The preparation and optical properties of two new tin‐activated calcium metasilicate phosphors are described. The more efficient of these is a phosphor emitting in the visible at about 520 mµ. The optical properties of the tin‐activated species are compared to their lead‐activated analogues; and from considerations of crystal structure as affected by firing temperature and activator concentration, it is concluded that changes in spectra result from changes in crystal structure and not from the degree of aggregation of the activator ions. It is suggested that the emission spectra correspond to the and the transitions of the lead‐ and tin‐activator centers.