In this paper, a novel scheme of extracting multiple illuminant directions from an image of a Lambertian sphere of known size is proposed. The illuminant direction detection process is based on the concept of critical points introduced in the paper. We show that the illuminant directions have a close relationship to those critical points and that, by identifying those critical points as many as possible, illuminant directions may be recovered if certain conditions are satisfied. Our preliminary experimental results show that illumination information can be obtained accurately.