural light experienced fewer health effects. [See Wilkins, A. J., Nimmo-Smith, I., Slater, A. & Bedocs, L. (1989). Fluorescent lighting, headaches and eye-strain. Lighting Research and Technology, vol. 21, 11-18]
The older type that uses the light ballasts are typically the long glass type types (4 to 8 feet long). Modern compact spiral fluorescents use an electronic, not magnetic ballast.
Voltage changes can be caused by dimmer switches or when electrical equipment drawing heavy currents are turned on or when being used (e.g., resistance welding machines; motors in refrigerators, air conditioners; arc furnaces; medical imaging machines (x-ray, CAT scan, MRI); motors subject to variable loads; large capacity photocopiers). Resistance welding machines that repeats welding a rate of once or more per second can cause repetitive voltage fluctuations and may result in a noticeable light flicker.
Usually voltage fluctuations are small and do not have adverse effects on electrical equipment. However, in offices, for example, voltage fluctuations of just a few tenths of one percent can produce very annoying flickers in the lighting, especially if they are regular and repetitive in the 5-15 Hz range.
What kind of lighting is likely to cause a flicker problem?
Flicker is usually a potential problem only with lighting that require the use of ballasts, like fluorescent lights. Incandescent lights usually do not cause a flicker problem since the light filaments generally do not cool quickly enough (and make the light dimmer) during the "off" time as the voltage changes in the AC power line.
The type of ballast, which controls the electrical supply to fluorescent lights, affects the amount of flicker. "Magnetic" ballasts change the voltage supplied to the fluorescent lamps but do not alter the frequency - the power line frequency of 60 Hz. The ultraviolet (UV) light produced inside the fluorescent light tube also fluctuates 120 times per second. The phosphorescence (the fluorescent light) resulting from the UV shining on the phosphor coatings inside the light tube is sufficiently stable (i.e., lasts long enough) to even out the variations in the fluorescent light output.
What can be done to reduce or eliminate light flicker?
Some types of ballasts can reduce flicker considerably. New, energy-efficient electronic ballasts take the 110 or 60 Hz power and convert it to voltages at a much higher frequency (20,000 - 60,000 Hz). The resulting flicker frequency (twice the supplied power frequency, 40 -120 kHz) is so high that the human eye cannot detect any fluctuation in the light intensity - essentially flicker-free. An added benefit is that electronic ballasts produce less hum than that emitted by other kinds of ballasts.
It should be noted that there are other sources of light-related eye strain and headaches:
Glare on computer screens from any overhead lighting
Glare from overhead incandescent lighting
Solutions for these problems are:
Reposition the computer monitor so the light strikes it at a different angle, removing the glare.
Add a glare screen to your monitor.
Put a hood or shield over your monitor to keep light from hitting it from above and from the sides. A piece of cardboard works fine!
Turn off the nearby fluorescent lights