optical sensor system 19 the device 23 for generating a pneumatic stream is arranged which has a nozzle 23 a oriented in the direction of the clothing 17 a of the opening roll 17 in such a manner that a short duration powerful air stream flows to the clothing 17 a approximately tangentially thereto the sensor system 19 is coupled via the evaluating device 21 and the electronic control and regulating device 22 with the air blast generating device 23 which includes a valve control device 24 when the camera 20 based on comparison values or desired values detects foreign material in the fiber mass situated on the clothing 17 a the valve control device 24 sends a command to the device 23 to emit a short high speed air blast toward the clothing 17 a to remove the foreign material from the fiber layer on the clothing 17 a with a small number of fibers the sensor system 19 is accommodated in a housing 56 which as shown in fig4 a may be pivoted inwardly and outwardly about a stationary rotary support 57 turning to fig5 the two withdrawing rolls 16 a and 16 b are arranged obliquely above the rotary axis m of the opening roll 17 adjacent the clothing 17 a thereof as viewed in the rotary direction 17 b downstream of the withdrawing rolls 16 a 16 b a cover 28 a cover element 29 an opening 30 a cover element 31 an opening 32 and a cover element 33 are arranged in a circumferential series about the opening roll 17 the device 23 is coupled to a pressurized air source 25 u2032 the valve.