A combinatorial approach was used to systematically investigate the effect of trace Pr(3+), Tb(3+), or Sm(3+) on the VUV photoluminescence of Eu(3+) in the Pr(3+), Tb(3+), or Sm(3+) co-doped (Y(0.65)Gd(0.35))BO(3):E(3+)(0.05). We found that Pr(3+) and Tb(3+)increases the VUV photoluminescent efficiency, while Sm(3+) decreases the efficiency. The optimized composition was identified to be between 7 x 10(-6) and 3 x 10(-4), and the corresponding efficiency improvement is about 15%. Scale-up experiments confirmed the results in the combinatorial materials libraries.