To view this page ensure that Adobe Flash Player version
11.1.0 or greater is installed.
programmes not so good?
Flash-bulbs and disco lights are the most frequent
by Robin Palmer
E ver since the
hundreds of Japanese
children were struck with epileptic
fits provoked by a series bright red
flashes in a TV cartoon programme
broadcasters have become only too
aware of PSE.
Photo-Sensitive Epilepsy is a rare
condition affecting perhaps only 1 in
4,000 people where flashing lights
or images can cause a fit or seizure.
Much of the medical research on this
was done in the 1990s by Professor
Graham Harding and his name has
become colloquially associated with
this problem and (Harding) testing for it.
For the last 8 years or so, Ofcom
in its rules for British broadcasters,
has effectively mandated that all
programmes are checked as far as
possible for PSE triggers in the visual
content prior to transmission. Many
broadcasters also have their own
‘house rules’ on this subject. The odd
thing is that this stringency is peculiar to
the UK and Japan. The same PSE test
specifications are given as guidance
for Europe in ITU-R BT.1702. But this
is only a recommendation and, as
such, is ignored in Germany, France
and all the other partner states. In the
cinema, theatre or disco club there
are no such rules. In the USA, with
more lawyers than any other single
profession, broadcasters don’t need to
pre-test what is shown on public TV.
The FCC appears to have shown no
interest in this subject, unlike the CALM
mandated loudness issue.
Checking for PSE problems in video
content has to be an automated
process as the many dimensional
parameters are impossible to measure
by any manual means. The stated
requirements are to check for 3
categories of possible stimulation:
luminance-only flashes, red flashes and
spatial patterns. These specifications
are poorly drafted in the Ofcom rules
and BT.1702 gives cause for some
practical concerns for repeatability and
effectiveness. The luminance flash is measured
in screen area percentage, rate of
flash and change in brightness level.
Basically more than three flashes in
one second over 25% of the screen
area with a prescribed brightness
difference change is a fail. The first two
parameters are unambiguous but the
brightness is based on a voltage to
brightness gamma curve for a cathode
ray tube. We all know CRTs are a dead
technology and modern television
receivers have digits going all the way
to the LCD/plasma/OLED panel driver
with an image processor en route.
Also screen sizes and brightness are
growing by the year, making the original
assumptions in BT.1702 obsolete.
The Red flash specification is very
similar to luminance except we are told
to test for “red” instead of brightness.
No levels, hue angle limits or saturation
are given as test limits to define
provocative coloured flashes.
Spatial patterns are a third category of
image and not actually flashing. Any
static regular high contrast pattern or
design of sufficient area and so many
stripes could score positive on this.
For park railings, large plain captions
or like designs. the feted 2012 London
Olympics logo can give a false positive
fail. The logarithmic nature of the
brightness curve defined in the
given PSE recommendations makes
the testing process very sensitive
to any transcoding or conversion
of the programme content. This is
one reason’s for the widely reported
unreliability of PSE test results from
instance to instance, location or
At the moment the whole UK industry
is hampered by the imposition set of
‘rules’ on PSE not applicable anywhere
else except Japan. These make it
imperative that PSE testing is done.
This is not any guarantee that such a
passed programme or item could not
cause a seizure, just that due diligence
was done to try and protect the
vulnerable. It is time the specifications
were brought up-to-date by Ofcom
with the same detail given for loudness
testing when all the parameters are
properly specified for the digital era.
With this, there would be consistency
and repeatability across programmes,
equipment, operators and different
locations. Seizures caused by TV provocation
are so very rare the 1 person in 4,000
minority who is susceptible should be
advised to only watch small screen
television sets in brightly lit rooms with
the contrast and brightness controls set
low. Perhaps the time is right for some
de-regulation and the Ofcom rules
should really be reduced in application
to ‘guidance’ only as in the ITU-R
BT.1702 version of the very same thing.
Meanwhile broadcasters, programme
makers, editors and will all have just to
Robin Palmer is Managing Director of
Cel-Soft and is usefully involved with
solutions for 3D & TV quality control
and measurement technology.
30 | TV-BAY MAGAZINE