ˈfin(t)sən-, -nzən- noun
Usage: usually capitalized F
Etymology: after Niels R. Finsen died 1904 Danish physician
: a mixture of blue, violet, and near ultraviolet light that is produced by a lamp using a high-temperature carbon arc or a mercury arc and that is used in the treatment of lupus and certain other skin conditions and in testing paints and other protective coatings