EXAMPLE OF SOME OF THE MATERIAL AVAILABLE IN TE MEMBERS AREA: THIS IS NOT REQUIRED READING BUT PRESENTED FOR YOU TO HAVE AN IDEA AS TO THE SCOPE OF AVAILABLE MATERIAL
This page is a a short exert from "In the MATTER OF the ADMISSIBILITY OF MOTOR VEHICLE SPEED READINGS PRODUCED BY SPEED DETECTION SYSTEM" (from case law that comes with The Tipmra, There are more pages in The Tipmra Members area with the rulings that make laser conviction improbable)
A laser is an artificially generated and amplified light which is in the infrared light section of the electromagnetic wave spectrum. It is not visible to the naked eye. It is very concentrated. The laser speed detector fires a series of laser pulses at a selected remote target. When the laser light strikes the target, a portion of the light is reflected back to the detector. Since the speed of light is a known constant, by measuring the time which it takes for the laser pulse to travel to the target and back, the detector is able to calculate the distance between the detector and the target. Each laser pulse which is fired and reflected back establishes one distance reading. The laser speed detector fires 43 laser pulses every time the trigger on the detector is squeezed. These 43 pulses are fired in a total period of approximately one-third of a second. If the target at which the laser pulses are fired is a stationary target, each of the 43 pulses will give the same distance reading to the target, and distance will be the only thing that the detector can tell us about the target. However, if the target is moving, each of the 43 pulses will give a slightly different distance reading and the detector can then compute the velocity or speed of the target from the changes in distance divided by the known elapsed time between the firing of each of the laser pulses. In
simplest terms, this is the basic theory underlying the use of lasers to calculate speed, and there can be no dispute about its fundamental validity.
There are, however, both conceptual and practical problems which have to be overcome in designing and constructing a reliable laser speed detector. The detector works by measuring the time it takes a laser pulse which it transmits to go out to a target and come back. However, there are many other pulses in the environment of the detector which can interact upon it, and the detector must be programmed to distinguish between those "false" pulses and the "true" pulses which it has transmitted.
As mentioned above, the laser is very concentrated and has a characteristically narrow beam. At a point 1,000 feet away from the detector, a laser beam is about three to three and one-half feet wide and has a height of about three feet. This may be contrasted with the beam of radar which is about 320 feet wide at a point 1,000 feet away from the radar transmitter. Although the laser beam is much more concentrated than the radar beam, it is far from being a true pinpoint. As the three by three and one-half foot laser beam strikes the very irregular surface of a moving motor vehicle, it does not hit a single, highly-reflective point on the vehicle. In effect, it splashes over a portion of the vehicle. This is true even though operators are trained to fire at the front license plate area of the vehicle, because the beam is considerably larger than the license plate. Indeed, depending upon the angle at which the beam hits a vehicle, and depending upon the vehicle's location with respect to other vehicles on the highway, particularly a multi-lane highway, it is conceivable that a portion of the beam splashes onto another vehicle. For reasons which will become apparent when I discuss possible sweep error, there is a sense in which it is important for the detector to be programmed so that it can distinguish the point on a vehicle from which the return impulse is coming.
It is important for the laser detection device to measure distances between it and a motor vehicle at the same point on the
motor vehicle. In traffic law enforcement, the ultimate use of the detector is to fix the speed of a vehicle. However, the primary measurement made by the detector is distance. (In this respect, the detector contrasts with the Doppler radar units employed in police work. The Doppler radar units compute speed from differences in frequency between microwaves. For them, distance is not significant.) This distinction is important, because, although the entire vehicle travels at the same speed, not every point on the vehicle is the same distance away from the detector. A vehicle traveling along a highway at 60 miles per hour travels 88 feet in a second. In the one-third second which elapses while the laser speed detector is firing 43 pulses at the vehicle, the vehicle will travel 29.33 feet. If the laser pulses being fired by the detector were allowed to sweep [714 A.2d 374] from the front grille of the vehicle to the windshield of the vehicle during the one-third of the second the pulses were being fired, the pulses reflecting back from the windshield would be four feet farther away than the pulses reflecting back from the front grille. Unless the detector has an appropriate error trapping program built into it, the detector would conclude that the vehicle traveled 33.33 feet instead of the 29.33 feet which it actually traveled, and the detector would show the speed of the vehicle as being 68 miles per hour instead of the correct 60 miles per hour. This would be what is known as "sweep" error. If the detector were allowed to sweep for ten feet along a vehicle, that would lead to about 20 miles per hour being erroneously added to the calculation of the vehicle's speed. If the detector were allowed somehow to pan from one vehicle to another vehicle in a way which would lead to a distance differential of 30 feet, that would convert to a speed reading 60 miles per hour too high.
The inventor of the laser speed detector is clearly aware of many of the conceptual and practical problems involved and has designed computer programs and hardware mechanisms designed to trap a variety of errors. The State's expert, Dr. Gezari, submitted a report which had this to say on the subject of error trapping:
One of the basic operating features of the LTI 20-20 is an error trapping algorithm which tests the integrity of the received data. Thirty to forty data pulses received must have the correct pulse shape, rise time, duration, color and basic time sequence to be considered valid data fit for analysis. If at least thirty received impulses do not fit the criteria the data is rejected and no velocity is calculated. Other criteria are also used by the manufacturer to identify valid pulses having to do with actual vehicle characteristics--acceleration/deceleration parameters, change in target direction, etc. The LTI 20-20 speed gun will display error messages if no velocity is calculated informing the operator as to the reason no answer was given or no velocity was calculated. These error messages do not indicate errors were made; they simply identify the reason that no calculation was made. The error trapping approach used in data analysis further insures that factors such as steadiness of the gun during the measurement, weather conditions, motion of other objects nearby, etc. do not affect the accuracy of the speed reading calculated.
The electrocomputer circuitry of the LTI 20-20 speed gun uses sophisticated techniques including pulse stretching algorithm, statistical data analysis techniques, etc. to provide fully adequate timing accuracy in the generation and detection of laser pulses resulting in typical velocity measurement accuracies of ± 1 mile per hour or better for typical highway speed measurements.
The inventor and designer of the detector, also testified fairly extensively with respect to his efforts to eliminate inconsistent data and to trap error. One of the mathematical techniques to screen out inconsistent and erroneous readings is a procedure called the "average of least squares". That procedure was discussed at some length by Mr. Dunne and by three of the defense experts, and an exhibit setting forth a partial test program for least square speed error was admitted in evidence. The average of least squares is a common procedure which can eliminate inconsistency and error in a variety of applications, but it is only a limited part of the error trapping techniques which are purportedly built into the laser speed detector.
Click here to return to Debunking_the_Myth