the display of the operating and display unit 13 14 in addition it is also possible to regulate the cleaning elements 3 1 to 3 n until a match is obtained such discrepancies may come about despite the fact that in a case of repetition the same material is being processed again this is due to the fact that the material being processed is a natural product which always is subject to certain variations in respect of consistency colour dirt content etc a method according to the invention is accordingly capable of automatically carrying out reproducible adjustments and also moreover of automatically recognising material specific variations and making a correction 15 if corrections are made with respect to the originally determined adjustments those new adjustments can likewise be stored and then on the next change over can again be taken as guide values by this means a continuous and automatic process of adaptation to the product in question can be carried out in the embodiment of fig5 a cleaner is of generally similar construction to that of fig1 a and parts indicated by reference numerals 2 a 2 b 3 a and 3 b have the meanings given with reference to fig1 a arrows 17 18 indicate the movement of fibre material into and away from cleaner 1 a transparent window 23 is arranged in a side wall of the trash collecting device 4 through which window the electronic camera 12 records from the outside the trash 19 u2033 which has been collected in the internal space 21 two illumination devices 20 a 20 b are associated with the camera 12.