It's been almost five years since our last radar/lidar detector test ( C/D, April 1997). Since all the major manufacturers have either updated their wares or introduced all-new models, the time is right for a fresh look at the high-end segment of the detector market.

Radar is still the most popular form of speed detection-some 100, 000 guns are in use, and roughly 20, 000 new ones are sold each year. A radar gun works by transmitting a microwave beam at your car. When that beam reflects off the moving vehicle, it changes frequency, and the reflected frequency is used to calculate speed. Traffic radar, which is regulated by the Federal Communications Commission (FCC), operates on three frequency ranges. The oldest is X-band, from 10.500 to 10.550 gigahertz (GHz); about 10 percent of all radar guns use this band. The biggest chunk, about 60 percent of guns, operates on K-band, at 24.050 to 24.250 GHz. Increasingly popular is Ka-band, which spans a wide range of frequencies from 33.400 to 36.000 GHz; Ka-band accounts for 30 percent, including photo-radar units.

Photo radar involves a camera set up at the side of the road that automatically photographs the license plates of speeding vehicles. These devices are popular in Europe, although their use in the U.S. has stalled-just nine police jurisdictions in four states have deigned to rely on them.

Conventional radar can be used in either a stationary or moving patrol car and can transmit its signal coming or going, front or rear. And don't count anymore on using that slow-moving semi-truck in the right lane to shield your smaller but faster-moving Corvette. This technique used to work with older radar guns that would only display the speed of the truck, the stronger reflected signal. Most newer radar guns can clock both vehicles at once and pick out the faster-moving one. The state-of-the-art Stalker Dual DSR radar unit is especially lethal; it can clock cars in opposing lanes or in the same lane the patrol car is in, whether the target is oncoming or moving away. With other units, the officer has to determine whether the gap to the target is opening or closing. The Dual DSR does all this automatically and makes clocking speeders easier than shooting rats in a barrel.

All radar guns can also be set in a steady-state mode and used to monitor traffic continuously. Or the gun can be switched to an instant-on mode in which the operator flicks the unit on and off to instantaneously measure the speed of passing cars. This "instant on" mode is much more difficult to distinguish because the detector must sniff out the occasional brief zap intended for another vehicle in front or behind. Detecting and correctly identifying these brief signals is your only defense.

Because the FCC allows many other devices to operate on the police radar bands, detector warnings do not always signify the presence of police radar. Automatic door openers at markets and malls, burglar-alarm motion sensors, and other devices broadcast on X-band and to a lesser extent on K-band. Some radar detectors even emit a weak signal on a frequency in the Ka-band spectrum, thereby sending false alarms to fellow motorists.

Lidar is another, especially fearsome speed-enforcement tool. A lidar gun works by firing a series of laser light pulses (with a wavelength of 904 nanometers) at a targeted vehicle. The device times the return of the reflected pulses and uses that number to compute the vehicle's speed. The lidar gun sends a narrow beam at its target; even at a distance of 1000 feet, the most intense portion of it is only six feet wide-narrow enough to pick a single car out of a crowd.