The issue of Photosensitive Epilepsy comes up often in television - back in the late nineties there was an episode of Pokemon that featured flashing images that provoked kids in Japan to have seizures. Since then Ofcom have been very keen to avoid this on British television and since 2003 have produced the following guidelines;
OFCOM Guidance(extract).pdfThis is an extract from the document but it does include the important details which hinge around the following;
3. A potentially harmful flash occurs when there is a pair of opposing changes in luminance (i.e., 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 (see
notes 1 and 2). 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.
3.1.1. Isolated single, double, or triple flashes are acceptable, but a sequence of flashes is not permitted when both the following occur:
i. the combined area of flashes occurring concurrently occupies more than one quarter of the displayed (see note 3) screen area; and
ii. there are more than three flashes within any one-second period. For clarification, successive flashes for which the leading edges are separated by 9 frames or more are acceptable, irrespective of their brightness or screen area.
These parameters are well defined and so anyone who can understand them can build a PSE detector that will with certainty detect when a violation occurs. This is how the situation should be as it avoids any one manufacturer of test equipment having a monopoly. Unfortunately this is just the situation that has nearly developed with the Harding FPA detector. Their machine is a PC with SDi capture card that you digitise the video sequence to be checked into and it runs an analysis. The other popular unit is the GordonHD which is more like a traditional piece of equipment in that it sits in the signal chain and gives an alert when it sees a violating sequence go past.
We have a couple of customers who like the idea of realtime performance that the Gordon gives and don't like having to capture (the Harding doesn't support standard codecs so no Quicktime reference export from Avid!), analyse (in slower-than-realtime) and then get a report - only to repeat it all after you've corrected the offending clips (because the broadcaster likes to see a full 10:00:00:00 - 10:54:00:00 report!). The Gordon on the other hand is cheap (£3k against £13k) and just sits there taking a feed of HD/SDi video and Timecode and firing a GPI when a violation is detected (and even entering the TC into a file) which means you can have it hanging off the Avid (or whatever) and the editor can rock'n'roll over a piece of footage adjusting his edit point over the flash frames (it's mostly paparazzi footage with all those camera flashes that cause it) until he gets a sequence that doesn't cause a problem.
Anyhow - you can tell which machine I think is best. Harding is a great self-publiciser who gives you the idea that he alone knows the secret-sauce of PSE. The guys at Tektronix tell me it's on the way as an upgrade for their WVR-series 'scopes but they are worried that Harding has all the patents stitched up.
Anyway - a quick once around pals revealed the following;
Ascent Media check all of five's output (including the two daughter channels) on a GordonHD, ITV's QC department at Upper Ground use a Gordon as their first-pass analyser and Channel Four specify it as well. However, talking to everyone in facilities reveals that they (almost) universally believe the Harding to be the only machine capable of doing the job.
I had several earlier-model Gordon's at Resolution and they were superbly fitted to the job (and cheap enough to have one in every suite). We never had a tape sent back that had gone through it and that included many more hours of terrestrial television than most facilities ever turn out (including quick turn-around stuff with lots of potential trouble - think the Friday night eviction show for Big Brother).
John Emmett of
BPR is a gentleman of the old school (they make the Gordon) and you can find an interesting paper on the subject of PSE he co-authored
here.