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!