Method and device for separating letters by transporting a pile of non-separated and unordered letters by a conveyor band in a continuous flow of essentially non-superposed letters which are largely distributed on said conveyor band. The winding action is not stopped while the motor is accelerated even when the current exceeds the predetermined value. A data store 6 stores several software libraries 100 and library management software 110. Selection of peak wavelength and bandwidth can be accomplished using mechanical interrupters, mechanical shutters, or electro-optical devices including liquid crystal device.