WO2008104238A1 - Offline error inspection device for transparent plastic samples on the basis of a consumer flatbed scanner having a transmitted-light unit - Google Patents

Abstract

The invention provides an error inspection system for plastic samples on the basis of a commercially available consumer flatbed scanner.

Description

 Offline defect inspection device for transparent plastic samples based on a consumer flatbed scanner with transparency unit

Field of the invention

The invention relates to a consumer flatbed scanner with incident light and transmitted light unit and a modified transmitted light illumination, which is used as an optoelectronic system for the simultaneous detection and quantitative analysis of light-absorbing and light-deflecting optical defects in the offline defect inspection of transparent plastic samples. Examples of suitable transparent plastic samples are polymethyl methacrylate films (PMMA films), polymethyl methacrylate molded articles, polycarbonate films (PC films) and composite films made from the above matehals.

description

The invention relates to the development of an offline defect inspection device for transparent plastic samples (FIG. 2, no. 5) on the basis of a consumer

Flatbed scanner (1) with a transparency unit (4) in the scanner cover (3). A consumer flatbed scanner is understood to mean the flatbed scanners offered in the popular electronics wholesale markets, such as Canonscan LIDE and similar models, Epson Perfection, Epson V 10, Epson V 100 and similar models, or the Hewlett Packard Scanjet models. The scanners have a USB or Firewire connection to the PC. The plastic sample (5) is located in the recording field of the transparency unit (4) and the incident light unit (2). With this recording / lighting situation, light-absorbing defects (6) in the material (5) can be detected in transmitted light or incident light, and classified in size and type.

Due to a modification in the transparency unit (FIG. 3, no. 6) located in the scanner flap (5), the flatbed scanner (1) can also provide transparent light deflecting defects (3) in addition to the light absorbing defects (9) called gel body, in the plastic material (8) are located and analyzed. For modification, the translucent cover (5) must be raised by means of an attachment (4) on the scanner (1). Furthermore, the transparency unit (5) must be moved in a defined manner, so that the sensor unit (2) and the transmitted-light illumination (6) deviate from their original positional arrangement. In the entire field of view of the line sensor (2) is now the non-luminous area (7) of the transparency unit.

State of the art:

In the production of plastics, especially in the case of transparent plastic parts, there is a great demand for optical purity. As a result, material samples for impurities and optical defects are tested offline prior to fabrication or parallel to the ongoing process. From the results a qualitative statement about the optical purity of the material results.

Optoelectronic systems of well-known image processing companies exist for this purpose, a description of such a system can be downloaded for example under the following link: (eg http://www.ocsgmbh.com/cn/bt4.php). These off-line fault inspection devices use asymmetric illumination over an edge that is not in the depth of field of the shot for simultaneous detection of light-absorbing and light-deflecting flaws. The sample testers are equipped with modern lighting and sensor units and can be used in a variety of applications. Commercially available flatbed scanners are only used for the digitization of texts or image data (photos, slides), but these have not yet been used for purposes other than as error inspection equipment or even modified in the lighting.

The high performance of the sample testers from the industrial environment is matched by a correspondingly high financial outlay and operating expenses. Furthermore, these systems are often overdesigned for simple problems and thus do not justify their acquisition and use.

Task and solution:

The invention has for its object to develop an offline fault inspection system on the hardware basis of a consumer flatbed scanner and thus to have a cost-effective and problem-adequate solution. In particular, the scanner should be able to act as an error inspection device in transparent

Plastic samples also detect transparent, light-deflecting gel bodies. With the device according to the invention shown in Fig. 3, these tasks can be solved surprisingly simple. The consumer scanners in the field of sensors and optics are now very mature and high-quality systems and their field of application need not be limited to the digitization of text and image material.

What is needed is a scanner with a transparency unit (6). The area scanned by the transparency unit limits the area to be examined. In order to detect transparent, light-deflecting gel bodies (3) in the sample material (8) in addition to the absorbent impurities, a simple modification of the recording conditions is required. Based on experience from lighting tests and industrial example systems, asymmetric lighting, ie lighting over a light and dark area, is realized. For this, the edge is to bring the transition between the light and dark area out of the depth of field of the sensor, since the edge is not to be imaged. By placing the scanner lid (5) raised on the scanner (1) with an attachment (4), this requirement is realized. In order to achieve an asymmetric illumination, the scanner cover (5) must be defined in the horizontal, parallel to the scanning direction, shifted. Consequently, the positions between the transmitted light illumination (6) and the sensor unit (2) of the scanner are defined changed during the scanning process. This leads to a darkening of the recording scene. Is there now a light-absorbing error (9) in the material and the sensor unit (2) is located with the modified

Transmitted light unit (6, 7) in the error range, the light intensity drops markedly at the corresponding pixels in the sensor unit (line sensor) (2). Depending on the size of the defects, the pixels on the sensor unit (2) are significantly attenuated in their signal. Lichtablenkenden error, gel body (3) now form the edge situation between the transmitted light illumination (6) and a non-luminous part (7) of the transmitted light slide, like a lens sharply. The corresponding pixels on the sensor now register lighter and darker areas than the uniform background. These feature differences allow a classification between light and dark imperfections and gel bodies, which manifest themselves as imperfections with a light and dark part. The resolution of the scene is determined in the solution of the task by the number of pixels of the sensor unit (2). The depth of field of the system also determines the sample thickness, which can be studied optimally, but not only. The scanner is connected to a host computer for data processing and performing the classification of errors in type and size.

Implementation of the invention

The components of the error detection system:

To carry out the present invention, the consumer flatbed scanner (1) used was the Epson Perfection V 750 Pro. Technical data (excerpt from the data sheet):

Flatbed scanner with DinA4 transparency unit specified physical resolution: 6400dpi ^* 9600dpi (scanning with sample in film holders) optical density: 4.0 DMax

Interface: USB 2.0 (Type B female x 1) High Speed, IEEE 1394 (6-pin male x 1) Photoelectric device: 6-line Color Matrix CCD with Epson Microlenses

6400 dpi at 113,280 pixels (18,880 x 2 lines x 3 colors) Light source: White cold cathode fluorescent light

The device is priced in the upper range of consumer flatbed scanners, but still offers a significant price advantage over industrial solutions (-Factor 100). The high resolution of the device justifies the price, with these manufacturer's claims are still reduced to lower, realistic values. In order to get the optimal resolution of the system, the samples had to be clamped in brackets, which, however, rather suited the solution of the problem.

As sample material (8) 0.3 mm thick plastic film was examined for impurities. In the case of extruded material, it has been noticed in the invention that the edge direction and the extrusion direction should be parallel, so that the surface irregularities do not become optically effective and an optimal error detection is ensured.

The scanner attachment (4) is for example made of Plexiglas ^®, wherein the material does not matter. The scanner lid (5) is attached with this attachment (4) raised 5 cm and defined in the horizontal shifted until the inclusion of the gel body with the described characteristics to advantage. The scanning and image processing software used is SilverFast ^® Ai from Lasersoft Imaging ^® . The evaluation device is a standard PC with USB 2.0 interface on which the analysis software is installed.

Advantages of the invention

The inventive method offers a tremendous financial advantage over industrial sample testers and yet represents a powerful error inspection device, with the different impurities can be found in transparent material.

For the illumination modification, there is no need to modify the existing lighting units or re-integrate a different lighting method.

Furthermore, consumer scanners are known due to their distribution in households and offices and insofar easy and intuitive to use.

The inventive implementation of the well-known asymmetric

Illumination for the detection of transparent, light deflecting defects can be applied to any flatbed scanner with a transparency unit, even if the scanner in the light slide next to the luminous surface (6) has no dark, non-luminous area (7). The method according to the invention can be realized here with the generation of an edge (FIGS. 4, 7) in the transmitted-light region (6). This non-luminous region (7) can be realized, for example, with a black adhesive strip. The system otherwise corresponds to the present invention. The cover (5) of the scanner (1) must be positioned elevated for absorbing the sample (8) on absorbent (9) and transparent, refractive errors (3) by a cap (4). By horizontal displacement of the

Scanner lid (5) is again necessary for the detection of the gel body (3) necessary position position between the sensor line (2) and the transmitted light unit (6) with cover (7). The invention also offers the possibility to examine samples with 3 lighting arrangements. These are the examination in incident light, in normal transmitted light and in modified transmitted light according to the method of the invention. The lights can be used individually or combined for fault inspection.

The minimum size of the defect to be detected is approximately 20 μm in the case of a gel body in a P M MA film.

The minimum size of the defect to be detected is approximately 20 μm in the case of an optical disturbance in a PMMA foil.

Examples:

Example 1 :

Error detection on polymethyl methacrylate film (PMMA film)

A 0.3mm thick polymethyl methacrylate film is used with the mounts in the

Scan area adjusted. The modified transmitted-light illumination inspects the material for absorbing and refractive errors. The errors become visible with the method according to the invention and can be classified in type and size. By "defects" in a polymethylmethacrylate film is meant cracking particles, gel bodies, foreign material inclusions, and surface defects.

Example 2:

Detection of inclusions in mobile phone displays

For the analysis of contamination levels in injection molded mobile phone displays, the samples should be positioned in the scanner's recording field. Subsequently, the sample is examined in normal transmitted light. With this method you get good morphological information about the inclusions and can then make statements about their size.

Example 3:

Detection of inclusions in incident light

For the analysis of impurities in extruded materials, this is examined in incident light. The resulting image results in an error contrast maximization of the light-absorbing impurities.

Description of the OFIS_CR software (offline fault inspection system, developed by Christian Rosner) for the fault inspection device

In the OFIS_CR software, the steps of the image processing chain have been implemented (see FIG. 1). In addition, the communication with the hardware and the user takes place. The software is written under IDL (Interactive Data Language). IDL is produced by the company Research Systems, Inc. (RSI) and distributed in Germany by the company Creaso GmbH. Using the IDL Virtual Machine (IDLVM), the program is platform independent, (free download: httß _ι ; // www _ι .ittyis. _N To the

To perform error inspection with a scanner, it must be connected to the computer and the software for controlling the recording must be installed. To control the scanner, additional software must be available. As part of the development, the scanner software SilverFast ^® from LaserSoft Imaging ^{® was} used. The control of the scanner and the possibilities of image preprocessing of this software to program itself would not have been expedient.

The resulting image can be examined with the program and the user controls the detection and classification of the contaminants via the entered parameters. The program provides the relevant results after the classification and evaluations. FIG. 1 shows the chronological sequence of the OFIS_CR software.

Required hardware and software

Hardware:

^■ PC with USB port and common peripherals (monitor, mouse, keyboard) ■ Epson Perfection V 750 Pro flatbed scanner with accessories

Software:

^■ IDL Virtual Machine (IDL VM)

^■ scanner software SilverFast from LaserSoft Imaging ^{® ®}

The software must be installed before the fault inspection and the hardware must be connected.

Procedure for error inspection with a flatbed scanner and OFIS CR software

The communication between the software and the user is based on entering values or pressing buttons.

After the start screen of the software, the user can select the image source for error inspection via an appearing dialog.

Subsequently, the OFIS protocol appears in the form of a text file. Here, the user can enter all data relevant to the examination (eg name of the examiner, sample material, process-relevant numbers). Furthermore, the resolution of the image (dpi), the path to the scanner software and the crack threshold can be changed here. These values are based on the default values in the OFIS Protocol set. The standard values in this context are the values that have turned out to be useful in system development.

If the scanner is selected as the imaging medium, the image acquisition is controlled via the scanner software called up. For the simultaneous detection of light-absorbing and light-deflecting defects, the illumination modifications mentioned in the patent must be carried out. Image preprocessing functions can be performed with the scanner software. After scanning the sample, the external program is terminated.

In addition to this functionality, image data can be imported from files. This offers the advantage that older samples can be examined with new parameters without the need for a new scan. For error inspection, the image data must be in tif format.

For examining the gray values in the image, the user can then interactively view the test image. In addition to the test image displayed on the screen, the gray value profile of the current mouse position is displayed. Here, the user can determine whether he would like to have the row or column displayed in the gray value profile of the current mouse position.

With this pre-information, the user must enter the gray value thresholds for the classification in the next step. Gray values are used as parameters for the detection and classification of impurities in the material. The light threshold, dark threshold and crack threshold classify each impurity into exactly one class. The light and dark threshold are intended to limit the background noise, so that only values count as contamination that exceed these thresholds. The basis for this simple classification strategy is the explained illumination modification of the scanner.

The classification follows the following rules: ^■ Bright blemishes: These blemishes are brighter than the background of the shot and thus exceed the bright threshold.

^■ Crack particles: These contaminants have much smaller gray values than the background noise and thus fall below the dark threshold set.

^■ Gel body: The contaminant shows gel body characteristic. Part of the detected blob is lighter than the background, and a part is darker. However, the smallest gray value is not below the crack threshold.

^■ Cracking particles with optical effect: The inclusions with gel body characteristics differ from the gel bodies on the basis of the smallest gray value. This is due to the cracking particles below the cracking threshold.

The crack threshold retains the default value or the value entered by the user in the OFIS log. During development, it has been shown that

Inclusions are generally below the gray value of 30, gel bodies, however, do not fall below this value.

For orientation in the setting of the classification parameters, the average image column is displayed as a gray value profile, since the gel body characteristic can only be observed in the column direction due to the edge direction during the scanner recording. This column is marked in white in the test image also shown. As information about the gray value dynamics in the picture, the mean gray value and the scatter of the gray values are shown in the picture. The thresholds are drawn after input into the gray value profile.

Through a final dialog, each threshold can be re-entered, the software can be started or aborted at this point of the fault inspection.

If the fault inspection is started with the set classification parameters, the number of impurities and a progress display are displayed during the classification process. This allows the user to track the temporal evolution of the classification. The end of the investigation is announced by a beep and information screens.

Note on the classification strategy:

The illustrated method with the gray value thresholds also works when the sample is examined in reflected light or normal transmitted light. Since light-absorbing defects in the material are detected with these lighting techniques, the set dark threshold is decisive in this case.

Result files of the software

In order to be able to make a qualitative statement about the investigated sample quality on the basis of the detected and quantified contaminants, these data are prepared after the examination for the user.

In addition to the examined test image, the label image of the impurities is stored. Each contaminant is provided with a number here. With this number, the values of each impurity can be specifically examined and the single-fault image can be viewed.

The result image of the defect inspection contains the result of the detection and classification. Each contamination is marked with a detection frame in this picture. The detection frames around the contaminants have a different color depending on the classification result. With this visual information, the user gets a quick overview of the sample quality.

In the "OFIS result log" are those of the user before and during the

Error Inspection entered data listed. In addition, the number of different impurities is shown. For each cracking particle and gel body, the label number and the result of the size evaluation are displayed. The features are considered: Circular equivalent diameter, maximum Feret diameter, size of the impurity in column direction. The log also stores the test time.

In the folder "Error images", the individual error images are stored for each type of contamination.The size of the impurity is also noted in the error image.The file name with the included label number can be used to back the error in the test image.

For processing the evaluations of gel body sizes, these are also stored in nxm format. The 2D number format (gel body sizes, absolute frequency of different sizes) stored in this format can be loaded into a spreadsheet program, such as Excel, for further processing.

These relevant quantities are already plotted as diagrams in the "Diagrams of evaluation" folder and can be viewed in advance.

Error messages:

During the error inspection with the OFIS_CR software, the user is informed with defined error messages about incorrect entries.

LIST OF FIGURES FIG. 2

LIST OF REFERENCES FIG. 3 and FIG. 4

Claims

claims:

1. Use of a lighting device of scanner (Figure 3, 1), sensor unit (Figure 3, 2), attachment (Figure 3, 4) and transmitted light unit (6) for the

Detection of light-absorbing and light-deflecting defects integrated in a low-cost consumer flatbed scanner with a transparency unit.

2. Device according to Figure 3 or 4, characterized in that the integration is carried out by simple modification of the transparency unit.

3. Combination of the three lighting options, incident light, normal

Transmitted light and modified transmitted light for fault inspection.

4. Use of a modified according to claim 1 consumer scanner as error inspection device.