US3051841A - Printing and photography - Google Patents

Description

Aug- 28, 1962 J. F. cRosFlELD ETAL 3,051,841

PRINTING AND PHOTOGRAPHY 3 Sheets-Sheet 1 Filed NOV. 25. 1957 mddoml/ A torne y Aug. 28, 1962 J. F. cRosFlELD ETAL 3,051,841

PRINTING AND PHOTOGRAPHY Filed Nov. 25, 1957 f 5 Sheets-Sheet 2 sPon. DETECTOR A tem e y Aug. 2s, 1962 PRINTING AND PHOTOGRAPHY 5 Sheets-SheecI 3 Filed Nov. 25, 1957 SUESLY GATE LIINTERGRTOR r A 2/ 90 `94 l/ I H-E REsET PHlsING GATE SWITCH swlTcH L l -I DIFFERENCE W A AMPLlFlER k GATE D.C. RESTORER INTERGRATOR AMPLIFIER..4

PHOTOMULTIFLIER United States Patent Gilice 3,051,841 Patented Aug. 28, 1962 3,051,841 PRINTING AND PHTGRAPHY `ohn Fothergill Cros'iield, Gordon Stanley James Allen, and Peter Brimelow, London, and Peter Ralph Price, Longhton, Essex, England, assignors, by direct and mesne assignments, to l. F. Crosiield Limited, London, England, a British company, George Edmond Brackenbury Abell, London, England, and Peter Ralph Price and Henry Loveridge Chadder, Loughton, Essex, England Filed Nov. 25, 1957, Ser. No. 698,665 Claims priority, application Great Britain Nov. 28, 1956 12 Claims. (Cl. Z50-220) This invention relates to a method and apparatus for checking the quality or uniformity of a number of twodimensional visual representations in the form of reproductions of an original. The reproductions may be formed in any manner, for example they may be obtained photographically of by the deposition of ink.

At present in the high quality printing industry, trained ystati check the quality of reproductions by visual examination. A disadvantage is that in large-scale production this may necessitate the employment of large numbers of examiners, and a further drawback is that the standards of the examiners are different and the standard set by any one examiner varies from time to time. The object of the present invention is to enable the quality of the reproduction to be checked without the need for a staff of trained observers and without the disadvantages arising from the personal equations of the examiners.

The invention consists in a method of checking of the purpose of controlling the manufacture of reproduced copies, in which two visual representations of the same subject, one of which may be a standard copy or an original, are scanned simultaneously and in synchronism by means of systems including light-sensitive devices to produce two electric signals corresponding to the light reflected from or transmitted through the scanned elements of the representations. These two signals are used to provide an indication of the ditference between corresponding scanned elements of the two representations, or to bring into operation auxiliary apparatus, for example, a marking mechanism, which marks the faulty reproductions when the difference exceeds a given amount.

The invention also provides apparatus comprising a scanning system whereby the two visual representations can be scanned simultaneously and in synchronism, the scanning system including two light-sensitive devices arranged to receive light from the scanning beams after reflection by, or transmission through, the scanned elements of the two representations respectively. The apparatus further comprises means wherein the output signals from the two light-sensitive devices are used t provide an electric signal represe-nting at any moment the difference between the intensities of the light received by the -two light-sensitive devices, and means for providing an indication of the diiference, or if the difference exceeds a predetermined value, for bringing into operation the auxiliary apparatus.

In an alternative form of the apparatus the output from the light-sensitive device associated with one of the representations is used to control the intensity of the light source which is the origin of the two scanning beams, in such a manner that the output of this light-sensitive device is maintained substantially constant. If the second representation is a good copy of the rst, the output of the light-sensitive device associated with the second representation will also be constant.

The scanning Asource is preferably a cathode ray tube.

One or both of the representations which are compared may be moving with respect to the scanning system, for

example, on `a moving web or sheet of paper. Furthermore two of the reproduced copies which require to be checked may be used as the two visual representations which are scanned land compared. While this arrangement will be less accurate in detecting variations from a standard than the method in which a single reproduced copy is compared with a standard or the original, it is found in practice that in a `suitable system it will show up nearly all the faults which occur, for example, in a system in which the compared images Iare printed by different printing plates arranged side by side across the plate cylinder. It has the -advantage that it is much simpler to build because the images printed on a web or sheet can be arranged to be in register with one another by careful lay-down of the printing plates yon the plate cylinder, thereby `facilitating synchronised scanning of the two images.

If desired, more than two representations, each associated with a light-sensitive device, can be scanned in synchronism, the output signals from the light-sensitive devices being compared to provide an indication of faults in any of the compared representations.

In order that the invention may bel better understood two embodiments thereof will now be described, by Way of example, with reference to the accompanying drawings. In these embodiments two adjacent printed images on a web or on a sheet are checked against one another. In the drawings:

FIGURES l and 2 show diagrammatically apparatus for comparing two adjacent images on a moving printed web;

FIGURE 3 shows the form of the inking mechanism used in the apparatus of FIGURES l and 2;

FIGURE 4 is a circuit diagram of an integrator unit which may be used with the system of FIGURES 1 and 2;

FIGURE 5 shows diagrammatically the application of the invention to a sheet feed system, in which the sheets are printed on both sides, and

FIGURE 6 shows a modiiied form of control for the apparatus.

In FIGURE 1 a web 6 having printed thereon a succession of transverse rolws of reproductions of the same subject, each row consisting of two such reproductions arranged side by side, passes over rollers S and lti and under a white lbacking plate i2. As each pair of printed images pass under the backing plate 12 they are separately scanned by light beams originating at a common light spot on the face of a cathode ray tube ld. Two separate optical systems arranged one behind the other `and represented in FIGURE l by the lens 16 and mirror 17 are used to focus the light beam from the cathode ray tube on to the two printed areas. The light spot on the face of the cathode ray tube is caused to Scan repeatedly along 4a single line by the application of line-scan voltages to the deflection coil I8 from a time base system 2li controlled by a square-wave generator 21, the line-scan on the web 6 in FIGURE 1 taking place in a direction perpendicular to the palne of the paper. The movement of the web causes the successive scanning lines on the printed areas to be progressively displaced in the direction of movement of the web, so that a scanning raster is built up on each of the two adjacent printed areas ,as they pass under the backing plate l2. In practice, the cathode ray tube is also provided with frame-scan voltages, but these cause only a slow movement of the scanning line over the `screen in order to utilise the whole of the phosphor coating. The frame-scan voltages are not in themselves responsible for the longitudinal movement of the scanning spots over the two images, this being caused by the movement of the web.

The coil 22, on the cathode ray tube represents the 3a focusing coil and is connected to a focusing control circuit 24. The grid or cathode of the cathode ray tube is pulsed at a high frequency from an HF. pulsing unit 26, the output of which is combined with that of the square-wave generator in a mixer circuit 23. The purpose of the I-LF. pulsing unit is to ensure a more efiicient utilisation of the phosphor coating on the screen of the cathode ray tube which is activated for only half the time of operation of the system. The square-wave genereator 21 blacks out the screen during the line ily-back.

Two light-collecting devices 28 are arranged one behind the other beneath the web to prevent scattering of the light reflected from the two adjacent printed areas respectively only one of these devices being seen in FIG- URE l. Two photomultipliers 30 and 3l, also arranged one behind the other, look into the two light-collecting devices respectively, and receive the light reflected from the two printed areas. The signals from these two photomultipliers are applied over conductors 32 and 34 to two amplifiers 36 and 38 and through D.C. restorers 37 and 39, which clamp the black levels of the signals, to a common difference amplifier 40, the output of which is applied by way of a conductor i2 to an inking device 44 (FIGURE 2). Details of the inking device are shown in FIGURE 3. When the signal from the dierence amplifier is sufficiently great the magnetic coil (i6 is energised to a degree sulhcient to attract the armature 48, causing the pen 50 to move against the margin of the travelling web 6. Any convenient marking system can be used to distinguish the row containing the faulty image from the remainder of the images on the web.

The web after passing the inking device passes through the drive rolls 52 and below a web cutter 54 which separates each row of printed images from the remainder of the web. The leading edge of the separated portion of the web is gripped by the fingers of a gripper chain 56 which draws the separated portion along beneath a spoil detector 58 consisting of a lamp and photocell arranged above the margin of the printed areas. When an ink mank is detected in the margin the output of the photocell, after amplification, is used to cause the fingers of the gripper chain which engage the faulty portion of the web to release the prints above the reject bin 601. If there is no ink mark in the margin the separated portion of the web continues until it reaches a point above the normal delivery bin 62, at which point the fingers of the gripper chain open to release the separated portion of the web.

Each photomultiplier circuit channel is provided with a kind of automatic gain control system to ensure that the passage of identical copies of the printed material will produce signals of substantially equal amplitudes and to reduce the effect of variations in factors unrelated to the transmittance of the scanned images, To achieve this result the output signals from the amplifiers 36 and 38 are applied to circuits 64 and 66 each of which consists of a peak rectifier and comparator circuit. rIlhe circuits `64 and `66 also receive a DLC. voltage from an adjustable device 68 which may be a potentiometer. The D.C. voltage from the device 68 is compared with the peak rectified signal in each channel, which will be the white signal obtained when the margin of the printed area is scanned. The `output of the circuit 64 represents the differenjce between this peak signal and the voltage derived from the device 68 and is applied to a gain control amplifier 70 in which a varia-ble voltage is added to the volt-age generated by a power unit '72, the resultant voltiage constituting the power supply for the photomultiplier 30. The circuit 70 may consist of a cathode follower the cathode load of which is in series with the power supply unit 72 as far as the photornultiplier is concerned, the grid of the cathode follower receiving the output of the circuit 64. Thus if the amplitude of the signals obtained `from the margins of the printed areas decreases owing to `ageing of the photomultiplier a greater voltage will be added by the control amplifier 70, resulting in an increase in the voltages applied to the photornultiplier 30. In a `similar manner the output of the circuit 66 represents the difference between the peak rectified voltage in the second photomultiplier channel and the voltage derived from the device 68, this difference signal being used to control the output of an amplifier M- by means of which the supply voltages for the second photomultiplier are adjusted.

In the system which has been described the two printed areas in each transverse now on the web are compared not with a standard copy but with each other. If therefore .thesame fault is present on each `of the two copies, for example if the ink supply to the plates responsible for printing the two rows was stopped during the printing, there will be no difference signal generated and consequently the inking mechanism will not be operated. To detect a fault of 'this kind the circuit shown in FIG- URE 4 can be used. The signal from the D.C. restorer 37 or 39 (FIGURE l) is applied to an integrator circuit '76 (FIGURE 4). An adjustable -device 78 provides a DiC. voltage which is applied to a second integrator 80, the device 7S being preset to give a voltage representing the average reflectance of the printed area. The `output signals from the two integrating circuits 76 and 80 are applied to two gate circuits 82 and 84 respectively which are periodically opened by an electronic yswitch 86 to -apply the two integrated signals to a difference amplifier 88. The output of the latter is used to operate the inking device shown in FIGURES 2 and 3, or a sepa-rate inking device.

The electronic switch 88 is controlled by a gate circuit 9i) which receives the signal from the D.C. restorer 37 or 39, by way of conductor 92, together with a signal from the square-wave `generator 26 by way of conductor 93. The gate circuit 90 is a coincidence gate which opens when `the amplifier signal from conductor 92. has a value above a given level, considering white level as minimum, and at the same time the signal from the square-wave generator is most positive, that is to say when the cathode lray tube is brightened. The gate circuit 90 thus opens for the dunation of the signals corresponding to a printed area but is close-d between linescans and also during the scanning of the white paper between successive printed areas. During scanning of the printed areas the signals from the gate circuit serve to charge a capacitor in the electronic switch 86, which capacitor prevents -a trigger circuit in the latter from operating. If there are no signals from the gate circuit for a period corresponding to, for example, two linescanS, the capacitor discharges sufficiently to allow the operati-on of the trigger circuit. This `opens the gates 82 and 84, as described above, and at the same time operates a reset switch 94 which discharges the two integrating circuits to return them lto their zero condition. Thus, the apparatus described integrates the signals .obtained from one of the scanners during the scanning of the whole of a printed area and compares the integrated value at vthe end of the raster, with a preset value. When the difference is greater than the predetermined value the inking mechanism is operated causing the row of printed areas to be rejected.

The reason for integrating a standard voltage from the device 78, instead of comparing the output of the integrator 76 directly with a standard voltage, is to enable the 'apparatus to be used :at different press speeds.

If the web 6 is printed on both sides, a second scanning system employing a further cathode r-ay tube and two optical systems and photomultipliers, can be placed above the web, the output signals from the two photomultipliers being 4led to a further difference amplit'er.

The apparatus which has been described in connection with a web feed system is also applicable to a sheet feed system. FIGURE 5 shows diagrammatically a system for use with sheets which are printed on both sides, each sheet having on each side two similar images arranged .side by side. The sheets are fed on to a conventional Istream feed device 96, which may comprise a number of travelling tapes. When a sheet reaches the end of the stream feed device it is gripped by the fingers of gripper devices on a first cylinder 98, round which it passes in the direction of the arrow. As it passes between the cylinder 9S .and 4a cylinder 100 it is released by the gripping lingers of the cylinder 93 and simultaneously gripped by the fingers of the cylinder 100, around which it passes in a clockwise direction. As it passes round the cylinder lili) each of the two printed areas on the sheet is scanned by a light spot originating from 'a common cathode ray tube 102, on the face of which a yline-scan is generated in lthe manner indicated in FIGURE l, the light from the cathode ray tube being `focused .on the two printed areas by two optical systems arranged one behind the other and represented in FIGURE 5 by the lens 104. The direction oi the line-scan on the travelling sheet on the cylinder 10d in FIGURE 5 is perpendicular to the plane of the paper. The movement of the sheet `causes the line-scan to move over the paper, thereby forming a scanning raster on the latter. Photomultiplier and light-collecting `devices are arranged in a manner similar to that of FlGURE l to collect the light reflected from the two printed areas, and the circuits which receive the output signals from the two photcmultipliers are also. similar to those shown in FIGURE l. The printed sheet continues to rotate with the cylinder 19h until it reaches the point between this cylinder and a cylinder 1%, at which it is released by the gripping fingers of the cylinder lili) and gripped by the lingers of the cylinder 106, which rotates in an anticlockwise direction. The effect of this transference is that 'the side of the sheet which was adjacent to the cylinder 160 is now the outer side, and vice Versa. The sheet now passes under a second vscanning system represented in FGURE 5 by .the cathode ray tube 10S and the lens 110, which scans the reverse of the sheet. This scanning system includes two photomultipliers and two light-collecting devices, and is associated with a set `of circuits similar to those shown in FIGURE yl. If the difference signal from either `of the difference amplifiers associated with the tubes 102 and 10S is too great, the inking device will be caused to mark the defective sheets.

The sheet continues to rotate with the cylinder 106 until it is released by the gripping fingers of .the latter at a point where the periphery of the cylinder 166 meets the periphery of a sprocket drive [112, at which point it is seized by gripping fingers mounted on bars between two chains 114 which pass round the sprockets. The printed sheet then passes round the sprockets 112 and 'along the lower run of the gripper chain 114. The margin of the [sheet is examined by .a spoil detector device 116 consisting of a lamp and a photocell, which causes the grip-ping lingers to release the sheet when a mark is detected in its margin. The sheet then falls into .a reject container 1113. If there are no such marks the sheet continues to move with the gripper chain until it reaches the normal delivery container 126 at which point it is released by the gripping fingers.

The systems which have been described with reference to FIGURES l5 have been suitable for scanning two printed areas arranged side by side on .a web or sheet. It will be clear, however, that more than two printed areas can be scanned simultaneously and compared, and that the areas which .are scanned need not be transversely displaced but can be displaced in the direction of movement of the web .or sheet. However, om the point of View of fault detection it is preferable to compare printed areas arranged transversely to the movement of .the web or sheet. As an example of the simultaneous comparison of more than two adjacent areas or sheets, a web having printed thereon three adjacent images could be used, the three images being scanned simultaneously by light beams derived from a common cathode ray tube. Three photoparts of the prints.

liaults in more than one parameter of the print.

multipliers would be provided to receive the light reflected from the three images, and the signals from these would be compared in pairs, or alternatively the signal corresponding to each outside image could be compared wi-th that `represen-ting the centre image.

If `one .or more images on a moving web are to be compared with a standard copy, a length of paper on which are printed .a number of standard copies can be wrapped around a cylinder the circumference of which is equal to the distance between an integral number of prints `on .the web. The cylinder carrying the standard `copies is driven from the same source as the web driving rolls, the gearing between .this cylinder and the driving source being such that the circumferential speed of the cylinder is equal to the linear speed of the web. Preferably the paper bearing the standard copies is printed from the plate cylinder which prints the web, and has a length equal to the circumference of the plate cylinder. It can then be `arranged that each image on the web is compared with a standard copy which has been printed by the same printing plate. This also removes difliculties due to uneven spacing `of the plates on the plate cylinder.

A difficulty which is experienced is that of obtaining uniform brightness over 4t-he whole `of the elemental area illuminated by the light spot at any given moment, the term elemental area being intended to mean the area |adopted as the smallest resoluble area of the represent tion for the purpose of the comparison. To overcome this difficulty a smaller light spot can be used, .the smaller light spot being `made to scan each elemental area before it passes on to the next. Thus, :the spot can be caused to wobble in such a manner that it :scans over the Whole area of each `elemental :area or it can be given a rapid twodimensional scan7 at a size suiiicient to cover `one elementar area, superimposed on the main scan. The output signal from the photomultiplier is -then integrated over the period corresponding to the 4scanning of each elemental area.

An `alternative arrangement of .the apparatus which does not use a difference -ampliiier is shown diagrammatically in FIGURE 6. rl'he output signal from the photomultiplier 122 associated with a stmdard copy 124 is fed back through a high-gain amplifier i126 to the grid of the `cathode ray tube 128. 'Ilhis has the eilect of producing on the face of :the latter a light spot, the intensity of which varies inversely with the reflection characteristics `of the picture, so that the value of light picked up by the photomultiplier `122 associated with the standard copy is maintained approximately constant, the departure from the lconstant value being just sufiicient to provide the required output signal from the feed-back amplifier for opposing the absorption variation. If the print 130 to be checked is correct, its refletcion characteristics cancel out the light variations on the face yof the cathode ray tube 128, and there is no variation in the output -of the second photomultiplier 132. If .the tone or colour of the print on the web is incorrect, the photomultiplier 132. produces a signal which shows the diflerence between the scanned elements of the two representa-tions.

A scanning raster is formed on the face of the cathode ray tube insead of a single scanning line if the representations to be scanned `are stationary. If the opacity of the representation is not high and remains fairly constant, transmitted light can be picked up by the light-integrating units instead of reflected light.

The system may also be used to give an indication of As an example, in the case of multicolour printing, the scanning of the print can be elected through a number of different filters and in this way faults in any particular colour can `be differentiated from faults in other colours.

In some cases it may be desirable to provide one or more gating devices for rendering the comparison system inoperative during the :scanning of a selected part or If, for example, the images are numbered serially the portions of the images bearing the `successive numbers should not be compared. To operate such a gating system, a mark can be printed in the margin in line with the printed number, and a photoelectric system responsive to the marginal marks can be arranged to actuate the gating device after a suitable delay.

It will be evident that the apparatus which has been described can be made extremely sensitive and will provide a uniform standard of rejection of faulty prints.

We claim:

l. A method of checking for the purpose of controlling the manufacture of reproduced two-dimensional reprresentations of a subject comprising checking against each other two such represent-ations of the same subject Iformed on a single record material member to determine dilerences arising from Afaults in either of said two representations including the steps of simultaneously scanning in two mutually perpendicular directions successive corresponding elemental areas o-f the two representations with light, sensing the light received from the scanned elemental areas to produce two independent electric signals by continuously collecting, for each representation, light which is not absorbed by the second element of that representation and applying it to a photoelectric device, comparing the two signals to obtain an indication of the diterence between the corresponding elemental areas of the two representations.

2. Apparatus for checking reproduced two-dimensional vsual representations of a subject comprising light generating means in the form of at least one cathode ray tube for producing two light beams for scanning two such representations of the same subject, both requiring to be checked and provided `on a single record material member, means for deiiecting said light beams to scan identical areas of said two representations simultaneously Iand in synchronism, element by element, each representation being scanned in two mutually perpendicular directions by the corresponding light beam, two photoelectric devices and two light-collecting devices arranged so that that part of the light `from each scanning beam which is not absorbed by the scanned elements of the corresponding representation is continuously collected and applied to that `one or said photoelectric devices which is associated with Ithat representation, and means responsive to the output signals from said two photoelectric devices to provide an indication of the difference between the intensities of the light received by the two photoelectric devices.

3. Apparatus according to claim 2 in which the scanning light spot is small compared with an elemental area of the scanned visual representation, and in which the.

said light spot scans each elemental area in two mutual- -ly perpendicular directions before passing to the next elemental area.

4. Apparatus according to claimv 2 for use when said pair of representations are arranged side by side on a moving record material member, in which said deflecting means ydeflects each of said light beams to scan repeatedly `over substantially the same line in the plane of the record material member during the passage of the pair of representations, the said line extending transversely to the direction of movement of the representations.

5. Apparatus for checking reproduced two-dimensional representations of a subject comprising a cathode ray tube and means ttor producing a scanning light spot on the face of said tube, optical means for splitting light from said light spot -into two beams for scanning two representations of the same subject, reflection means for continuously scanning said light beams over identical Iareas or" said two representations simultaneously and in synchronism, two photoelectric devices and two lightcollecting devices arranged so that that part of the light from each scanning beam which is not absorbed by the scanned elements of the corresponding representation is continuously collected `and `applied to that one of said photoelectric `devices which is associated with that representation, and means responsive to the output signals ffrom the two photoelectric devices to provide an indication of the difference between the intensities of the light received by said two photoelectric devices.

6. Apparatus for checking reproduced two-dimensional representations of a subject comprising light generating means including at least one cathode r-ay tube for producing two light beams for scanning two representations of the same subject, means for deecting said light beams to scan said two representations simultaneously and in synchronism, element by element, each representation being scanned in two mutually perpendicular directions by the corresponding light beam, two photomultiplier idevices arranged to receive that part of the `light from the scanned beams which is not absorbed by the scanned elements of the two representations, respectively, two ampliiiers receiving output signals from said photomultiplier devices, and means controlled by the -output signals from each of .the ampliers for autotmatically controlling the gain of a respective photomultiplier, whereby the operation of the scanning systems is rendered less subject to variations in factors unrelated to the density of the scanned elements of the two representations, and means responsive to the output signals from said amplifiersV to provide an indication of the difference between the densities of the scanned elements of the representations.

7. Apparatus for checking repro-duced two-dimensional visual representations of a subject comprising means for producing two light beams for scanning two such representations of the same subject, both requiring to be checked and provided on a single record material member, means for `deflecting said light beams to scan said two representa-tions simultaneously Vand in synchronism, element by element, two photomultipliers arranged to receive that part of the light from said scanning beams which is not absorbed by the scanned elements of said two representations, respectively, means responsive to the output signals from said two photomultipliers to provide an indication o-f the difference between the intensities of the light received by the two photomultipliers from corresponding elements of the two representations, and means controlled by selected output signals Vfrom each of the photomultipliers for automatically controlling the gain of the respective photomultipliers whereby the operation of the scanning system and the associated circuits is rendered less subject to variations in -factors unrelated to the density of the scanned elements of the two representations.

8. Apparatus yaccording :to claiml 7 in which the means for automatically controlling the gain includes a peak rectifier circuit coupled to the output of each photomultiplier and a comparator circuit in which the peak rectilied signal is compared with a preset value, the difference being used to modify the operation of the photomultiplier.

9. Apparatus according to claim 8, in which yan electric signal representing the diiference between the peak rectified and the preset value controls the amplitude of the supply voltages `applied to a photomultiplier constituting lthe light-sensitive device.

10. Apparatus for checking reproduced two-dimensional visual representations of a subject comprising means for producing two light beams for scanning two such representations of the same subject, both requiring to be checked and provided on a single record material member, means for deflecting said light .beams to scan said two representations simultaneously and in synchronism, element by element, two photomultipliers arranged to receive that part ofthe light from said scanning beams which is not absorbed by the scanned elements or said two representations, respectively, means responsive to the output signals from said two photomultipliers to provide an indication of the difference between the intensi- .'ties of the light received by the two photomultipliers from corresponding elements of the two representations, yand `an integrating circuit coupled to the output of one of said photomultipliers and a comparator circuit coupled to the output of the integrator circuit for comparing its output voltage with a predetermined voltage.

11. Apparatus Iaccording t-o claim 10, comprising an adjustable electrical device, land an integrating circuit coupled to said adjustable electrical device, the output of said integrating circuit constituting said predetermined voltage.

12. Apparatus for checking reproduced two-dimensional visual representations of a subject comprising a light source, means for deriving two light beams from said light source for scanning two representations of the same subject, both requiring to be checked and provided on a single record material member, means for deflecting said light beams to scan said two representations simultaneously and in synchronism, element by element, two photoelectric devices arranged to receive lthat part of the light from said .scanning beams which is not absorbed by the scanned elements of said two representations, respectively, modulating means for said light 10 source coupled to the output of one of said photoelectric devices, whereby the output of said photoelectric device is maintained substantially constant and the variations in the output of the other photoelectric device represent the difference between corresponding scanned elemental areas of said two representations.

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