behind the start marking with reference to the running direction of the conveyor belt according to one variant not shown here the address of the segment marking and the start marking 6 form a uniform marking system for example in the form of a code in stripe form the common scanning unit is then a read head the precise location determination takes place using an encoder that is driven by the conveyor belt 1 itself for example by means of friction wheel coupling within the framework of the exemplary embodiment presented here the encoder 10 is driven by way of the axle of a non driven drum 2 the encoder produces a certain number of electrical pulses for a certain path distance these pulses are acquired in the process computer 15 by means of a counter and together with the segment marking and the address of the belt segment yield precise location data for every point of the conveyor belt to be inspected the precision of the location determination depends on the selection of the encoder and the precision of the determination of the segment marking and can be very high precision values of a few tenths of a millimeter are possible the encoder can be for example a multi pole encoder de 203 12 808 u1 or an optoelectronic encoder in this regard reference is made to the general state of encoder technology the process computer 15 is coupled with the following device parts namely with the radiation source 4 including its control device 12 the line sensor 5 with.