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Opinion: warning signs

Is the digital signage industry aware of the risks of photosensitive epilepsy, asks George Boath of Telestream?

At ISE 2017 in Amsterdam I was impressed by the size, quality and brightness of the displays shown in the Digital Signage hall, and further pleased to see that they are using many of the leading-edge technologies seen in broadcast: high dynamic range, 4K resolution and so on.

As the digital signage business uses more full-motion video, as opposed to the simpler animated graphics that are currently common, there are some important good practices – already used in the broadcast industry – to protect viewers from unwanted harmful effects of certain visual patterns, which should be considered by those producing content for these signs.

Avoid harm

The most important consideration should be to avoid actually causing harm to those that see the signs. It is not well known that flashing video, flashing patterns and bright red can trigger photosensitive epilepsy (PSE) seizures in a small proportion of the population (1 in 4,000). It is for this reason that broadcast TV has a legal requirement to test that these are below an acceptable threshold – with large fines if video is transmitted that contravenes. (Alternatively an explicit verbal warning must be given beforehand, as is common in news programmes.)

While triggering seizures is rare, it does happen from time to time. It most recently occurred in late 2016 in the UK, and there are some better-known examples:

  • In Japan in 1997, an episode of the children’s cartoon Pokémon triggered over 750 admissions to hospital.
  • In the UK in 2012, a TV commercial for Citroen cars caused several viewers to have epileptic seizures and resulted in numerous complaints to regulators.


So, what is it technically that triggers these seizures, and how easy is it to avoid producing offending content?

Without going into too much detail, the PSE seizure is triggered by three or more bright flashes or transitions from bright to dark in a 1s period. Moving strong patterns can trigger a seizure, as can rapid transitions from bright red to dark. Examples of offending content could be a typical ‘red carpet’ scene with lots of camera flashes, or a sequence shot driving past a row of trees with the sun breaking through the gaps.

The PSE phenomenon was originally researched by Professor Graham Harding of Cambridge University, and as a result of his early work the test for content that could trigger this condition is colloquially known as the ‘Harding test’, though it has now been formalised under UK Ofcom Guidance Notes Section 2, Rule 2.12 and Annex 1 and ITU BT.1702.


Checking for potentially harmful content manually is unreliable, subjective and laborious. Fortunately, technology suppliers have developed automated tools that can test for the existence of offending sequences, and identify the locations of these; or they can produce a test certificate to show that the content complies with the regulations, as required by UK broadcasters. Offending material needs to be re-edited and re-tested before transmission. Failure to comply with the regulations leaves the broadcaster exposed to potential legal liabilities. A few QC products (such as Telestream Vidchecker) can automatically alter the flashing patterns so that they are within the Ofcom guidelines.

As digital signage adopts more of these high-intensity displays in public places such as sports stadia, shopping malls and public transport, the companies that are delivering content to the displays should be using the same best practices that have been developed by broadcasters to avoid the potentially dangerous effects that some kinds of video content can have on their audience. This would not only position advertisers as good citizens, but will avoid potential legal claims and damage to the their brand image.

George Boath is director of channel marketing at Telestream