紫外线辐射的特性和益处

　　紫外线辐射的特性和益处

　　一个简短的历史

　　德国物理学家Johann Wilhelm Ritter发现紫外线辐射的1801。他观察到了氯化银浸泡过的纸加速变暗，当它暴露在不可见的光线时，就在紫色末端的可见光谱之外。为了区分这些射线与前一年在可见光谱的另一端发现的“热射线”（IR），他称紫外线辐射为“氧化射线”，这强调了他观察到的化学反应。这一点很快被“化学射线”所取代，在十九世纪余下的时间里它仍然很受欢迎。最后，化学和热射线项被替换为现在常用的紫外和红外名称。

　　什么是紫外线？

　　在电磁光谱中，紫外线（UV）辐射被归类为波长从10纳米到400纳米。这些波长比可见光短，比X射线长。有三种：UVA紫外线具有315和400 nm之间的波长，UVB具有280和315 nm之间的波长，和UVC 100和280 nm之间的波长。

　　Figure 1： The UV wavelengths are those immediately below what the human eye can see. （Image courtesy of W.S. Badger Company， Inc.）

　　Our sun emits UV radiation across the 10 to 400 nm spectrum. At the edge of Earth‘s atmosphere sunlight consists of approximately 50% infrared （IR）， 40% visible， and 10% UV light. By the time it reaches the Earth’s surface， with the sun at its highest point， the composition of the sunlight is 53% IR， 44% visible， and 3% UV. Of this 3% of UV that reaches the ground， approximately 95% is UVA and 5% is UVB. Of course， these percentages vary somewhat with cloud cover and other atmospheric conditions.

　　Much of the UVC wavelengths are absorbed by oxygen in the upper atmosphere which then generates ozone in the ozone layer. The ozone layer blocks most of the UVB and the remainder of the UVC not already absorbed by oxygen.