They use the same technology as light pens.  There is a phototransister inside
the gun.  The TV scans the picture, and when the scan line gets to where the
light gun is pointed at, the phototransister turns on.  The controller, knowing
where the scan line is at any given time, can give the position of the light gun
to the processor.

Many VGA chips still made today have a light pen input.  Hook that up to a
phototransister with a lense and you've got a light gun.

-Adam

Andrew Seddon wrote:
>
> I`ve been meaning to ask this for a while now as it`s realy getting on my
> nerves. I saw a review of light guns for computers, the kind that allow you
> to shoot at your TV and most were selling for under 40. Now how the hell do
> they do this??
>
> I thought maybe it had a sensor that detected the color of what it was
> pointed at and correlated it to the screen but that would be completely un
> reliable. I can`t possibly see how they could implement inertial tracking to
> that degree of accuracy for such a low cost product. So I figure it must be
> something to do with the linescan on the TV, as I had a look on my
> playstation and sure enough the bit that you plug one of the guns connectors
> into breaks out from the scart lead.
>
> Anybody shed any light on this!
>
> Andrew Seddon