Abstract: We present a fast algorithm for rendering animated hair under a dynamic lighting environment at interactive frame rates. We use spherical harmonics (SH) to represent the environmental lighting. Since SH functions are orthogonal, the environmental light is converted into an SH representation in real time. To compute hair shadows efficiently, the transmittances between a point on a hair strand and the light sources are also represented by SH functions. The intensity at a point on the hair is then efficiently computed by a dot product of the SH coefficient vectors between the environmental light and the transmittance. However, the computation of the transmittance is very time-consuming. Our method reduces the computational cost by using a voxel-based approach to compute the transmittance.