Process and apparatus for measuring the optical quality of a glass sheet.

The invention relates to techniques for measuring optical defects of the glazings and especially automobile windows.

In general, the industry is looking increasingly to control the quality of products it manufactures. This is true in particular of the optical quality of glass. To achieve this goal, is maintained in very narrow ranges all parameters of production. But, nevertheless, an incident is always possible and even if it is detected, its effect on quality is not always known so that a quality control on 100% of production is indispensable in many case. As regards the glazing, it is often necessary to assess the optical quality constantly. It may, in particular, wish to select the outgoing glazing production lines to be intended for a particular purpose, such as a mirror for scientific applications or a thin flat glass intended to be transformed into a highly raked windshield . In general, moreover, the windshields of modern cars are closely monitored in terms of their optical quality. This criterion affects in fact the problem of the safe driving of motor vehicles and, in addition, the shapes of the windshield, their inclination, the materials from which they are manufactured - very thin glasses or similar transparent polymers - require very careful monitoring the optical quality control that you often perform to 100%.

This is necessary to be sure that the windshield we just manufacture is in compliance with existing standards. These are either official standards valid whatever the windshield type, for example, the international standard ECE-R43 which measures the distortion of the image of a multitude of circles or specific requirements of a given automaker. One of them requires, for example, a pattern projected onto a screen through the windshield (the plane of symmetry coincides with the optical axis) shows bands at any point of width of less than 8 mm, whereas in the absence of default, the screen shows the parallel black and white stripes with a width of 12 mm. In addition it is required that, on a 300 mm strip length, the variation in width does not exceed 4 mm. The measure is to make twice, first with horizontal stripes and a second with vertical stripes. Another manufacturer uses the same device and the same pattern but with a windshield whose symmetry plane is turned 55 ° to the optical axis with the tapes of the slide that was rotated until obtaining the maximum effect. The variation in the absolute width of the strips tolerated on the screen is only ± 3 mm, while a requirement that in any square of 5 cm side, the projected line not vary by more than 4 mm.

The previous three control methods to assess the optical power or its variation. In general, the three physical quantities that seeks to measure on a glazing is the angular deviation, the angle of the multiple images and the optical power. The first variable is the consequence of the fact that a light beam passing through the glass, can undergo a change of direction which "see" objects observed in a different place than they actually occupy. The appearance of multiple images is observed in the case of strong contrast between a bright object and its environment and consists of the appearance of a ghost in the vicinity of the main image. The third variable, whose measure is the object of the invention, the optical power, it is also called distortion because it distorts the observed images, it is mathematically expressed by the derivative of the angular deviation from space and its measure is either to study the variation of deviation of a spot beam which explores the glazing or to observe the distortion of the image of a known object defined and located on one side of the glazing and that is observed through the same glazing from a point on the other side.

existing control methods concretely use processes that belong to a principle or the other, but they are always throwing techniques, or with a localized beam such as that of a laser, or by projecting a pattern to through more or less of the windshield. The technique of localized beam, like that of US Patent 4,398,822 or US 4,453,827 which relate to airplane windshield, allows accurate measurement, in particular the angular deflection of the light rays at the crossing of the place concerned by the narrow beam, but it requires very long measuring time if we want to have an overall assessment of the windshield. And that is precisely the case when you want to be certain that a default limit is reached by any auto windshield allowed to control output of a production line.

Other known methods are global methods. That of US Patent 4,299,482 that performs a Fourier transform of the image of a striated screen as seen through the windshield requires carrying a photograph and is, therefore, unsuitable for continuous control in production. The method described in the patent application DE 36 00 199 uses it, the moiré between a striated screen and projection through the windshield of an identical pattern that rotated slightly relative to the screen so as to obtain, in the absence of defects, rectilinear moire fringes. The deformation of the fringes is observed in the eye and measuring the most important information on the distortion optical defect the biggest of the field concerned. But this is a visual method it would be very difficult to automate.

Other methods propose to illuminate a transparent object with a substantially point source of light and observe on a screen projected image; this technique, known as "shadowgraph" can identify the optical power of the object of defects, in particular a glazing. Indeed, where there is a converging lens, the light rays gathered and illuminate more strongly the region of the concerned screen while, on the contrary, the affected area is darkened in the case of a diverging lens. In this type of method, the visual assessment can only be qualitative because the eye is unable to quantify the differences in brightness. Thus, when these methods are used in the windshields of production units, this can only be to perform a quality control during production or end of the chain.

A measurement method, based on the same principle was proposed. Patent application EP 0342127 assesses the ombroscopic image of a glass ribbon obtained by reflection when illuminated under grazing incidence. But its evaluation process, which uses a measure of illumination, is very sensitive to ambient lighting. It requires spotless windows tested and it causes problems when using colored glass.

The last type of measurement methods and automatic optical quality control glazing use patterns of projection through the glass on a screen. And FR 2,556,097 proposes to automate traditional visual method. through the windscreen is observed, using a special camera, regular rectilinear scratches carried by a screen. The scan is performed with a fixed speed, allows to "measure" the width of the dark and light rays in the direction of scanning and to deduce information about the corresponding default.

In FR 2,556,097, it specifically proceeds as follows: a screen consisting of parallel strips alternately light and dark inclined at 45 ° is located vertically at a distance from the windscreen to test. The other side of the windscreen, is placed a CCD camera which is aimed at the screen therethrough. Camera and screen are fixed, but the windshield moves horizontally, perpendicular to the optical axis of the camera. This saves the image of a vertical line of the display - view through the windshield - that provides its goal. The same line will be observed the next moment through a parallel section of the windscreen to the first and which is distant from 10 mm. This method therefore allows a measurement in the scan direction of the camera (it is however possible to rotate so as to perform the measurement in a defined direction other than the vertical). If the optical power defect has no rotational symmetry (astigmatism) or if a fault is cylindrical in a direction different from the scanning direction, the measured default is not the real fault but only the component in the direction concerned. Furthermore, the measured optical defect is averaged over a width of 10 mm. The resolving power of the method is low and well below that required by government regulations or customer recommendations. For these two reasons, unidirectional measurements and insufficient resolution, the method of FR 2556097 can not be a method of controlling 100% of a production windshield.

The invention proposes to provide automatic measurement of optical power across a glazing, particularly a windshield, to measure the value of the real optical defect whatever its direction and comparing the measured value at predetermined tolerance limits. Moreover, the method of the invention must allow an assessment of the windshield according to the criteria of the usual standards and requirements in the automotive industry.

The invention provides a method for measuring the distortion of a glazing in which analysis is carried out of the image of a pattern consisting of parallel rectilinear strips, alternate light and dark, with the aid of a detector light scanning and which are stored as light-dark and dark-light transitions in the scanning direction. The analog signal from the detector is digitized into "one" if it is above the average signal and into "zero" if it is less. The pattern has two bands networks inclined with respect to each other, the scanning direction making a zero angle with the direction of each network. Moreover, the pattern has clear unit polygonal areas isolated in a dark single field or, conversely, dark unit polygonal areas isolated in a clear single domain.

In a variant of the invention, the isolated areas are squares.

The method of the invention further provides that it is the intersections of light-dark transitions, that is to say, the tops of isolated polygons, whose positions are compared to their theoretical positions.

In the case of isolated domains squares, are the lengths of their diagonals is compared with the same lengths in the absence of defects.

The method also provides that the peaks of all the isolated squares are associated in fours to form a continuous network with a square mesh of which comparing the length of the diagonal to the same lengths in the absence of defects.

The invention also relates to a device for implementing the method of measuring the distortion and wherein the scanning of the image is provided by a linear camera provided with a diaphragm and associated with a rotating mirror. In the device, the test pattern is carried by a screen which is observed by the scanning light detector through the glazing. Moreover, in a variant, the bright parts of the screen are transparent and illuminated from behind. Advantageously, these are Fresnel lenses which are placed against the screen and which are themselves illuminated by point sources, whose position is conjugate of that of the opening of the camera diaphragm relative to the lens in question.

In another variant, the bright parts of the screen are equipped with retro-reflecting elements and illuminated by a point source in the vicinity of the opening of the camera diaphragm.

Finally, to adapt the measuring direction to the required specifications can be turned to the rod.

The invention also concerns the application of the method to measure the auto windshield.

The method of the invention thus makes possible an industrial line control of all production unit of a modern automobile windshield manufacturing. It allows either to evaluate the maximum default regardless of its direction or to measure distortion in a preferred direction to conform to a particular standard or prescription of a customer's specific requirements. Measurements can be performed in the atmosphere of the workshop without requiring a work in obscurity as is the case with ombroscopiques methods. The glazing of the cleanliness condition is not critical. The method of the invention also allows simultaneous visualization of defects in an area of ​​the window whose size is selected - if necessary global vision - making it an important tool for the development of a new manufacturing or correction a production problem.

The description and figures will understand the operation of the invention and appreciate the advantages. Among the figures:<ul><li>■ <b>Figure 1</b> shows the installation which enables measurement,</li><li>■ <b>Figure 2,</b> presents an embodiment with a linear detector camera,</li><li>■ <b>in Figure 3,</b> is shown the pattern of the preferred embodiment of the screen of the invention,</li><li>■ <b>Figure 4</b> shows a variant of the illumination system,</li><li>■ <b>Figure 5</b> shows another variant of this system,</li><li>■ <b>Figure 6,</b> one shows the flowchart of the sequence of operations in electronics and computing.</li></ul>

The installation required for the performance of the method of the invention comprises three main elements. They are shown schematically in Figure 1. A camera 1 equipped with its target 2 can observe, through the glass 3, the display 4. The camera allows an evaluation of the light intensity at any point in his field. In general, it is large enough to allow a comprehensive observation of the glazing. When the camera view does not allow it, it is necessary to perform two or more partial steps, one after the other, which requires a higher data acquisition rate to maintain the same rate control. The display 4 has made a pattern of parallel black stripes it is easier to perform all of the same width. The width of the intermediate white stripes may also advantageously be the same. The test pattern has two networks of the preceding strips, inclined with respect to each other so as to constitute a continuous black area 5 trapping many white 6 identical domains. In the simplest case, these are diamonds.

In the figure, it is the white areas 6 which are isolated. We get the same result if it were the black areas that were isolated in the middle of a white single domain. However, in this case, an excess of light can be detrimental by causing the camera saturation and optical heating.

FIG 2 shows a practical embodiment of the method. The screen 4 is made of a frosted glass mate sand. It is backlit by a large number of lamps that illuminate 7, of substantially the same manner, all areas 6. The glazing 3 is an automotive windshield supported by a device 8 that can orient both about a horizontal axis as a vertical axis. The first orientation allows, for example, to find the position of the windscreen mounted in the car, the second reproduces the conditions imposed by certain standards. The camera 1 of Figure 1 is replaced here by the system placed on the table 9. It is massive and mounted on supports 10 for damping vibration. The system comprises a CCD camera 11 with a vertical linear detector. The tests were carried out with a camera CA-2 type manufactured by TA DESIGNS LTD in MAIDEN NEWTON (Dorset, UK). Its detector 3456 includes unit cells 7 x 7 mμ² spread over a strip of a length of 24.2 mm. 12 The objective of the CCD camera enables it to form on the strip the image of a vertical line segment with a length of 460 mm in the screen plane 4. The overall objective distance is 12-screen 4 here 4 meters. In general, there should be a goal glazed distance is the same as that provided in the standard that we want to comply. In the case of the ECE-R43, for example, it should be 4 meters. The horizontal movement of the beam during its scan is obtained by rotating a mirror 13 located in front of the lens 12. The mirror is rotated about a vertical axis by a DC motor 14 and a worm drive system in the housing 15. the motor 14 is equipped with a tachometer and an incremental encoder. On experimental design, the amplitude of angular displacement is such that it allows the measurement of a half windshield. In addition, a diaphragm 16 of a diameter of 8 mm is placed on the lens 12. Again, the choice of the diaphragm will depend on the standard to be met. The electronic and computer equipment, located on a nearby table 17 includes a CCD camera control system and an interface for controlling the motor 14 according to the indications of the tachometer and encoder. In the housing 18 are the processor of a microcomputer PC 80386 and the circuit connecting the previous control circuit and the microprocessor interface. A CRT 19 and a keyboard controller, not shown, complete the package.

The optical control glazing - in this case, the windshield - takes place as follows:

The windscreen 3 is installed on the holder 8 in the position (inclination relative to the horizontal and rotating with respect to the mirror axis center 13 of the screen 4) chosen for the measurement. A clear area 6 of the screen 4 is lit by (the) light (s) located 7 (s) behind him. With glass diffuser, the light that illuminates is rebroadcast on the other side. At a given time, is the elongated area 20 of the screen which was analyzed by the Bar with the unit cells. Its dimensions are 0.13 x 460 mm. In fact, the camera lens 11 forms an image of the 4 two-dimensional screen, but only at the time in question, the area 20 is visible to the array of cells. When light rays, from the zone 20, through the windshield, they are deflected as a function of optical defect located at this location and the camera image is not in conformity to the object (the corresponding region of the sight), she underwent a distortion. This is what will be measured.

The table that bears the mirror 13 rotates about a vertical axis situated in the reflector plane. Each time the mirror has rotated 0.002 °, the encoder performs an incrementation by one unit. The elongated zone 20 has, also, moved 0.28 mm. It is that which is measured by the bar.

The principle of the measurement consists in pinpointing the position of the four vertices of the diamond constituting the clear areas 6 of the test pattern, to involve four by four to form a continuous network with a square mesh, measuring the distances of the nodes of the meshes and to the compared to what they were in the same place, in the absence of the glazing. This assessment is made between diagonal corners of a part (2 steps) and between neighboring vertices of the other (4 measures).

The measure of a window begins with the acquisition of data: when the system is switched on, the PC the software moves the mirror 13 until a special mark, at one end of the test pattern , becomes visible to the camera detector. At this time, the motor is reversed, the encoder is reset and measurement begins. The mirror rotates in a new direction. Whenever he rose 0.002 °, the encoder is incremented by one and when the rotational direction is reversed (that is to say when the counter count rose by a specific number and the 'reverse order was transmitted to the motor), the total is reduced by one after each rotation of 0.002 °. The other function of the encoder is due to the impulses it generates, giving the playing order of the cells of the array of the CCD camera detector. One then obtains an analog signal whose amplitude is proportional to the light intensity received by each of the cells of the array. Each value is compared to a level corresponding to an intermediate brightness between light and dark and becomes "zero" if lower or "a" if it is greater. The reference level is dynamically adjusted to a value that exactly matches the average of the two "white" signals and "black" precedents. It follows that the illumination changes, both of the test pattern as the atmosphere, do not disturb the measurement. By digitizing the analog signal from the cells, was decreased greatly the amount of information to be stored.

Another circuit called "pixel location" is used to further reduce significantly the amount of information stored. The system consists of analyzing the "zeros" series and "ones" of a bar, identify transitions 0 → 1 and 1 → 0 and raise their x-axis and the direction of the transition. This information (extremely small compared to the original analog signal) are stored in a FIFO (First In, First Out). Its content will be "read" by the CPU of the PC already has in memory the value of the encoding of the mirror 13, that is to say, the ordinate of the point where the black-white-black transition or white that comes from being recorded has occurred. The information stored in the memory of the microcomputer are thus very limited: at the end of a complete scan of the pattern, only the coordinates of each vertical transition, black-white one hand, the other white-black hand, are stored. This feature of the invention is particularly advantageous. Indeed, in the prior art, the number of information to be stored and compared with reference values ​​in memory too, was such that their treatment using a time machine with inconsistent production rates. The only solution compatible with industrial rates would be to limit the optical system resolution for reducing the number of information to be processed.

Following acquisition, the data is processed by software developed especially. Its functions are:<ul><li>■ connect the corresponding transitions of two adjacent vertical lines, so as to reconstruct the boundaries of white of the pattern areas;</li><li>■ for each blank field, determine the four vertices and midpoints of the sides;</li><li>■ for each white area, calculating the distortion in four directions, for example in the case of square areas vertical diagonals, the four directions are: vertical, horizontal, 45 ° right and 45 ° left. This measurement is performed by comparing the distance between the above characteristic points (vertices and midpoints of the sides) with the theoretical values, that is to say measured at the same place in the absence of glazing;</li><li>■ calculate the distortion on the same terms where possible on the black squares, using the same reference points.</li></ul>

The following operations differ depending on the purpose of the operation - Either one wishes to study the quality of the glazing, determine the critical areas so as to act on the production parameters such as, for example, on the shape of the press in the case a windshield, or we want to control only the tested glazing is in accordance with specifications defined.

If there is a qualitative expertise, the sequence of operations is:<ul><li>■ display on the screen 19 of the image of the pattern, possibly enlargement of a portion of the image. It is either the image, obtained from the analog signal from each cell after it is scanned or a reconstruction obtained from the peaks areas. Simultaneous display possible distortions in each area (the quantities and units are those that intend to: differential deviation in milliradians or optical power in diopters or focal distance of defect in meters or any other quantity required by a standard;</li><li>■ develop, with colors corresponding to different levels of defects, areas of glazing;</li><li>■ stored in the PC hard drive, all data for later statistical analysis or an assessment according to other rules.</li></ul>

In the case of the type "stop or go" control the entire production of glazing for example windshield, the coordinates of each field are returned to their position in the windshield and, depending on the area concerned, the optical values ​​measured in the field will be treated differently. If it is the peripheral area of ​​the windshield, the measures will be "forgotten" but if it comes to central areas where the limits are set by official regulations or customer requirements, measurement will be compared with the imposed limit and glazing accepted or rejected as appropriate. Here too, it is possible to store the information in a statistical evaluation purpose.

In Figure 3, preferred pattern of the form is shown of the invention. The white areas 6 are squares whose diagonals are oriented in the direction where the measurement must be made with the best accuracy. For example, if the standard method that we want to comply uses black bars and horizontal alternating white, diagonal will be vertical (and possible measures in other directions than the vertical will not be considered). If the method recommended by a car manufacturer imposes a given rotation of the windshield around a vertical axis and the measure in a direction that depends on the shape of the windshield, the installation will be such that the windshield march past the camera with the right guidance, as to the pattern we choose with diagonal white areas in the recommended direction (which has been empirically determined once and for all for each type of windscreen).

The invention can be implemented with various patterns. It has already been discussed above in white areas with a diamond-shaped or even square, any polygonal shape that isolates clear areas in the middle of a dark field single appropriate. (The symmetrical situation, dark polygonal islands in a clear single domain is equivalent). The choice of the polygonal shape of the islands meets two criteria, firstly, we need to make islands and, secondly, we need to also exploit areas outside the islands. Thus, in Figure 3, open squares four neighbors (and together defining a square themselves nine times greater than each) enclose between them a black cross consists of five dark squares and all vertices (twelve in total) this cross are the same as those of the four light squares. The space is fully squared and without any "terra incognita" remains. The system's resolving power is at least one corresponding to the grid network summits clear squares. (Actually it's better because of additional points on the sides of the square clear that further improve the mesh). Other polygonal shapes, equilateral triangles are not suitable because the bright areas are never isolated in the middle of a single dark field (or vice versa). With rectangles or squares, the condition to be met is that the scanning directions make a nonzero angle and not straight with the sides of the fields, the principle of the measure, in fact, that we have transitions clear- dark or dark-light in the scanning direction.

The measuring system described above, in the case of square unit areas, provides a good resolution power. Indeed, at the windshield, each surface dimensions 4 mm x 4 mm is evaluated individually. For half a windshield which has a width of 0.75 m and a height in the position of use of 0.46 m, the number of measurement points is, for example, of more than 23000. The time available for collect information on each cell of the vertical web of the camera is thus very short, for example 2 msec if the rate control half windscreen is of one every 15 seconds. To double the pace (a windshield every 15 seconds) or the optical system is doubled, or it is necessary to increase the processing speed thanks to a more powerful microprocessor.

A consequence of the above is the need for sufficient lighting for an electrical signal of sufficient intensity. Figure 2 arrangement is satisfactory, however, the required electrical power is important. Figures 4 and 5 provide more energy efficient systems. The screen 21 of Figure 4 is the same as that of Figure 2 with one difference: instead of diffusing the glass support of the pattern is here transparent. It is backlit using intense point light sources, namely quartz-halogen lamps 22. These are placed behind the Fresnel lens 23 at a point which is the conjugate point of the entry of the diaphragm 24 of the camera lens. This system allows all the light from the lamp 22, which crossed the white areas of the screen to be in the plane of the diaphragm 24.

A variant of the previous system uses a single very intense point source; it is represented in Figure 5. The lamp 25 with a reflector 26 and condenser 27 illuminates the half mirror 28 which reflects the light towards the screen 29 that carries the pattern. White fields 30 thereof are made of a catadioptric material, eg a paint such as reflectorized beaded screens. The incident light is thus returned to its original direction and returns in part penetrate the diaphragm 24.

Figure 6 depicts the flowchart of electronic and computer operations for the acquisition and processing of data.

Seen in the CCD camera 31 having a vertical scanning array. We also see three frames, each of which consist of different logic functions. The frame 32 is designed to control the camera, the frame 33 is the scanning device and the frame 34 corresponds to the processing of data in the computer. Items 32, 33 and 34 are associated with each other through an interface card 35.

The element 33 comprises, at 36, the engine rotating mirror, 37, tachometer, the movement of the mirror is controlled by the speed control card 38. Between 36 and 37 is found in 39, the encoder incremental.

The element 32 comprises for its part, in 40, the image acquisition circuit, supplied directly to the output of the camera 31, the analog / digital conversion circuit 41 and then, at 42, the pixel location circuit and therefore white-black transitions, black-white.

The functions performed by the microprocessor, shown schematically at 34, are transferred to RAM memory 43, the comparison with the reference values ​​and the default calculation 44 and the alphanumeric output and / or graphic 45 and finally control General process in 46.

The method just described is to look through the glass a pattern to the precise geometry. A different device where it is a specific slide that is projected through the glass on a screen, the screen being watched by the camera, allows the implementation of the same method with the same characteristics and the same benefits, is simply a variant of the device of the invention.

The advantages of the method of the invention, besides the speed, accuracy, ease of use mentioned above, also lie in its flexibility. Depending on the software equipping the computer (part 34 of Figure 6) can either make a thorough study of the optical quality of a glazing, for example in the development of tools for bending to manufacture the lens WINDSCREENS or verify the intrinsic quality of each glazing made - possibly to the specific requirements of a customer - and, in addition, if desired, follow statistically production so as to prevent abuses.