My Off-and-on-and-off-and-on Love Affair With Computers

At work there’s a computer room that I take great pains to avoid spending much time in (to erm, avoid great pains). Don’t get me wrong – I love computers, in general. It’s not so much the noisy CPUs (which in this case are tolerable) but rather the old monitors which flicker, every last one of them. If I have to spend more than 20-30 minutes staring at one, I am setting myself up for risk of a migraine. What’s curious (if not outright frustrating) is that not everyone understands what I’m talking about when I mention that I can’t really spend much time working with some computers because of the screen flicker. Either people can see it, or they can’t.

The whole issue of monitor flicker is due to the Refresh Rate setting, which I used to know how to change in old Windows OS, but not in the current one. Whether or not you can see the flickering depends upon the way your brain is wired. But regardless of your ability to vouch for this phenomenon, there is some basic science that is commonly accepted in the computer industry (and elsewhere), so you don’t have to take my word for it!

The annoying/tiring flicker of monitors and fluorescent lights is related to Flicker Fusion Frequency (FFF). You have seen and hopefully played with “flip-books”, little booklets of cartoons, when you flip the pages, at the right speed of flipping the pictures appear to get animated. This works just like a motion picture (movie) film is a long serious of still shots that are run quickly by, giving the illusion of motion. The “flicker-fusion frequency” is when the stills flicker by at a speed fast enough that your mind fuses them together.

This kind of action is measured in Hertz ( Hz ); 1 Hertz is one cycle per second. For example fluorescent light fixtures run at a rate of 50 Hertz in Europe and 60 Hertz in the US. Fluorescents, unlike incandescent lights (ordinary lamp bulbs) do not emit continuous light. Rather, they flicker


Each OFF-ON is one cycle. But because the off-on is a sine wave function, they go off and on 100-120 times per second.

Most average people cannot consciously discern that flickering, because the “average” human FFF is only 25 Hz. (I should note that although nearly all humans with adequate vision have the sensing ability, not all have the perceptual ability to discern the flickering — perception is how the brain interprets the sensory inputs.)

The flickering effect is most noticeable outside the corners of eyes, where the rods (light sensing) parts are most sensitive; the cones (color sensing) are most sensitive in the middle. That’s why you notice movement or faint stars outside the corners of your eyes. Theatre movies run at a slower rate of speed (24 Hz) because they are shown in dark surroundings. When something is brighter, it requires an even faster flicker rate to not be noticeable.

Old computer monitors and CRTs (Cathode Ray Tube — those old TV-like green print on black screen terminals) ran at 50 Hz, too. If you happen to recall getting CRT headaches that would be why. In contrast, LCD panel monitors have a refresh rate around 200 Hertz!

When a visual input goes higher than an organisms FFF rate, it has reached the Critical Fusion Frequency (CFF) and is no longer perceived as flickering, but as steady. The human CFF is about 50-100 Hz. Apparently some people (including many people with ADD and autistics) have a higher CFF threshold than the neurotypical human. So things like fluorescent lights are more bothersome. (Insects have an even higher FFF than humans; flies have a FFF of 300! You gotta wonder what it does to insect colonies kept in incubators under artificial illumination.)

A Swedish ergonomic study found that individuals with a higher critical fusion frequency experienced more stress and decreased accuracy under fluorescent light conditions. Their recommendation was better ballasts, rather than using incandescent or natural lighting. ::rolls eyes::

To prevent this problem on your computer, either get a plasma screen, or if not budgeted for such, then set your refresh rate to its maximum capacity — German researchers recommended 70 Hz for the general population (sorry, reference link now broken).

UPDATE:  The older fluorescent lamps had magnetic ballasts, and those seemed to be the problematic sort.  Newer fluorescent lamps have electronic ballasts, and fewer issues – they also seem quieter!


  1. Mike Stanton said,

    5 January 2007 at 1:46

    I just bought a new laptop. no flicker but a lot of stress from losing data on dead PC and dealing with telphone helplines when my credit card got chewed by the online system.

  2. 4 January 2007 at 20:04

    Nought new to me there, I see flicker at the Cinema too.

    I did an experiment with one of my cameras recently to see how brief a flash of light had to be before my consciousness could not detect that I had seen it.

    Well I could still see the light through the lense of my camera, looking through the back at a shutter speed of 1000 second.

    I had a very strange effect in the near dark in Wales the very faint light reflecting from the sky on the white parts of my landrover, in an overcast nighttime sky seemed to be strobing.

  3. natalia said,

    4 January 2007 at 14:43

    I never saw the screen flicker, but it (combined with spending 10+ hours at the computer per day, whenever circumstances allowed) gave me a tic in my left eye that disappeared instantly that I got my (now old) laptop with the flat panel screen. I would never go back to CRT monitor.

  4. mcewen said,

    4 January 2007 at 14:27

    My new Mac [sorry to boast] is infinitely better for that, but I still notice it if I go somewhere like the library where there are a collection of computers. I think it’s the collectivity that emphasizes it, but I don’t think that they’re deliberately ganging up on us.
    Cheers dears

  5. 4 January 2007 at 13:46

    I can’t see monitor flicker, but it drives my husband nuts. Several years ago, our monitor got on the wrong refresh rate and I didn’t notice; he was incredulous that I could actually sit there looking at it.

  6. qw88nb88 said,

    4 January 2007 at 4:48

    I can hear computer hard drives, too. It was quite discouraging to be telling our department techie that a couple of the computers sounded bad, and she couldn’t hear them unless she put her ear right against the CPUs, and even then couldn’t tell that they were getting “sour”! Of course, they died a few months later. Working in another lab with 15 computer CPUs set atop the tables gave me raging headaches. Hyperacusis sucks.

  7. bamagirl said,

    4 January 2007 at 4:18

    To edit the nest to the last sentence in my previous post, SORRY!

    No, I can’t speak of what you see, but feeling what I feel based on what my ears sometimes hear, there is no doubt you get migranes based on what you are seeing. I believe it is really there.

  8. bamagirl said,

    4 January 2007 at 4:17

    Oh, Yes! I can relate, well not to seeing the flash that you do. But I can attest to migranes brought on my noises that other people swear they can’t hear. In my classroom, I have a computer that has to be 10 years old (good ol public education for you!) and makes a horrible noise that no one else in the room can here. It drives me nuts and some days I just have to say that we have to turn it off and not use it. The teacher I share a room with doesn’t care too much for this. I don’t care too much for getting a migrane. Once the migrane is brought on my the noises I hear, things do start effecting my vision.

    No, I can speak of what you see, can feeling what I feel based on what I hear, there is no doubt you get migranes based on what you see.

    I hope that you respond well to migrane medication. I don’t.

  9. Ooi said,

    4 January 2007 at 2:59

    Gosh, this is a good info especially for me. Thanks.

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