Passage 4

In the wake of 11 September, Visionics, a leading manufacturer, issued a fact sheet explaining how its technology could enhance airport security. They called it "protecting civilization from the faces of terror". The company’s share price skyrocketed, as did the stocks of other face-recognition companies, and airports across the globe began installing the software and running trials. As the results start to come in, however, the gloss (光滑表面) is wearing off. No matter what you might have heard about face-recognition software, Big Brother it ain’t. The concern was based largely on an independent assessment of face-recognition systems carried out in 2000 in the U. S. by the Department of Defense. These tests found that to catch 90 per cent of suspects at an airport, face-recognition software would have to raise a huge number of false alarms. One in three people would end up being dragged out of the line and that’s assuming everyone looks straight at the camera and makes no effort to disguise himself. Results from the recent airport trials would seem to justify that concern. Most face-recognition systems use some kind of geometric technique to translate a picture of a face into a set of numbers that capture its characteristics. Once it has identified these boundaries, the software calculates their relative sizes and positions and converts this geometry into what Visionics calls a "faceprint". Feed the software a series of mug-shots (通缉犯), and it’ll calculate their faceprints. Then it can monitor live CCTV images for the faces of known suspects. When it finds a match, it raises an alarm. Even if the system does manage to capture a face, the problems aren’t over. The trouble is that a suspect’s faceprint taken from live CCTV is unlikely to match the one in the database in every detail. To give themselves the best chance of picking up suspects, operators can set the software so that it doesn’t have to make an exact match before it raises the alarm. But there’s a price to pay: the more potential suspects you pick up, the more false alarms you get. You have to get the balance just right. Despite the disappointing tests, some people insist that face-recognition technology is good enough to put terrorists off. After all the claims and counter-claims, with no one able to discern (洞察) the truth, the industry may soon have to face up to reality.

What is the fatal defect of the system according to the U. S. Department of Defense