GB2347740A - Detecting surface flaws in films - Google Patents

Abstract

Method and apparatus to detect surface flaws in cinematographic film stock (13) such that the film stock is irradiated using radiation to which it is transparent, eg. using an infra-red or ultra-violet or x-ray radiation source (14), an image of the film stock is captured and processed, preferably using a CCD camera (18), to detect discontinuities resulting from scratches, opaque surface flaws or adhered matter and means are provided to indicate the result.

Description

DETECTING SURFACE FLAWS The present invention relates to the detection of surface flaws, and is particularly concerned with the inspection of cinematographic film stock during the production and processing of copies of a film, for cinema distribution.

In the preparation of copies of cinematic films for distribution, many copies of a film are required for distribution to cinemas for projection. High standards of quality are required in the copying process. Such copies are made from a master film, using a multi-stage process whose process steps have remained essentially unchanged for decades.

The first step in the copying process is to copy the master film, which is a"negative", using the duplicate negative process. In this process, the master film is placed in contact with unexposed stock and both are passed through a bath of fluid whose refractive index (u) is the same as that of the film stock. The master film and unexposed stock are passed through an exposure station within the fluid bath, and the unexposed stock is exposed to make a contact copy. This part of the process is normally conducted with the film and unexposed stock travelling through the exposure station at approximately 60 feet (18.28 metres) per minute.

The now exposed stock is separated from the master film and processed to produce a"duplicate positive", which is an intermediate product. The equipment used to process the stock is known to those skilled in the art as a"wet printer", and conventional wet printing equipment can process the film stock at a rate of approximately 60 feet (18.28 metres) per minute to produce the duplicate positive.

The next stage in the process is to use the duplicate positive to produce a duplicate negative which is in all respects identical to the master film.

The master film is used to produce a number of duplicate positives, and each duplicate positive is then used to produce a number of duplicate negatives. In the production of the duplicate positives and duplicate negatives the wet printing process is used since the immersion of the film stock in the fluid bath serves to wash off any dirt or dust particles adhering to the film stock or to the master film, and also to fill any scratches in the master film or film stock. Since the refractive index of the fluid is the same as that of the film stock, any scratches or surface irregularities in the master film are not transferred to the duplicate positive during the contact printing process. However, since the film stock is passed through the fluid bath, the speed at which processing can be carried out is limited to approximately 60 feet per minute, due to the need to remove liquid adhering to the film stock after the film stock emerges from the bath.

Once a number of duplicate negatives have been produced, each duplicate negative is used to prepare the final copies of the film for distribution, using a dry contact printing process. The dry contact printing process is essentially the same as the wet process, in that the duplicate negative is placed in contact with unexposed stock and is passed through an exposure station. However, the exposure takes place in air.

The absence of the liquid bath allows the dry contact printing process to be carried out at approximately 1000 feet per minute (305 metres per minute). In this process, the duplicate negative and new film stock are placed in contact with each other in air, and are passed through an exposure station. The duplicate negative and exposed film stock are then separated, and the soundtrack is added to the exposed film stock. The exposed film stock is then processed to reveal the final copy of the film, with soundtrack. The soundtrack is added at this stage to provide flexibility, in that the same duplicate negative may be used to produce copies of the film with different language soundtracks.

Conventional dry contact printing equipment stores the unexposed stock and the duplicate negative on open supply and take-up reels, and conducts the duplicate negative and film stock to and from the exposure station along open runs, in a completely darkened room. The personnel monitoring the equipment are required to change reels of film stock after the duplicate negative has run through the contact printing machine and this activity within the dark room, as well as the moving parts of the machinery, can cause air currents to transport dust particles which adhere to either the negative or to the film stock during its passage along the open runs from the supply reel to the exposure station and thence to the take-up reel. Since the duplicate negatives are repeatedly used for copying, a gradual deterioration in the reproduction quality of the duplicate negative is observed as it accumulates dust particles and/or surface imperfections due to handling. Eventually, the level of imperfections in the duplicate negative is such that acceptable final copies of the film can no longer be produced, and the working life of the duplicate negative is at an end.

The detection of these surface imperfections has hitherto been carried out by a visual inspection of the duplicate negative and/or of the final copies of the film prior to despatch, but this is unsatisfactory for two principal reasons. The first is that visual inspection of the negative or of the film is extremely time consuming, and labour intensive. The second is that if inspection of the final copy proves it to be unsatisfactory, the entire copy must be scrapped and this necessarily involves large costs in wasted materials and processing equipment and inspection time.

An objective of the present invention is to provide a method and an apparatus for detecting surface imperfections such as adhered dirt or surface flaws in film stock without the need for visual inspection of the stock.

A further objective is to provide a monitoring apparatus which can be used in association with either a wet or a dry contact printing machine or a processing machine to monitor the condition of film stock as it passes through the contact printing or processing machine.

A further objective of the present invention is to provide a monitoring apparatus which can measure and record information relating to the condition of the surface of a length of film stock.

A further objective is to provide a monitoring apparatus which can compare a detected surface condition with a quality threshold, and optionally can control a treatment device such as a cleaner when surface quality falls below the threshold.

An embodiment of the present invention will now be described in detail, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a system for detecting surface flaws; Figure 2 is a schematic sectional view of a sensor used in the system used in Figure 1.

Figure 3 is a schematic view of a length of film stock and a frame of a video image thereof.

Figure 4 is an example of a graphical representation of the image analysis results.

Referring now to the drawings, Figure 1 illustrates the system for detecting surface flaws according to the present invention. The system comprises a central processing unit 1 to which are connected a display 2, and an input device such as a keyboard 3 or mouse 4, and a printer 5. RAM memory 6 and ROM memory 7 are also provided to the processor 1 and an external disc drive 8 capable of receiving a removable disc 9 is also connected to the processor 1.

The processor receives inputs from one or more sensor devices 10 which are shown in more detail in Figure 2.

Referring now to Figure 2, each sensor 10 comprises a housing 11 formed at the left-hand end (as seen in the Figure) with a recess 12. A length of film stock 13 extends through the recess 12. In the sectional view seen in Figure 2, the film stock 13 extends in a horizontal plane in a direction perpendicular to the plane of the Figure. Within the housing 11, a light source 14 emits light on to a diffuser 15, the light passing through the diffuser 15 and striking a first mirror 16. The light then passes through the recess 12 to a second mirror 17, where it is reflected into the lens 18 of a video camera 19. A light shield 20 is positioned so as to prevent light from the source 14 escaping directly to the recess 12 or mirror 17 without passing first through the diffuser 15 and to the first mirror 16.

The light source 14 is preferably an infra-red light source, and the video camera 19 may be correspondingly equipped with an infra-red sensitive CCD device to capture the video image. Infra-red light is preferred because the film stock is transparent to infra-red light, irrespective of whether the stock has been exposed and processed, or is new stock. However, dust particles on the surface of the film stock 13 and surface flaws such as scratches, affect the passage of the light from the source 14 and are thus detectable by the video camera 19.

As an alternative to infra-red radiation, it is foreseen that ultra violet radiation or X-rays may be used, with the appropriate CCD devices in the video camera. It is essential that the material of the film stock 13 should not impede the passage of the radiation, but only surface imperfections and dust particles on the surface of the film stock should affect the passage of the radiation through the film stock. To enable the same device to accommodate both exposed and unexposed film stock, a radiation source may be chosen which emits radiation to which unexposed emulsion on the film stock is insensitive.

The output of the video camera 19 is a conventional 50 frames per second video output, and it is preferable that the radiation source 14 is strobed to emit pulses of radiation at the same rate as the frame rate of the video camera 19. Preferably the pulse length of the strobing is less than one micro-second. There is no necessity to synchronise the strobing of the radiation source 14 or the video output of the camera 19 with the movement of film stock 13 through the recess 12, since the picture"frames"of the film stock 13 are invisible to the radiation from source 14.

When an infra-red source 14 and corresponding CCD device in the camera 19 are used a so-called"cold mirror"18a may be fitted to the front of the lens 18.

The"cold mirror"serves to admit infra-red radiation to the lens but to reflect any visible radiation. Likewise, if ultra violet or X-ray radiation is used, a corresponding filter may be attached to the lens 18 in order to admit only the radiation required.

Although the sensor 10 shown in Figure 2 utilises first and second mirrors 16 and 17 to"fold"the optical axis, it will be clearly understood that the light source 14, diffuser 15, lens 18 and camera 19 may all be arranged in a linear array in an appropriately-shaped housing. The advantage of the arrangement shown in Figure 2 is that the sensor lies substantially to one side of the run of film stock 13, and can be conveniently attached to existing processing equipment so that the film path passes through the recess 12 and the sensor does not project from the equipment to cause obstruction to the operators. Since the recess 12 is open at one side, threading up of the processing machine is not impeded.

The physical orientation of the device with respect to vertical and horizontal directions is unimportant, and the sensor may be mounted in any orientation provided that a run of film stock passes through the recess 12, or passes through the light path from the source 14 to the camera 19.

It will be observed from the sectional view of Figure 2 that no part of the sensor comes into physical contact with the film stock 13, and thus the sensor itself cannot cause physical damage to film stock passing through the recess 12.

The output from the video camera 19 is a conventional 50 frames per second video signal, each frame being composed of a number of pixels corresponding to the cells of the CCD array of the camera 19. It has been found in practice that satisfactory video signals are obtained using an infra-red radiation source 14 and a conventional visible light video camera 19, with a "cold mirror"18a positioned to prevent visible light from entering the camera lens. The CCD array of a visible light video camera is sufficiently sensitive to the infra-red radiation to generate a usable video signal. This video signal is sent to the processing unit 1, where it is processed under the command of processing programmes stored in the memory 6 and 7 of the system.

Figure 3 shows, on the left side of the Figure, a length of film stock 30. The film stock has sprocket holes 31 along its edges, and its central area is exposed to form a number of picture frames 32. A dust speck 33 and a scratch 34 are illustrated.

An image of an area A of the film stock is captured by the camera 19, and is represented by the pixel array 35 illustrated on the right side of Figure 3. In the image, the picture frame 32 of the film stock is invisible, and only the dust speck 33 and scratch 34 appear, at areas 33a and 34a respectively, as dark pixels. In the example shown, area 33a is 2 pixels wide and 2 pixels high while area 34a is 1 pixel wide and 40 pixels high.

In this example, the camera is arranged so that the columns of pixels in the video image frame 35 are aligned in the longitudinal direction of the film stock 30. It will however be understood that the camera 19 may be so positioned that the rows of pixels of the video image frames are aligned with the length of the film stock.

If the film stock 13 has no surface flaws or adhered particles, the camera 19 will send a completely"white" picture to the processing unit 1. Flaws in the surface of the film stock, such as scratches, show up as dark areas in the video signal, as do dust particles on the film surface.

The processing of the video signal consists in examining each frame of the video signal for boundaries between white and non-white pixels, and recording how many such boundaries occur in each frame. This may be done by examining each pixel of the frame in turn, and comparing it with the previous pixel. If the brightness level at the pixel in question is the same as the previous pixel then the examination moves on to the next pixel. If the level at the pixel in question is different from, i. e. higher or lower than that of the previous pixel, then a first counter is incremented by one. At the end of processing each frame, the total of the first counter is recorded. Simultaneously with the processing described above, a second counter operates whenever a light to dark pixel transition is detected to count the number of sequential dark pixels before the next bright pixel. This may be done by timing the interval between the signal dropping from the"white"or "bright"level and regaining that level, or by counting the pixels as they are sequentially processed. Thus, each time the first counter is incremented by one on detection of a light to dark pixel transition, the second counter counts the number of sequential dark pixels and stores this as a sub-total. When all of the pixels of the frame have been examined, the first counter will show the number of transitions from light to dark in the frame, and the second counter will store a number of sub-totals, each indicating the respective run length of a dark pixel sequence. An average of the sub totals is computed, and stored in a memory. The memory thus contains for each frame of the video signal a first count indicating the number of light to dark pixel transitions, and a second count indicating the average run length of dark pixels following each dark pixel transition. This data may be displayed graphically by plotting the averages of the two totals over, say, each fifty sequential frames of the video signal, and displaying these averages by a graph whose vertical axis corresponds to the count totals, and whose horizontal axis essentially corresponds to distance along the film stock 13 passing through the sensor 10.

Each frame of the video signal comprises an array of pixels in rows and columns. Preferably the camera 19 is aligned with respect to the path of the film stock 13 so that the rows or the columns are aligned in the longitudinal and transverse directions of the film stock.

The sequence in which the pixels of each frame are examined may then be so arranged that examination proceeds in the longitudinal direction of the film stock; i. e. if the columns are aligned in the length direction of the film stock, then the pixels of each column are sequentially examined and when the end of the column is reached examination passes to the next column.

Due to the winding of film from one spool to another, and the passage of the film over guide pulleys and the like, physical damage to the film stock is most often manifested as scratches extending in the length directions of the stock. By examining the pixels sequentially in this direction, these scratches are detected as long sequences of dark pixels. Isolated dust particles, on the other hand, are generally non-linear in shape and show no preferential orientation relative to the film's length direction. Such imperfections show up as short runs of dark pixels irrespective of the orientation and scanning direction of the video frame pixels relative to the film length.

Using the processing method described above on the video frame 35 of Figure 3, and examining each pixel in turn along successive rows of pixels, the first counter will yield a total of 42 transitions from light to dark pixels (40 from area 34a and 2 from area 33a), and the second counter will yield an average length of the runs of dark pixels as 1.04 { [ (40 x 1) + (2 x 2)]-42}. By comparing the totals of the counters for a number of video frames with the results of visual inspection of the film stock, threshold values may be arrived at which indicate whether or not the film stock is of satisfactory surface quality by correlating the results of the image processing computation with the visual determinations of whether the quality is adequate or not.

Alternatively or additionally, the video image frame 35 may be processed in a similar manner to that described above, but taking the pixels in each column sequentially rather than working across each row. In this case, the first counter will record three transitions from light to dark pixels, and the average run length of dark pixels will be 14.66 { (40 + 2 + 2)-3]. If both calculations are performed, then by comparing the results of the two calculations an indication as to whether the detected surface irregularities are elongated in the direction of the film can be arrived at. For example, if the number of light to dark transitions is greater when the pixels are analysed row by row than when they are analysed column by column, this indicates that the detected irregularities are elongated in the length direction of the film and are therefore likely to be surface scratches.

If the number of transitions are roughly equal, the irregularities do not exhibit any particular directionality and are therefore likely to be dust particles.

The results of such analysis may be used to determine whether simply cleaning the film stock or negative to remove the dust particles will prolong the useful life of the stock or negative, or whether the stock or negative is scratched and therefore irrepairable and should be discarded.

The data may be further processed by comparing the totals for each set of sample frames with preset thresholds so that if the totals exceed the thresholds the film can be considered as being unsuitable for further use.

Figure 4 shows an example of a screen display of the results of processing data from a large number of video images taken sequentially as a length of film passes through a sensor 10. The graph lines are distinguished from one another on the video display by colour. In the example shown, analysis of the video signals has been carried out over samples of 50 frames, and for each 50 frames the average value of the number N of light to dark transitions in the frame is designated Navet the maximum number of transitions N in one frame of the 50 frames is designated Npeak ; the highest average run length of dark pixels in one frame is designated Wpeak ; and the number of frames in the sample of 50 frames which have a value of Npeak greater than 10 is also computed, and displayed as Npeak > 1 0- In the display shown in Figure 4, the vertical axis of the graph represents the numerical values Npeak, NaVe, Wpeak, and Npeak > 10. The horizontal axis represents successive samples of 50 frames numbered so as to indicate their approximate location along the length of a strip of film stock.

For an arbitrary sample Q, the completed values are seen as Q1, Q2, Q3 and Q4, representing the numerical values of Npeakt Navet Wpeak, and Npeak > 10, respectively.

The traces show results from a length of film stock F between two leader sections, the leader sections Li and L2 being severely damaged due to their having been reused several times. In these areas of the trace, the values of Npeak and N... are both 100 or more and the number of frames in each sample of 50 having Npeak > 10 is 50. Wpeak values lie at about 7 or 8, indicating the maximum of the average dark run lengths in the sample.

In the film stock section, values of Wpeak are close to zero, and values of Npeak are about 25, with Navre at about 12. In each sample, the number of frames wherein Npeak > 10 is between about 40.

By comparing traces such as that shown in Figure 4 with the results of visual inspection of the same film stock lengths, values of the parameters Npeakt Navex Wpeak and Npeak > 10 which correspond to acceptable stock quality can be determined. Such acceptable quality values may be shown as horizontal lines on the graphical display, with indications of the values to which they relate and whether they represent maximum or minimum acceptable values. A visual examination of the display will then enable even an untrained operator to determine whether the measured parameters lie within acceptable limits and the film stock is of adequate quality in a fraction of the time required for a full visual inspection of the film stock by conventional methods.

In yet a further development, the acceptable ranges or acceptable maximum or minimum values of the parameters may be stored in memory in the processor 1, and as the computed values for each sample of video frames are produced, they may be compared with the preset acceptable ranges or values to produce a simple output indicating whether the film stock is or is not of acceptable quality. Different ranges and values may be stored for comparison, depending on whether the film stock in question is intended for distribution to cinemas or for the production of further copies, since the inspection apparatus and method is applicable not only to finished copies of a film but also to the master film, duplicate positives, duplicate negatives, and even to unexposed film stock.

The results of inspection may be printed as hard copy, or stored on magnetic media or transmitted to remote locations via a network. Likewise, software for programming a processor such as a personal computer to carry out the image processing steps of the method may be provided on magnetic media or as a signal downloadable from a source via a communication network.

Claims (16)

1. Claims: 1. Apparatus for detecting surface defects in film stock, comprising: illumination means for illuminating the film stock with radiation to which the film stock is transparent; sensor means for producing an image signal representative of an image of the illuminated filmstock; image processing means for processing the image signal to detect image characteristics corresponding to surface defects of the film stock; and output means for outputting an indication of the result of the processing means.
2. 2. Apparatus according to claim 1, wherein the film stock is coated with unexposed photosensitive emulsion, and the illumination means produces radiation to which the emulsion is insensitive.
3. 3. Apparatus according to claim 1 or claim 2, wherein the radiation is infra-red or ultraviolet or X-radiation.
4. 4. Apparatus according to claim 1, wherein the illumination means emits radiation intermittently.
5. 5. Apparatus according to any preceding claim, wherein the sensor means inlcudes a photoelectric sensor.
6. 6. Apparatus according to claim 5, wherein the sensor means is a video camera.
7. 7. Apparatus according to claim 5 or claim 6, wherein teh output of the sensor means comprises a monochrome video signal.
8. 8. Apparatus according to any preceding claim, wherein the image processing means is capable of detecting edges in the image signal output by the sensor means, and provides an output corresponding to the detected edges in each frame of the image signal.
9. 9. An apparatus according to any preceding claim, wherein the image processing means is capable of detecting the length of a sequence of like pixels in the image signal output by the sensor means, and provides an output corresponding to the detected sequence lengths in each frame if the image signal.
10. 10. A sensor for an apparatus for detecting surface defects in film stock, comprising: a housing having a path through which a length of film stock may pass; illumination means for illuminating the path; and imaging means for forming an image of a length of film stock on the path;
11. 11. A sensor according to claim 10, wherein the illumination means, the path and the image forming means are arranged on an optical axis.
12. 12. A sensor according to claim 10, wherein radiation from the illumination means is deflected by one or more mirrors before entering the imaging means.
13. 13. A sensor according to any of claims 10 to 12 wherein the housing defines a recess through which the path passes.
14. 14. A sensor according to claim 13 wherein the recess has an open side.
15. 15. A sensor according to any of claims 10 to 14, wherein the imaging means is a video camera.
16. 16. A sensor according to any of claims 10 to 15, wherein the illumination means is an infra-red light source.