Abstract: A method of making a haloapatite phosphor having a general formula represented by (Ca(1?x?y)MnxSby)5(PO4)3(F(1?z?y)ClzOy), where O<x<0.05, 0.004<y<0.01, and O<z<O.1. is provided. The method involves (a) formulating a blend of ingredients to make the haloapatite phosphor, the ingredients including CaHPO4 as a primary source of calcium and phosphorus, (b) partially substituting an amount of an equimolar combination of an ammonium phosphate compound and calcium carbonate for an equal number of moles of CaHPO4 in the formulation, (c) combining the ingredients in their formulated amounts to form the blend, and (d) firing the blend to form the haloapatite phosphor. The partial substitution has been found to increase the 100-hour brightness of the haloapatite phosphors.