GB2340943A

GB2340943A - Sheet material inspection apparatus and methods

Info

Publication number: GB2340943A
Application number: GB9825716A
Authority: GB
Inventors: Martin John Webb; Frank Thomas Dawson
Original assignee: BANK OF ENGLAND GOVERNOR A; Governor and Co of Bank of England
Current assignee: BANK OF ENGLAND GOVERNOR A; Governor and Co of Bank of England
Priority date: 1998-08-28
Filing date: 1998-11-25
Publication date: 2000-03-01
Anticipated expiration: 2018-11-25
Also published as: GB9825716D0; GB2340943B

Abstract

A printed sheet inspection apparatus is described which possesses a mounting 52 for locating a detector 54 above the sheet material path. The mounting comprises a thin elongate resilient member, one end of which is attached to a fixed point 50 above the path, extending therefrom in the direction of movement of the sheet material. The other end lightly contacts the sheet material and comprises a point of attachment 60 for a detector. The mounting ensures that a constant gap is maintained between the detector and the sheet and reduces any rippling effect present as the sheet moves beneath it. Methods for validating the properties of the magnetic strip in or on the sheet material, and for quality control of the printed sheet are also disclosed.

Description

2340943 Title: Improvements in and relating to sheet material inspection

apparatus and methods

Field of invention

This invention concerns apparatus and methods for inspecting printed sheet material, and the invention is particularly applicable to inspecting sheet material on which bank notes, gift vouchers, postal orders, bank cheques, passports, labels for high value goods, lottery tickets and the like have been printed. Such items are generally referred to herein as security documents.

Backqround to the invention Various techniques have been adopted to assist in validating security documents. One such technique involves the step of inserting during manufacture into a web of sheet material which is to be printed to form security documents, especially bank notes, a thin strip, or thread of material. The strip or thread is encoded with machine readable information as by a repetitive pattern of magnetisable regions along the length of the thread (as described in GB 2098768B). In the case of bank notes this thread can be seen by viewing the note in transmitted light although of course the magnetic regions remain concealed.

A continuous process is normally involved in the manufacture of a web of paper, and large rolls of paper containing such threads, accurately spaced apart across the width of the roll are produced. The subsequent web of paper is cut into sheets suitable for printing in a printing press. By accurately registering the sheets of paper in a printing machine, front and back faces of a security document such as a bank note can be printed on top and bottom surfaces of each sheet in correct 2 registry, not only with the sheet but also with the lines of thread, so that one thread extends across each resulting security document.

Since the printing plates employed in such processes are of finite size, the process for producing banknotes, is arranged to print the sheets in a regular matrix or block containing 40 or so banknotes on each sheet. The sheets are stacked and can be inspected individually and either accepted as good or rejected as flawed before the sheets are guillotined into separate notes.

Historically the printing was checked by visual inspection of each sheet by specially trained staff. In recent years, using image analysis techniques, the print inspection process has been automated.

Whilst image analysis techniques permit the printing process to be checked, they do not allow the magnetic properties of the thread to be checked and validated. For example it is valuable to be able to check that it is continuous across each bank note impression, that the code is the correct code for the denomination of note under inspection, and/or that the code runs in tAl-Le correct direction across the note, etc.

To this end a checking method has been proposed in which the sheet material containing the thread is moved relative to a magnetising device upstream of a high resolution magnetic field detector or reading head, from which electrical signals are derived as the magnetised thread passes below the reading head. The latter responds to the magnetic field associated with the residual magnetism in the magnetic regions of the thread after it has left the influence of the magnetising device. A checking device containing such a read head, and method as aforesaid is described in our co-pending UK Patent Application published under Serial No. 2316521, which contains details of the reading head and signal processing circuits required to i 3 allow the proposed validation method for such magnetisable thread to be performed. Where a magnetisable thread is included in sheet material from which other security documents are formed, a similar procedure can be adopted as described above for bank notes. A read head, especially one containing a magneto-resistive sensor, is described in UK No. 2316521 and will be referred to as a "read head of the type described".

other validating techniques such as including lines of special printing, the use of specially responsive or absorbing inks, reflecting material in or on the sheet material and many other devices have been proposed to be incorporated as security document validation devices. The invention can be used to advantage in connection with the inspection and checking of any such devices.

Problem with nrior art vroi)osal The movement of sheet material along a transporter such as a vacuum conveyor has been found to produce rippling or uncontrolled movement of the sheet material. Where this occurs variation occurs in the gap between the sheet material and any detector. Where this is a magnetic read head of the type described in UK Patent Specification No. 2316521, this results in errors occurring in the checking of the magnetic thread.

Different but equally unwanted errors will arise if some other security feature is being checked by another type of detector.

The problem therefore exists that printed sheets containing a security validating feature, (such as magnetisable thread) cannot be checked reliably for the feature during the high speed passage of the sheets through existing print checking machines.

The problem is of particular relevance to the checking of a magnetisable thread extending through cut sheets of paper on which blocks of security documents as aforesaid have been 4 printed, after the sheets have been print checked and before being guillotined into separate documents.

Either a separate lower speed checking process of the printed sheets is required, or alternatively the security feature (such as the thread) has to be checked after the sheet material has been guillotined into the numerous separate documents, such as bank notes or the like, which is time consuming.

Object of the invention It is a primary object of the invention to provide for mounting a detector (such as a thread checking device) to allow a reliable and reproducible checking process to be performed on moving sheet material.

It is another object to provide an improved apparatus for checking and validating a security feature such as magnetisable threads or other information incorporated in security documents printed onto sheet material containing the feature.

It is a further object to enable such checking to be performed without interrupting the production process.

It is a further object to enable the checking to occur in such a way and at such a point in the process as to allow flawed material to be identified and discarded with minimal interruption to the production process.

Summary of the invention

The invention allows any detector such as a read head of the type descrbed, but not limited thereto, to be mounted above a path along which printed sheet material passes at high speed, so as to enable a characteristic of the sheet material either printed therealong, or formed therein as by the inclusion of elongate strip material into the sheet material during its manufacture, to be checked on the fly. The invention therefore is suited to the checking of sheet material containing elongate security features, such as magnetisable thread.

Mountinq of the detector readinq head According to a first aspect of the invention, a detector such as a read head of the type described is attached to an end region of a thin elongate member, the other end of which is attached to a fixed point above a conveyor along which sheet material containing magnetic security thread to be inspected is to pass, the elongate member being mounted so as to lightly contact the upper surface of the conveyor and therefore any sheet material moving therealong, and the material of the elongate member possesses natural resilience which by virtue of its mounting to the fixed point, causes the elongate member to flatten the sheet material and reduce any rippling effect introduced into the sheets as they progress along the conveyor, the attachment of the detector to the free end region of the elongate member causing it to rise and fall therewith due to any ripple or irregularity which is not eliminated from the passage of the sheet material under inspection, so as to maintain a substantially constant gap between the detector and the surface of each sheet under- inspection.

A preferred point of attachment is the extreme downstream free end of the elongate member so that it is secured thereto at a paint along the length thereof which is furthest from the fixed point to which the elongate support member is attached.

A preferred form of elongate member comprises a strilD of springy metal.

Preferably the metal strip is bent intermediate the fixed point to which it is attached and its free downstream end, to form a curved underside surface between a first portion which extends in a generally vertical or near vertical manner from 6 the fixed point towards the conveyor and a second larger, flat portion which extends parallel to the run of the conveyor, the curvature forming with the conveyor a converging mouth for receiving the leading edge of each sheet travelling along the conveyor.

The fixed point is typically on a bridge which extends over the conveyor.

A plurality of such elongate members may be mounted at points regularly spaced apart across the width of the conveyor.

Preferably the spacing and positioning of the elongate members corresponds to the linear regions of the sheets which contain the security features which are to be checked, such as lines of magnetisable thread in the sheets, and a detector is mounted on each of the elongate members.

This aspect of the invention may be employed to mount a variety of types of detector not only of a magnetic nature but also optical and radiatIon detectors designed to dec-ect optical effects such as absorption or reflectance, or a response to electromagnet ic radiation such as radio waves or microwaves or to detect radioactivity, electrostatic, charge or fluorescence. Furthermore the mounting could be used to support a detector adapted to generate signals for supply to character recognition apparatus to enable number, bar-codes or text to be detected and read electronically.

A mounting embodying the invention may be employed for supporting a detector for inspecting a continuous run of webbased printed material or individual cut sheets of printed material.

Positioning-the detector According to another aspect of the present invention when 7 applied to the checking of individual cut sheets of printed material, a detector such as a thread checking read head of the type described is mounted so as to check the feature under investigation which extends through the blocks of security documents contained on the individual sheets, after the first guillotining and before the second guillotining in which the sheets are cut up to produce the individual security documents.

Thread checkinq method According to a further aspect of the invention, a method of checking the validity of magnetisable thread or strip in sheet material containing printed impressions of security documents thereon, comprises the steps of subjecting the sheet material to a magnetic field to magnetise the magnetic regions of the thread therein, producing relative movement between the sheet material and the magnetic field, and at a point remote from the influence of the magnetising field, sensing any residual magnetism in the thread by a high resolution magnetic field detector (read head) which generates electrical signals indicative of the said residual magnetism, processIng the electrical signals so produced to determine if the residual magnetic field pattern is correct for the document through which it extends, generating a warning signal if the signal processing indicates that the magnetic field pattern is incorrect or incomplete or non-existent, and generating a routing signal in response to the warning signal and thereby operating a sheet diverter to change the destination to which the printed sheet is to pass after the magnetic thread check, from a first destination reserved for sheets which have passed the magnetic thread check to an alternative destination for sheets which have failed the magnetic thread check.

Modifications to the above methods and aiDparatus The signal processing may provide signals indicative of the types of flaw detected in the parameter under test, such as the 8 security thread in the case of banknotes.

The apparatus may include sheet marking means, and the method may include the step of initiating the said marking means, to mark any sheet identified as containing a flaw, such as a flaw in either the security thread in the case of banknotes and/or in the printing on the sheet.

The method may involve the step of diverting flawed sheets to different destinations depending on the nature of the flaw in the parameter being checked.

The invention also lies in apparatus for performing any of the above methods.

Illustrated examvle The invention will now be described by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic side elevation of apparatus for checking printed sheets of bank notes, fitted with a magnetic field detecting read head of the type described in accordance with the invention, for checking the validity of magnetisable thread in the sheets, and

Figure 2 is an enlarged side view of part of the apparatus shown in Figure 1.

Detailed description of the drawings

In Figure 1 the apparatus comprises an elongate housing 10 within which is mounted a drive motor 12 for a conveyor 14 which passes around rollers such as 16, 18 and over rollers such as 20. Below longer horizontal upper runs of the conveyor are located suction boxes such as 22, 24, 26 and 28 and the conveyor is perforated so that printed sheets of bank notes are 9 held down in contact with the conveyor as they are moved thereby.

A stack of printed sheets awaiting checking is shown at 30 carried in a tray 32, and a sheet feeder (not shown) lifts the sheets from the top of the stack one by one and places them on the conveyor 14 where it extends between rows 34 and 1G. A further suction box 36 is located below this run of the conveyor to hold the sheets into contact with the conveyor.

A housing 38 comprising an inspection station and containing one or more high resolution television cameras (not shown) and signal processing circuit means (not shown) and a computer based control system (not shown), extends across the machine housing 10 above and below the run of the conveyor between rollers 16 and 20, and illumination means is provided (not shown) to enable images of the printed sheets to be formed in the cameras. The signal processing circuit means provide signals indicative of the imaged areas of printing and logic and/or signal processing circuits determine from pre-loaded data if the printed areas are flawed. The computer-based control system generates destination routing signals in synchronism with the conveyor movement so as to route each inspected sheet to one of three hoppers 40, 42 and 44. The routing signals are determined from the decision made by the logic and/or processing circuits as to whether any of the inspected printing on the sheet is flawed or not.

Sheets whose printed images pass the test applied by the control system to the processed camera signals, are routed to hopper 40 by momentarily rotating a conveyor sect-Lon 46 from a horizontal position into a downwardly inclined position by operating a motor 47, to deliver such sheets to the hopper 40.

If the sheet fails the camera check, the control system instead delivers the sheet to hopper 42 by rotating conveyor section 48 into a delivery mode by driving motor 49 instead of 47, at the appropriate time.

Each conveyor section 46, 48 etc is returned to its upper position as soon as it has delivered a sheet to its associated hopper. This allows sheets to pass over the lowerable conveyor sections such as 46, 48 etc to reach a later lowerable conveyor section if the routing signal requires the sheets to by hopper 40 (or 42) etc.

Beyond the camera inspection part of the machine, a bridge 50 spans the width of the machine and so extends across the conveyor 14. A plurality of springy metal strips, one of which is shown at 52, are attached to regularly spaced apart points across the bridge 50.

Each strip 52 is bent upwardly at both ends, the bend at the left hand end having a sharp right angle to provide a mount 4 Lng surface to which a high resolution magnetic field detecting read head of the type described 54 is attached. The bend at the other end (the right hand end as shown) being rather more gradual, but nevertheless continues until the metal strip is generally vertical, to enable it to be secured to the bridge 50.

The radius of curvature at the right hand end is greater than that at the left hand end to produce a smooth gently curved underside, and this smooth curved underside defines with the generally flat conveyor 14 a mouth 56 that converges in the direction of movement of the conveyor, and also therefore the movement of the sheets of bank notes, as denoted by arrow 58 in Figures 1 and 2.

As better seen in Figure 2, magnetic field detecting read head 54 is mounted to an upwardly bent left hand end 60 of the strip 52 and lightweight cables one of which is denoted by reference numerals 62 and 66 connect the output 64 of the read head 54 to the signal processing circuit means and control system in

1 the housing 38.

The control system generates a further routing signal if the magnetic detector signals denote an invalid, damaged or nonexistent IMT, so as to drop the third conveyor section 68 (by means of drive 69) and deliver the sheet to hopper 44, instead of hopper 40 or 42.

After a run of sheets, the three hoppers 40, 42 and 44 will have between them all the sheets which have been checked, those in 40 having passed both camera and thread inspections, those in 42 having failed the camera inspection test, and those in hopper 44 having failed the thread check by read head 54.

As described, sheets which fail both camera and thread checks 'led will be routed to hopper 42 as well as those which have fa-L only the camera check. A f ourth hopper (not shown) may be provided if desired, and logic circuits provided to generate a countermanding signal to nullify the operation of the motor 49 if a sheet has failed both tests, so as to route such sheets to the fourth hopper (not shown) instead.

Where each sheet contains for example eight rows and five columns of bank note impressions, so that each row contains five bank notes, and each bank note contains a magnetisable thread, five metal strips 52 are provided, spaced apart across the bridge 50, so that each strip registers with the position of a magnetisable thread in one of the five columns of bank notes.

However the invention is not limited to the provision of only just the number of strips corresponding to the number of read heads, and purely for flattening purposes additional metal strips may be mounted between and beyond those carrying the read heads. The additional strips do not need to have the upturns at their downstream free ends.

12 Where timing pulses are needed for the circuits associated with the read head, these may be obtained from the conveyor or from a sensor set to monitor movement of the sheets under -inspection so as to provide synchronising pulses and timing pulses indicative of the approach of sheets to the read head, and the speed of movement of a sheet past the head. A shaft encoder can be used to indicate speed of the conveyor and therefore the relative speed between the sheets and the read head.

A device, similar to item 52, could be mounted on any machine through which material is transported as individual sheets, such as a printing press.

A device similar to item 52 could be mounted on a machine adapted to handle a continuous web of material.

The invention is not limited to magnetic detectors and the detector adapted to respond to a property such as optical absorption (eg colour) or reflectance; fluorescence; electrostatic charge; radioactive emissions; and radio frequency emissions or absorption, could be mounted on a device such as 52 for detecting the appropriate property in sheet material passing therebelow.

1 1 13

Claims

1. A mounting for locating a detector above a path along which sheet material passes at high speed comprising a thin elongate I J ence one end of which is attached member having a natural res-LLto a fixed point above the said path to extend therefrom in the direction of movement of sheet material along the path so that its other end will lightly contact the upper surface of the path and therefore any sheet material moving therealong, said other end of the elongate member comprising a point of attachment for the detector, the natural resilience of the elongate member maintaining contact between it and the sheet material moving thereunder to flatten the sheet material and reduce any rippling effect introduced therein as it progresses along the said path, whilst permitting the free end of the elongate member to rise and fall due to any ripple or irregularity which has not been eliminated, and wherein the detector is attached to the elongate member so that a substantially constant small gap exists between it and the sheet material passing below the end of the elcngate member.

2. A mounting as claimed in claim 1, wherein the point of detector attachment is the extreme downstream free end of the elongate member so that the detector is secured thereto at a point along the length thereof which is furthest from the fixed point to which it is attached.

3. A mounting as claimed in claim 1 or 2, wherein the elongate member comprises a strip of springy metal.

4. A mounting as claimed in claim 3, wherein the metal strip is bent intermediate its two ends to form a curved underside surface between a first portion which extends in a generally vertical or near vertical manner from the fixed point towards '-he said path, and a second larger flat portion, which extends 14 parallel to the path, the underside thereof forming with the path a converging mouth for receiving the leading edge of a sheet travelling along the path.

5. A mounting as claimed in any of claims 1 to 4, wherein the fixed point is on a bridge which extends over the path.

6. A mounting as claimed in any of claims I to 5, comprising a plurality of elongate members mounted at points spaced apart along the bridge across the width of the path.

7. A mounting as claimed in claim 6, wherein the positioning of at least some of the elongate members corresponds to spaced apart linear regions of sheets which contain elongate security features which extend therealong parallel to the direction of movement along the path, and a detector is mounted at the end of each of at least some of the elongate members, for interrogating the elongate security features extending therebelow.

8. Sheet checking apparatus having mounted thereon a plurality of detectors each carried by a mounting as claimed in any of claims 1 to 7.

9. Sheet checking apparatus as claimed in claim 8, for checking elongate security features which extend along each sheet parallel to the direction of movement along the path, wherein each detector is adapted to produce an electrical signal in response to relative movement between it and the security feature in each sheet passing therebelow.

10. Sheet checking apparatus as claimed in claim 9, wherein each security feature has magnetic properties and the detectors are sensitive to magnetic f-lux associated therewith.

11. Sheet checking apparatus as claimed in claim 9, wherein the security feature is printed on the sheet material and the detectors respond to absorbtion of light by or reflection of light from, the security printing on the sheets.

12. Sheet checking apparatus as claimed in claim 9, wherein the security feature has a property such that it emits or 1 absorbs electromagnetic radiation, or is radioactive, or is -rost capable of supporting an elect t_atic charge, or fluoresces on appropriate stimulation, and the detectors are adapted to respond to the particular property of the security feature which is to be checked.

13. Sheet checking apparatus as.claimed in claim 9, wherein the detectors are adapted to respond to printed characters on the surface of the sheet material and generate signals corresponding thereto for supply to character recognition apparatus, or bar code decoding apparatus, to enable the characters or bar codes to be detected and read electronically.

14. Sheet checking apparatus as claimed in claim 9, wherein the sheet material is in the form of a continuous run of webbased printed material.

15. Sheet checking apparatus as claimed in claim 9, wherein the sheet material comprises a sequence of separate cut sheets.

16. Sheet checking apparatus as claimed in cla-1m 15, wherein the detectors are positioned along the sheet material path after a first guillotining stage but before a second guillotining stage.

17. A method of checking or validating the properties of an elongate magnetisable element contained in or on sheet- material containing printed matter defining a security document comprising the steps of, subjecting the sheet material to a magnetic field to magnetise the magnetic regions of the element. therein, producing relative movement between the sheet material and the magnetic field, at a point remote from the influence 16 of the magnetising field sensing any residual magnetism in the element by a read head comprising a high resolution magnetic field detector, generating electrical signals indicative of any residual magnetism detected, processing the electrical signals so produced to determine if the residual magnetic pattern producing them is correct for the document through which it extends, generatAng a warning signal if the signal processing indicates that the magnetic pattern is incorrect or incomplete or non-existent, generating a routing signal in response to a warning signal and operating a sheet diverter to change the destination to which the printed sheet is to pass after the magnetic check, from a first destination reserved for the sheets which have passed the magnetic check to an alternative destination for sheets which have failed the check.

18. A method as claimed in claim 17, wherein the signal processLng step also generates signals indicative of the t _e Vp of flaw detected in the security parameter under test.

19. A method as claimed in claim 17 to 18, further comprising the step of marking any sheet identified as containing a flaw.

20. A method as claimed in claim 19, further comprising the step of diverting flawed sheets to different destinations depending on the nature of the flaw in the parameter being checked.

21. Apparatus for performing the method of claims 17, 18, 1.9 or 20.

22. A method of checking the accuracy and quality of printed characters on sheet material comprising the steps of: illuminating the sheet material, imaging the printed characters onto a photosensitive detector carried above the printed surface by a mounting as claimed in any of claims 1 to 7, generating electrical signals indicative of the- images presented to the detector, producing relative movement between 17 the sheet material and the detector to present different regions of the printing to the detector, processing the signals from the detector and comparing them with reference signals or subjecting them to mathematical analysis or a mathematical algorithm and generating a signal indicative of the result of the processing, generating a warning signal if the result of the processing is outside predetermined limits thereby indicating a fault in the printing, generating routing signals in response to a warning signal, and operating a sheet diverter by the routing signals to change the destination to which the printed sheet is to pass from one reserved for acceptable sheets, to one reserved for sheets which have failed the test.

23. Apparatus and methods for mounting one or more detectors above a path along which sheet material passes to allow checks to be made on one or more characteristics of the sheet material or printed matter contained thereon, substantially as herein described with reference to and as illustrated in the accompanying drawings.

1

23. A method as claimed in claim 22, wherein the signal processing step also generates signals indicative of the type of flaw detected in the security parameter under test.

24. A method as claimed in claim 22 or 23, further comprising the step of marking any sheet identified as containing a flaw.

25. A method as claimed in claim 23, further comprising the step of diverting flawed sheets to different destinations depending on the nature of the flaw in the parameter being checked.

26. Apparatus for performing the method of claims 22, 23, 24 or 25.

27. Apparatus and methods for mounting one or more detectors above a path along which sheet material passes to allow che cks to be made on one or more characteristics of the sheet material or printed matter contained thereon, substantially as herein described with reference to and as illustrated in the accompanying drawings.

181 Amendments to the claims have been filed as follows CLAIMS 1. Apparatus for inspecting printed sheet material comprising a conveyor on which the sheet material is supported to be transported along a linear path, a thin, elongate, resilient member one end of which is attached to a fixed point above the said path to extend therefrom in the direction of transport of sheet material along the path with its other end in use lightly contacting the upper surface of the sheet material moving along the path, and a detector mounted to the said other end of the elongate member, the resilience of the elongate member maintaining contact between said other end and the sheet material moving thereunder to flatten the sheet material and reduce any rippling effect introduced therein as it progresses along the said path, whilst permitting the free end of the elongate member to rise and fall due to any ripple or irregularity which has not been eliminated, and wherein the detector is attached to the said other end of the elongate member so as to maintain a substantially constant small gap between the detector and the sheet material passing therebelow.

2. Apparatus as claimed in claim 1, wherein the detector is attached to the extreme end of the elongate member remote from the fixed point to which the elongate member is attached.

3. Apparatus as claimed in claim 1 or 2, wherein the elongate member comprises a strip of springy metal.

4. Apparatus as claimed in claim 3, wherein the metal strip has a first portion which extends downwards from the fixed point normally towards the conveyor, and then curves into a longer, flat second portion, which extends substantially parallel to the said conveyor, in the direction of travel of the sheet material, the strip thus forming with the conveyor 19 a converging mouth for receiving the leading edge of a sheet travelling along the path.

5. Apparatus as claimed in any of claims 1 to 4, wherein the fixed point is on a bridge which extends over the conveyor.

6. Apparatus as claimed in any of claims 1 to 5, comprising a plurality of elongate members mounted at points spaced apart along the bridge across the width of the conveyor.

7. Apparatus as claimed in claim G, wherein the spaced positions of at least some of the elongate members across the width of the conveyor correspond to spaced apart linear regions of sheets which contain elongate security features which extend therealong parallel to the direction of movement of the sheet material along the path, and a detector is mounted at the end of each of at least some of the elongate members, for interrogating the elongate security features passing therebelow.

8. Sheet checking equipment having mounted thereon a plurality of detectors each forming part of apparatus as claimed in any of claims 1 to 7.

9. Sheet checking equipment as claimed in claim 8, for checking elongate security features which extend along each sheet parallel to the direction of movement of the sheet material along the path, wherein each detector is adapted to produce an electrical signal in response to relative movement between it and the security feature in the sheet material passing therebelow.

10. Sheet checking equipment as claimed in claim 9, wherein each security feature has magnetic properties and the detectors are sensitive to magnetic flux associated therewith.

11. Sheet checking equipment as claimed in claim 9, wherein the security feature is printed on the sheet material and the detectors respond to absorbtion of light by or reflection of light from, the security printing on the sheet material.

12. Sheet checking equipment as claimed in claim 9, wherein the security feature has a property such that it responds to electromagnetic radiation, or is radio-active, or is capable of supporting an electrostatic charge, or fluoresces upon radiation, and the detectors are adapted to respond to the particular property of- the security feature which is to be checked.

13. Sheet checking equipment as claimed in claim 9, wherein the detectors are adapted to respond to printed characters on the surface of the sheet material and generate signals corresponding thereto for supply to character recognition apparatus, or bar code decoding apparatus, to enable the characters or bar codes to be detected and read electronically.

14. Sheet checking equipment as claimed in claim 9, wherein the sheet material is in the form of a continuous run of webbased printed material.

15. Sheet checking equipment as claimed in claim 9, wherein the sheet material comprises a sequence of separate cut sheets.

16. Sheet checking equipment as claimed in claim 15, in combination with first and second guillotining stages, wherein the detectors are positioned along the sheet material path after a first guillotining stage but before a second guillotining stage.

17. A method of checking the accuracy and quality of printed characters on sheet material comprising the steps of: illuminating the sheet material, imaging the printed characters onto a photosensitive detector carried above the printed surface at one end of a thin elongate resilient member attached 21 at its other end to a fixed point above a path along which the sheet material can move, the thin elongate member engaging and flattening the sheet material and supporting the detector thereabove so that a constant small gap exists between it and the sheet material, generating electrical signals indicative of the images presented to the detector, producing relative movement between the sheet material and the detector to present different regions of the printing to the detector, processing the signals from the detector and comparing them with reference signals or subjecting- them to mathematical analysis or a mathematical algorithm and generating a signal indicative of the result of the processing, generating a warning signal if the result of the processing is outside predetermined limits thereby indicating a fault in the printing, generating routing signals in response to a warning signal, and operating a sheet diverter by the routing signals to change the destination to which -the printed sheet is to pass from one reser-ved for acceptable sheets, to one reserved for sheets which have failed the test.

18. A method as claimed in claim 17 for additionally checking or validating the properties of an elongate magnetisable element contained in or on sheet material containing printed matter defining a security document comprising the steps of, subjecting the sheet material to a magnetic field to magnetise the magnetic regions of the element therein, producing relative movement between the sheet material and the magnetic field, at a point remote from the influence of the magnetising field sensing any residual magnetism in the element by a read head comprising a high resolution magnetic field detector, generating electrical signals indicative of any residual magnetism detected, processing the electrical signals so produced to determine if the residual magnetic pattern producing them is correct for the document through which it extends, generating a warning signal if the signal processing indicates that the magnetic pattern is incorrect or incomplete or non-existent, and generating a routing signal in response to a warning signal, wherein the sheet diverter is operated to change the destination to which the printed sheet is to pass, after the magnetic check, from a first destination reserved for the sheets which have passed the magnetic check to an alternative destination for sheets which have failed the check.

19. A method as claimed in claim 17 or claim 18, wherein the signal processing step also generates signals ind-icative of the type of flaw detected in the security parameter under test.

20. A method as claimed in any one of claims 17 to 19, further comprising the step of marking any sheet identified as containing a flaw.

21. A method as claimed in claim 19 or claim 20, further comprising the step of diverting flawed sheets to different destinations depending on the type of flaw in the parameter being checked.

22. Apparatus for performing the method of any one of claims 17 to 211.