First of all, the ‘confidence factor of 97%’. They’re 97% confident of what? As an arts graduate occasionally called upon to struggle with statistics, I’m no authority on scientific method, but I know hand-waving when I see it. The basic assertion here is that the light coming from the suspect’s screen matched a programme being broadcast at that time, proving that they were watching that programme, which required a TV licence.

But does ‘97% confidence factor’ mean that 97% of the data captured matched the data for the simultaneously broadcast programme? Surely not so simple, and we’d still need to decide what to think about the 3%. Or does it mean that whatever the nature and degree of similarity identified, this was calculated by some method to represent a 97% likelihood that what was being watched was indeed this programme? More likely, but introduces unknown variables. Is the method valid? Has it been published? (Of course not. The BBC doesn’t talk about how detection works, ‘for obvious reasons’.)

An explanation of the detection system is also provided, but it’s not, shall we say, massively helpful:

5. A television display generates light at specific frequencies. Some of that light escapes through windows usually after being reflected from one or more walls in the room in which the television is situated. The optical detector in the detector van uses a large lens to collect that light and focus it on to an especially sensitive device, which converts fluctuating light signals into electrical signals, which can be electronically analysed. If a receiver is being used to watch broadcast programmes then a positive reading is returned. The device gives a confidence factor in percentage terms, which is determined by the strength of the signal received by the detection equipment and confirms whether or not the source of the signal is a “possible broadcast”.

This reminds me of the worst kind of patent application. Why are we told that the light is ‘at specific frequencies’? All light is at specific frequencies. A colour TV picture, by design, includes pretty much all the frequencies visible to humans, which isn’t very specific. The optical detector uses a ‘large lens’? Not all large lenses are fast and not all fast lenses are large, so meh.

It focuses the light ‘on to an especially sensitive device’ — oh yawn, this is going to be a CCD or CMOS image sensor that you could get for a fiver in Maplin’s; it’s more sensitive than the one in the packet on its left and less sensitive than the one on the right. It ‘converts fluctuating light signals into electrical signals, which can be electronically analysed’ — yes, that’s what an image sensor does, congratulations again on not using a house brick.

And thus… ‘the device gives a confidence factor’. Yay! Wait, was there a bit missing there? You know, where it actually DID THE THING?

To summarise:

Point camera in general direction of TV A little app that somebody wrote, of which we haven’t seen the source code, compares the aggregate RGB levels (we’re only going to be dealing with the screen as a whole here, not reconstructing an image pixel by pixel, because this is not CSI: Miami) with those of a current live TV broadcast, at some unspecified time interval The app uses a formula we haven’t seen to give you a percentage confidence that something or other is the case (that the data matches? That the data matches and it wouldn’t match any other programme’s data? That the data matches and it wouldn’t match some other programme’s data and the data isn’t erroneous?) You take this number to a judge and they go ‘97%, well, sure’*

*This is apparently what happened, because TV Licensing got the warrant. As a result, they were able to enter Steve Heather’s home (accompanied by police) against his wishes, ‘test’ his TV set by turning it on, and briefly see something that they claimed was a live BBC One broadcast but the defence claimed was the Sky EPG that Steve used to select radio programmes. The end result was that TV Licensing lost the case and Steve didn’t have to buy a TV licence to not watch TV.

I wonder why they needed to point a ‘detector camera’ at the window at all? Why could the surveillance person not just look through the window with their eyes, see Antiques Roadshow on the TV and make a sworn statement to that effect? Was the suspect’s television screen invisible to the naked eye from the window through which its reflected light could be detected? It seems like bad luck if so. I could tell you what most people in my street were watching at any given time just by walking around.

I probably wouldn’t think it was worth designing a whole system specifically for the couple of houses with their TVs in exactly the wrong place. Especially if I already had a database of all the people who had TV licences, narrowing down the list of potential infringers by about 95%. (We’ll return to this later.)