Photosensitivity is sensitivity to flickering or intermittent light stimulation and visual patterns. It affects approximately one in four thousand people. A number of people have this sensitivity but have not yet had a seizure and therefore have not been diagnosed with the condition of photosensitive epilepsy. The most common trigger for photosensitive epilepsy in Europe is the domestic television set. Almost fifty percent of patients are sensitive to the 50Hz flicker of television, and some seventy five percent of patients are sensitive to the 25 Hz flicker from the line raster which can be observed with close viewing. The onset of photosensitive epilepsy in an individual occurs typically around the time of puberty; in the age group 7 to 20 years the condition is five times as common as in the general population. Three quarters of patients remain photosensitive for life.

In response to a Pot Noodles advert in 1993 which induced photosensitive epileptic seizures in 3 people, the ITC introduced its Guidelines for Flashing Images and Regular Patterns. The sequences to be avoided sound relatively simple - repetitive bright or red flashes and spatial patterns - but the details are complicated.

In December 1997 a children's Pokemon cartoon episode in Japan produced 685 admissions to hospital. 560 cases were shown to have had proved seizures, triggered by four seconds of alternating saturated red and blue light used in the programme. Of those patients, 76 percent had no previous history of seizures. The Guidelines have since been updated.

Professor Graham Harding, an expert on Photosensitive Epilepsy who assisted with the drafting of the Guidelines, also collaborated in developing the Harding FPA.

Epilepsy Action is the working name of the British Epilepsy Association, an information and support organisation for sufferers of epilepsy.