Ultraviolet light animal tests. Researchers for the United States Department of Agriculture carrying out ultraviolet (UV) light therapy on white rats. Ultraviolet light is the portion of the electromagnetic spectrum between visible light and X-rays. UV rays occur in sunlight and are responsible for sunburn. They have beneficial effects when used to treat skin conditions (a form of phototherapy). This photograph, from 13 January 1926, is from the National Photo Company Collection, consisting mostly of photographs taken in Washington DC, USA.