Optoelectronic sensors using infrared light-emitting diodes and photo-transistors have been used for measuring velocities in snow avalanches for more than 10 years in America, Europe and Japan. Though they have been extensively used, how they should be designed and how the data should be processed has received little discussion. This paper discusses how these sensors can be applied to measure two-dimensional velocities. The effects of acceleration and structure in the underlying field of reflectance are carefully accounted for. An algorithm is proposed for calculating the continuous velocity vector of an avalanche, and a sketch of the mathematical analysis given. The paper concludes by suggesting design criteria for such sensors.