Why do theaters warn audiences when a strobe is used in a play?

Dear Cecil: Why do theaters put signs in their lobbies warning the audience when a strobe light is used in the performance of a play? C.G., Chicago

Cecil replies:

Because they’re afraid it might induce a seizure in an epileptic, of whom there are maybe 2,000,000 in the U.S. today. The phenomenon is called a “photoepileptic seizure,” and it most often occurs when the light source has a frequency of 15 to 20 cycles per second. Why it happens is not well understood, but it’s quite common, and in fact strobes are often used to diagnose epilepsy. In extremely sensitive individuals seizures can even result from the play of light through the trees or on water. Photoepileptic seizures often result from TV viewing, particularly when you get close to the screen to change the channel. It may be going a bit far to say that TV causes brain damage, but it sure doesn’t help things any.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.