The electromagnetic spectrum of ultraviolet light can be subdivided in a number of ways. The draft ISO standard on determining solar irradiances(ISO-DIS-21348) describes the UV-A (400~315nm), N-UV (400~300nm), UV-B (315~280nm), M-UV (300~200nm) , UV-C (280~100nm). Most of the UV-A radiation emitted from the sun passes through the ozone layer and, thus, it is the most abundant form of UV. UV-B and especially UV-C radiation are the most dangerous to people as well as plants. Fortunately, the ozone layer absorbs most UV-B and all UV-C radiation.