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For example, the strip of paper observed is %2 of an inch in the direction of movement of the paper and is ‘1.5 inches at right angles to the direction of movement of the paper. The paper sections observed by adjacent photo electric cells overlap so that the row of photoelectric cells views and inspects the entire width of the paper sheet. The paper is ?ooded by strong light, for example by two D.C. ?uorescent lamps placed close to the inspect ed section of the paper. The axes of such tube shaped lamps are arranged parallel to the line of photoelectric cells. The said photoelectric cells are enclosed in and supported by a light-tight housing the lower side of Which is provided with an aperture forming structure. The con struction and arrangement of the aperture is such that only the di?used light re?ected from the inspected paper can reach the photoelectric cells. The ?uorescent lamps are also surrounded by a housing which serves as a light shield against external illumination and light intensity changes of the inspected paper section due .to changes in external illumination which could simulate defects and cause rejection ‘of non-defective sheets. One or more of the photoelectric cells, for example a group of 10 cells, feed a common cathode follower in adder type circuit arrangement. Light intensity changes due to defects in the paper modulate the output current of one or more photoelectric cells of the group. Such‘ 3 modulation appears as a“ voltage variation '(defect signal) at the output of the common cathode follower stage. Such defect signal is ampli?ed in a constant-gain amph ?er, the output of which operates via two channels a pulse producing stage, one for example, a Schmitt trigger stage, which is well known in the art. The ?rst of the two said channels is a direct link be tween the ampli?er and the trigger stage and uses the signal amplitude levels, proportional to' the modulation of the photoelectric cells, for the initiation of the output pulse. The second channel consists of one or more integrating stages the output of which can build up to a critical voltage level which can also initiate the trigger action, The two said channels are used for the detection of faint defects in the paper. When a faint defect extends at right angles to the direction of movement of the paper, the ?rst channel is effective because the percentage of modulation of the photoelectric cell increases with the size of the defect and a signal is produced which is above the noise level of the paper. When a faint defect on the paper is in the direction of movement of the paper, the percentage of modulation of the photoelectric cell is low and substantially constant and is not proportional to the extent of the defect. Such ‘a weak unidirectional signal can be built up to the re quired voltage level by the integrating stage of the second channel which will operate the trigger stage, even though such unidirectional signal is of the order of the paper background noise.
In one embodiment of the inven tion the noise signal being of alternating polarity does not raise the output of the integrating stage. Several integrating stages with various time constants can be used to respond to defects of various sizes and intensities. After a required delay, the output trigger pulse operates the grading switch by means of relays and actuators, the grading-switch de?ecting the cut and defective sheets into a second grade compartment. The required delay between the instant a defect is detected and the instant the grading-switch must operate is determined by the construction of the paper cutting machine. Having passed the inspection roller, the paper sheet usually travels be tween draW-rolls, past a circular cutter and then by means of carrier tapes moves to the grading-switch. Thus the delay is de?ned by the characteristics of the par ticular machine to which the photoelectric inspecting means and the sorting apparatus is attached. Further, the percentage modulation of the photo electric cells associated with one ampli?er is low when a full-black 1/32" diameter defect passes under the inspec tion head. Interfering disturbances to the system which can modulate the photoelectric cell currents by only a fraction of one percent must therefore be avoided. The embodiments of my invention as hereinafter dis closed serve the elimination of the said disturbances and enhance the reliability of the subject inspection or sort ing machine. In particular there are two critical require ments of the inspection head and the surface above which the inspection takes place which must be satis?ed in order to avoid the production of interfering signals which can simulate defects which are not present on the sheet material inspected. , When inspection takes place above the inspection roller, esscentricity of the roller increases or decreases the dif fused re?ected light reaching the photoelectric cells throughthe aforementioned aperture. When ?uorescent lamps are employed, such lamps can ?icker or oscillate, even when the DC. supply potentials are highly regulated. In order to substantially reduce modulation of the photoelectric cells due to the said effects, one or more cells are connected to one common load resistor or im pedance and the same number of photoelectric cells (one or more) are connected to another common load re sistor or impedance. The value of the load resistors are equal. The two sets of photoelectric cells are matched 10 15 20 25 30 35 40 45 50 55 60 65 70 75 4 to give substantially equal output. Ideal matching can be accomplished by slightly adjusting the width of the aperture associated with one. set of cells relative to the width of the aperture associated with the second set of cells, so that the voltages developed across the two said load resistors due to the currents in the photoelectric cells are equal. Such load resistors are then connected through cathode or anode followers to the two control grids of asymmetrical push-pull ampli?er. Alternately, the resistance values of the load resistors (or impedance values when load net-works are used consisting of re sistors, capacitors and coils) at the input or output of i the cathode or anode‘ followers, can be adjusted‘relative to each other to give identical in-phase output voltages. The amplifying stages of the push-pull ampli?er con sists preferably of cathode coupled, longtailed pairs of’ amplifying means which are well known in the art. In such ampli?ers in-phase signals simultaneously reaching the two said control grids are not ampli?ed, while the signal reaching only one of the two control grids or anti-phase signals reaching both grids are ampli?ed. Sig nals due to the eccentricity of the inspection roller or due to intensity changes of the lamps are in-phase signals and are not ampli?ed. ' Signals due to defects of the sheet material are not in-phase signals and therefore are ampli?ed.