We propose an image motion constraint equation based on a model which allows the brightness of a scene point to vary with time, unlike the case in the brightness constancy model. Using this model, we describe a method for the computation of optical flow and investigate its performance in a variety of conditions involving brightness variations of scene points, due to illumination nonuniformity, light source motion, specular reflection, and/or interreflection. We show that in the application of this method, care must be taken in the estimation of image derivatives using finite difference methods to prevent biases in the solution. We suggest a simple modification to overcome them. Comparison with two other models, including the classical brightness constancy, is made through results from experiments with real images.