Al Qaeda soldiers may know the terrain better and they may be able to navigate hidden networks of underground tunnels. But once night falls, any American troops in Afghanistan will have at least one advantage:

They can see in the dark

The U.S. Navy Commanding Officer watches aboard the USS Enterprise as aircraft prepare for a second day of night strikes in Afghanistan, as seen through the green filter of a camera's night vision lens on Oct. 8, 2001. [Reuters]

Taliban and al Qaeda forces may have access to some night vision equipment, bought from other countries in the past. But experts believe that any equipment al Qaeda forces may have is scarce, and inferior to U.S. technology.

Night vision devices were invented during World War II for use by American, British and Soviet soldiers and pilots. Since then, the technology has evolved from bulky devices that amplify light about 1,000 times to compact equipment that can amplify any light source (including faint starlight) up to 50,000 times, and eyewear that allows soldiers to see in complete darkness (such as in caves) by detecting heat differences.

Already, U.S. pilots have used night vision equipment to navigate and find targets during night bombings of Afghanistan. Infrared lasers are also used to illuminate targets with a light invisible to the naked eye, but visible to those using infrared detection technology. Infrared images are portrayed in shades of color onto a TV screen in the cockpit.

Amplifying Light, Seeing by Temperatures

Night vision equipment falls into two major categories: image intensification systems and thermal devices.

Even when a night appears completely dark, near-infrared light is emitted by the moon and stars. A night vision device amplifies this light to visible levels. The light, which is made up of photons, is converted into electrical energy and then accelerated through a thin disk. As the converted photons strike a phosphorus screen as electrons, they are perceived through an eyepiece in shades of green. The reason it's in green is because when you put the unit down, you want your eyes to remain dilated so you can see in dim light. Use most any other color and your pupils will constrict when you take off the unit.

Infrared technology measures fraction of a degree differences of heat given off by objects. All living things and many objects - people, animals, recently used cars - emit heat in the form of infrared radiation. Infrared devices read heat by absorbing infrared light, converting it into a grid of video signals and creating a picture the viewer can see.

Effective in Winter

While viewing through an infrared device, you'll see varying shades of gray or black, with the whitest segments representing those giving off the most heat. Some reports have suggested that infrared technology will become more effective as winter arrives in Afghanistan, since contrasts between body temperatures and the external temperatures will increase. But the contrast doesn't necessarily enhance infrared images, and once snow falls, the opposite is true.

(Agencies)