Johann Wilhelm Ritter discovered ultraviolet radiation in February 1801. He observed that invisible rays beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more quickly than violet light itself.
Ultraviolet A covers 315 to 400 nanometers while Ultraviolet B spans 280 to 315 nanometers and Ultraviolet C extends from 100 to 280 nanometers. These ranges are recommended by ISO standard ISO 21348 for subdividing the electromagnetic spectrum of ultraviolet radiation.
Sunlight at the top of Earth's atmosphere contains about 10% ultraviolet light but the atmosphere blocks about 77% when the Sun is highest in the sky. At ground level with the sun at zenith sunlight consists of only 3% ultraviolet because almost no UVC reaches the surface due to blocking by diatomic oxygen or ozone.
Mercury-vapor lamps consisting of fused quartz tubes filled with mercury and argon emit ultraviolet light with peaks at 253.7 nm and 185 nm. From 85% to 90% of the UV produced by these lamps is at 253.7 nm which serves as a very effective germicide.
A committee of the Second International Congress on Light decided unanimously on the 17th of August 1932 to divide UV into UVA UVB and UVC. This decision was made at the Castle of Christiansborg in Copenhagen.