The system works much like a TV remote control: A beam of light is projected across the road at the level of the car's tailpipe. A computer then determines how much light is blocked by a car's pollutants. If enough is blocked, the car is flagged as a high emitter and a photograph is snapped of the license plate. The systems can be operated by computer, without any personnel. Soon after, the car's owner would receive notification that the vehicle failed inspection and must be repaired. Drivers would, in theory, return to a central inspection station to prove the car was repaired. Clean cars would never have to report to a station. "If what we care about is high on-road emissions, we have a tool that finds high on-road emissions," said Don Stedman, an inventor of the test. "Inspection programs are supposed to reduce on-road emissions; at least, I thought that's what they were for." But Zinger of the EPA said the test is not accurate enough to identify light polluters. And he points to another flaw: How do you make sure all cars drive through the checkpoints? "We like remote sensing and think it has a lot of potential," he said. "We don't think that it has been shown technically to be a replacement for traditional tailpipe testing." Instead, the EPA sees it as a way to gauge how well the regular inspection program is working, by taking on-road samples of exhaust. The agency does advocate using the test in another capacity -- to "clean screen." Drivers would voluntarily go to a designated street in town where their cars would be tested without stopping or slowing down. If they passed, the car would be exempt from inspection. If they failed, they would have to go to an inspection station. California has experimented with such a system, but not yet embraced it. "It's a good way to use it," Zinger said. "You get a letter saying you've passed and don't have to bring your car in this year." John Elston, head of the DEP's Office of Air Quality, said it would cost too much to use it as a "clean screen" in New Jersey. One system can cost more than $100,000. He did say the state was considering on-road testing to verify the effectiveness of its regular inspection program, but only in a limited capacity, possibly at one or two locations. Both the EPA and the state DEP are waiting for the technology to get better at finding high-polluting vehicles before deciding on further uses for the system, officials said. Model-year exemption New Jersey drivers are exempt from getting a new car inspected for two years. Other states, such as California and Colorado, have exemptions of four years. Arizona has a five-year exemption. Elston said there were several reasons behind the New Jersey rule. First, he said, the two-year rule allows drivers to get failing cars fixed under warranty. "There is also an equality issue," Elston said. "If we are just looking at older cars, we are, in fact, stratifying the system for those with economic well-being." Elston also cited the safety -- brakes, headlights, etc. -- and insurance checks that are done during car inspections as reasons to keep the exemption at two years. In California, discussions are under way to extend the exemption beyond four years. "There is no doubt that a lot of evidence suggests we can go longer," said Mike McCarthy, head of the advanced engineering division for California's Air Resource Board. "Six or seven years would not be unrealistic. "The only bad thing about going longer is that it's kind of nice to have one inspection while you are still in your warranty period," he said.