紫外線輻射的特性和益處

　　一個簡短的歷史

　　德國物理學家Johann Wilhelm Ritter發(fā)現(xiàn)紫外線輻射的1801。他觀察到了氯化銀浸泡過的紙加速變暗，當它暴露在不可見的光線時，就在紫色末端的可見光譜之外。為了區(qū)分這些射線與前一年在可見光譜的另一端發(fā)現(xiàn)的“熱射線”（IR），他稱紫外線輻射為“氧化射線”，這強調(diào)了他觀察到的化學反應。這一點很快被“化學射線”所取代，在十九世紀余下的時間里它仍然很受歡迎。最后，化學和熱射線項被替換為現(xiàn)在常用的紫外和紅外名稱。

　　什么是紫外線？

　　在電磁光譜中，紫外線（UV）輻射被歸類為波長從10納米到400納米。這些波長比可見光短，比X射線長。有三種：UVA紫外線具有315和400 nm之間的波長，UVB具有280和315 nm之間的波長，和UVC 100和280 nm之間的波長。

　　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.