A processing unit for measuring and controlling a rotary-driven tool, wherein the processing unit is connectable to a light barrier arrangement, which comprises a light-transmitting unit and a light-receiving unit, wherein the processing unit is configured to receive from the light-receiving unit signals that are at least approximately proportional to shading generated by the rotary-driven tool and/or at least one cutting edge of the rotary-driven tool at a first measuring position. The processing unit is further configured to evaluate the signals received and to transmit control signals to the light barrier arrangement, wherein the evaluation of the signals received by the processing unit comprises the following steps: determination of an interference signal component and/or a useful signal component of the received signal; and provision of information about the useful signal component, the interference signal component and/or the received signal for forwarding to a numerical controller of a machine tool.