  Viscously damped objects driven through a periodically modulated potential
energy landscape can become kinetically locked in to commensurate directions
through the landscape, and thus can be deflected away from the driving
direction. We demonstrate that the threshold for an object to become
kinetically locked in to an array can depend exponentially on its size. When
implemented with an array of holographic optical tweezers, this provides the
basis for a continuous and continuously optimized sorting technique for
mesoscopic objects called ``optical fractionation''.
