Owning the Night--For U.S. Troops in Afghanistan, Night Vision Could Be Essential Edge

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.

　

战争中，阿尔-凯达恐怖组织的武装分子也许比美军更熟悉阿富汗的地形，也许他们在暗藏在地下的四通八达的通道中行走如履平地。，但是每当夜幕降临，就该是美军的夜视技术大显身手的时:

夜视仪，让黑夜中作战成为可能

塔利班和阿尔-凯达恐怖组织军队过去可能从别的国家购买过一些夜视设备，但是专家认为阿尔-凯达目前的夜视设备非常匮乏，而且在技术上也要比美军落后得多。

夜视仪是在二战期间发明的，供美国、英国和前苏联的士兵和飞行员使用。这项技术发展至今，已经由当初仅可以将光源增强1000倍的大型仪器改进成为现在可将任何光源（包括微弱的星光）增强5万倍的小巧的精密仪器了。戴上可以感应热量的夜视目镜，士兵们即使在完全黑暗的环境里（例如山洞里）也可以将周围的物体看得清清楚楚。

在这次对阿富汗的夜间轰炸中，美军战机就是使用了夜视技术进行导航和确定轰炸目标。美军还使用了红外线激光仪对轰炸目标进行照明，这种红外线肉眼是看不到的，但是红外线探测技术却可以感应得到，并且可以在战机驾驶舱中的电视屏幕上形成彩色的影像。

夜视仪，放大光源和感应温度

夜视仪主要分为两种：影像清晰化系统和热感应仪器。

在漆黑的夜里，月亮和星星所发的光与红外线很接近。夜视仪将这种光增强到可视的程度，并把这种由光子构成的光转变为电子能量，然后通过一种很薄的磁盘增加电子能量的强度。这样，戴上一种绿色的目镜就可以看到由这种电子在磷屏幕上形成的影像。这种目镜之所以是绿色的，是因为当使用目镜者摘下目镜时，瞳孔可以继续保持放大状态，这样就可以微弱的光线中保持"夜光眼"。但是如果使用其他颜色，当摘下目镜时，瞳孔就会缩小，也就不再能看见黑暗中的东西了。

红外线技术可以探测到物体散发热量的轻微变化。所有生物和其它一些没有生命的物体，包括人类、动物、使用过不久的汽车，他们都是以红外线辐射的方式散热。红外线探测仪通过吸收红外线感应热度，将红外线转变为一系列视频信号，然后形成可视的图像。

夜视技术，数九寒冬也适用？

在使用红外线探测仪的时候，你眼前的景象是灰色或黑色的，其中散热最多的物体呈白色。有报道说，当冬季到来的时候，红外线技术在阿富汗战场上会更加有威力，因为人体和外界之间的温差变大了。但是，这种温差并不能使红外线影像更加清晰，事实上如果下雪的话，红外线技术的功效反而会下降。