A topic on Apple discussion forum is dedicated to a problem faced by some owners of an iMac 27". The following video describes well the problem:
For some unknown reason, the display delivers flashes of light. If the issue does not seem to affect many units, it already collected 477 answers on this dedicated topic, indicating that we are not speaking of isolated cases neither. If you are experiencing this type of problem, please provide your contribution in the forum topic dedicated to this news.