A variable wavelength ultraviolet lamp. An apparatus for selectively producing one or more of a plurality of wavelength distributions of light is provided, comprising a primary light source having a primary wavelength distribution, at least one wavelength-transforming material that, in response to illumination by the primary light source produces secondary light having a wavelength distribution different from the primary light wavelength distribution, the wavelength-transforming material being disposed on a substrate external to the primary light source, and a wavelength-transforming material selection mechanism for placing at least a portion of one or more selected wavelength-transforming materials in front of the primary light source, in a selected preferred direction of light emission from the apparatus, such that the selected wavelength-transforming materials emit from the apparatus light having a wavelength distribution different from the primary light wavelength distribution.