The experimental data concerning the processes of brightness relaxation of phosphors either in vacuum fluorescent displays or in a testing system are presented and discussed. Both increase and decrease of cathodoluminescence (CL) brightness of phosphor screen were observed after the first few tens minutes of first period of CL device operation. It is understood that the relaxation behavior results from the change of electrical conductivity of the phosphor particle surface. The root cause of the electron-stimulated change in electrophysical characteristics of a luminescent phosphor particle is the contamination of phosphor surface by thin layer of carbon. A physical model of a CL device, which describes a change of both luminescent and electrophysical characteristics of the phosphor screen under low-energy electron excitation, is proposed. It describes well the relaxation processes of brightness in the wide period of a CL system operation. There is good agreement between the results of theoretical simulation of the model and experimental data. (C) 2002 American Institute of Physics.