The brilliant beam of light from the police helicopter slices through the sultry night air to illuminate the backyard of a Logan Circle row house. From the air the picnic table, barbecue grill and brick patio appear stagelit in a steady circle of white light.

On the ground, police officers search the light-drenched yard for a gun tossed by a suspect in a street mugging. The whackety-whack of the helicopter blades and the dramatic "Night Sun" searchlight create a movie-like setting.

For the past 12 years, "Juno," as helicopters are known to police radio dispatchers, hasprovided a palpable--and often dramatic--police presence in many of Washington's neighborhoods. Swooping from the sky to tail stolen cars and hovering over the city's rooftops in search of burglars, the department's four tiny Bell 47s function as flying squad cars and give District police an added dimension in fighting crime in the city.

Every day from about 7 a.m. till 10 p.m., two crews alternate in keeping a helicopter in the air over Washington. Sometimes both crews are up, landing only when they need gas about every two hours. After 10 the helicopters fly only when they are specifically called on because of sensitivity to complaints about noise.

Just how effective the helicopters are is difficult to measure. The number of arrests the helicopters have assisted in--242 in the 1982 fiscal year and 113 so far this year--is one indication.

Another more subtle effect is the psychological impact of their presence overhead in preventing crimes from taking place.

Three years ago the helicopter squad almost fell victim to departmental budget cuts. Today the squad, with its 18 uniformed police, three mechanics, and a $840,000 budget for the coming fiscal year is viewed with respect both within and without the department.

"We are very pleased with those helicopters," says Leroy Coad, president of the Woodridge Citizens Association in northeast. "They keep the undesirables out of here. When they see that helicopter coming, they leave. We can rest with a little more peace knowing those helicopters are overhead."

"When I first heard about the helicopter unit, I didn't think too much of it," says Carl Profater, deputy chief in charge of the Fifth District in Northeast. "I thought they couldn't possibly help. When I became the commander of 5D in 1976, I realized their benefit. I went up in the helicopter. I made my own observation. I came back and told my crime analysis people to call them. We can't do without them for crime patrol and traffic control. They are great for finding stolen autos."

The helicopter unit is still viewed with a good deal of skepticism by some cops on the street.

Because the unit is located at National Airport across the river in Virginia and the flying officers are just voices from the sky, there is a feeling that they aren't really working as hard as the officer in his squad car.

"In my ten years on the street the only thing Juno did for me was to fly over 14th and U streets and tell me to skip an auto accident report because both parties had left," said one unconvinced officer.

Another officer, a ten-year veteran, was more positive. "It is hard to articulate the need for Juno when you have never seen them catch anyone. But they are a necessary avenue to criminal investigation. It gives us peace of mind that we have checked out everything."

While the pilots and observers who fly with them follow the same shift rotation that governs most of the police department, the nature of their assignment does set them apart from other officers.

When they come to work, for example, they wear bright blue flight suits which zip up the length of the front.

And they don't carry any arms, the result of a crash here in 1979 in which rescuers were unable to pull two police officers out of the burning helicopter because the heat caused the bullets in their gun belts to explode.

There has been only one other accident, in 1973, that resulted in injuries to an observer. Three other accidents damaged or destroyed the craft.

Almost all of the seven pilots in the squad are helicopter nuts--they'd rather be flying than doing anything else.

They often show up on days off and stay long after their shift is over to keep an eye on things.

The observers, some of whom aspire to be pilots, tend to be less fanatical about flying and concentrate on the actual police work. They talk to the dispatcher to give guidance to the street units and they are the ones who go to court to testify after arrests are made..

One of the most valuable functions of the helicopters, according to the police officials, is pursuit.

"We have the capacity to observe a large area," says Lt. Robert Morris, the squad commander. "When one officer on the ground is chasing two people and they split, he can not watch both them. We can and then we direct the ground units."

Four months ago, pilot Stanley Roberts recalls, he got a call about some bank robbers crossing into Washington from Virginia.

"We were asked to look for a green Ford Thunderbird in Southeast Washington," he says. "We found it on Stanton Road. They noticed us right away and they abondoned the car. They ran into an apartment building on Pomeroy Road, and we notified police. The money was recovered. There were three of them but the FBI thought there were only two. If it weren't for us, they wouldn't have known about the third guy."

Once the observer spots the suspect or car, the helicopter stays as close as possible, checking for hiding places and radioing instructions to cars on the ground.

Anne Scott, the first female observer in the unit, got a call recently about a robbery on Georgia Avenue. As the helicopter arrived over the address, police officers were running in the front door as three youths went out the back. Two were captured but one got away.

"The kid tried to outrun Juno," says Scott. "He ran all the way from the 3300 block of Georgia to the 500 block of Columbia Road . . . I could tell he was tiring by the way he was running. He made the strides but he didn't have no push in him. He looked up because he could hear us. We just kept after him till we got him arrested," she said.

A lot of the unit's success stories involve rescue work. Morris says that every pilot and observer has emergency technician training plus a senior life-saving badge.

Before Kevin Graulty became a pilot, he worked as an observer and had a chance to rescue two canoeists at Great Falls.

"We landed the helicopter on a little clearing and I was able to get out and get to the woman who had a head injury," he recalls. "I carried her back to the C and O Canal to the ambulance. A boat just couldn't reach them but we did."

Despite such exploits, the reception the helicopters receive is not always warm.

"No one likes the noise," says Morris. Although he says he has received only nine letters and six calls complaining about the noise in the past three years, unit members say they get several calls a week.

Most complaints come at night says 15-year-veteran, Sanders (Sly) Williams. "The helicopter noise blends in with the traffic and construction sounds during the day. We seldon get complaints during the day. At night time, we are the only things up there and we stand out."

The unit recently was given two military surplus Huey helicopters which Morris says will only be used for rescue work because of the noise they make.

In between calls for assistance, the helicopters are free to roam the city looking out for criminal activity, admiring L'Enfant's graceful city plan and waving to friendly boaters on the water. Sometimes the reception can be surprising.

Earlier this month, on a 90-degree weekday, Roberts was cruising over the Potomac River near the Woodrow Wilson Bridge when he spotted a woman waving franticly from a crowded power boat. It was not a friendly wave like other boaters had given the low-flying helicopter but a two-armed distress wave.

Roberts dropped down lower and circled the boat. Suddenly the woman leaned over, pulled up her dress and bared her bottom.

On the next circle over the boat, she looked up laughingly at the pilot who gave her a snappy salute.