In 1903, Niels Ryberg Finsen was awarded the Nobel Prize for his invention of
light therapy for
skin tuberculosis (
lupus vulgaris). The mechanism of action has not been shown; thus, we wanted to elucidate the mechanism of Finsen's
light therapy. We measured radiation that could be transmitted through his lens systems and absorption of the
stain solution filters in the lamps, and related the obtained results to the possible
biological effects on Mycobacterium tuberculosis. Judged from transmission characteristics all tested lens systems were glass
lenses (absorbing wavelength < 340 nm). The tested filters likewise absorbed wavelengths < 340 nm. The
methylene blue solution used to absorb heat, blocked out wavelengths below 340 nm and between 550 and 700 nm. Furthermore, fluorescence of M.
tuberculosis indicated the presence of
porphyrins and HPLC analysis of sonicated M. marinum showed that
coproporphyrin III was present, which highly justified that
porphyrins were present in M.
tuberculosis. Production of
singlet oxygen through radiation of
porphyrins with light of e.g. 400 nm seems to be a most plausible explanation why Finsen's
therapy worked in spite of the lack of shortwave ultraviolet radiation, which Finsen believed was the most effective radiation for treating
skin tuberculosis.