A cerium activated lutetium borate phosphor of the following formula (I): (Lu.sub.x,Ln.sub.y)BO.sub.3:aCe,bA (I)in which Ln is at least one rare earth element selected from the group consisting of Y, La and Gd; A is at least one element selected from the group consisting of Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Zr; and x, y, a and b are numbers satisfying the conditions of 0.5.ltoreq.x<1, 0.ltoreq.y<0.5, 0