Q I use a station clock that has large, red LEDs. I’ve noticed that if I am chewing on something (a mid-contest snack!) and happen to glance at the clock, the numbers seem to be jumping or flickering. Assuming that this isn’t the symptom of some dreaded disease, what really causes the flickering?
A If you’re chewing on something hard (crunchy potato chips, candy, etc) you set up vibrations in your jaw that propagate to your eyes, shifting their positions ever so slightly. The LED segments are “refreshing” themselves at a high rate of speed and, because of the movement of your eyes, the bright “moving” segments are in different places from where the visual centers of your brain expect them to be. You may see the same effect while watching your computer monitor.
This phenomenon involves something called the critical fusion frequency, which is the point where we begin to perceive things that are flickering as if they are solid. Different factors influence that frequency, including the size of the object, its brightness, and which part of the retina it is seen by. The brighter the background, for example, the greater the flicker. The action of chewing jars the visual axis and changes your line of sight relative to the particular point you are focused on, moving it far enough off the central retina to change your ability to perceive a flickering image as a stable one.
From QST November 2000
