No moon was out to illuminate the Afghan desert beyond the reach of Kandahar's dim and undependable city lights.

Blinded by the night, I fumbled up the hand-lashed wooden ladder and climbed the guard tower overlooking the lavish palace of the Taliban's one-eyed spiritual leader, Mullah Muhammad Omar -- or what had been his lavish palace until it was blasted by cannons from AC-130 flying gunships, then seized by the advancing militias of America's Pashtun allies and, finally, secured as a base by Army Special Forces, the Green Berets.

The desert of southern Afghanistan looks like pictures sent back from a Mars lander, and standing watch atop the tower that night was what looked like, well, a Martian: a faint green glow came from behind his eyes, and a long snout -- too high for a nose, too low for a horn -- extended six inches straight out from his face.

''Have a look,'' a quiet voice behind the mask said. ''Here's how we own the night.''

The Special Forces sergeant removed his night-vision goggles: two eye cups attached to a single tube; electronics package and two AA batteries, sufficient for at least 30 hours; straps that criss-cross the skull and snap under the chin; all less than two pounds.

Once in place, the goggles transformed the black slate of the desert night into a distinct panorama of what had been the final battlefield before the fall of Kandahar. Vehicles in the shadows were now easily identifiable: Humvee, armored car, S.U.V. Invisible shapes emerged as individual soldiers moved around the outpost.

Night-vision goggles have become a small but essential piece of America's advantage over its enemies, just as their design, with an array of closely guarded technology packed inside, has become part of the iconography of the modern American warrior.

In the campaign to halt global terror, against an enemy who has proven ingenious at exploiting seams in America's defenses, the nation's military still possesses a few asymmetrical advantages of its own. The bat-wing Stealth bomber can slip undetected past enemy radar. Precision-guided munitions, dropped from altitudes safely above antiaircraft fire, find targets at the end of an invisible track laid out by satellites.

But night-vision goggles state a most eloquent case for American prowess over adversaries who cannot mount major military operations once the sun has set. Night offensives proved decisive in Afghanistan, and this mismatch will be relied upon in whatever future military actions may be ordered by the president.

The United States likes to fight at night.

And it can carry out those missions because of a cylinder called the image-intensifier tube, which is the heart of night-vision devices. The cylinder has three parts: the photocathode, the microchannel plate and the phosphor screen. Tiny amounts of ambient light, reflected from stars or the moon or a distant city, are gathered by the tube. When the light energy, called photons, bangs into the photocathode, it is transformed into electrical energy, or electrons. These electrons fly into the microchannel plate, which has several million microscopic holes that release hundreds or even thousands of electrons each time they are hit just once. The growing mass of electrons slams onto a phosphor screen, like a tiny TV, which converts the electrons back into photons of light energy. The screen glows in the same pattern as the image that entered the tube, but highly intensified. Green is the color chosen by scientists for night-vision images, since it is the hue recognized with the most acuity by the human eye.

The Defense Department relies on ITT Industries and Northrop Grumman for most of its night-vision equipment. America's image-intensifier tubes are so superior to the competition's that their export is controlled by the Departments of State and Commerce, in consultation with the defense secretary and the Joint Chiefs of Staff.

The primary night-vision device used by the armed forces is the AN/PVS-7. It is called a biocular device, using one image tube to feed two eyepieces. The AN/PVS-7 is preferred for ground maneuvers and driving. It costs about $3,500, and more than 215,000 of a planned purchase of 400,000 are spread throughout the military.

A newer device, the AN/PVS-14, is a monocular system, covering one eye. It is the favorite of light and mobile units -- Special Operations forces, the Rangers, surveillance teams and those sent forward to spot targets or call in air strikes. Ten ounces lighter than AN/PVS-7 goggles, the monocle is preferable for urban warfare, too, where a soldier may move swiftly from a dark hallway to a bright room and needs a keen eye at both light levels. Since one eye remains unobscured, it offers a broader field of vision. And from the Department of Weird Science, troops say their hearing becomes sharper as their vision adapts to night, so they like to keep one eye in the dark. It costs about $3,300, and approximately 40,000 of a planned purchase of 53,000 are now in use.

''In terms of the ergonomics and performance, this by and large has not been a revolutionary technology,'' Charles Bradford, a team chief with the Army's Night Vision and Electronic Sensors Directorate, says. ''It has been incremental improvements over technology begun by the Germans in World War II. One piece at a time, we've tried to make it better and better.''

The photocathode systems of the 1950's were too large for the military -- and initially produced only images that were upside-down. By the mid-1960's, starlight sniper scopes were developed with Generation I light-intensification tubes and fielded in Vietnam. In the 1970's, night-vision devices were developed that relied not on image intensification but on new technology to sense variations in heat generated by a target compared to its surroundings. These heat pictures -- called thermal imaging -- led to what were called Forward Looking Infrared systems, which were too large for an individual soldier on the front lines. But when mounted in vehicles or aircraft, they could see where image-intensification devices could not: through dust and smoke, even when there was no ambient light.

The demands on these complicated optical devices, of course, are extreme. Military and industry officials describe a testing regimen that sounds like a torture chamber. The equipment must withstand desert heat and arctic temperatures and must operate in blowing sand after being carried through saltwater. Simulations include shock tables that bounce and pound the gear and a bizarre ''freezer oven'' that alternates scalding temperatures with frost.

The current Generation III image-intensification goggles are about four times more sensitive than the devices of the first era. Today, the military's research-and-development focus is on making them more comfortable for the soldier, who is running and jumping and crawling and rolling while under the great physical and emotional stress of combat for long periods of time in the harshest climates on earth. Researchers are pushing to create night-vision devices that are flatter, smaller and lighter.

''We are now working on very low-profile goggles, which sit very close to the eye,'' Jim Harris, vice president and director of engineering for ITT Industries' night-vision division, says. ''Some we are working on now do not protrude past the tip of the nose. The sensor is mounted on the side of the head or on top of the helmet. We have put a very thin display right in front of the eye.''

The Army also wants its contractors to develop ''fusion sensors'' for its future soldiers. Lt. Col. Cynthia M. Bedell, product manager for Soldier Sensors, the Army office responsible for all personal night-vision devices, said fusion technology may offer the best of two worlds by combining the crisp picture from light-intensification systems with that from thermal imaging, which is less sharp but can identify targets in total darkness and through fog and smoke and dust.

She said the biggest challenges facing researchers designing these fusion goggles, called Enhanced Night Vision goggles, are weight, battery life and, in particular, poor alignment of the two images when dropped into the viewfinder by prisms. Current prototypes produce a fuzzy image like newsprint photos with the colors out of register. The system also will be costly, possibly about $8,000 or more apiece.

The Air Force has its own night-vision goggles in development, to meet the needs of pilots and crew members who say they prefer a wider field of view. Their Panoramic Night Vision goggles will use a system of four optical tubes feeding images to the pilot, offering a 100-degree field of vision, greater than the 40-degree angle of view from standard goggles. Because of its highly complicated combination of cylinders, the device will probably cost $50,000 per unit.

The problem, of course, is that each new development brings new risks for the military. As technology advances, it often becomes cheaper and proliferates into the civilian world -- and potentially, to enemies. Unlike, say, a B-2 bomber, night-vision goggles are an item the average consumer might be able to use and afford. In fact, in recent years, night-vision devices have steadily migrated from the military to the commercial market and are a favorite with campers, astronomy buffs, hunters, boaters and even high-end burglars.

Many goggles in private hands are Soviet surplus and are of quite low quality, and the photosensitive material at the core has burned out. Camping stores and ''I Spy'' shops offer Generation I and II night scopes and even some models constructed with Generation III image-intensification cylinders -- but only rejected Generation III tubes that failed muster at military standards, according to industry officials. (Military-quality Generation III scopes can be sold to American law-enforcement agencies, but that remains a tiny market.) On the low end, there are hand-held Generation I devices designed to look like camcorders; they start at less than $200. Hand-held binocular-style devices made from Generation I tubes sell for about $500. At the other end of the price spectrum, night-vision devices constructed from Generation III-class tubes begin at several thousand dollars and can cost up to $10,000 with all the extra lenses, magnifiers, compasses, helmet mounts and assorted bells, whistles or glowing internal displays.

Pentagon and industry officials decline to describe the exact difference in quality between their models and those sold to civilians, although it is at least a 25 percent gap. But senior military officials are quick to express great concern that as night-vision devices improve and are sold more broadly around the world, the free market will diminish the Pentagon's advantage, and adversaries will match American capabilities with off-the-shelf optics.

One official at the United States Special Operations Command, the installation in Tampa, Fla., that oversees the nation's elite shadow warriors, told me, ''The civilian market continues to grow, and as technology evolves, the price of these devices continues to shrink.'' The military, he added, ''tries to maintain a several-generation lead on technology available for commercial sales. However, as production methods improve and the devices become easier to manufacture, this gap could narrow. This puts the commercial manufacturers in a very difficult position, trying to balance profit margins to investors versus sitting on a technological breakthrough for several years while it is fielded for military use.''

American consumers have long benefited from military technology -- at least military technology that has worked its way through the slow and difficult declassification process. Radar began as one of the military's top secrets but now controls civilian air traffic, predicts rainstorms and even alerts drivers of luxury sedans when the car up ahead is slowing down. Some current winter fashions are a direct descendent of cold-weather research to dress troops warmly during the Korean conflict. Global positioning systems used by hikers, boaters and, again, certain luxury-car owners, followed a similar course.

In the case of night-vision technology, defense-industry officials say their business plans focus on satisfying the American military, which is by far the largest customer, especially for the most expensive models.

''With the exception of a few niche, high-end consumers, the civilian market does not support the high-performance, high-value products that ITT Industries makes,'' Harris, the company's vice president for night vision, says. ''The civilian market today is less than 10 percent of our overall night-vision business.''

For the time being, then, the military will maintain its edge. No night scopes on the civilian market would allow someone to take a midnight drive, as I did, across the desert at highway speeds -- with headlights switched off -- to rendezvous with a C-130 transport scheduled to take off after dark to avoid antiaircraft fire from Taliban holdouts still dug in around Kandahar.

The rotors of the Air Force's workhorse cargo plane kicked up big dust clouds, and the static electricity of the talcum-fine sand whipping through the blades generated an image of a great green halo around the C-130. I was flying out of Afghanistan on angel's wings -- or at least that's how it appeared in my final view through night-vision goggles.

As the lumbering cargo plane banked and turned north, I unstrapped and went forward for a last look at Kandahar below. All I saw was darkness.

Thom Shanker covers the Pentagon for The New York Times.

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