US3748482A

US3748482A - Device for the detection of irregularities in a material

Info

Publication number: US3748482A
Application number: US00150646A
Authority: US
Inventors: Cock E De
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 1970-06-08
Filing date: 1971-06-07
Publication date: 1973-07-24
Anticipated expiration: 1990-07-24
Also published as: BE768215A; JPS5433152B1; DE2127999C3; FR2096032A5; SU393849A3; CA949152A; DE2127999B2; GB1343114A; DE2127999A1

Abstract

An apparatus for the detection of irregularities in a moving sheet material. The apparatus comprises a roller for guiding the sheet material along a non-rectilinear path. The roller has sections of reduced diameter so that the sheet is self-supporting at the corresponding areas, and the detection of irregularities in the sheet occurs in said unsupported areas.

Description

United States Patent 1 [111 3,748,482 De Cock July 24, 1973 DEVICE FOR THE DETECTION OF 3,566,138 2/1971 IRREGULARITIES IN A MATERIAL 3,001,080 9/1961 3 206,606 9/1965 [75] Inventor: Etienne Marie De Cock, Hamme, 2,244,22 1941 Belgium 3,479,518 11/1969 [73] Assignee: AGFA-Gevaert N.V., Mortsel,

Belgium Primary Examiner-James W. Lawrence [22] Filed: June 7, 1971 Assistant Examiner-D. C. Nelms {21] pp No: 150,646 Attorney-W1ll1am J. Daniel [30] Foreign Application Priority Data June 8, 1970 Great Britain 27,617/70 ABSTRACT [52] US. Cl. 250/219 DF, 209/111.7, 250/219 WE An apparatus for the detection of irregularities in a [51] Int. Cl. G01n 21/30 moving sheet material. The apparatus comprises a [58] Field of Search 250/219 DF, 219 WE, roller for guiding the sheet material along a non- 250/219 FR, 223; 209/1l1.7; 356/237, 200, rectilinear path. The roller has sections of reduced di- 202, 203, 204, 205, 207 ameter so that the sheet is self-supporting at the corresponding areas, and the detection of irregularities in [56] References Cited the sheet occurs in said unsupported areas.

UNITED STATES PATENTS 3,419,723 12/1968 Germans 250/219 WE 9 Claims, 12 Drawing Figures PAIENIEU JUL 2 M975 SHEEI 2 0F 7 PATENTEBJULZMSH sum 8 0F 7 Fig. 9

DEVICE FOR THE DETECTION OF IRREGULARITIES IN A MATERIAL The present invention relates to an apparatus for the detection of irregularities in a moving sheet material- ,e.g., film or web, of the type comprising means for transporting the sheet material from a supply station to a take-up station, an electromagnetic energy source for directing energy on a part of a surface of the sheet material, wherein the energy is influenced in a different way by irregularities in the sheet material than by uniform parts of the sheet material, and a detector element for measuring the radiation modified by the sheet material. I

More particularly, the present invention relates to a apparatus for sensing imperfections in light-sensitive photographic material, such as films, plates and papers.

Light-sensitive photographic materials may be affected by various kinds of faults such as an uneven thickness of the emulsion coatings, small bubbles, opaque particles or the like and it is of paramount importance that before marketing and preferably even before sizing of webs all serious faults are detected in the manufactured photographic material and that the faulty material is cut out and/or sorted.

For tracing such faults it is common practice to examine films and plates visually. For preventing fog formation it is a necessity to carry out this visual examination with light to which the material was insensitive. Panchromatic materials, however, are sensitive to all regions of the visible spectrum and can visually only be inspected by means of e.g., an infrared viewer so that the examination becomes very difficult and uncertain thereby. Further, it has appeared that the eyes acuity at low intensity levels is so small that faults under a certain finite size are completely invisible.

It is known to scan the photographic material by means of an inactinic light source wherein the light reflected by the photographic material or the light that has passed the photographic material impinges on a photosensitive detector, see e.g., US. Pat. No. 2,393,631.

It is equally known to use static means, wherein for example a series of in-line photocells are positioned over the full width or part of the width of the sheet-like material which has to be examined for faults.

The signals which are obtained in these ways are, however, not always a true measure for faults in the material on account of disturbances which are due to vibrations of the machine on which the measuring device is mounted, fluttering of the film, cloudiness of the film and electric disturbances.

The vibrations of the machine can be limited by arranging the measuring device vibration free with the aid of damping means known in the art.

The signals which are due to the cloudiness of the emulsion and of the support layer, which cloudiness per se'is not a disturbing phenomenon for many applications of the photographic material, can be filtered electrically for the greater part since the frequency spectrum of the cloudiness of the different kinds of photographic material can be defined by way of tests and differs from the frequency spectrum produced by faults in the material.

The electric disturbances can be subdivided in three groups viz. disturbances which are due to magnetic fields, disturbances which are due to electrostatic fields and disturbances originating from the electric mains. The influence of magnetic fields can be reduced by protecting the lines with a ferromagnetic material. The influence of electrostatic fields can be limited by copper protections, whereas the disturbances originating from the electric mains can be reduced by introducing filters in the feed line.

The fluttering of a film constitutes a problem for which the present invention offers a solution. It has been attempted to limit vibration or fluttering of a sheet-like material by guiding the sheet over two adjacent small rollers between which the photosensitive detector or detectors were arranged. It has, however, appeared that even with a minimum distance between said rollers and a sufficient tensioning of the film vibration or fluttering of the film material still occurs.

Further a process for detecting faults in the upper surface of sheet-like material is known wherein the web is moved over the upper surface of a roller and light rays are directed onto a part of the web, normal to the rotation axis of the roller, and wherein the reflected light is measured.

The latter process shows the drawback that in measuring light transmitting sheet-like material all faults which are present on the roller surface supporting such material give rise to signals which may be interpreted as faults in the sheetlike material.

The object of the present invention is to provide a device for the detection of irregularities in a sheet material which does not show the drawback of the known devices.

According to the invention a device for detecting irregularities in a moving sheet material comprises means for transporting the sheet material from a supply station to a take-up station, an electromagnetic energy source for directing energy on a part of a surface of the sheet material, wherein the energy is modified in a different way by irregularities in the sheet material than by uniform parts of the sheet material, a detector element for detecting energy modified by the sheet material, and at least one roller for guiding the sheet material over a non-rectilinear path and supporting the sheet material only over part of its width, the said energy source and the detector being arranged in such a way with respect to the roller, that the energy which is detected is limited to a part of the sheet materialpassing over the roller which is not directly supported by the said roller.

For the arrangement according to the invention the incident electromagnetic energy is reflected from or is transmitted through the surface of the sheet material.

The present invention will now be described by way of some examples with reference to the accompanying drawings of which FIG. 1 is a diagrammatic view of a sorting machine in which a first embodiment of the present invention is applied,

FIG. 2 is a top view of part of the machine according to FIG. 1,

FIG. 3 is a cross sectional view in part of FIG. 2 on line 33,

FIG. 4 is a diagrammatic elevation of a detail of the device according to FIG. 1,

FIG. 5 is a view of a roller portion of the device according to FIG. 1,

FIG. 6 shows a second embodiment of the device according to the invention,

FIG. 7 is a diagrammatic elevation of the embodiment according to FIG. 6,

FIG. 8 shows a detail of the third embodiment of the device according to the invention,

FIG. 9 shows a device for detecting faults in cine-film wherein use is made of the embodiment according to FIG. 6,

FIG. 10 shows diagrammatically the driving mechanism of the device according to FIG. 9,

FIG. 11 shows a detail of the device according to FIG. 10,

FIG. 12 represents a device which can be used together with the device according to FIG. 1 for the sorting of sheets.

The invention will be described hereinafter with reference to a device for detecting faults in photographic material. The device according to the present invention can, however, also be used for sensing other materials such as paper and plastic webs, plates, etc. for imperfections.

In FIG. 1 a sheet cutting and sorting machine for photographic films, such as e.g. X-ray film, is represented in which use is made of a device according to the invention for detecting irregularities in the material.

In said FIG. 1 a photographic film 30 is pulled from a roll 21 by the action of a motor 22 the roll 21 being braked by a constant torque motor 20. The film 30 is guided over a balance roller 23 of small diameter, and over rollers 24 to 28 which will be described more in detail hereinafter. The balance roller 23 is pulled downwardly by means of spring means 46. The shaft of roller 23 is connected over a rod with a potentiometer 45, the terminal of which are e.g. fed with a voltage of respectively V and 15 V. From the slider contact of the potentiometer 45, a voltage is taken off which is a function of the position of the roller 23. The voltage taken off from the potentiometer 45 is differentiated and controls the speed of the motor 22, so as to ensure a constant tension of the film over the rollers 24 to 28.

After guiding the film 30 over the rollers 24 to 28 it is passed over a speed controlling roller 29 which controls the speed of the film 30. The film speed is maintained to a constant value and may amount to about 30 m/min.

The

rollers

25 and 27 are common guide rollers, and they may be e.g., idle rollers. The

rollers

24, 26 and 28, preferably also idle rollers, are so-called measuring rollers and will be further described hereinafter with reference to FIGS. 2, 3 and 5. Beyond the roller 29 the film 30 is tightly tensioned between a pair of rollers 31 and is cut into sheets of a determined length by the knife 32. Succeding to the cutting-off action the sheets are received on a conveyor belt 19 running at a speed which is higher than the speed of the film 30 so that an intermediate space is formed between two successive sheets. The conveyor belt 19 delivers the sheets to a deflecting element such as a sorting trap 33 known per se which permits the sorting of the sheets. The trap 33 is controlled e.g., by a digital computer 36 which stores the data detected by the detectors 38 (see also FIGS. 3 and 4) which are located in the

measuring rollers

24, 26 and 28. The data of the detectors are transferred synchronously with the advance of the film material to the sorting station 33.

When sheets with irregularities arrive on the sorting trap 33 this trap is controlled by the computer 36 to open and the imperfect sheets are deflected towards a stack 35. The sheets containing no irregularities are passed by the trap 33 to a stacking station 34.

The arrangement of the measuring

rollers

24, 26 and 28 will be further described hereinafter, with reference to FIG. 2. According to this figure the measuring rollers show a crenelated profile which means that each roller consists of successive portions of locally smaller and larger diameter. The smaller diameter portions of the

rollers

24, 26 and 28 are displaced from each other according to the axis of the respective rollers, so that the sectors of the film 30 passing over the smaller diameter roller portions of the

successive rollers

24, 26 and 28 contact or slightly overlap.

FIG. 5 represents an enlarged part of such a crenelated roller. Preferably the portions with larger diameter at their extremities, i.e., the regions where the larger diameter passes into a smaller diameter, are rounded off with a certain minimum deflection radius, e.g., 5 mm, to avoid fogging caused by pressure and also scratching when the film is passing over the sharp edges of the roller portions. Further, the portions with larger cross section are tapered or beveled, e.g., at about 20 mm from their extremities, at an angle of about 1 with respect to the axis. The length of the portions with larger diameter is e.g., mm. The length of the portions with smaller diameter is e.g., 48 mm. The diameter of the portions with larger diameter is e.g., 158 mm, and the diameter of the portions with smaller diameter is e.g., 65 mm.

FIG. 3 represents in cross section the measuring roller 24 on line 33 of FIG. 2, wherein the arrangement of the radiant energy source 37 and of the detector 38 is illustrated. A detector 38 is supported by a stationary arm 43 and is arranged in the space between two portions of larger diameter of the roller 24. The film 30 is curved on the roller 24 and passes at about 6 mm from the detector. It will be understood that actually for each roller portion of smaller diameter a radiant energy source and a detector are provided.

As represented in FIG. 4 the radiant energy source 37 comprises a tubular incandescent lamp 42, an infrared transmitting filter 49 and a cylindrical lens 44. The lamp 42 comprises a wire filament 40 which is tensioned by an incorporated tension spring 41. The power which is consumed by the radiant energy source may be e.g., 15 W. The energy level of the radiation transmitted may be controlled by adjusting the feed voltage of the lamp 42. The lamp 42 emits light towards the cylindrical lens 44 which focusses the light according to a line 45 of non-actinic light on the surface of the film 30. The radiation transmitted through the film is received on the detector 38 located at the side of the film opposite to the radiation source 37.

It will be understood that it is also possible to measure the film in reflection rather than in transmission. This is illustrated by the radiant energy source 17 and the detector 18 shown at the right upper side according to FIG. 3. Preferably the source and the detector are positioned to cover a sector of the film which is between the roller portions of greater diameter. In this way the surface structure of the roller cannot possibly interfere with the reflection measurement of the film, e.g., in the case the density of the film is small, so that part of the radiation may penetrate through the film and be reflected on the surface of the supporting roller.

As may be seen in FIG. 4, the detector 38 is composed of a plurality of in-line photosensitive cells 16. In this way it is possible to define the exact situation of a fault. Moreover the use of a series of in-line photocells 16 allows a series of signals to be obtained which can be treated electronically and which permit to define the nature of the fault, that is to say which allow to make a distinction between e.g., large and small faults, transverse streaks, etc. The electronic circuit which is used for treating the signals obtained with a series of photocells 16 is described in our co-pending United Kingdom Patent Application 27,618/70 filed on even date as the present application. The photocells 16 may be phototransistors, such as the MRD 200 type transistors, marketed by Motorola, Arizona, United States.

It is possible to treat film sheets of different width in the device according to FIG. 1. If the film width is inferior to the effective axial length of the

rollers

24, 26 and 28 then the film 30 may run at one extremity over the measuring rollers at a region where the measuring rollers have all a portion with larger diameter (as indicated at the left hand side in FIG. 2). The other film extremity is then not supported at the places where it passes on the roller portions with smaller cross section but it is stabilized by the curvature in the longitudinal direction of the film.

The portion of the roller section of smaller diameter which is not covered by the film must be screened since otherwise a number of the photocells of the corre sponding detector 38 receive the radiation from the source 37 directly. This screening may occur by an opaque plate 47 which may be made from a metal and may be displaced laterally as a function of the film width. The plate matches the curvature of the rollers 24, as shown in FIG. 3.

A second embodiment of the invention is illustrated in top view in FIG. 6. A film 30, being drawn as fully transparent, is moved in the direction of the arrow 50 and subsequently guided over two

rollers

51 and 52, which each support the film for only half its width. In this way it is possible to check the non-supported film half for faults by means of detectors 38. The arrangement is illustrated in side view in FIG. 7 wherein one half of the film width is checked for faults by means of a detector 38 and a lamp 37 at the locus where the film 30 is curved in its longitudinal direction around a roller 51. The other half of the film width is sensed for faults at the region where the film is curved in the longitudinal direction around the roller 52.

The device according to FIGS. 6 and 7 shows the disadvantage that there may be fog formation by pressure of the central part of the film width on the rim portion of the extremities of the

rollers

51 and 52. This fault is, however, without importance if the film 30, which has a width of e.g., 32 mm, must be split at a later stadium to obtain [6 mm film so that the fault will appear at the edge of the 16 mm film.

FIG. 8 represents an arrangement for measuring faults in e.g., 35 mm film. A film 30, being drawn as fully transparent, is supported at its margins on disc portions of a measuring roller 14 and curved in the longitudinal direction. The detector 38 is located within the angular region where the film is being supported on portions 15, i.e., at the area where the film is curved. Damaging of the film by pressure-fog formation at the margins is unimportant since the margins of the film will be perforated at a later stage.

FIG. 9 illustrates a device which enables to automatically position the part of a film 30 in which a fault has been detected, e.g., by means of the measuring arrangements according to FIG. 6 or 8, before a light source 93 for non-actinic light, so that the fault can be inspected visually for decision of acceptance or refusal of the fault. According to the arrangement of FIG. 9 the film 30, e.g., a 32 mm cine-film is unwound from a delivery spool 92 braked off by means of a motor. The film is guided over the measuring

rollers

51 and 52 according to FIGS. 6 and 7, whereafter the film is wound on a take-up spool 90. A speed controlling roller 91, e.g., a so-called vacuum roller, determines the speed of the film 30. Between the roller 91 and the measuring roller 52 a light source 93 for non-actinic light has been positioned. The operation of a device according to FIG. 9 is as follows. In normal operation, i.e., when the detectors 38 do not detect a fault in the film 30, the roller 91 drives the film 30 at a constant speed of the order of magnitude of 30 to 60 m/s. The roller 91 is driven, as diagrammatically represented in FIG. 10, over a reduction gear 95 and a hydraulic variator 96 by means of an asynchronous motor 97 provided with a brake. The starting speed of the hydraulic variator 96 is adjusted with the help of a servomotor 98 which adjusts the control shaft of the hydraulic variator. The control shaft of the hydraulic variator is provided with a disc 99.

At the moment a fault is detected by a detector 38 located in a section of roller 52, a friction disc 100, see also FIG. 11, carrying a cam 103 is coupled by a soleniod 101 to the shaft of the roller 91 which extends through reduction gear 95. At the same time the servomotor 98 is switched on and the rotation thereof makes the rotation of the output shaft of the hydraulic variator 96 slacken to zero and inverse thereupon. The travel ling direction of the film 30 is reversed thereby, and the servomotor 98 is arrested when the reverse speed of the film is e.g., l0 m/min, in the negative sense e.g., by means of a microswitch on the disc 99 of the control shaft of the hydraulic variator 96. The film movement at the reverse speed of 10 m/min is arrested as cam 103 on the friction disc 100 closes a microswitch 104. The microswitch controls the brake of motor 97 so that at said very movement the asynchronous motor 97 is arrested. The arrangement is adjusted in such a way that the area of the film where the fault has been detected is located right before the light panel 93 at that moment, so that the operator can evaluate the fault visually.

At the moment of arresting the magnetic coupling 101 is de energized and the friction disc 100 is returned by a spring 105 to its starting position, where it will be ready for a next operation.

If a fault is detected by a detector which belongs to the roller 51, then this signal is delayed in,e.g., a flipfiop circuit whereafter the delayed signal is used in the same way as the signal of a detector belonging to the roller 52.

The device according to FIG. 1 may be combined with the device according to FIG. 12 for the selection of already cut sheets or strips, i.e., the separation of good sheets or strips from faulty sheets or strips. For this purpose the sheets are guided subsequently over a measuring roller according to the invention by means of e.g., five sets of five endless belts each, which are all driven'at the same peripheral speed. The fault signals are passed to the computer 36, which controls the sorting trap 33 at the moment the sheet containing the fault arrives thereon, as described above. FlG.l2 is a crosssectional illustration of the additional elements necessary for the sorting of the sheets by means of the device according to FIG. 1 together with the measuring

rollers

24, 26 and 28. The

reference numerals

111, 112 and 113 represent each a set of five rather narrow endless belts running each over an angular portion of the peripheral surface of the portions with larger diameter of the measuring

rollers

24, 26 and 28. Additional sets of belts 115 and 117 lie between adjacent portions with larger diameter of the measuring

rollers

24 and 26.

The operation of the arrangement according to FIG. 12 occurs in the following way. The sheets to be sorted are introduced one after the other in the nip indicated by the arrow 120 between the measuring roller 24 and the belts 111, e.g., by means of an automatic sheet delivering device. The roller 24, which may be an idle roller, is driven by the belts 111 and sheets is carried along, passes the roller 24 and is received on the set of belts 115. At the same time the sheet is passed under the set of belts 112 and under a roller 118, the belts of the sets 111 and 112 running over said rollers 118.

Beyond the roller 1 18 the sheet is moved into the nip between the measuring roller 26 and the belt 112 and further between the set of belts 113 and 117, under the roller 119, which is arranged analogously with the roller 118, and in the nip between the roller 28 and the belts 113.

The sheet emerges after it has left the roller 28 and is e.g., received on a transport belt and carried to the sorting trap which is controlled by the computer as described hereinbefore.

The fault detecting device of the present invention is not only suited for the detection of faults before the film is cut or packed but can also be used as a measuring device during the manufacturing of photographic material. In the manufacture a random test may, however, suffice and therefore only a portion of the film surface is examined. Use can be made e.g., of one single measuring roller, such as the roller 24 according to FIGS. 1 and 2. The measuring roller, together with the detectors and the light sources, is moved according to its axis over a distance which is equal to or somewhat greater than the length of the portions with larger diameter of the measuring roller 24. This measuring roller 24 is e.g., displaced at a constant speed over 170 mm according to its axial direction during the time that the film 30 covers a distance of 30 m. This scanning technique thus permits to examine the full film width with a measuring roller the sectors of which cover only part of the film width. Practice has proved that the axial movement of the measuring roller 24 does not affect the light-sensitive emulsion of a photographic film.

The invention is not limited to the above described embodiment but embraces all variations which are obvious for one skilled in the art.

We claim 1. A device for detecting irregularities in a moving sheet material comprising a. means for transporting the sheet material from a supply station to a take-up station along a predetermined path,

b. at least two rotatable rollers for guiding the sheet material along non-rectilinear portions of said path, each such roller comprising at least one large diameter section having a generally continuous surface for supporting contact with a corresponding longitudinal band of said material and at least one axially contiguous section of significantly reduced diameter leaving a corresponding band of said film wholly unsupported, said rollers being spaced along said material path and having the large and reduced diameter sections thereon arranged in generally staggered relation with the aggregate of the axial extent of said reduced diameter portions substantially equalling the overall width of said material,

c. at least one electromagnetic energy source disposed adjacent said non-rectilinear path portions for impinging said energy on at least each unsupported longitudinal band of said material at each such roller, said energy being modified in one way by the normal material and in a detectably different way by irregularities in said material, and

d. radiation detecting means disposed adjacent each non-rectilinear path portion for scanning each unsupported band, at a locus on said path portion, whereby the material is ultimately scanned by said detecting means over substantially its entire width so as to detect substantially all of the irregularities therein.

2. Device according to claim 1 wherein each roller comprises alternating large and reduced diameter sections.

3. Device according to claim 1 wherein the aggregate axial extent of said large diameter sections is equal to approximately one-half of the width of said material.

4. Device according to claim 1 comprising an electromagnetic energy source impinging said energy against a side of the sheet material, said detecting means is situated on the same side of the sheet material as the said energy source in order to detect the energy reflected from said side.

5. Device according to claim 1 wherein the sheets are curved over part of the peripheral surface of the roller by means of a number of endless belts.

6. The device according to claim 1 including energy focusing means interposed between said source and said material to focus said energy substantially along a line.

7. The device according to claim 1 wherein the corners of said large diameter roller sections are rounded on a small radius.

8. The device according to claim 1 wherein the margins of said large diameter roller sections are slightly beveled at an angle of about 1.

9. A device for detecting irregularities in a moving sheet material comprising a. means for transporting the sheet material from a supply station to a take-up station along a predetermined path,

b. a rotatable roller for guiding the sheet material along non-rectilinear portions of said path, said roller comprising at least one large diameter section having a generally continuous surface for supporting contact with a corresponding longitudinal band of said material and at least one axially contiguous section of significantly reduced diameter having a corresponding band of said film wholly unsupported,

c. means operable during the movement of said material to displace said roller cyclically in an axial diby irregularities in said material,

e. radiation detecting means disposed adjacent said non-rectilinear path portion for scanning said unsupported band, at a locus on said path portion, and

f. means for moving at least one of said energy source and said detecting means transversely of said path portion in synchronism with said roller displacement.

Claims

1. A device for detecting irregularities in a moving sheet material comprising a. means for transporting the sheet material from a supply station to a take-up station along a predetermined path, b. at least two rotatable rollers for guiding the sheet material along non-rectilinear portions of said path, each such roller comprising at least one large diameter section having a generally continuous surface for supporting contact with a corresponding longitudinal band of said material and at least one axially contiguous section of significantly reduced diameter leaving a corresponding band of said film wholly unsupported, said rollers being spaced along said material path and having the large and reduced diameter sections thereon arranged in generally staggered relation with the aggregate of the axial extent of said reduced diameter portions substantially equalling the overall width of said material, c. at least one electromagnetic energy source disposed adjacent said non-rectilinear path portions for impinging said energy on at least each unsupported longitudinal band of said material at each such roller, said energy being modified in one way by the normal material and in a detectably different way by irregularities in said material, and d. radiation detecting means disposed adjacent each nonrectilinear path portion for scanning each unsupported band, at a locus on said path portion, whereby the material is ultimately scanned by said detecting means over substantially its entire width so as to detect substantially all of the irregularities therein.

8. The device according to claim 1 Wherein the margins of said large diameter roller sections are slightly beveled at an angle of about 1*.

9. A device for detecting irregularities in a moving sheet material comprising a. means for transporting the sheet material from a supply station to a take-up station along a predetermined path, b. a rotatable roller for guiding the sheet material along non-rectilinear portions of said path, said roller comprising at least one large diameter section having a generally continuous surface for supporting contact with a corresponding longitudinal band of said material and at least one axially contiguous section of significantly reduced diameter having a corresponding band of said film wholly unsupported, c. means operable during the movement of said material to displace said roller cyclically in an axial direction to traverse said roller portion of reduced diameter across substantially the entire width of said material, the rate of axial displacement of said roller being not more than about one-tenth the rate of movement of the material, d. at least one electromagnetic energy source disposed adjacent said non-rectilinear path portions for impinging said energy on at least the unsupported longitudinal band of said material at such roller, said energy being modified in one way by the normal material and in a detectably different way by irregularities in said material, e. radiation detecting means disposed adjacent said non-rectilinear path portion for scanning said unsupported band, at a locus on said path portion, and f. means for moving at least one of said energy source and said detecting means transversely of said path portion in synchronism with said roller displacement.