How is TV made safe for people with epilepsy?

Launch footage for the London 2012 logo sparked seizures, but how is television checked for footage that could harm people with epilepsy? For the organisers of the 2012 Olympics vilification over its logo has taken a turn for the worse with reports that people with epilepsy suffered seizures as a result of watching an animation in the launch video.

One section, featuring a diver causing ripples in a pool, has led to 18 people reporting ill effects. It is believed to be the biggest episode of triggering of photosensitive epilepsy in British broadcasting history.

Episodes like this are now rare in the UK because of a gadget called the Harding Flash and Pattern Analyser, used by broadcasters and advertising agencies to avoid triggering seizures.

Only 5% of people with epilepsy suffer from photosensitivity, but this still amounts to 23,000 people in the UK, Epilepsy Action says. For them flashes, strobing and flickers, typically at rates of 16 to 25 times a second but as low as three and as high as 60, are a danger.

Two incidents in the 1990s intensified research into the way television triggers photosensitive epilepsy. A Pot Noodle advert in 1993 sparked three reports of seizures and prompted concern and a ban on the advert.

But nothing in Britain has come near the episode sparked by a Pokemon cartoon in Japan in 1997 when more than 600 children were admitted to hospital after suffering epileptic seizures. Three-quarters of those had never suffered from symptoms of epilepsy before.

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Based on research by British expert Professor Graham Harding, Cambridge Research Systems released the Harding FPA in 2001. Before the machine, work was done by hand to analyse pieces of contentious video, frame by frame. Don Jackson, of CRS, explains that the £12,000 analyser is looking for phenomena that break Ofcom guidelines. "The machine is looking for changes in luminance, red flashes, red is a particularly difficult colour, a known trigger for epileptic seizures. It looks for stationary patterns."

The Ofcom guidelines say:

A potentially harmful flash occurs when there is a pair of opposing changes in luminance (ie an increase in luminance followed by a decrease, or a decrease followed by an increase) of 20 candelas per square metre (cd.m-2) or more. This applies only when the screen luminance of the darker image is below 160 cd.m-2. Irrespective of luminance, a transition to or from a saturated red is also potentially harmful. There are regulations over how much screen area a flash can use, how many flashes can be in a sequence (three a second) and what patterns and particularly stripes can be displayed.

But Mr Jackson insists: "Nothing can guarantee to stop all seizures."

The guidelines are not absolute rules and material that presents a low risk, such a repeated flash photography, is often broadcast with a warning.

Advertising agencies regularly produce video that needs to be checked by an analyser, with many big agencies having access to a machine or using the Broadcast Advertising Clearance Centre. "Adverts are a major problem as they are trying to attract attention. One way is to make the material very active. Sixty per cent of photosensitive epileptics have their first seizure while watching television."

Mr Jackson thinks the "secrecy" of the logo project may have led to the need for analysis of the risk being forgotten. But running it through an analyser and hearing "computer says no" would have saved a lot of pain and anguish.