The time evolution of the charge carrier density, ionized donor density and space‐charge field under a sinusoidal light pattern have been calculated by solving numerically the nonlinear differential equations for a photorefractive material (such as BSO). The time evolution for optical erasure starting from the steady‐state solution has also been obtained. Arbitrary modulation depths have been considered for two relevant cases: pure diffussion and under an applied dc electric field of 5 kV/cm. ©1995 American Institute of Physics.