The Science Behind A Laser Speeding Ticket

In order to understand how to fight a speeding ticket issued as a result of a laser device, you need to understand basic mathematics and the science behind laser technology.

You may recall from grade school that rate multiplied by time equals distance (R x T = D).  Similarly, distance divided time equals rate (D/T = R). Laser devices use this latter formula from data it receives to accurately determine the speed of a moving object.

In regard to science, laser devices use two well accepted principles in the scientific community.  The first principle is that the speed of light is known and constant. Second, a laser beam emitted from a laser generator is very narrow in width, will not spread significantly after emission, and is emitted in a narrow frequency band.

A laser device consists of two primary parts.  A laser emitter which “shoots” the light beam and a photodiode which is a clock and a computational device. In operation, when a short laser beam burst is emitted from the laser emitter toward the reflective surface on a vehicle (usually a license plate), the time at which the reflected beam is received back at the photodiode is determined. Based upon the time between the laser beam emission and return, and the known speed of light, the distance between the object and the laser device is determined by simple arithmetic calculation (T/R = D).

Knowing one distance alone, however, is insufficient to determine speed.  Therefore, laser devices actually emit two (or more) sequential light beams at a moving vehicle, and the photodiode measures two (or more) return times.  The difference in the distances measured for each emission is used to determine the difference between the locations of the vehicle at two points (D).  By knowing the difference in time between each of the emission returns (T), and then dividing the difference in distance by the time elapsed between the two (or more) returns, the velocity of the vehicle between the two points is determined (D/T = R).

For example, one popular laser device is the LTI 20-20, which, when activated, emits 60 pulses within about one third of a second. The first several pulses are used to establish a range to the target. The next 40 pulses are then transmitted and the reflections received back by the photodiode. In order for a reading of speed to be displayed on the unit’s digital read-out, 30 pulses must return to the photodiode and result in speed determinations within 1 mile per hour. The emissions of the pulses are computer generated. The computer also does the calculations and insures that the calculated velocity values fits within the allowable tolerance (i.e. +/- 1 mile per hour at a speed of 60 miles per hour).

In order to determine whether the laser device is operating properly, the user is required to conduct certain tests periodically during each day of  use.  These tests are: 1) a read-out display test is done by turning on the device to ensure that the display is working properly; 2) an alignment test for the scope attached to the device to insure that the bulls-eye in the scope is properly centered on an object, both vertically and horizontally, when the device’s “on-target” tone is heard; 3) a distance read-out test is done by standing at two pre-measured distances away from an object and then measuring the distances with the laser device (Note: this measures both the laser operation, the clock action and the computer since all must be synchronized for the correct response); and 4) a velocity test is done when the operator applies the laser gun to the two same preset distances and then the device displays a velocity reading as if the device was measuring two locations of a vehicle. This latter test shows that the calculation means are properly functioning.

In order for a police officer to use a laser device, he or she must be trained and certified including the use, handling and calibration testing of the laser device.  Knowing the science is the only way to effectively be able to fight a laser speeding ticket.