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Lasers: In the Legal Crosshairs

Overcoming ‘sweep error’ and meeting the legal standard, police and scientists made a strong case for the speed laser.

February 01, 2002  |  by Craig Peterson

Students of history - including police and prosecutors - are well aware that when a new type of speed-measuring technology hits the street, court challenges to its accuracy won't be far behind. And until a case reaches a high-profile state superior court and is granted judicial notice, prosecutors are expected to produce expert witnesses at each trial to testify to the validity of its theory of operation.

At the time radar was introduced in the early 1950s, the Doppler Principle that formed the cornerstone of its operation had long been accepted by the scientific community. But it would still take a few years until judicial notice was finally taken in the case of New Jersey v. Dantonio. Another decade rolled by before tuning forks finally passed judicial muster in 1966. And the last precedent case of any real significance wasn't handed down until 1984, more than a quarter century after cops started using radar.

After its introduction in 1991, the speed laser appeared to be somehow exempt from the decades-long vetting process that greeted radar. By the spring of 1992, courts in six states had already granted judicial notice to speed laser. At that time, lasers were the darlings of the technological world. Lasers were commonplace in everything from precision munitions used in Operation Desert Storm, to CD players, to medical equipment. And with so much positive press coverage of laser technology, it seemed that court approval of lasers could only continue.

But the legal honeymoon was about to end.

Fittingly, the speed laser's big day in court occurred in the same state that was first to take judicial notice of radar: New Jersey. A defendant - popped with a laser by a state trooper - contested his speeding ticket. The laser in this instance, a Laser Technologies Inc. (LTI) 20-20 Marksman, had passed the required NHTSA tests and was on the CPL approved-product list, but Superior Court Judge Reginald Stanton wanted hard evidence proving the laser's reliability.

Arguments for reliability in that case took a major hit when, during a recess, one of the defense's expert witnesses hefted a Marksman and squeezed the trigger as he panned it across a back wall. A 4-mph target speed appeared. He repeated the feat twice more with the same results. The implication was clear: if laser would display target speeds for a stationary wall, what other errors were possible?

The prosecution's expert witnesses cried foul, knowing that the phenomenon, technically known as "sweep error," can be induced in any properly designed speed laser. All modern speed lasers determine target speeds by directing a burst of electromagnetic pulses at a target, and a portion of that burst is reflected back to the laser. With the round-trip elapsed time noted and the knowledge that the pulses travel at the speed of light (about 186,000 miles per second), range can be calculated.

When the target is in motion, the distance to the laser is changing. If a constant elapsed time between laser pulses is maintained, the laser can calculate speed by comparing the target's range from one pulse to the next. The entire process takes about one-third of a second.

But suppose the laser wobbles slightly during this calculation, shifting the point of aim from a vehicle's front bumper to the windshield header, a distance of perhaps four or five feet in a passenger car. The variation in distance will result in an incorrect target speed. Laser designers recognize this and use error-trapping algorithms to guard against the possibility. For example, the LTI laser's microcomputer does not display speeds and issues an error message when sweep error is detected. Other lasers collect a longer sampling before displaying a speed as a way to combat sweep error.

Trouble is, a laser in the hands of an expert, being panned very smoothly along a wall, may accept the relatively constant change in distance as legitimate and display a speed, as it did in the New Jersey case. Prosecution experts explained this phenomenon to the judge and described how error-trapping software prevents its occurrence in the real world.

Fine, said the judge, and he invited LTI to submit its error-trapping software for review. This posed a dilemma for LTI; the company was confident of the superiority of its software but also aware that by handing over proprietary data for public scrutiny, that data could be accessible to rival firms. LTI declined the offer.

Too bad, said the judge, who declared that the unit's software had not been scientifically proven to his satisfaction. "After considering all of the proofs and the analysis and the argument presented, I am satisfied that the general concept of using lasers to measure speed is widely accepted in the relevant scientific communities and is valid," he said.  "I am, however, not satisfied that the laser speed-detector device is accurate and reliable enough to be used for law enforcement purposes."

Judge Stanton went on to note that he may have placed less weight on the software had there been more operational testing of the laser speed detector.

"Under actual highway conditions, we might be able to accept the detector as being reliable even though we did not have complete details about the way in which the error-trapping procedures are designed and programmed," he wrote.

Comments (1)

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Hans U. Meyer @ 7/23/2018 9:28 AM

A "sweep error" could also occur without human intervention when the laser is on a tripod off the road and aimed at a constant angle to the road (0.1 rad or more). The fixed beam would intecept the far side of the front of an approaching car first, then "sweep" across it to the nearby side during the measurement. The succession of the spot positions being longer than the length of the car's travel, increase the measured speed by the angle's inversed cosine, always larger than one. This "sweep error" would thus be detrimental to the driver, contrarily to the microwave radar's known "cosine error", always smaller than one, in the driver's favor. "Experts" seem to disagree by saying that this "cosine error" applies also to lidars, but physics don't lie.
However, these "experts" are probably not American, as such unattended laser speed traps are only seen in Europe, as far as I know.

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