GB2130362A - Optical registration sensor. - Google Patents

Abstract

An optical scanner for detecting registration marks 17 printed on a sheet of labels 14 comprises a light source 23 for illuminating an inspection area and a light sensor 25 responsive to the magnitude of the light reflected therefrom, detecting means having a plurality of outputs and responsive to the light sensor for providing a plurality of parallel signals varying in number in proportion to the magnitude of light reflected from the inspection area, and a display means 31 connected to the plurality of outputs of the detecting means for providing a plurality of visible linear-array signals in response to these signals and means connected to one of the plurality of outputs for providing an output signal EDS in response to the corresponding one of the plurality of parallel signals being applied in response to a registration mark. Threshold means 30 also provides a visible signal (32) in response to EDS. <IMAGE>

Description

SPECIFICATION High speed optical sensor This invention relates to an optical sensor for detecting registration marks printed on a sheet of labels and providing an output signal in response thereto and, more particularly, to a method and apparatus therefor.

A printed-label cutoff machine comprises a pair of cutting wheels which severs individual labels from a sheet of printed labels at registration marks printed therebetween. The registration mark is e readable indexmarkprintedon acontrasting background. Such a mark is read or detected by an optical scanner which provides a signal to a registration controller that causes the cutting wheels to sever an individual label from the sheet The optical sensor comprises a scanner and a scanner circuit, theformertypically comprising asourceoflightforilluminating an inspection area on the sheet and a photosensor responsive to the light reflected from the inspection area. The optical sensor can be, forexample, the type disclosed in U.S. Patent No. 4,266,123 granted to N.

Friberg and assigned to the assignee of the present invention. Although such sensor can be used in many detection applications, it is not responsive at high production speeds because of the slow response time of its scanner circuit which automatically adjusts the intensity ofthe light source to prevent saturation of the photosensor. A modification ofthe optical sensor making it responsive at higher production speeds would necessitate more complicated circuitry which would be more expensive and less reliable.

The instant invention is based on the discovery of apparatus for detecting registration marks printed on a sheet of labels and for providing an output signal in response thereto. The optical sensor comprises a scanner including a light source for illuminating an inspection area and a light sensor responsive to the magnitude of light reflected from the inspection area, and detecting means having a plurality of outputs and responsive to the lightsensorfor providing a plurality of parallel signals varying in numberin proportion to the magnitude ofthe light reflected from the inspec tion area.The optical sensoralso comprises a display meansconnected to the plurality of outputs of the detecting means for providing a plurality of visible linear-array signals in response to the plurality of parallel signals. The optical sensorfurther comprises output means connected to a desired one of the outputs ofthe detecting means for providing an output signal in response to the corresponding one of the plurality of signals being applied in responseto a registration mark detected in the inspection area.The optical sensor finally comprises threshold means connected to the output means for providing a visible threshold signal in response to the output signal and a control means responsive to the lightsensorand adjustable for changing the responsiveness of the detecting means to the light sensor so that the output signal is provided only when the registration mark is present in the inspection area.

Rather than designing more sophisticated circuitry to make the optical sensor responsive at higher production speeds, the instant invention reverts to a more simple sensor comprising fewer components.

The optical sensor is more simple because it incorpo ratesthe control means which provides the scanner circuit with a manual sensitivity control ratherthan a more complicated automatic control. The display means and the th reshold means provide visual information to facilitate the manual adjustment of the control means. It is, therefore, an object of the invention to provide an optical sensor capable of detecting registration marks at high production speeds without producing spurious output signals due to extraneous markings between the registration marks.

Fig. is a partially schematic view of a printed-label cutoff machine including an optical sensor in accordance with the invention.

Fig. 2 is a partially schematic pictorial view of a sheet of printed labels moving through the cutoff machine and being scanned by the optical sensor of Fig. 1 showing the scanner and display ofthe optical sensor in accordance with the invention.

Figs. 3(A) to 3(F) area series of pictorial views ofthe display of the optical sensor with respecttc the corresponding view ofthe sheet of printed labels of Fig. 2 showing the display responses in accordance with the invention.

Fig. 4 is an electrical schematicofthe optical sensor of Fig. 1 including a scanner and a scanner circuit in accordance with the invention.

Referring to Fig. la schematic perspective view of a printed-label cutoff machine is indicated generally at 11.Aweb 12 provides a sheet 13 of material, having a plurality of labels 14 printed serially thereon, to a pair of feed rollers 15. The sheet 13 then passes through a pair of cutting wheels 1 6where individual labels 14a are severed by a cutting blade 1 6a mounted on the peripheryof one ofthe cutting wheels 16. The particular point at which the labels 14 are to be severed is defined buy a registration markwhich is a readable index mark printed on a contrasting background.The registration mark can be,forexample, a dark mark printed on a light background or a light mark on a dark background, the light registration mark commonly being referred to as an eyemark. Although the instant invention is designed to detect either dark or light registration marks, its operation will be described in terms of detecting light registration marks to facilitate the description. A series of light registration marks 17 is printed along the bottom edge ofthe sheet 13. The light registration marks 17 are read or detected individually byan optical sensor 18which can be of the type disclosed in U.S. Patent No.

4,266,123 granted to N. Friberg and assigned to the assignee ofthe present invention. The sensor 18 is electrically connected to a registration controller 19 which is electromechanically coupled to the feed rollers 15 and the cutting wheels 16, as indicated by dashed lines 19a and 19b respectively, to cut the labels 14 in register in response to a signal from the sensor 18. One type of controller 19 is disclosed in U.S. Patent Application No. 208,712,filed November20, 1980, in the name of N. Friberg, assigned to the assignee of the present invention and hereby incorporated by reference.

Referring in more detail to Fig. 2,the sensor 18 comprises a scanner 21 and a scanner circuit 22. The scanner21 typically comprises a source of light 23 for illuminating an inspection area 24 on the sheet 13 and a photosensor 25 responsive to the light reflected from the inspection area 24. Although the sensor 18 disclosed in U.S. Patent No. 4,266,123 can be used in many detection applications, it is not responsive at high production speeds because ofthe slow response time of its scanner circuit which automatically adjusts the intensity of the light source 23to prevent saturation ofthe photosensor25.Rather than designing more sophisticated circuitry to make the sensor 18 responsive at higher production speeds, the instant invention reverts to a much moresimplesensor 18 comprising less components and having a manual adjustment. Consequently, the instant invention is also cheaper and more reliable. The scanner21 ofthe instant invention comprises a light source 23 that is an incandescent light bulb and a photosensor 25 that is photovoltaic cell, such as, for example, type 1 4C30 available from the Hird-Brown Company located in Bloomfield, New Jersey. The light bulb 23 is focussed bya lens (not shown) through a hole in the center of the photocell 25 to illuminate the inspection area 24 as indicated by the dotted/dashed line L.Light impinging on the inspection area 24 is reflected back to the photocell 25 which provides a voltage directly proportional to the intensity of the reflected light. Thus, light reflected from a light registration mark 17 is sensed by the photocell 25 which provides a voltage signal V3 that is utilized by the scanner circuit 22 to detectthe passage of a registration mark 17.

The scanner 21 is connected to the scanner circuit 22 bythree wires 26, 27, and 28. The circuit 22 comprises a display which can be, for example, a multisegment bar graph circuit 29 in combination with a threshold detector 30. The bar graph circuit 29 can be, for exampie, type LM3914from National Semiconductor which includes a linear array 32 of N segments, one through ten, positioned one on top ofthe other as illustrated, each segment being a light-emitting diode.

The numberofsegments M illuminated is proportion al to the voltage signal V3 from photocell 25. The bar graph 29 is wired to detect when the voltage signal V3 exceeds a predetermined threshold voltge Vtor, more specifically, when a predetermined threshold segmentTS, or one higher, illuminates. When the bar graph 29 detects an illuminated threshold segment TS, the circuit 22 provides an eyemark-detected signal EDS to the registration controller 19. For example, if the bar graph 29 is wired so that the sixth segment is the threshold segmentTS, the circuit 22 will provide an eyemarksignal EDS when the sixth segment illuminates.The eyemark signal EDS is also provided to the threshold detector 30 which comprises a threshold indicator32that is a light-emitting diode.

The light-emitting diode 32 is positioned adjacent the threshold segmentTS ofthe linear array 31 so that it provides a discrete and stationary visible indication of a detected light registration mark 17.

The content ofthe visual information is best illustrated in Figs. 3A-3C. When the scanner 21 inspects the area between the light registration marks 17 as shown in Fig. 3A,the voltage signal V3 is low as indicated by only one illuminated segment, the first segment, on the linear array 31. When the light registration mark 17 moves into the inspection area 24 ofthe scanner 21, the voltage signal V3 is high because the amount of reflected light is increased as indicated by seven illuminated segments, the first through seventh segments on the linear array 31. Furthermore, thethreshold indicator 32 illuminates to provide a discrete visible indication that the bar graph circuit 29 detected an eyemark 17 and caused the circuit 22 to provide an eyemarksignal EDS to the registration controller 19.Even when there are extraneous mark ings 33, e.g.fihe print, between the light registration marks 17 as shown in Fig. 3C,the voltage signal V3 is not high enough to illuminate the threshold segment TS causing the bar graph circuit 29 to trigger an eyemarksignal EDS if the sensitivity ofthe scanner 21 is properly adjusted.

The linear array 31 and threshold indicator 32 provide enough visual information to an operator to facilitate manual adjustment of the sensitivity of the sensor 18. Thus, the scanner circuit 22 further comprises a manual control 34for adjusting the sensitivity ofthe scanner circuit 22 by reducing the voltage signal V3 being provided by the photocell 25to an acceptable sensitivity value V3,.With the manual control 34, the operator can adjustthe sensitivity of the scanner circuit 22 so that a voltage signal Vse produced by a light registration mark 17 is greater than the threshold voltage Vtwhile a voltage signal Vsn produced bythe extraneous markings 33 between the light registration marks 17 is less than the threshold voltage, i.e.: Vsn < Vt < Vse.

Referring morespecificallyto Figs. 3B and 3C, the operator performs the adjustment so thatthe highest segment HS illuminating in response to the eyemark voltage signal V33 is higher on the linear array31 than thethreshold segmentTS (i.e.,the seventh segment is higher than the sixth segment) while the highest segment LS illuminating in response to the ex traneous voltage signal V,, caused by the extraneous markings 33 is lower on the linear array 31 than the threshold segmentTS (i.e., the second segment is lowerthanthesixth segment).

The same type of adjustment is required at high production speeds. However, the linear array 31 looks more like a two-phase display with the upper phase appearing dimmer as shown in Fig. 3D. In such case, the operator adjusts the manual control 34 so that the highest segment HS (seventh segment) illuminating dimly in response to the eyemarkvoltage signal V33 is higher on the linear array 31 than the threshold segmentTS (sixth segment), while the highestseg- ment LS (second segment) illuminating more brightiy in response to the extraneous markings 33 is lower on the linear array 31 than thethreshold segmentTS. If the scanner circuit 22 is too sensitive so that spurious eyemarksignals EDS are triggered by the extraneous markings 33 as shown in Fig. 3E, the operator must adjustthe manual control 34to reducethe adjusted voltage signal V,' to obtain the display as illustrated in Figure 3D.If, on the other hand,the scanner circuit 22 is too insensitive so that the eyemark signal EDS is not triggered even by the light registration mark 17 as shown in Fig. 3F, the operator must adjust the manual control 34to increase the adjusted voltage signal V,' to obtain the proper sensitivity level as shown in Fig. 3D.

Referring in more detail to Fig. 4, an electrical schematic of the sensor 18 is shown. In the scanner circuit 22, the light bulb 23 is connected from ground via the wire 27 to a source of positive voltage V via the wire 28. The source of positive voltage V is approximately 5.0 volts. The photocell 25 is connected from ground via the wire 27 to the manual control 34, a grounded variable resistor, via the wire 26. The wiper 314a ofthe variable resistor 34 provides the adjusted voltage signal V3, having a magnitude corresponding to the reflected intensity sensed by the photocell 25.

The operator uses the wiper 34a to control the sensitivityofthe scanner circuit 22 as described above The wiper34a is connected to one end of a capacitor41, other end of which is connected to the input of a preamplifier 42. The capacitor 41 is used to decouple DC from the preamplifier 42. As shown, the preamplifier42comprisesan operational ampli fier 43, an input resistor44 connected between the otherendofthecapacitor4l andtheinverting input of the operational amplifier 43, and a feedback resistor 45 connected between the inverting inputand the output ofthe operational amplifier 43.The preampli fier 42 further comprises a source resistor 46 con- nected between the noninverting input ofthe operating amplifier 43 and a source of positive voltage V/2.

The source of positive voltage V/2 is approximately 2.5 volts. The values ofthe input and feedback resistor44 and 45, respectively, are selected to provide a gain somewhere between about 50 and 100 depending on the characteristics of the photocell 25. The output of the operational amplifier 43 isthe output ofthe preamplifier 42 which is connected to the input of an absolute-value amplifier 47.

The absolute-value amplifier47 comprises an operational amplifier48, an input resistor 49 connected to the inverting inputof the operational amplifier48, and a feedback resistor 51 connected between the inverting inputandtheoutputoftheoperational amplifier 48. The absolute-value amplifier 47 further comprises a first diode 52 having its anode connected to the other end ofthe input register 49 and its cathode connected to the noninverting input ofthe operational amplifier 48 and, a second diode 53 also having its cathode connected to the noninverting input of the operational amplifier 48 and its anode connected to the source of positive voltage V/2.The absolute-value amplifier 47 also comprises a ground resistor 54 connected to the aoninverting input ofthe operational amplifier 48so thatthe diodes 52 and 53 are sufficiently forward biased. The output ofthe operational amplifier 48 is the output ofthe absolute-value amplifier 47 and provides a quiescent voltage of approximately2.5 volts varying up to approximately 5.0 volts due to the presence of V3. As mentioned above, the scanner circuit 22 accommodates either a dark or light registration mark.This feature is provided by the diodes 52 and 53 in the absolute-value amplifier 47 so that it provides a voltage change from the quiescent voltage to 5.0 volts whetherthe voltage at the output ofthe preamplifier 42 swings low from its quiescent voltage of 2.5 volts in response to a detected light registration mark 17 or swings high in response to a detected dark registration mark.

The output ofthe absolute-value amplifier 47 is connected through a pair of serially connected resistors 55 and 56 to ground. The resistors 55 and 56 function as a voltage dividerforthe bar graph circuit 29to provide a reflection signal RS at the junction therebetween. The junction between the resistors 55 and 56 is connected to the input of the bar graph circuit 29. The bar graph circuit 29 also comprises a controller 57 and the linear array often light-emitting diodes 31.

The controller 57 ofthe LM 3914 bar graph circuit 29 can be wired to provide a linear display in the form of eithera bargraph ora moving dot,theformerbeing the preferred embodiment. As described above, the number of illuminated segments M ofthe linear array 31 varies in proportion to the adjusted voltage signal V3 or, more specifically, the reflection signal RS applied to the input SIG ofthe controller 57. The REF. O terminal and the RLO terminal of the controller 57 are wired to ground through a control resistor 58 which controls the brightness of the light-emitting diodes of the linear array 3, as well as the lower value of the voltage range of the bar graph circuit 29. The REF.A terminal and the V(-) terminal are both grounded, whiletheV(+)terminal and the RHI terminal are connected to the sources of positive voltage, V and V/2, respectively. The values ofthe resistors 55 and 56 are selected so thatthe voltage range ofthe reflection signal RS matchesthe voltage range ofthe bar graph circuit 29 as defined bythe voltages at the RHI and RLO input terminals. The voltage atthe RHI terminal is the quiescent voltage having a value of approximately 2.5 volts,whilethevoltage atthe RLOterminal represents the swing so that the voltage range extends from about 1.7 volts to about 3.4 volts.

All of the outputs 1 and 10-18 ofthe controller 57 are individual and provide DC regulated currentto each one of the corresponding light-emitting diodes (the first through the tenth segment) ofthe linear array 31.

Since successive light-emitting diodes ofthe array 31 are energized as the reflection signal RS increases, any ofthe outputs ofthe controller 57 can be used to detectthe threshold voltage Vt as well as drive the corresponding threshold segmentTS. In the preferred embodiment and as described above in Fig. 3, the output terminal 14 is connected to the threshold segment TS, the sixth segment, and provides a threshold signal each time the reflecting signal RS exceeds a predetermined th reshold voltage V,of approximately 2.5 volts, the quiescent point. The output terminal 14 of the controller 57 is also connected to the input of a monostable multivibrator 61 which comprises a NOR gate 62, a first inverter 63 connected between the first input 62a of the NOR gate and the outputterminal 14 of the controller 57, a timing capacitor having one end connected to the output ofthe NOR gate 62, and a timing resistor 65 connected between the other end ofthetiming capacitor 64 and the source of positive voltage V. The multivibrator also comprises a second inverter 66 having an input connected to the junction between the timing capacitor 64 and the timing resistor 65 and an outputconnected to the second input 62b of the NOR gate 62. The output ofthe second inverter 66 is also the output of the muttivibrator 61.The val ues of the timing capacitor 64 and the timing resistor 65 are selected so that he pulse width ofthe multivibrator 61 is approximately 40 milliseconds. Athreshold resistor 67 is connected between the output terminal 14 ofthe controller 57 and the cathode of the sixth lightemitting diode ofthe linear array 31. This resistor 67 guarantees a minimum decrease in voltage, at least 2.4 volts, necessary to switch the first inverter 63 ofthe multivibrator 61.

The output ofthe mulitvibrator 61 is connected to the input of a driver circuit 71. The driver circuit 71 comprises an operational amplifier 72, a diode 73 having its anode as the input of the driver circuit 71 and its cathode connected to the noninverting terminal ofthe operational amplifier 72, a resistor 74 connected between the noninverting terminal ofthe operational amplifier 72 and the source of positive voltageV/2, and a feedback resistor 75 connected between the output and the inverting input of the operational amplifier72.The driver circuit71 further comprises a loading resistor 76 and capacitor 77 connected in parallel between the output of the operational amplifier 72 and ground, an NPN transistor78 having its base connected to the output ofthe operational amplifier 72 and its collector connected to the source of positive positive voltage V, and a grounded resistor79 connected to the emitter of the transistor 78. The output ofthe driver 71 is the emitter ofthetransistor78which provides the eyemarksignal EDS to the registration controller 19. When the multivibrator 61 applies the 40-millisecond pulseto the driver 71 so that the applied voltage exceeds approximately 2.5 volts, the diode 73 becomes for wardbiasedsothattheopertional amplifier 72 provides an output ranging from about 2.5 volts to about 5.0 volts.Such an outputfrom the operational amplifier 72 causes the transistor 78 to turn on and provide a five-volt eyemark detection signal EDS of approximately 5.0volts and 40-millisecond duration.

The output of the driver 71 is also connected to the input ofthe threshold detector 30 which also compris esacomparator82 having an inverting inputasthe input ofthethreshold detector 30 and a noninverting input connecting to the source of positive voltage V/2, and an output resistor 83 connected between the output ofthe comparator 82 and the cathode ofthe threshold indicator 32. The anode ofthe threshold indicator 32 is connected to the source of positive voltage V. When the eyemark signal EDS is applied to the inverting input of the comparator 82, the output of the comparator 82 goes low and causes the threshold indicator 32 to illuminate and provide a visible indication thatthe eyemark signal EDS has been applied to the registration controller 19.As indicated bya bidirectional arrow 84, the threshold indicator 32 is positioned adjacentthethreshold segment TS, the sixth segment, to provide additional visual informa tion that the adjusted voltage signal V3, is provided by the photocell 26 exceeded the threshold voltage Vt indicating the presence of a light registration mark 17.

The foregoing disclosure is the best mode devised bythe inventorfor practicing this invention. It is apparent, however,to one skilled in the pertinentart that various changes may be made in details of construction from those shown in the attached drawings and discussed in conjunction therewith without departing from the spirit and scope ofthis invention. The detail in theforegoing disclosure is intended to enable one skilled in the pertinent artto practice the instant invention. Therefore, it is to be understoodthatthisinvention isnottobelimitedto the specific details shown and described.

Claims (18)

1. An optical sensorcomprising: a scanner including a light source for illuminating an inspection area and a light sensor responsive to the magnitude of light reflected from said inspection area; detecting means having a plurality of outputs (N) and responsive to said light sensorfor providing a plurality of parallel signals (M) varying in number in proportion to the magnitude of light reflected from said inspection area; and output means connected to one ofthe plurality of outputs (N) of said detecting meansforproviding an outputsignal (EDS)in responsetothecorresponding one of said plurality of parallel signals (M) being applied in responseto a registration mark detected in said inspection area.

2. An optical sensor as recited in Claim 1 further comprising display means connected to the plurality of outputs (N) of said detecting means for providing a plurality of visible linear-array signals in responseto said plurality of parallel signals (M).

3. An optical sensor as recited in Claims 1 or 2 further comprising threshold means connected to said output means for providing a visible threshold signal in response to said output signal (EDS).

4. An optical sensor as recited in Claim 3 wherein said visible threshold signal is adjacentthe corresponding one ofthe plurality of said visible linear-array signals associated with the output of said detecting means connected to said output means.

5. An optical sensor as recited in Claim 1 further comprising control means responsiveto said light sensor and adjustable for changing the responsiveness of said detecting means to said light sensor so thatthe output signal (EDS) is provided only when the registration mark is present in said inspection area.

6. An optical sensor as recited in Claim 1 wherein said light sensor is a photovoltaiccell providing a voltage signal (V3) proportional to the magnitude of light sensed and further comprising a preamplifier and an absoiute-value amplifier having an input connected to the output of said preamplifier, said amplifiers being connected between said photovoltaic cell and said detecting means, the input of said preamplifier being connected to said photovoltaic cell and the output of said absolute value-amplifier being connected to said detecting means to provide a reflection signal (RS)thereto having a magnitude varying in proportion to the voltage signal (V3).

7. An optical sensor as recited in Claim 6wherein said detecting means includes a linear array of N light-emitting diodes each of which being connected to a corresponding one of the outputs (N) of said detecting means to illuminate in response to a corresponding one of said one of said plurality of applied parallel signals (M), whereby said plurality of parallel signals (M) varies in numberfrom one to N in proportion to the magnitude of the reflection signal (RS) to provide a bar graph display on said linear array of light-emitting diodes.

8. An optical sensor as recited in Claim 7 wherein said output means includes a resistor connected between a desired one ofthe outputs ( N ) of said detecting means and a corresponding one ofthe light-emitting diodes of said linear array, a monostable multivibrator having an input connected to the desired output of said detecting means, and driving means responsive to said multivibratorfor providing an output signal (EDS) when the magnitude ofthe reflecting signal (RS) exceeds a predetermined threshold value (vet) causing the corresponding one of said pluralityofparallelsignalsto beappliedtothe desired output of said detecting meanstotriggersaid multivibrator.

9. An optical sensor as recited in Claim 8 further comprising control means connected between said photovoltaiccell and said preamplifierfor reducing the voltage signal (V3) of said photovoltaic cell to and adjusted voltage (V3,) so that a voltage signal pro ducedbythe registration mark detected in the inspection area is greater than the threshold (Vt) while the voltage signal produced between registration marks is less than the threshold (Vt).

10. An optical sensor as recited in Claim 8 further comprising threshold means connected to the output of said driving means for providing a visible signal in responseto the output signal (EDS).

11. An optical sensor as recited in Claim 10 wherein said threshold means includes a comparator having an inverting input connected to the output of said driving means and a noninverting inputconnected to a source of positive voltage, a resistor connected to the output of said comparator, and a light-emitting diode having a cathode connected to said resistor and an anode connected to a source of positive voltage whereby said diode illuminates in response to the output signal (EDS).

12. An optical sensor as recited in Claim 11 wherein said light-emitting diode is adjacent the light-emitting diode of said linear array connected to the desired output of said detecting means whereby a discrete visible indication of the output signal (EDS) is provided in conjunction with the bar graph display of saidlineararray.

13. A method for detecting the presenceofa registration mark in an inspection area,the method comprising the steps of: illuminating the inspection area; sensing the magnitude of light reflected from the inspection area; providing a plurality of outputs (N)with a plurality of parallel signals (M)varying in number in proportion to the magnitude oflightsensed; monitoring one ofthe plurality of outputs (N); and providing an output signal (EDS)whenthecorres- ponding one ofthe plurality of parallel signals (M) is applied to the monitored output as a threshold signal in response to a registration mark being detected in the inspection area.

14. A method as recited in Claim 13 further comprising the step of displaying the plurality of parallel signals (M) being provided to the plurality of outputs (N).

15. A method as recited in Claim 14further comprising the step of displaying the threshold signal when applied to the monitored output.

16. A method as recited in Claim 15wherein the threshold signal is displayed adjacentthe corresponding one ofthe plurality of parallel signals (M) being displayed.

17. A method as recited in Claims 13 or 16 further comprising the step of providing an adjustable control for scaling the magnitude of light sensed so that the output signal (EDS) is provided only when the registration mark is present in the inspection area.

18. An optical sensor as claimed in claim 1 or a method as claimed in claim 13, substantially as described with reference to the drawings.