laser guns - suggested mod............

[bryan35]

Having played about with my newly aquired pro-laser, (thanks CPTsideways), I can honestly suggest mods to the video kit that the police/scammers use.
CCD cameras can actually see infre-red quite well, but usually have a filter to block this out (as it can make images de-focussed and affect colour balance)
However, how usefull would it be to have the laser gun locked to the video camera so each laser pulse can be clearly seen on the recording?
That would prove pretty conclusively exactly what the laser was bouncing off, ie the numberplate as apposed to the crash barrier through the glass!.
As they stand, you've no idea what it's bouncing off because nothing can see it.
The error trapping clearly can't be screwed down that tightly otherwise you'd never get a speed reading of anything, so there is always the marging for eroneous readings - believe me! A house travelling at 57MPH I had last week! 'swept down the guttering'

SO, you can see what the gun can see. Would also help in making sure the thing is callibrated too, and a bit of slo-mo would reveal the pulses as they happened.

Think back to the dodgey motorbike incident...........

[smeggy]

bryan35 said:

However, how usefull would it be to have the laser gun locked to the video camera so each laser pulse can be clearly seen on the recording?

Good question, but there’s a good reason why this isn’t done.

The output power from typical Lidar speedmeters can be very high but the pulses are very very short (a few nanoseconds per pulse), so the maximum output energy is very small (up to 1 hundredth of the average power of a 5mW laser pointer). You should be able to see the reflected beam from an object when viewed close up, assuming dark ambient conditions and you managed to fool the lidar gun to operate at full power output close to the target (by partially covering the receiver window).

However, beam divergence combined with range of operation (most likely at less than full output power) will mean that the energy of the laser will be completely and utterly swamped by ambient light (when considering exposure time of the CCD imager). A shorter CCD exposure time will result with proportionally increased laser light compared to ambient, but the chances of capturing the image at exactly the right moment will diminish in proportion (you only get 50 chances per second). To overcome this problem the CCD would need to by synchronised to capture the image exactly when the reflection is expected (not easy but not impossible), but the required lack of exposure time would likely make the rest of the subject (target) disappear, unless (zoom) lenses the size of a bus are used.

For the setup to work, the Lidar gun would need to have a beam divergence of nought (and the laser fixed at full output power), but such a narrow beam will be impossible to keep on target (i.e. the VRM) thereby highlighting the poor aim of the operator – as well as inviting the dreaded slip!

well that’s what I think, try it anyway and see what you get

[james_j]

If it increases the chance of a "customer" contesting their charge or in any way looks as if it may decrease camera revenue, then I think the idea stands no change whatsoever.

[bryan35]

interesting.

I've just tried viewing the pro laser with my camera phone (which does see a typical remote control for the telly) - diddly squat.

Tried it in the dark (broom cupboard) and the camera sees a very VERY faint pea green glow, and that's with the gun firing straight into it.
I thought that you'd see a lot more than that!.
The sensor in the laser gun must be pretty sensitive.

[smeggy]

bryan35 said:

interesting. I've just tried viewing the pro laser with my camera phone (which does see a typical remote control for the telly) - diddly squat. Tried it in the dark (broom cupboard) and the camera sees a very VERY faint pea green glow, and that's with the gun firing straight into it. I thought that you'd see a lot more than that!. The sensor in the laser gun must be pretty sensitive.

My estimate of output power for a typical IR remote control is 1mW. The LTI will deliver <60uW absolute max.

A green glow is normal for a colour CCD stimulated with IR, don’t ask me why - perhaps that filter passes IR the best?

I’m not so intimate with the pro-laser, but I guess it won’t be greatly different from the LTI. I would think the gun aborted the measurement as it couldn’t get a lock onto a target surface with the required characteristics – you were too close. For the same reason, the laser power would quickly be wound down to minimum anyway. I guess the IR rejection filter in your camera would filter out most of what's left.

Yes, the sensor in the gun (the avalanche diode) is very sensitive - photon counter sensitive and they’re very fast. They aint cheap either.