Reflection, excitation, and emission spectra of and are reported, as well as measurements of the dependence of luminescence intensity on exciting intensity for and . It is shown that the data support the conventional concept of sensitization, where all the ultraviolet absorption is due to the sensitizer, and that Butler's picture, where the activator absorbs ultraviolet, is invalid. An interpretation of the data on the system is proposed, and the distance over which Pb+2 and Mn+2 interact in this phosphor is calculated on the basis of this interpretation.