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This law corre sponds quite Well to the mental effect whereby the human eye senses the magnitude of a defect. The dimensioning of the time constant CR of the inte grator in the above described manner is given only as an example. The time constant can be chosen to satisfy other integration requirements. For example, to inte grate a de?ned number of minute defects (each less than 1/32 inch diameter) falling Within a certain area of the sheet material. One or more integrator stages satisfying several programmes can operate simultaneously and can be coupled via capacitors such as 163 and diodes such as 164 to the output cathode fol-lower 1811. > When [the defect signal is smaller than the amplitude of the noise signals such as are indicated at 173 and 174, it is desirable that the integrator shall not respond to the noise signals. According to my invention this is accomplished by coupling the output plates 152 and 153 by diodes 159 and 160 as well as capacitor 163v to the input of the integrator. The cathodes of the diodes 159 and 160 are connected to the output plates while the anodes are connected to the capacitor 163. The mean potential of the output plates is fed to the anodes of the diodes 159 and 1-60by two equal resistors 161 and 162 so that the coupling diodes are not biased and only the negative excursions of the noise signals 173 and 174 pass through the coupling diodes and appear at the junc tion of the capacitor 163‘ and diodes 159 and 160. The resulting signal 195 is the recti?ed noise signal and thus its peak to peak amplitudes are halved. The time con stant of the capacitor 163 and the resistors 186 and 187 is chosen in such a manner that'it satis?es the low fre quency response requirement of the amplifying system and as an example it is made three seconds. Therefore, the recti?ed continuous noise signal charges in three seconds the capacitor 163, thus biasing the coupling’ diodes 159 and 160 by a voltage level which is of the order of the recti?ed continuous noise amplitudes so that such noise signals are substantially eliminated at the input of the integrator as shown by signal 197. The integrator input signal 196 does not have the said effect upon diodes 159 and 160 and therefore is not attenuated. According to the present invention, the screen potential of the integrator 1185 is so selected that its current is cut oil when only a few volts are applied to the control grid of the integrator. Various sheet materials have diiferent noise contents. Some sheet materials have a rather non uniform noise content in which case the sensitivity of the integrator has to be reduced thus limiting the sensitivity of the system for the detection of the defects extending in the direction of travel of the sheet material. Aword ing to the present invention, the potential pider 194» is adjusted to various levels to match the noise level of the various materials. When the screen potential of the integrator is raised, the grid cut-off potential is increased. Therefore, according to my invention the “noise sensi tivity contro ” member 194 is adjusted according to the noise characteristics of the various sheet materials. As an example of this, when the peaks of noise signals 197 are —2 volts, the noise sensitivity control 194 is set in such a manner that the control grid cutoff potential of the integrator is say minus 5 volts, then the plate potential of the integrator will not reach the +223 volts at which level the coupling gate, diode 164, opens. In accordance with the invention, the control grid, and hence the plate of the integrator 185 is switched to ground potential periodically by positive pulses 198 or 2% which are applied through coupling diode 192. Pulse 209 is initiated by the delayed output pulse 184. When, for example, the conditions of integration pro grammed are such that defect-information contained in each one-inch length of the sheet material (having a width de?ned by the sections viewed by the photoelectric cells 98 and 99) is to be integrated, it is possible that a one-inch length containing defects which raise the plate potential of the integrator above the trigger level of the output pulse generator is followed by a one-inch length of the sheet which, while containing minor defects, should not be discarded as defective. The characteristics of the integrator circuit are such that such sections con taining minor defects and which are not to be discarded could also be sensed as defective sections, unless the out put pulse 184 or “sampling pulses” 193 return the inte grator to its steady state, namely, when the potential of its plate is close to ground. This effect is better understood by reference to FIG. 11 and by assuming in the example which follows that speci?c voltage levels are applied to the integrator. Let the sheet velocity be 100 inches per second with a faint defect one-inch ‘long in the direction of the travel of the sheet material. Then let the signal v corresponding to this defect be ~10 volts at the integrator input and let the control-grid cut-o? potential of the integrator be -2 volts. The time constant of the integrator is so selected that when v=—10 volts, the integrator output . reaches the positive supply potential +350 volt-s in 10 milliseconds; 350 volts__l0 volts . 10-2 sec. _ CR Therefore, the trigger level of 250 volts is reached in 6.7 milliseconds and the resulting duration of the output pulse 134 would normally be 3.3 milliseconds while the defect passes under the aperture 7. Let the steady state bias of the control ‘grid, due to grid current, be —1 volt; after 10 milliseconds the control grid'reaches —2 volts which is the cut-off potential of the integrator‘. At the instant the defect passes, beyond the aperture (11 in FIG. 11), the output potential starts to fall slowly; the voltage drop corresponds to v=+2 volts, because when the defect signal drops to zero the potential across resis tor 186 is +2 volts while during the presence of the defect signal it was —9 volts. Thus for a period of ap proximately lSrn-illiseconds the integrator output volt age remains above the +250 volt trigger level and only 10 15 20 30 35 40 45 50 55 60 after a considerably longer time period can the initial potential (+10 volts) of the integrator plate be reached. Conditions are made worse when during the slow re covery of the integrator plate minor defect or noise sig nals, say of the order of ——2 volts occur, such signals should not be detected. Such signals however will retard the recovery of the integrator as seen in FIG. 11. Due to such slow recovery, the trigger generator is kept in its trigger .state longer than required and as a ‘result minor de?ects can cause retriggering. The slow recovery of the integrator after the termi nation of a defect is a basic and essential feature of an integrator but by its very nature it isv also responsible for the above described sluggishness of the trigger gen erator. By applying the positive pulses 198 or 200 to the control grid of the integrator such sluggishness is elimi nated. In the example given the “PRF,” or pulse repeti tion frequency, f of sampling signal 198 is 100, then each one-inch length of the sheet material isintegrated. The “PRF” of the sampling pulses f is adjusted to be proportional to the sheet velocity and to be inversely proportional to the sheet lengths to be integrated; that is as where W is the sheet velocity and L is the length of the sampled sections in the direction of travel of the sheet material. Alternatively, when defect signals raise the integrator output to the trigger level, the resulting trigger pulse is delayed and is used after conditioning in sampling pulse generator 199 as the resetting pulse at the control grid of the integrator. Such time relay (“Td” de?nes the Width of the output pulse 184. The delay Td is so se lected that the width of the output pulse 184 is of the shortest duration compatible with the satisfactory opera tion of the control circuitry of the grading switch 16 shown in FIG. 6, or is compatible with other applications of the output pulse 184. . The integrator resetting pulse or pulses, according to either alternative, are produced in “sampling pulse gen erator” 199 which is shown in block form only in FIG. 10. This generator can, for example, be a transitron Miller pulse generator associated with a phase inverter both of which are well known in the art. Such generator also produces either the “in-time” equally spaced sam pling pulses according to the ?rst alternative or accord ing to the second alternative is triggered by the leading edge of pulse 184, completes its Miller run-down de?n ing delay Td and at the end of such run-down generates the resetting pulse. 1 The width of the resetting pulse 200 or sampling pulses 198 can be varied by a multi-position switch in pulse generator 199 by conveniently switching a number of capacitors coupling the suppressor and screen grids of the transitron Miller pentode. The width-s of the resetting or sampling pulses are de?ned by the velocity of the sheet material under inspection and by the charg ing time of capacitors 189 or 190, the said pulse Width . must be long enough to allow the return of the plate 65 70 75 potential of the integrator to its initial condition close to ground level. The pulse width must also be as narrow as possible for the reason thatduring the resetting period (corresponding to the pulse Width) the integrator is in active. By arranging ‘that the positive excursions of pulses 198 or 299 are large enough and by feeding such pulses from a high energy sour-ice of low output imped: ance, such as represented by the cathode follower 201, the control grid of integrator 185 is speedily driven well into its grid-current ‘region, the resulting heavy plate current discharging capacitor 189 or 190 rapidly.