As they prepare to enter the streets of Baghdad for the final assault on Iraqi President Saddam Hussein's bastion, U.S.-led soldiers are counting on what many consider their ultimate weapon: a 500-gram, Multi-thousand dollar device that gives them the power to see through pitch blackness.

This is the AN/PVS-7B Night Vision Goggle, a state-of-the-art device that turns night into day. Through the PVS-7B, a soldier sees an image far sharper and brighter than the green-tinted nighttime combat scenes that run on television news. "It's incredible how well it works," says a U.S. army colonel who specializes in weapons and tactics. "It's as close as you're going to get to daylight. If you're fighting someone who doesn't have it, they have one hand tied behind their back."

In the view of many military experts, night vision is the single biggest technical advantage the coalition possesses. Because of the arms embargo imposed by the United Nations that began when Mr. Hussein invaded Kuwait in 1990, his army has an extremely limited supply of night-vision equipment, all of it outdated.

David Bercuson of the University of Calgary's Centre for Military and Strategic Studies considers the U.S. night-vision superiority an "incalculable" tactical advantage.

"You can see," Prof. Bercuson said. "They can't. That's very hard to overcome."

Although night-vision technology made its debut near the end of the Second World War when the army experimented with huge, truck-mounted infrared beams that illuminated the enemy and allowed officers to see through special binoculars, night vision didn't come into common use until the mid-1970s, in the Vietnam war. This was the era of the Starlight scope, which operated on the same principle as current systems, amplifying light from the moon and the stars.

Since then, improved sensor technology has produced better and better results. By the time of the 1991 war in the Persian Gulf, night vision worked so well that General Barry McCaffrey, who commanded the 24th Infantry Division, decreed it was the most important advantage he had: "Our night-vision capability provided the single greatest mismatch of the war," he said.

The technology has improved markedly since then. The latest systems, such as the AN/PVS-7B, are known as third-generation night vision and can amplify light up to 50,000 times, producing extremely clear images even on moonless nights.

But goggles are just one small part of the U.S. military's vision arsenal. Night-vision systems are now installed in almost every vehicle used by the U.S. forces, including Humvees and transport trucks, which use a device called a Driver's Vision Enhancement System that shows an image of the road ahead on a screen in front of the driver.

Combat vehicles such as the Abrams M1A1 tank and Apache attack helicopters are equipped with both light-amplifying viewers and thermal sensors, which let commanders see the heat emitted by vehicles or bodies.

"It's a very significant advantage," the colonel said.

The U.S. military has spent billions on seeing in the dark, and has set up a centre to study its use and development: the Night Vision and Electronic Sensors Directorate, at Fort Belvoir, Va. The Iraqi military, by comparison, remains in the dark ages.

According to military analysts, only a small number of Iraqi military units are equipped with night-vision equipment. Most of these, they believe, are outdated systems that have been bought through the military black market and civilian channels that sell to hunters and others who want night-vision systems. Military experts say the vast disparity in the number of tanks lost by each side -- the Iraqis have lost an estimated 100 tanks or more, while the U.S. has lost only a few -- is largely owing to the superior ability of the U.S. forces to see in the dark.

"They have a huge disadvantage," said Martin Rudner, a security and intelligence expert at Carleton University in Ottawa. "They're blind."

Night-vision systems

This device collects the tiny amounts of light - including the lower portion of the infrared light spectrum - that are present but hard for our eyes to see, and amplifies them so we can easily observe an image.

How the generation 3 night vision device works

Starlight, moonlight and infrared light enter through the lens.

The light strikes a photo cathode which has a high-energy charge from the power supply.

The energy charge accelerates across a vacuum and strikes a phosphor screen.

The eye piece magnifies and focuses the image.