How Laser Works: also caller lidar
Laser works by time-distance measured by a laser beam. A laser beam is aimed at a prominent flat spot on your car near the license area. It measures the distance and a split second later measures the distance again. The difference is the distance your car moved in that given time period. By using time distance formula's the laser gun comes up with a speed. I am certain that you can see the problems.
At 1,000 or even 500 feet a man holding the gun will have the beam moving all over the target. Although he is aiming at the license plate area the target at that distance is smaller then a dime. In fact the beam moves all over the target with one return coming from the front of the car, the next return from the back of the car and even from another car accidentally captured in the seep or a telephone pole. All these distances are different and only when the second return is from the exact same spot can a accurate speed be calculated. The beam does not know that the return was from the rear fender rather then the front grill. The distances it measures for speed calculation is often less then a foot. The difference between the front of the car and the rear is more then a foot. Problems.
For an accurate reading:
The unit must be held steady normally mounted on a tripod
There can be no visible moisture present (fog, rain even light drizzle) because the water droplets diffract the beam giving a wrong reading
The unit can not be used from inside the car shooting through the glass as the car glass diffracts causing errors
It can not be used from a moving vehicle
The shape of the car affects the reading, a sloped hood like on a sports car makes a accurate reading almost impossible
the unit can not be field tested for accuracy, it can only be tested on a pre-assigned fixed mount at a accurately measured distance
How Vascar Works: (and other devices similar)
Vascar and other devices similar, to many to name including the car speed measuring devices used by aircraft are all forms of vascar. It is a time distance measurement. Generally lines are placed across the road and the distance between these lines is measured. The time it takes you to cover that distance is feed to a computer or compared to a chart that then give your speed. The greatest variable is the device that triggers the on and off of the timing device. Newer units use a laser beam that is set at 90 degrees to the road. When your car passes through the beam the timer is triggered on and when it again passes through another beam it is triggered off. Most units are just a stop watch that is clicked on and off by a human observer. Where a human observer triggers the device off and on there is serious possibility of human error entering into the equation.
The only logical device to go after as to calibration is the stop watch and the measurement of the distance markers.
Vascar and similar devices are not that common. You will find it prevalent in Pennsylvania where the local police are forbidden by law to use radar or laser devices. This is because of notorious abuse and a screeching outcry of the populace to that abuse. Only the State police is permitted to use radar and laser for a speeding conviction. as such vascar is the revenue generating alternative they have turned to.
Because of the setup of vascar there are generally two officers involved. In the use of aircraft there are always two officers involved. One to take the speed reading and the other to pull over the victim. Whenever two officers are involved, both must appear in court.. the reason is that one officer may not testify as to what the other officer observed or did. This is hearsay evidence and inadmissible. Unless you object a judge will allow it. If only one officer appears and you object the judge has no choice but to dismiss the case.
The item subject to calibration is the timing device or stopwatch.
PACE: or pacing
This is where the officer sneaks up behind you and follows you in his patrol car and matches your speed. He then records the speed from his speedometer as being the speed you were doing. The device used it the speedometer itself.
Visual Speed Estimation:
This is where the officer testifies that he has attended some special course and has a certification that he can visually estimate speed. He may or may not present a certificate from some agency or another and state that he is trained to estimate speed to a 99% accuracy.
Do not confuse this with testimony in a radar or laser case where the officer says that he first estimated your speed and then used the laser or radar gun to verify your speed. This is proper usage and the speed he estimated is not considered as evidence you your speed. Confusing, yes. However you will recognize the difference when you see it. In the case where he wants to enter his observation as a definitive speed he will testify to some special training and certification. This is easy to overcome by stating that "no Jurisdiction has ever taken judicial notice of such an ability".