By his contemporaries Niels Ryberg Finsen (1860-1904) has been described as a highly dynamic personality and characterised as a sympathetic, kind, confidence-inspiring, conscientious, reliable, intellectual, mysterious and genious doctor, who took care of his patients' illness as well as their social and economic conditions. But he was known too as sharp, critical and able to defend his scientific viewpoints with a deep commitment. Niels Finsen introduced the universal light bath, but his great achievement was the local light treatment of lupus vulgaris for which he got the Nobel Prize in 1903. In the article the history of lupus vulgaris, its treatment and its social consequenses for the patients as individuals are described as an introduction to an analysis of Niels Finsen's scientific methods, his treatment and results and the reactions from the contemporary medical authorities, the public represented by the press and the patients themselves. Finally the discussions after Niels Finsen's death are researched.