GB2183332A - Cinefilm fault detector - Google Patents

Abstract

Tears, creases or other faults in a cinefilm (5) are detected whilst the film is being transported on rollers (1, 2, 3) during processing. An image of a slit (S) in a mirror (10) is focused onto the surface of the film at P. When the film surface lies exactly at the focal point the image of the slit is reflected back to its original position. However any deviation in the position of the film surface, due e.g. to a crease or tear (as shown exaggerated at 5') causes a broadening of the reflected image, which is therefore captured by the mirror (10) and reflected to a photodetector (12). The output signal of the photodetector is used to generate an alarm signal so as to disable drive (4) to the rollers (1, 2, 3). A beam splitter (9) and a further photodetector (11) monitor the total reflected optical intensity to enable variations in the output of a light source (6) to be compensated. Polarised light may be employed. <IMAGE>

Description

SPECIFICATION Optical position sensor The present invention relates to optical monitoring of reflective surfaces, and is particularly but not ex clusivelydirected to the problem ofdetecting creases, tears and the like in a moving web of material such as cinefilm for example.

Cinefilm is processed by passing it at high speed through baths of chemical developer and the like. It is guided by a multiplicity of rollers and consequ ently any imperfections such astears in the film may causethefilm to weaken or break on the rollers or other members in its path, with the result that a con siderable length ofthefilm may be spoiled.

Optical probes have been employed to detecttears in cinefilm but they have generally relied on interrup tion of a light path to generate an alarm signal. When the film is being conveyed through the rollers at high speed however, even minorcreases may cause the film to snag, and it is difficult to detect such creases by detecting interruption of a light path adjacent the surface of the moving film.

Accordingly, it is an object of the present invention to provide an optical sensor capable of detecting even minor changes in the position of a reflective surface.

In one aspect of the present invention provides a method for detecting the position of a reflective surface in which the reflective surface is used to define a real image and blurring ofthe image resulting from displacement ofthe surface from a predetermined position is detected by photosensitive means.

In an alternative aspect, the invention provides a device for detecting faults in a member in which light reflected from the member is directed along a light path by an optical system which is arranged so that light is deflected laterally ofthe path when a fault is present and a detector detects the laterally deflected light from thefault.

The invention isapplicabletothedetectionofboth specular and diffuse reflection from a surface and is not limited to sensors employing light of visible wavelengths; the term "optical" is to be construed accordingly.

Embodiments ofthe invention will now be described by way of example only with reference to Figures 1 to 5 ofthe accompanying drawings ofwhich: Figure lisa diagrammatic representation of apparatus in accordance with the invention for detecting creases ortears in moving cinefilm; Figure2 is a diagrammatic representation of further apparatus in accordance with the invention for detecting creases or tears in cinefilm; Figure 3 is a ray diagram illustrating the detection of a crease in a specularly reflective surface of the cinefilm by the apparatus of Figure 2; and Figure 4 is a schematic representation of a differentoptical detector system for use in the apparatus of Figure 2.

Referring to Figure 1 the apparatus shown comprises a set of rollers 1,2,3 overwhich cinefilm 5 is transported at high speed. An optical system comprising a light source 6, focusing lens 7, mirror 10 and objective lens 8 focuses an image P of a narrow slit S in the mirror 10 onto the surface ofthe moving cinefilm 5. The slit S is suitably approximately 3 mm long and 0.04 mm wide and extends parallel to the axis of roller 2. In general the minimum dimension of the slit or other aperture in mirror loins preferably equal to a small multiple of (e.g. up to five times) the image broadening caused by the aberration and diffraction properties of objective lens 8. Preferably said minimum dimension is slightly greaterthan e.g.

up to twice as long as said image broadening. The objective lens 8 preferably incorporates an anti reflection coating and is preferably a multi-element lens. Image P lies on a radius of roller 2, since at this position the film is held againstthe surface ofthe roller 2 and cannot therefore vibrate.

Under normal conditions when the image of slit S is precisely focused on the surface ofthefilm 5, substantially all ofthe light returning through the object ive lens 8 after reflection from the film is focused back onto the slit, and consequently minimal light is reflected from mirror 10. However, if the surface of the film 5 should move, as shown greatly exaggera ted at 5', the image ofthe slit broadens and is there- fore reflected by mirror 10, as illustrated by the dashed ray-lines. Such reflected light is detected by a photodetector 1 2,the output of which is amplified by an amplifier 13.The output of amplifier 13 is fed to a variable gain amplifier iSA beam splitter 9 reflects a fixed proportion of the total light intensity reflected from the film through objective lens8 onto a photodetector 11. The output of this photodetector is amplified in amplifier 14. The gain of amplifier 15 is arranged to vary inversely with the output ofampli- fier 14, and consequently its output is unaffected by the light output of source 6 and the orientation or reflectivity ofthe surface of film 5, but is a minimum when slit S is precisely focused on the surface ofthe film 5.Any deviations in the position ofthe surface of the film along the optical axis ofthe system (whether towards or away from source 6), such as might be caused by a crease in thefilm, will lead to an increase in the output of amplifier 15.Atrigger 16 generates an output when its inputfrom amplifier 15 rises above a predetermined level and causes controller4 to disablethedrive to the rollers 1 and 3. Thusthe system is immediately halted when a crease, tear or other surface defect is detected in the film SIt will be appreciated that the apparatus described is capable of detecting deviations in the position of any reflective surface, for example the reflective surface of a metal object, and may be used for detecting the position of such a metal object, for example.

With the apparatus of Figure 1 a ridge or crease on the shiny surface ofthe film as little as 6 microns high can be detected.

Figure2 shows apparatus similartothatshown in Figure 1. Howeverthe apparatus of Figure 2 is primarily intended for use with specularly reflective surfaces, such as the shiny surface of cinefilm forexample, and in the arrangement shown, is arranged to generate an output signal whose magnitude varies monotonically as the su rface of film 5 passes through the position of bestfocus. In orderto achieve this, two orthogonally polarised images of slit S are formed on the surface offilm 5 by means of a block 17 of calcite or other birefringent material.

Block 17 is cut along its optic axis, and the optic axis (not shown) is perpendiculartotheplaneofFigure2.

Hence rays of one polarisation (shown dashed in Figure2) are refracted to a greater extent than rays of the orthogonal polarisation (shown chain-dotted).

Co nseq uently there a re two poi nts P1 and P2 which are conjugate to the slit S for the respective planes of polarisation. In the position of best focus as shown in Figure 2,the two images of slit S on film 5 are broadened equally. Consequently light components of both polarisations are reflected by mirror 10 with equal intensity. It could be noted that this is true irrespective of the total reflectivity ofthe surface of the cinefilm 5, although the total reflected light inten sitywill depend on the reflectivity ofthe film. Con- sequently the accuracy ofthe arrangementwill not be affected by e.g. changes in the surface texture of the film, which might affect its reflectivity.However if appreciable diffuse reflection occurs at the surface of film S,the sensitivity ofthe apparatus will be redu- ced, owing to depolarisation effects.

The reflected light is directed onto a polarising beam splitter 18 which separates the respective polarisations and divides the reflected light between two photodetectors 12' according to polarisation.

The outputs ofthese detectors are amplified by amplifiers 13andcompared in a differential amplifier 19, whose output (displayed by meter 20) increases monotonicallywith the position ofthe surface of film 5 (as measured in the direction of P1 from P2).A reference circuit 21 detects when the output of amplifer 19 deviates in either sense from a reference value (corresponding tothe position of bestfocus) by morethan a predetermined amountand sends a triggersignal to controller4to disablethe driveto the rollers accordingly. Afurtherfocusing lens (not shown) may be interposed between calcite block 17 and roller2 in orderto increase the convergence angle ofthe light rays forming the images P1 and P2, thereby increasing the sensitivity of the apparatus to displacements of the film surface. Preferably this convergence angle is between 10 and 30 .

Figure 3 shows in more detail the optical behaviourofthe apparatus of Figure 2. In Figure 3 the Iightfrom slit S is assumed to be focused on the re flective side of the filmS. It is is assumed that the image of siitS is focused onto a crease 25 in thefilm.The ridge of the crease intersects the optical axis at the point P1 which is conjugate to the position ofthe slit with respect to the plane-polarised light indicated by the chain-dotted ray lines 26 and 27. Accordingly the image of slit Sin this polarisation is focused sharply on the crease at P1 and is reflected back along the paths indicated bythe ray lines 28 and 29to afocal region within the border ofslit S.The point P2 which is conjugate to the position of slit S for light ofthe complementary polarisation (indicated by the dashed ray lines 30 and 31) lies behind the surface of the crease. Accordinglythese rays are reflected back towardstheslitS along ray lines 32 and 33 which are parallel to, but displaced from, rays 30 and 31. These reflected rays 32 and 33 are therefore defocused from the focal region at slitS and are reflected from the mirror 10 as rays 34 and 35. These rays are directed by beam splitter 18 (Figure 2) to the appropriate photodetector 12'. Thus mirror 10 acts as a stop which preferentially intercepts the defocused reflected light from film 5.

Itwill be appreciated that a similar broadening of the image of slit S will occur if the surface of the film lies behind either of points P1 and P2. Under normal conditions the shiny surface of the film will be exactly midway between points P1 and P2 and therefore the reflected images of slit S corresponding to the respective polarisations indicated by the dashed and chain-dotted ray lines will be broadened equally, so that the photodetector 12' will detect equal reflected intensities from the mirror 10.

Block 17 may be of any suitable birefringent material cut so that its optic axis is perpendicularto the optical axis ofthe system. In an alternative embodiment the block 17 may be dispensed with and objective lens 8 may be composed of birefringent material.

Figure 4 shows a modification to the apparatus of Figure 2 in which a common photodetector 12" is used alternately to detect reflected light 44,45 of the two orthogonal polarisations. This arrangement has the advantage that both polarisations are detected with precisely equal sensitivity. A body of liquid crystal material 23 is interposed in the light path in front of photodetector 12" and passes the two polarisations alternately in synchronism with clock signals which are applied to electrodes 24 on op posite sides ofthe liquid crystal. These signals are generated by a signal generator 21, which sends corresponding timing signals to a reference circuit 22.

The output of photodetector 12" is amplified by amplifier 13" and fed to this reference circuit. Reference circuit 22 compares the amplitude of alternate pulses from amplifier 13" and transmits an alarm signal to controller4 (Figure 2) ifthe amplitudedifference exceeds a predetermined value.

Other polarisation switching means (for example a Faraday cell our a mechanically controlled sheet of polarising material) may be used in place ofthe liquid crystal material.

Although the apparatus of Figure 2 utilizes birefringent material to define points P1 and P2 which are conjugate with the position of slit S for two polarisations, other means may be provided for defining two such conjugate points and comparing the image broadening associated with each ofthem. For example, a block of material of high optical dispersion may be located in the optical path so thattwo conjugate points P1 and P2 are definedfor lightof respective wavelengths. Filters placed in the path of light reflected from the surface of the film 5 may be utilized in place ofthe beam splitter 18 to separate light of these two wavelengths so that the image broadening at the two wavelengths can be compared to give a position signal. However, such an embodiment is best suited to the detection of the position of a metallic or other surface whose colour does not vary.

Although the arrangements described above have utilized a slit S both to generate a line image and to analyse the reflection of that line image, the invention includes within its scope arrangements in which light reflected by a specularly or diffusely reflective surface is bought to a focus at a focal region which is displaced from the optical axis of the light source.

However the arrangements described above have the advantagethatthey are simpler and easier to set up for use.

Furthermore although the preferred arrangements described above focus an image onto the reflective surface of the film or other object whose position is to be measured, this is not an essential feature ofthe invention. Thus, a small area ofthe reflective surface may be illuminated by converging light rays so that light rays reflected from the illuminated area converge to a focal region in front of the reflective surface, and this focal region may be focused onto an analysing slit, for example. However such an arrangement has the disadvantage of increased complexityand is much more difficult to setup.

Although in the embodiments described above an image has been defined bythe borderofa mirror surface, namely a slitformed in the mirror surface, it will be appreciated that the border ofany othersuitable stop could instead be used to define the image.

The photodetector(s) could then be mounted on the surface ofthe stop facing the reflective surface ofthe film or other object whose position is to be sensed.

The apparatus ofthe invention is useful not only for detecting deviations in the position of a reflective surface but also for determining whether or not a reflective member or body (e.g. an object carried past the apparatus on a conveyor belt) is present.

Furthermore although the embodiment of Figure 2 described above is arranged to disable the roller drive carrying the cine film in response to a detected fault, the invention can also be used for inspecting cineflims priorto processing sothatsuspectfilms can be treated appropriately.

Claims (20)

1. Position sensing apparatusforsensingthe position of a specularly or diffusely reflective surface, comprising meansforilluminating a region of said surface, means forfocusing light reflected from said illuminated region to a focal region atthe border of a stop, and photodetector means arranged to detect light reflected from said illuminated region which is incident upon said stop nearsaid border.

2. Apparatus as claimed in claim 1, wherein said stop comprises a reflectorfacing said illuminated region and said photodetector means is responsive to light reflected from said reflector.

3. Apparatus as claimed in claim 1 orclaim 2, wherein an image defined by said border is focused onto said reflective surface such that a corresponding image is reflected backfrom said reflective surface to said focal region when said reflective surface is at or near a predetermined position.

4. Apparatus as claimed in claim 3, wherein said stop is located intermediate a lightsource and said reflective surface such that the surface of said stop opposite said reflective surface is illuminated by said light source.

5. Apparatus as claimed in any preceding claim, wherein said border is the border of a pinholeorslot in said stop.

6. Apparatus as claimed in any preceding claim, comprising means for monitoring the total light intensity reflected from said illuminated region and means for comparing the output of said photodetec tor means with the total monitored reflected light in- tensity.

7. Apparatus as claimed in any preceding claim in combination with powered roller means for conveying a web of material along a defined path, wherein said apparatus is arranged in usetoillum- inate a locaiised region of a moving surface of said web such that defects in said moving surface cause variations in the output of said photodetector means on passing through said localised region.

8. Apparatus as claimed in claim 7 comprising means responsive to variations in said output to halt said powered roller means.

9. Apparatus as claimed in claim 7 or claim 8 arranged to detect surface defects in cinefilm.

10. Position sensing apparatus as claimed in any of claims 2to 9, wherein an image defined by said border isfocused onto said reflectivesurfaceby focusing means whose focal length varies as a function of a characteristic ofthe illuminating radiation such that two corresponding images in light of respective characteristics are reflected back from said reflective surface to said focal region when said reflective surface is at or near a predetermined position, analysing means being interposed in the path of said photodetector means so asto separate the detected intensities corresponding to said characteristics.

11. Apparatus as claimed in claim 10, incorporating means for comparing said detected intensities.

12. Apparatus as claimed in claim 10 or claim 11, wherein said focusing means comprises a body of birefringent material and said analysing means comprises polarising means.

13. Apparatus as claimed in claim 12,wherein said analysing means is arranged to transmit light of differing polarisations to a common photodetector alternately.

14. Position sensing apparatus for sensing the position of a diffusely or specularly reflective surface comprising a light source, a reflectorwhich in use of the apparatus faces said diffusely or specularly reflective surface and which is provided with an aperture which is illuminated by said light source, meansforfocusing an image of said aperture onto said specularly or diffusely reflective surface, and photodetector means arranged to receive light reflected from said reflector for detecting broadening of said image.

15. Apparatus as claimed in any of claims 2 to 14, wherein said reflector is inclined with respect to the optical axis of said focusing means.

16. Apparatus as claimed in any of claims 3to 15, wherein the angle of convergence towards said reflective surface of the outermost rays defining said image is between 10 and 30 degrees.

17. A method of detecting surface defects in moving cinefilm, wherein a region ofthe cinefilm surface is illuminated, light reflected from said illuminated region is focused to a focal region which is relatively sharply defined when said illuminated region contains no significant surface defects, light re flectedfrom said illuminated region and incident upon a region immediately outside said focal region is detected, and a signal is generated in responseto such detected light.

18. A method as claimed in claim 17 utilising apparatus as claimed in any of claims 9 to 16.

19. Apparatus substantially as described here inabove with reference to Figure 1 of the accompany- ing drawings.

20. Apparatus substantially as described hereinabove with reference to any of Figures 2 to 4 of the accompanying drawings.