A theoretical expression for the brightness of powder phosphor is derived in order to investigate the effects of phosphor’s properties on low-energy cathodoluminescence. It contains the particle size, surface recombination rate, lifetime of holes, carrier diffusion coefficients, and anode voltage. As such, it can be applied to systematically optimize the design parameters of phosphors for maximum efficiency at low-voltage excitation. Use of the given expression is illustrated and a typical case is analyzed in this study. ©1997 American Institute of Physics.