Abstract: In an apparatus for sorting material into desired and undesired pieces or portions according to their optical characteristics, comprising a background, and a viewing means for viewing the material against the background; a method for removing debris from the background is proposed which comprises providing a jet of air or water which is moved relative to and across the background, and directed towards the background so as to remove debris and thereby clean the background. The jet is positioned out of the field of view of the viewing means so that debris removal can take place without interruption of the sorting process. A magnetic type air-cylinder, in which exhaust gas is fed back as the debris-removing air-jet, is preferably used.