Two men who robbed a New York bank not long ago wore ski masks to hide their faces and plastic gloves to leave no fingerprints. Surprise! They were caught by a laser beam at the FBI laboratory in Washington.

Laser light is the latest weapon the FBI has mounted in its fight on crime. Where chemical powders won't pick up fingerprints, laser light will. In the last year, the laser has found for the FBI no fewer than 215 sets of fingerprints at the scenes of crimes that could not be found by the time-honored powders method.

Among the first victims of the FBI's laser were the two New York bank robbers. When they fled the bank with their stolen money, the robbers whipped into a waiting stolen car that they drove to a second waiting car. On his way out the door of the first car, one of the robbers pulled off the plastic gloves he'd been wearing and dropped them on the floor of the car.

FBI agents retreived the gloves which they sent to the lab in Washington. Chemical powders saw nothing, mostly because the static electricity that builds up on plastic serves as a mask to fingerprints. In came the laser, whose light on the inside fingers of the gloves was like a spotlight shining on the robber's fingerprints.

"Bear in mind that if we didn't have the laser we never would have gotten these prints," the FBI's James Ridgley said the other day in an interview. "Not only did the laser give us the prints, it led us to the people who had committed the crime."

Of the 215 sets of fingerprints spotted by laser light in the last year, 45 produced positive identifications of the persons who'd left the prints behind. One led to a suspect who left his fingerprints on the plastic stock of a sawed-off shotgun, another to a man whose prints were on plastic electrician's tape in a false bomb threat.

There was the case of the purloined Bicentennial coin collection. In the middle of the coin collection had been a rare silver certificate, and under a strong light the outline of the missing bill could still be seen. The laser didn't pick up the fingerprints of the burglars but it did identify the serial number on the bill, which was then traced and recovered along with the rest of the stolen coins.

The laser's beauty in spotting fingerprints lies in the fact that it can get prints off plastic cups, bags and fibers that chemical powders can't reach. The blue-green light of the laser works by spotlighting an enzyme of vitamin B called riboflavin that is excreted in the perspiration that is a natural part of the body oils that stain the ends of everybody's fingers.

"We've been able to get prints off syrofoam coffee cups, cellophane bags and plastic seat cushions where we didn't get a thing with chemical powders," Ridgeley said. "The laser brought these prints out beautifully."

Once, Ridgley said, he trained the laser beam on a film can that had been picked up in Hawaii in a case that involved pornographic films. The laser saw nothing, he said, until he was called to the telephone. When he returned, he noticed the laser light had warmed the labels on the can so that the glue had melted and peeled back the labels. There, under the labels, were perfect sets of prints brought out by the laser.

THE FBI is now so happy with laser fingerprinting that it has put a second laser beam in its identification laboratory in the J. Edgar Hoover Building on Pennsylvania Avenue. Its main drawback is that it is too heavy (465 pounds) and isn't portable enough for use in the field. It also has yet to be tested in court but so far it hasn't had to be tested in court. Each one of the suspects the laser has led to has confessed his crime and pleaded guilty in court.