Originally Posted by Heckler
it was an LS20 (I think) laser speed gun and asked why. So I told him I had been reading up about a thing called 'slip effect' on those types of cameras and how easy they get confused, how the laser itself isn't sighted down the same direction as the viewfinder, but is manually set... thus leading to increased chance of the laser detecting the speed from other vehciles, through poor sigting or reflective echoes.
I think the device you are refering to is the LTI 20-20. That has been banned in several states in the US due to the 'slip effect' you mention.
In fact, it is NOT a laser device. It is infrared. Infrared beams 'scatter' as they leave the gun, and can produce a beam upto 1.5 metres wide at the range typically used in speed traps. The manufacturers know this, so there is software built in to the gun which averages the returning signals to give a 'best guess' of your actual speed.
You've probably seen many speed traps where the copper is using the gun without a tripod. Do they seriously expect us to believe he can keep it aimed at your number plate (the flat reflective surface they are supposed to aim at) while your car is moving, and his hand is also moving to keep the gun aimed?
There has been a famous example of just how useless these 'laser' guns are. During a recess in a court case the defence solicitor picked up the gun and pointed it at the wall 30 feet away. According to the gun the building was travelling towards him !