It lasted a thousandth of a second and touched off a technological revolution.

In 1960, nearly a generation before "Star Wars," a physicist at the Hughes Aircraft Research Laboratories in California took a four-inch rod of synthetic ruby and pumped it full of energy from a high-intensity photographic flash lamp.

What came out of the ruby cylinder was a brilliant pulse of crimson light -- the brightest, purest light ever seen. Physicist Theodore H. Maiman had produced the world's first laser.

"If someone had looked directly into the beam," recalled Maiman, "he would have seen -- briefly -- a light brighter than the sun."

Maiman's invention has spawned a multibillion dollar industry that reaches from the supermarket checkout counter to the operating room, producing spectacular light shows, three-dimensional holograms on credit cards and a bloodless, invisible surgical knife. A mile-long laser beam across New York Harbor lit the Statue of Liberty on its recent 100th birthday.

A laser is an extremely dense beam of light, so uniform and narrowly focused that its energy can shape diamonds or vaporize steel, yet so controllable that it can safely pierce a calcified stapes bone in the inner ear -- the tiniest bone in the body.

Maiman compares the difference between regular light and laser light to the difference between two types of women's shoes. For example, a woman wearing a pair of flat shoes can walk across a linoleum floor without leaving a trace, but if she then puts on spike heels and walks across the same floor, the heels leave little marks in the linoleum.

"She didn't weigh any more the second time," he said, "but the weight was more concentrated."

Ordinary light from a light bulb or the sun is a chaos of different wavelengths, including all the colors of the rainbow, radiating in all directions from its source. But in a laser beam, all the waves move "in unison" in the same direction, and they are all the same wavelength or color.

Some lasers are used to cause small-scale thermonuclear reactions in energy experiments in scientific laboratories. Others drill holes in baby-bottle nipples and put eyes in surgical needles. The laser pulse can last for hours or just trillionths of a second, and its target can be spread out over an area the size of RFK Stadium or concentrated on a single red blood cell.

Medicine, in particular, has been transformed by the laser's unique combination of power and precision.

About 1 million Americans were treated with lasers last year, the American Society for Laser Medicine and Surgery estimates. Most were treated for eye problems such as glaucoma or retinal bleeding, but a growing percentage of the cases involve other specialties -- from removal of growths on the vocal cords to neurosurgery to treatment of gastrointestinal ulcers.

On the horizon are a host of other possible uses: to pulverize kidney stones, "spot-weld" skin grafts and surgical incisions without sutures, diagnose and treat dental decay and blast the cholesterol buildup from artery walls.

"The medical field is about to undergo a revolution in surgical therapy and laser diagnostics," concluded a report on lasers by Arthur D. Little Decision Resources, a leading research and consulting firm. The report predicted that the world market for medical lasers, estimated at $243 million last year, will more than triple to $740 million by 1990.

"I've worked on a lot of new medical devices, but none that has the sex appeal of the laser," said Dr. David E. Fleischer, associate professor of gastroenterology and director of the endoscopy unit at Georgetown University Medical Center.

Gastroenterologists' fascination with the potential of the technology, he said, results from the futuristic mystique of the laser itself and the hope it offers in dealing with cancers that often are untreatable by conventional surgery, radiation or drugs.

Lasers have three basic uses in medicine: cutting, "welding" and "cooking." They make an extremely precise scalpel, or "knife of light"; they can seal off, or cauterize, leaky blood vessels or bleeding ulcers; and they can destroy unwanted or harmful tissue by heating its cells to the point where they evaporate or explode.

Initially, lasers were limited to treating easily accessible areas of the body, such as the skin, eyes and mouth. But when combined with fiberoptic endoscopes -- flexible, wire-thin, lighted tubes that allow doctors to see and treat internal organs directly without cutting the body open -- they can reach the lungs, the esophagus and most of the gastrointestinal tract.

Many lasers are the size of a small refrigerator, but a medical team from Johns Hopkins University recently carried a 65-pound portable laser by four-seat airplane north of the Arctic Circle to treat 44 Eskimos for glaucoma in remote Alaska, hundreds of miles from the nearest doctor's office.

Ophthalmology was the first medical field to put lasers to work. For years, eye doctors had been using light energy from xenon lamps to "spot weld" detached retinas, but with only limited success because the energy was too low. The laser offered a much more powerful source of energy.

"In ophthalmology, compared to when I started out 30 years ago, the laser has revolutionized how we treat patients," said Dr. Arnall Patz, director of the Wilmer Eye Institute at Johns Hopkins University Medical Center in Baltimore.

Lasers are used to treat several of the leading causes of blindness. They can repair leaky blood vessels in the eye, relieve the fluid pressure in the eyeball caused by glaucoma and open up the hazy membrane behind the lens of a patient who has complications after cataract surgery.

"The laser has made the greatest impact on prevention of blindness of any invention in the past three or four decades," said Patz, who built one of the first lasers used to treat the eye -- back in 1968, before commercial lasers existed.

Patz's original laser machine was dubbed the "ping-pong laser" because it used a ping-pong ball to help measure the beam's intensity. The ball glowed when the laser passed through it, and a light meter gauged its brightness.

"It was pretty crude, but it worked. Occasionally, the ping-pong ball fell off and we had to put it back on with Scotch tape, but it worked almost as well as this $60,000 instrument over here," he recalled, pointing to a state-of-the-art argon laser covered with dials and red digital readouts.

Today, there are many types of lasers, suited to different surgical tasks. They are usually named after the gas or solid that is the source of the light. Carbon dioxide, argon and a synthetic crystal abbreviated YAG are the most common examples, each producing a laser with a specific wavelength, or color.

The effect of a laser beam on body tissue depends on many factors, including the intensity, duration and wavelength of the laser and the color and makeup of the tissue.

The carbon dioxide laser is primarily used as a scalpel. It doesn't penetrate deeply because its energy is rapidly absorbed by water, which makes up more than 70 percent of human tissue. If the carbon dioxide laser is primarily a cutter, the YAG laser is a "cooker." It penetrates more deeply, and vaporizes tissue by heating it to cause a microscopic explosion.

The light of carbon dioxide and YAG lasers is invisible; the argon laser produces a blue-green light that is readily absorbed by its opposite color, red. The argon is often used to treat the eye, because it shines harmlessly through the eyeball until it strikes the red pigment of the retina in the back of the eye. When it hits the retina, its energy is absorbed and an instantaneous photochemical reaction occurs.

"It causes a tiny microscopic explosion in the tissue," said Dr. Douglas E. Gaasterland, director of the glaucoma and laser service at Georgetown's Center for Sight. "It's not painful, but the patient may hear a tiny snap."

As a surgical "knife," the laser has several advantages over a conventional scalpel. It cuts more precisely, with less disturbance of surrounding tissue. No instrument has to touch the tissue directly, which not only gives the surgeon an unimpeded view but also lessens the risk of infection. And as it cuts, the laser photocoagulates tissue, sealing off blood vessels and reducing bleeding.

The first non-ophthalmological medical application of lasers was in removal of papillomas -- benign viral warts -- and other growths from the vocal cords.

Conventional surgery on such growths was almost impossible, particularly in children.

"To try to get our heavy instruments in there below the vocal cords in an infant is well-nigh impossible," said Dr. Bernard Marsh, associate professor of otolaryngology at Johns Hopkins. "You can't both see and do. If you can get an instrument in there, you can't see what you're doing with it."

Lasers are also used to treat some early-stage cancers of the larynx, or voice box.

"You used to have to undergo radiation five days a week for six to eight weeks," said Dr. Michael E. Johns, chairman of head and neck surgery and chairman of the committee on lasers at Johns Hopkins. "Now we can cure that patient with one outpatient treatment and just one hour in the operating room." or all the laser's marvels, and despite continual technical improvements, doctors warn that it is not a panacea and may be overused and oversold. "In some cases, unfortunately, the laser has become gimmicky," said Dr. Stanley Shapshay, an otolaryngologist and director of the laser research lab- oratory at the Lahey Clinic in Burlington, Mass. "Some doctors say, 'Okay, we'll remove your nasal polyps with a laser and you'll do better.' Well, it isn't necessarily better."

Lasers are also unnecessary for most tonsillectomies, he said.

"It takes me half an hour to remove tonsils without a laser ," Shapshay said. "Why would I want to take an hour with a laser and not do any better job?"

A recent article in the New England Journal of Medicine, titled "Selling Surgery," criticized some ophthalmologists for misleading advertisements.

"Although some advertisements blatantly imply that cataracts are removed with lasers," wrote Dr. Curtis E. Margo, an ophthalmologist in Tampa, Fla., "others merely suggest it by using subtle techniques of design. Advertisements that list surgical procedures often place the word 'laser' close to statements about cataract surgery. It is therefore easy to understand why someone unfamiliar with cataract surgery could be confused."

Some experts worry that training of physicians is lagging behind the rapid advance of laser technology. Such a lag poses a twofold danger. Some doctors may be unaware of the latest capabilities of lasers; others may overestimate the laser's abilities and underestimate its limitations and risks.

"While the laser has permitted some remarkable achievements, it is not a miracle instrument," cautioned a recent report of the American Society for Laser Medicine and Surgery. "It is simply a tool, and like any tool, it has both its good points and its bad points . . . Physicians who want to adapt the technology for new medical uses, therefore, must make sure that it won't create more problems than it solves."

Chief among the laser's risks is the possibility of the light hitting an area for which it was not intended. Like any beam of light, the laser can be reflected inadvertently by a metal instrument. Both the patient and the medical team must wear protective glasses during surgery to stop a stray beam.

Another concern is that because of some remarkable, highly publicized successes, patients may assume that the laser is a cure-all. For many of the conditions it treats, it is not.

For example, lasers increasingly are used to remove blockages in the airway of a patient with lung cancer or in the esophagus of a patient with gastrointestinal cancer. Those treatments are palliative -- they make the patient more comfortable, and they may even extend the patient's life by restoring natural breathing or swallowing. But they can't alter the underlying tumor, which remains largely untreatable, and they may not change the long-term prognosis.

"Everybody thinks there's something magic about it," said Johns Hopkins' Johns. "Patients tend to think it's going to solve the whole problem and they're not going to feel a thing.

"In fact, in most cases, that's not true." One of the most promising, though still experimental, uses of the laser involves intravenous injection of a light-sensitive dye, or "marker." The dye, called hematoporphyrin derivative, or HpD, is rapidly excreted by normal tissues of the body but remains in cancerous tissues. When the tumor is exposed to red laser light from an argon laser whose wavelength can be precisely "tuned," the dye absorbs the laser's energy and the tumor is destroyed. Normal tissues remain relatively unaffected, because they do not contain the dye.

"It's a very sexy concept," said Lahey Clinic's Shapshay.

Controlled studies are under way at Lahey and other medical centers to test the technique on several types of tumors -- including lung, head and neck, bladder, cervix and uterus -- but the results are years away. The same principle could be used to diagnose cancer promptly -- say, in the lung of a smoker -- by injecting a dye that will be absorbed by the tumor and then fluorescing the area with a lower-energy laser. The laser would highlight possible tumor tissue, which could be seen by a surgeon wearing special glasses and peering through an endoscope.

"It's very experimental," Shapshay said, "but very exciting.