NEW YORK (REUTERS) - New York City's police commissioner unveiled body cameras on Thursday that his department will start testing soon.

Bill Bratton demonstrated the equipment by playing a short video showing a notably good-natured traffic stop from an officer's perspective.

"I'm going to issue you a summons for that red light, OK?" the unseen officer can be heard politely telling the driver he has just pulled over. "Give me a second, OK?" the officer adds in a reassuring voice, which the driver appears to think is fair enough. "I'll be right back."

It is the sort of genteel encounter between an officer and a civilian that Bratton, the city's police commissioner since January, said he hoped would become more common if both parties know they are being recorded.

Other departments already using the cameras have found they tend to bring out the best behaviour on both sides and "de-escalate" encounters with police, Bratton said at a news conference at the headquarters of the nation's largest police department.

"I think clearly the officer, knowing they are being recorded, will affect the behaviour of the officer in a good way," Bratton said.

Only 60 of the city's 35,000 officers will be asked to volunteer to wear the cameras in the pilot program.

The death of Michael Brown, the unarmed Missouri teenager shot in the street by a police officer in disputed circumstances last month, has brought renewed attention to the technology. The officer who shot Brown was not wearing a camera.

Bratton said the cameras would provide a more objective record of a police encounter than contradictory hearsay.

Two uniformed sergeants stood near Bratton, modeling the two types of cameras the department will test. The first is made by Taser International Inc, best known for its eponymous stun guns. The second is from Vievu, a Seattle-based company specialising in wearable cameras.

Bratton has said he is a fan of the technology and would have launched tests even if a federal judge had not ordered the department to do so last year as part of a ruling that found the city's use of so-called stop-and-frisk tactics unconstitutional.

The Centre for Constitutional Rights, one of the advocacy groups that successfully sued the city over the stop-and-frisk tactics, criticised the way the police department had gone about launching the pilot program.

"This kind of unilateral decision on the part of the NYPD follows the non-transparent, go-it-alone approach to police reform we saw with the prior NYPD and mayoral administration,"Darius Charney, one of the center's lawyers, said in statement, referring to the New York Police Department and the former mayor, Michael Bloomberg.

The city's largest police union has expressed skepticism about the cameras. "Police officers have nothing to hide but there are many unanswered questions as to how this will work practically," Patrick Lynch, the president of the Patrolmen's Benevolent Association, said in a statement on Thursday. "We await the answers."