CA1241405A - Measuring the diameter and detecting surface defects of moving wire - Google Patents

Abstract

TITLE
OPTICAL DEVICE FOR MEASURING THE DIAMETER AND DETECTING
SURFACE DEFECTS OF MOVING WIRE

INVENTORS

Paolo Cielo Ghislain Vaudreuil ABSTRACT OF THE DISCLOSURE
An apparatus using optical techniques for simultaneously detecting surface defects and measuring the diameter of a wire coming out from an extruder. A combination of cylindrical lenses is used for projecting, perpendicularly across the longitudinal axis of the wire, a sharply focused laminar beam. This apparatus includes an optical source system for emitting optical rays, an optical system for focusing the rays on the wire, an optical detector system for receiving the rays and generating a detector signal to be processed by a signal processing system, the processing system generates an electric signal indicative of the amplitude of the defects and another electric signal indicative of the diameter of the wire.

Description

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Field of the invention This invention relates to an automatic inspeetion systeM for moving wires. It is a non-eontaet type apparatus for simultaneously deteeting the magnitude of defeets distributed over the surfaee of the wire and measuring the diameter of the wire. The apparatus makes a real time inspeetion of the wire and it can be used at high wire temperatures.
Plastic-eoated wires are extruded at speed of 10 to 20 m/see through a die in a plastic-melting eell. Surface defects such as blisters produced by the infiltration of gas, moisture in the cell, unfused pellets caused by insufficient extruder temperature, inadequate pressure in the cell or other defects are unaceeptable to the eustomers. Typieal amplitudes of the deFeeks to be detected are in the 50 ~m range and spatial periods of the defects over the surface of the wire are typieally 500 ~m.
Deseription of the prior art Optieal teehniques are a good ehoiee for such an applleation beeause they are non-eontact and fast; they have a high degree of resolution and they ean deteet exactly the kind of defects which can be visually detected by the customer. There have been a number of devices constructed or proposed for optically measuring the dimensions of an objeet. Some of these deviees use a seanning light beam projeetor, sorne others use a scanning light deteetor and stlll others use a skationary beam teehnique.
Known in the art i3 an apparatus for measuring the dla1r1eter of ar1 eloneated mernber (U.S. Patent No. 3,765,77~1, Octol)er 16, 1973 1'etrohL10s). The apparatus uses a scanning light bearn method where a l1Kht beam is scanned,aeross the wire. Rotating rnirrors reeeive a narrow Laser beam from a light souree and project a rotary scanning beam to a ]ens which converts it into a parallel scanning beam. An article to be measured is positioned in the path of the parallel scanning bearrl and the interruptions of the beam, as produced by the article, are sensed by a photodetector.
Also known in the art is a non-eontaet type dimension measuring deviee (U.S.Patent No. 3,901,606 Augus~ 26, 1975 Watanabe et al). The apparatus uses the seanning light deteetor method where eollimated rays . ,

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are proJected across the obJeet to be rneasured. A convex lens, situated on the opposite side of the obJeet receives the eollirnated rays and pro~ects them on an array Or photodeteetors. The photodeteetors are eleetronically seanned in order to deteet the prbjeeted shadow of the objeet.
Also known in the art is an opto-eleetrleal system for monitoring fl]arnents of flrle eonf`i~uratloll (IJ.~. patent 1l,166 214 Augu3t 28, 1979 Fuehs-Vinizay et ~al). The system uses a statiollary be~m teehnique where a light beam is projeeted aeross the filament and is reeeived by a light guide situated on the opposite side of the filament.
The light guide is eoupled to a photodetector.
Although all the above systems are suitable for measuring the dlameter of the wire none of these systems ean deteet the surraee defects mentioned above without problems. The apparatus dise]osed in the U.S. Patents 3.765 771l and 3 901 606 don't ~ave the measuring scanning speed f'or the detectlon of the defects at extrusion speeds of more than 10 m/see The apparatus disclosed in U.~S patent 4 166 214 uses the sehlieren teehnlque which limits its use to small objeets such as flbers it is also very delicate to align and it require3 expensive optieal instrumentation.
There are needs for a non-contact type apparatus for slmultaneously deteeting the maenlturle of the def`eets and Ineasuring the diameter of the wire rapidly and with a hlgh df3grec o~` senslbL'Ilty.
Another nerld 19 to aohleve the above rneasur-emel)t by a slmple and lnexpcrl3lve tecllrllqlle.
Surnmary of the lnvcntion Aeeordin~ to the present Lnvention there is pro~ided a non-contaet type apparatlJs for slrnultaneouqly deteeting surf'acé defeets ar)d mea~urlng tlle dlameter of an elonK;Ited objcct moving aLong lts longltudinal axls comprising:
- an incoherent optical source system for emltting incoherent optical rays;
- an optical system situilted between the optical source system and the object for focussing tlle opticill rays to form a substantially converging uniform laminar beam lying in a plane substantia1ly perpendicular to the longitudinal axis of tlle object and for projecting tl~e said la~ninar beam towards the said object; and --3~
- an optical. detector system allgned with the optical system on the opposIte side of' the obJect for receivLng the optlcal rays to generate a detector signal indicative of surface derects and the diameter Or the object.
Brief description of the drawings The accompanying drawings illu~strate, by way of example, embodiments of the present invention, in which Figure 1 is a top view of a non-contact type apparatus for simultaneously detecting surface defects and measuring the diameter of an elongated object, according to one embodiment of ti1e present invention, Figure 2 is a side view of the apparatus shown in Flgure 1, Figure 3 is a schematic diagraIn in b].ock fotm of signal processing electronics for processing the detector signal generated by the apparatus shown in Figure 1, an~ appears on a drawing sheet containLng s 8 and 9, Figure 4 is a Eront view of the se~ond cy1in~rical lens ~f the ap-paratus shown in Figure I and appears on a drawing sheet containing Figures 1 and 2, Figure S is tlle t~p view of anotller embodirnent o~ the non-contact type apparatus for simultaneously detecti.ng surface defects and measurlng the dialneter of an elongated object, Figure 6 is the ~ide view of the apparatus shown in Flgure 5, Figure 7 is an alternative ernbodIment of the optlcal ~ource system of the apparatus shown in Figure 5, Flgure 8 ia a typlcal prorile of' lIgtlt Inten~lty, obtaLned wIth a ~slrIgle sour(-e of' hIghly oohererlt la;er beam, a.Ior1g the bearn 'llne after proJeotIorl throlJgh the ol)Joct, Flgure 9 is a typlcal prof'l:Le of lIght intensity, obtalned wIth three multlrnode diode lasers, along the beam ].ine after projection through the object, Figure 10 is a waveform diagram 9t~0wir1g, by way of example, AC
signal generated by the apparatus shown in Figure 1 and Figure 2 with motionless wire, Flgure 11 is a waveform di.agram showir1g, by way of example, AC
signal generated by the apparatus shown in Figure 1 and Figure 2 with wire of acceptabl.e surf'ace quality, and Figure 12 is a waveform diagram showing, by way of example, AC
signal generated by the apparatus shown in Figure 1 and Figure 2 with wire of unacceptab].e quallty.

Detail descrLptlon of' the preferred ernbodlment In i'igure 1 and in Figure 2, there i~ shown a non-contaet type apparatus for sirnultaneously detectin~ surface defeets and measuring the diameter o~ an elongated objeet moving along its longituclln~l axis cornprising:
- an optical souree system 2 ~or emitting optical rays;
- an optlcal systerrl 4 situate~ between the optieal souree system 2 and the objeet 16 for l'oeu-;1rl~ ~lle oi~tlca1 ray~ as ~ lallllrlar beam ~ub~tantially perpendLcular to the longitudin~l ax1s of the objeet 16;
and - an optical detector system 6 aligned with the optical system 4 on the opposite side o~ the objeet 16 for receiving the eptical rays and to generate a deteetor sigrlal indieative of surrace defeets and t1~e diameter o~ the objeet 16.
The optieal souree system 2 includ~s a 11near fi1ament lamp 7 oriented with its lonsitudinal axis perpendicular to the longitudinal axi~ of the object 16. The filament lamp 7 emits optical rays in the visible range so that the aligr~ent of the optical rays toward the object 16 can be done easily. The rilament lamp 7 produces optieal rays of short coherent length whieh eliminates speekle problems observ~d wlth source of eoherent lLght. The3e speckle problerns are produced by the presence of dust or grease partLcles on optical wlrl~ows and lenses. '1'he optlcal .system 11 1nclu~les ~ rlr~t cylln(irieal L~n~ 12 an(l t.l 8eCOIld cyl1ndrical ]ens 14, whici~ L~ prer~rably a plarlo-conv~x one, ~or prolectlng the Lrllage of the F;11~mellt ln the form of a larminar benrn per-pcndLc~ r to tl~e Lorlgitllcl11)al axls of the obJect 16. '1'he f1rst cyllndrical 1ens 12 is orlerlted w1ttl its longltuclinfll axls parallel to the filament lamp 7 and perpendicu1nr to the longitudinfll axis of the object 16 wlllle the second cylindrical lens 14 is oriented perpendicularly to the first lens 12.
The portion ol the lamlllar beam tllat ls not b10cked by the obJect 16 is collected by the optical detector system 6.
The first cylindrical lens 12 1s used to converge the optical rays to focus the imflge of tl1e filament lamp 7 on the elong~ted object 16 and the second cylindrical lens 14 ls used for projecting a collin~ted beam in a plane perpendic-1~r

3 ~3~r ~

to the axls of the e1.ongate(i ob~eot 16. 111e oombLned actlon of the two cylindr.ical lense3 produces a collLmatr3d br3arn line perpendicular to the longitudlnal axis of the object 16. The optical detector systeM 6 includes a spherlcal lens 18 and a slngle silicon detector 22. The spherical lens 18 receives the optical rays which have passed across the obJect 16; the action of this lens 1O is double, first it coLlimates the rays in a first p:l.ane whlch is parallel to the longitudinal axis of the ob~ect 16 and secorld it rocuses the rays ln a second plane perpendlcular to the first plane. The single silicon detector 22 receives the rays which are unobstructed by the object 16 and generates a detector signal the DC component of the detector signal depends on the diameter of the object 16 along tl1e pro~ected laminar beam. Sudden variations of the detector signal produced by the presence of surface derects on the moving object 16 produce an AC signal component proportional to the amplitude of the surface defects independently of the object 16 position or vibration.
The portion AB of the filarnent lamp 7 is diaphragmed by the aperture of the detector through the second cylindrical lens 14 and the spherlcal ].ens 18 having rocal lengths ~1 and F2 respectively.
The relatlon AB =(__~2_~ d where d i~ thr aperture dlamr3ter r~f tl1e detector holds betwern SllCh par.lrn/3ter3. A~.. L tll(3 por-tior1 of the fLlamerlt :Larlll) 7 betweerl the poLnt3 A
and B contribute to the illufrlin1tion of each portlon of the proJected laml.nar beam r~ tlle object 16 '30 tllat eventual hot or cold spots along the filament lamp 7 arc averagf!rl out. The lateral oscLllation of the obJect 16 will affect ttle sip~nal only Lf the angular dLstributiorl a of the lamp emission across the aperture of the secon(i cy1.indrical lens 14 is non-uniforrn. If this is the case the second cylLndrical lens 14 may be convenlently rna3ked a~ s~1o~n in the figure 4 wt1ere the shadowed areas 3 ~ 2~

represent the masked portion~ and the whlte area 5 represents the tran3parent portion of the lens. The gradua1 accumulatlon of specks or molten plastic droplets on the second cy11ndlica1 lenses 14, on the spherical lens 18 or on any window situated in the beam path w11l hardly affect the level Or the detector sienal as a function of the elongated obJect transverse posltion because Or the avelaging action along the focussed lamlnar beclm. A decrease in tile D~ signal leve.l. produce~ by excessive rl1rt acculnulatloll or by 11gnt intellsity fading because of lamp agin~ would tri8ger an alarm requesting mainterlarlce.
The amplltude of the AC component Or the detector signal fluctuates in the presence of the surface defects at a frequency F of the order of F = p (1) where v is the speed of the object 16, typic.alLy between 10 m~s ~nd 20 m/s, and p i9 the .spatial period of the surface defect~ typically 500 ~3m.
In order to properly detect the defects, the width w of the laminar beam intersect.ing the object 16 should be smaller than p.
Such a width i9 in this case:
w ~ M x D (2) where M i9 the rnagn1rLc~tiorl of the flrst cyllndrlcal .1ens 12 and D i.s tile wl~lth Or th~7 fllamt3rlt l,arnp 7:
I'he magllltlJ(3r3 M ls M = _ i (3) SO

where Si is the dlstarlce between the focused lmage of the filament lamp 7 and the first cylindrical ].ens 12 and SO Ls the distance between the filalrlent lamp 7 and the rlrst cyl.1ndrical. lens 12.
The equatLon (2) provldes a cutofr trequency Fc ` c w (4) above which the detector signal is rapidly attenuated.
Referring now to Figure 3, there is shown a signal processing system for establishing simultaneously the magnitude of the surface defects and the diameter of the object 16. The single silicon detector 22 sends the detector signal to a band-pass filter 26 via a buffer 24 and to a low-pass filter 36 via the same buffer 24. The band-pass filter 26 should be centered around the frequency given by equation 1 which is based on the spatial period of surface defects to be detected. The output of the band-pass filter 26 is an AC signal relating to the magnitude of the surface defects. The AC signal is fed to an amplifier 28, rectified by a rectifier 30 and then integrated by a gated integrator 32 to avoid isolated transient which may be produced by 20 ambient noise. The output of the gated integrator 32 is fed to a first meter 34 for displaying the magnitude of the detected surface defects.
The lower frequency limit of the band-pass filter must be determined to avoid low frequency noise due to ir-reeular intensity along the focused beam ]ine. The output of the low--pass fllter 36, a DC slgnal ls fed to 25 a second meter 38 for displaying the diarneter of the obJect 16. Ihe low--pas) filter 36 is tnonltored by a computer 42 which suppLic~ a proper rererence slenal. PcrLoclLcaL callbration could be performed on the referellce slenaL to 1VO:ld ahsolute Slgrlll variatiorls CclllSe(l by aging Orthe optical sour-ce system and the optlcal detector systeal 6 or dirt deposLt; on the lens surfaces. Such an operation would be performed very quickly the operator naving to reset the system with a standard diameter elongated object held in front of the optical detector system 6. The gated integrator 32 is also monitored by the computer 1l2 via a logic circuit 40.
Referring now to Figure 5 and Figure 6, there is shown another embodiment of the non-contact type apparatus for simultaneously detecting surface defects and measuring the diameter of an elongated object. The apparatus shown in Figure 5 and Figure 6 comprises an optical source system 52J an optical system 54 and an optical detector system 56. The

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-r~-optIoal source syYtem 52 Lnclude9 three multirrJode dLo~e laserr-~ 5~, each diode is individually coupled to a multlmode optical fiber 60. The diode lasers 58 emit a near infrared beam of 2 mW, the typical wavelength of the infrared beam i9 830 nm. The use of multimode diode lasers 58 with

5 multimode optical fibers 60 produce optical rays, emerging from the fiber ends after multiple internal reflections within the fiber, that are no longer coherent. A multimode optical fiber having a length of 1 meter would produce enough internal reflections to generate incoherent optical rays. The incoherent optical rays eliminate interference effect produced by dust or grease particles on lenses.
The optical system 54 includes a first cylindrical lens 62 and a second cylindrical lens 64 for projecting a focused laser line perpendicular to the longitudinal axis of the object 66. The first and second cylindrical lenses have their longitudinal axis perpendicular to the lon-gitudinal axis of the object 66. The ~irst cylindrical lens 62 focuses the rays of the optical source 52 as a sharp laser line which is the image of the source as projected by the first cylindrical lens 62, the second cylindrical lens 6~1 projects the image of the first lens 62 on the object 66 as a sharp laser line wh.Lch is substantially perpendicular 20 to the longitudinal axis of the object 66. The portion of the laser line that 1~ not blocked by the object 66 is coll.ected by the optical detector system 56.
The optical detector system 56 inclucles a sphcrical :1.ens 6E~, a thlr~l cy:1.indrif3a]. lens 70, whioh is prererclbly a plano-convex one, an(1 a ,.'5 3.1.nE~ .3 slLlaor1 deteotor 72. '1'he r~pherioal :1.en~3 6~3 rece.Lves the optLoal ray~ wh1.ch have passe(1 acrof~s the obJect 66 and direots the rays on the thlrd lens 70. The action of the spherical lens is double, first it oollimates the rays in a first plane whioh is parallel to the longitudLnal axis of the object, and second it focuses the rays in a second plane whioh is perpendicular to the first plane. The third cylindrical lens 70 focuses the rays in the first plane on khe single silicon detector 72. The silicon deteotor 72 generates a deteotor signal responsive to the magnitude of the defects distributed over the obJect ciroumference and also responsive to the diameter of the objeot 66. If the intensity along the focused laser line is constant, a short range excursion of the object laterally such as the vibration of the obJect _9_ 66 comlng out of` an extruder do not a~'fect the detector slgnal.
In order to properly detect ths derects the wldth w of the laser llne lntersectlng the obJect 66 should be smaller than p the spaclal perlod of surface defects.
Sucil a width is in this case:
w = ~ (5) a,a2 where a, is the dlstance between the output ends of the optical flber~ 60 and the flrst cylindrical Iens 62 a2 ls the distance between the image Or the ~Lrst lens 62 and the second lens 6~1 il is the distance between the first lens 62 and the image of the first lens 62 i2 is the distance between the second lens 6~1 and the object 66 and d is the diameter of the optical fibers 60 typically 50 ~m.
The first cylindrical lens is not absolutely necessary because one lens is sufficient for focuslng the laser beam as a sharp laser line.
However lf al > 1~ lt may help lowering w as shown in equation 5 maklng it possible to increase the ratio i2/a2 and thus the field's depth of the pro~ected laser llne. Thls is possible at the expense of a lower intenslty of the laser line.
Ref`errlng now to Figure 7 there is shown an alternative embodlment of the optical source systern for producing incoherent optical rays. The alternatlve embodlment includes an incoherent lLght source 25 .such as a ]amp 411 followed by an opaque menlber 118 havlrlg a sLit 116 orlented toward the optlcal sy~tem ll whereby optlcal rays are proJected towar~l tho ohJeat l6. 'I`he avallable llght lnterl3lty would be much ~maller Ln this case.
EXPERIMEN'rAI. RESUI.'I'S
In E'lgure 8 and Flgure 9 there are showr) graphs where the vertlcal axls repesent~ l~ght intensity and the horlzontal axls represents dlstance along t~le wi~th of the laminar beam after pro-~ection through the ob~ect.
A problem was observed when generating a line uslng a slngle laser source as a lle Ne laser. In theory the line intensity shown in Flgure 8 should be perfectly smooth. In practice the presence of minute du~st particles or fingerprints in the lenses or windows which are unavoidable in the real environment, produces considerable speckle noise ~ 1 0 -visible as closely-spaced irregularities along the curve of the kind shown~in Figure ~. Such a noise is produced by the lnterference of the highly coherent laser beam randomly scattered by the dust or erease particles. Such line irregularities as seen on Figure 8 are low in amplitude but high in spatial frequency, so that they strongly aE`fect the local value of the curve slope near the two cutoff points. l'his results ln important AC signal fluctuations when the wire vlbrates with a low amplitude but at high frequency.
l'he line obtained with the array oi' the laser-diodes, Figure 9, is much less affected by such fluctuations. The dot line represents the individual laser light intensity and the continuous line represents the cumulative light intensity. The reason for this is that the diodes are multimode and are transmitted through a length of the order of 1rn of multimode fibers, so that multiple reflections within the fibers completely destroy the initial coherence of the laser beam. Speckle effects are thus no longer present, and the projected line is much smoother.
The apparatus shown in Figure 1 and Figure 2 was experimented.
The roughness-monitoring prototype was set with XLPE wire. On Figure 10, 11 and 12, the vertical axis represents the magnitude of the detected AC
signal and the horizontal axis represents the time, one hori~ontal division corresponds to 5 ms on Figure 10 and 50 ms on Fieure 11 and 12.
Figure 10 shows the ~C signal obtained with a motivnless ~lire, one vertloal dlvision corresponds to 15 ~m. l'he amplitude of' .suoh a sLgrla:L
r-epre:3elltc) the intrln~io noL~se intcrlsity gencrated by thc apparatus, suoh nolse ls eqllivalent to a wire-diameter variation of approximately 5~m.
Figure 11 shows the AC signal obtained with a wire of acceptable surface roughness sliding at .1 m/s. One vertical division corresponds approximately to 15 ~m.
Figure 12 shows the AC signal obtained with a wire of unacoeptable surface roughness sliding at .1 m/s. One vertical division corresponds approximately to 60 ~m.

Claims (16)

1. A non-contact type apparatus for simultaneously detecting surface defects and measuring the diameter of an elongated object moving along its longitudinal axis comprising:
- an incoherent optical source system for emitting incoherent optical rays;
- an optical system situated between the optical source system and the object for focusing the optical rays to form a substantially converging uniform laminar beam lying in a plane substanti-ally perpendicular to the longitudinal axis of the object and for projecting the said laminar beam toward the said object; and - an optical detector system aligned with the optical system on the opposite side of the object for receiving the optical rays to generate a detector signal indicative of the surface defects and the diameter of the object.

2. The apparatus as defined in claim 1 wherein the incoherent optical source comprises a linear filament lamp.

3. The apparatus as defined in claim 1 wherein the incoherent optical source comprises at least one CLAIMS (cont.)

3. (cont.) multimode diode laser and one multimode optical fiber where each optical fiber is individually coupled to each diode laser at one end of the fiber and aligned in a plane perpendicular to the longitudinal axis of the object at the other end.

4. The apparatus as defined in claim 3 wherein the incoherent optical source comprises three multimode diode lasers and three multimode optical fibers.

5. The apparatus as defined in claim 2 wherein the optical system comprises:
- a first cylindrical lens being situated between the optical source system and the object for focusing the imaye of the lamp on the object, the first cylindrical lens being oriented with its longitudinal axis parallel to the longitu-dinal axis of the lamp filament; and - a second cylindrical lens being situated between the optical source system and the object for col-limating the rays and oriented perpendicularly to the first cylindrical lens.

6. The apparatus as defined in claim 3 wherein the optical system comprises a cylindrical lens being situated between the optical source system and the ob-ject, for projecting said laminar beam substantially perpendicularly to the longitudinal axis of the object.

CLAIMS (cont.)

7. The apparatus as defined in claim 3 wherein the optical system comprises:
- a first cylindrical lens for focusing the optical rays coming from the optical source system, the first cylindrical lens being oriented with its longitudinal axis perpendicular to the longitu-dinal axis of the object; and - a second cylindrical lens being situated between the first cylindrical lens and the object, for focusing the image of the first lens on the object.

8. The apparatus as defined in claim 2 wherein the optical detector sytsem comprises:
- a spherical lens being aligned with the optical system on the opposite side of the object for simultaneously collimating the rays in the first plane which contains the center of the lens and the longitudinal axis of the object and focusing the rays in a second plane which is perpendicular to the longitudinal axis of the object; and - an opto-electrical detector being aligned with the rays for generating a detector signal respon-sive of the surface defects and the diameter of the object.

CLAIMS (cont.)

9. The apparatus as defined in claim 3 wherein the optical detector system comprises:
- a spherical lens being aligned with the optical system on the opposite side of the object, for simultaneously collimating the rays in a first plane which contains the center of the lens and the longitudinal axis of the object and focusing the rays in a second plane which is perpendicular to the longitudinal of the object;
- a third cylindrical lens being aligned with the rays for focusing the collimated rays in the first plane; and - an opto-electrical detector being aligned with rays for generating a detector signal responsive of the surface defects and the diameter of the object.

10. The apparatus as defined in claim 6 wherein the said second cylindrical lens has masked areas.

11. The apparatus as defined in claim 1 further comprising a signal processing system having:
- a band-pass filter connected to the buffer for extracting the AC component signal from the de-tector signal, the band-pass filter is centered around the frequency F according to the following CLAIMS (cont.) ll. (cont.) equation: F = where v is the speed of the object and p is the spatial period of the surface defects;
- a first display means system connected to the band-pass filter for displaying the amplitude of the surface defects;
- a low-pass filter connected to the buffer for extracting the DC component from the detector signal;
- a second display means system connected to the low-pass filter for displaying the diameter of the object; and - a computer means connected to the first and second display means systems for providing to each display means system a proper reference signal.

12. A method for simultaneously detecting surface defects and measuring the diameter of an elongated object moving along its longitudinal axis comprising steps of:
i) emitting incoherent optical rays from light source means;
ii) focusing the optical rays to form a substantially converging uniform laminar beam lying in a plane substantially perpendicular to the longitudinal axis of the object and for projecting the said laminar beam toward the said object;

CLAIMS (cont.)

12. (cont.) iii) directing a portion of the optical rays, which has not been blocked by the object, toward an opto-electrical detector;
iv) generating an electric signal, by the detector, which is indicative of the surface defects and the diameter of the object.

13. The method as defined in claim 12 wherein the light source means is a linear filament lamp and the step (ii) of projecting the optical rays comprises steps of v) focusing the image of the lamp on the object by means of a first cylindrical lens which is oriented with its longitudinal axis parallel to the longitu-dinal axis of the lamp filament;
vi) collimating the rays from the lamp by means of a second cylindrical lens which is oriented perpen-dicularly to the first cylindrical lens.

14. The method as defined in claim 12 wherein the light source means is a linear filament lamp and the step (iii) of directing the rays comprises steps of:
vii) collimating the rays in a first plane which is parallel to the longitudinal axis of the object and contains the center of the detector;
viii) focusing the rays in a second plane which is per-pendicular to the longitudinal axis of the object.

CLAIMS (cont.)

15. The method as defined in claim 12 further com-prising steps of:
a) filtering the electric signal, for extracting an AC component signal with a band-pass filter centered around the frequency F according to the following equat ion. F = ?
where v is the speed of the object and p is the spacial period of the surface defects, the AC
component signal being responsive to the magnitude of the surface defects; and b) low-pass filtering the electric signal, for ex-tracting a first DC signal being indicative of the diameter of the object.

16. The method as defined in claim 15 further com-prising steps of:
a) converting the AC component signal to a second DC signal;
b) comparing the second DC signal with a first refer-ence signal provided by a computer means for dis-playing the amplitude of the surface defects;
c) comparing the first DC signal with a second refer-ence signal provided by the computer means for displaying the diameter of the object.