Sorting apparatus has a conveyor belt or equivalent mechanism for moving particles at a speed sufficient to generate a stream of particles in air, which particles can be graded such that unacceptable material can be removed. The grading or sorting is conducted by a primary scanning system for analyzing light reflected from particles in the stream in a plurality of wavelength ranges. Ejectors for removing particles from the stream are disposed downstream of the scanning system, and are instructed in response to signals received from the scanning system. An auxiliary scanning system is also included to establish the presence of material in the stream, and in the event that a void is detected in a given region, then the analysis of that region by the primary scanning system and any corresponding activation of the ejectors is inhibited.