Finsen was born into a prominent Icelandic family that was involved in the administration of the Faroe Islands. He attended schools in Denmark and Iceland before entering the University of Copenhagen (M.D., 1890), where he became interested in the effects of light on living organisms. In 1893 Finsen found that lengthy exposure of smallpox sufferers to the red light formed by exclusion of the violet end of the spectrum prevents the suppuration of the pustules, or formation of characteristic pockmarks. Aware of the bacteria-destroying effects of sunlight, he developed an ultraviolet treatment for lupus vulgaris, a form of skin tuberculosis, which met with great success. Although phototherapy has largely been superseded by other forms of radiation and drug therapy, Finsen's work did much to encourage the radiation therapy then being developed and led to the use of ultraviolet sterilization techniques in bacteriological research. Finsen's Medical Light Institute (now the Finsen Institute) was founded in Copenhagen in 1896. Faced with declining health in his early 20s, Finsen spent much of his later life as an invalid.