andys280176 wrote:
Oh it's also interesting to note how the cameras are set to flash oncoming drivers on other side of CW exceeding the limit though no penalties issued.
I've not heard about that before. You sure? I've never been flashed by Gatsos this way, and I usually go past at... er, let me put it like this, I should have been flashed more than once if they operated this way.
Or does this relate to those reversible Truvelos? (None near me luckily).
About the power of the flash on scameras. I found
some technical details for Gatsos. Now that doesn't actually give any sort of standard output for the flash like in joules or anything. But it does tell us that the lens is focused at 20m and that its maximum aperture is f3.8. Any photographers reading this will know where I'm going here.
There's a very basic way of describing the flash power of a camera flash (built in or separate flashgun) called a guide number, or GN. It's really used as a rule of thumb for working out correct exposures for flash photography, but it is related to the actual output of the flash. The GN is the distance to the thing you want to photograph mutiplied by the aperture you want to use, allowing for whatever the film speed is. So for example, if I was standing on the pavement trying to take a flash photo of a car passing 20m away using f3.8, I would need to use a flash with GN 76 (assuming ISO100 film). This is what Gatsos have to do, and GN76 is a bloody powerful flash. You really really wouldn't want to be looking that way at close range when the thing goes off.
That's a worst case situation, as it assumes they always use ISO100 film. If you use a faster film speed (ie a more sensitive film) you can shoot further or use a weaker flash. So it does depend on the film in the Gatso too, but too fast would mean an unacceptable loss in quality. If it was me I'd hedge my bets and use medium speed most of the time, say ISO200 or 400. That would mean Gatsos wouldn't have to use a flash any more powerful than you could buy for a few hundred quid in any camera shop, which are about GN40 give or take (and if anyone would give me a few hundred quid I'll be happy to go buy one and demonstrate it
). I'd still be happier if it wasn't pointing at me while I'm driving though.
On top of that, we have something called the inverse square law. This says that light intensity from a point source, such as a flash, is inversely proportionally to the square of the distance. To put it another way, double the distance is quarter the intensity, half the distance is four times as much. In Gatso terms that means the flash is 4 times as bad at 10m than it is at 20m. At 5m it would be like me firing my flashgun in your face from about 2 feet.
would be handy, but then you'd get pulled for driving with them at night
. Andy, were you fairly close to the thing when it went off?
Of course, the other thing is how sensitive the human eye is to a flash going off, especially at night when you've adjusted to dark conditions. Andy, you don't say when you were flashed but I'm guessing it was at night. I think it's safe to say that when our eyes have adjusted to darkness flash will appear much stronger than at the same distance in daylight. How much I have no idea. Anyone know any eye experts who could go into this? Mad Moggie, any where you work?
One final point. All this is just about how much (or not) flashes from Gatsos can dazzle drivers. It's just as important to consider how much of a distraction they are. Even if it's not strong enough to actually affect your visions, some people might insticntively look for the source of the flash. A momentary distraction, sure, but sometimes a moments distraction is enough to create a dangerous situation.