The present invention relates to a mass-flow sensor that measures the mass flow of conveyed reflective materials, such as cotton, in a stream of air or non-opaque fluid. In particular, the mass-flow sensor of the present invention may be used with a GPS receiver as a cotton yield monitor when mounted on a cotton harvester. It can also be used to measure mass flow of the various cotton component streams in a cotton gin. The mass flow measurements may be made non-intrusively and in real time. The present invention also relates to a method for measuring mass flow using the mass-flow sensor of the present invention.