DE102014105742B4 - Optical quality control method and quality control device - Google Patents

Abstract

A method of optical quality control by taking, by means of a lens (10), comprising a lens arrangement (101, 102) and an aperture stop (103) with an aperture aperture, from test objects (20) imaged on an image sensor (30) and comparing the recordings and / or values derived therefrom with stored reference data, a color channel change being performed between individual recordings from an output color channel in which the immediately preceding recording was made to a target color channel at which the immediately following recording is to take place, characterized in that With each color channel change, a change in size of the aperture diaphragm opening takes place automatically, the size of the aperture diaphragm opening to be respectively set being determined according to predetermined rules as a function of the respectively current target color channel.

Description

Field of the invention

The invention relates to a method for optical quality control by recording, by means of a lens, comprising a lens arrangement and an aperture diaphragm with an aperture aperture, of test objects imaged on an image sensor and comparing the recordings and / or values derived therefrom with stored reference data, wherein between individual recordings Color channel change from an output color channel, in which the immediately preceding recording was made, to a target color channel, in which the immediately following recording is to take place, is performed.

• - eine Fördervorrichtung zur Förderung von Prüfobjekten in einen Kontrollbereich,
• - eine Bildaufnahmevorrichtung mit einem Objektiv und einem Bildsensor zum Aufnehmen der jeweils im Kontrollbereich positionierten und mittels des Objektivs, umfassend eine Linsenanordnung und eine Aperturblende mit einer Aperturblendenöffnung, auf den Bildsensor abgebildeten Prüfobjekte,
• - eine Auswerteeinheit zum Vergleichen der Aufnahmen der Prüfobjekte und/oder daraus abgeleiteter Werte mit hinterlegten Referenzdaten, sowie
• - eine Steuereinheit zur Ansteuerung von Farbkanalwechseln zwischen einzelnen Aufnahmen, jeweils von einem Ausgangs-Farbkanal, bei dem die jeweils unmittelbar vorangegangene Aufnahme erfolgte, zu einem Ziel-Farbkanal, bei dem die jeweils unmittelbar folgende Aufnahme erfolgen soll.

The invention further relates to a quality control device comprising

• a conveying device for conveying test objects into a control area,
• an image pickup device having an objective and an image sensor for picking up the test objects respectively positioned in the control region and being imaged onto the image sensor by means of the objective, comprising a lens arrangement and an aperture stop with an aperture stop,
• an evaluation unit for comparing the recordings of the test objects and / or values derived therefrom with stored reference data, as well as
• - A control unit for controlling color channel changes between individual shots, each from an output color channel, in which the respective immediately preceding recording was made, to a target color channel, in which the immediately following recording is to take place.

State of the art

Such quality control methods and apparatus are well known in the industry. Engineered products, referred to herein as "inspection objects" in view of their quality control function, are fed by means of a conveyor to a control area under observation by an image capture device. The image recording device essentially comprises an image sensor and a lens with which the test objects in the control area are imaged onto the image sensor. The objective typically comprises a lens arrangement and an aperture stop, through the aperture aperture of which light from the test object falls onto the image sensor. The recorded images or values derived therefrom, such as length or distance values determined, for example, by image processing methods, are then compared in an evaluation unit with corresponding reference data in order to determine differences between realized actual values of the test objects and predetermined desired values. Depending on the size of the deviations, the test objects can then be qualified as proper or reject, and in the latter case in particular suggestions for a change in production parameters can be derived.

Especially in mass-produced products such quality control is largely automated. Purely by way of example here is the quality control in the chip production called. The very small and filigree structures that occur here, which must be checked in the context of quality control, require a very high-quality optics of the image recording device, i. after a very well corrected lens. In addition, due to the automated process, no comprehensive adjustment of the lens can be done for each shot of a test object. To compensate for process tolerances, such as temperature variations, an adequate depth of field is required, which is sufficiently large to compensate for said tolerances, but should not be greater than necessary in view of the associated difficulties in lens design.

A particular difficulty arises in cases where the quality control in different color channels must be performed. As color channel, the limited spectrum of the light contributing to the respective recording is generally understood here. A color channel change can be carried out in different ways. For example, the spectrum of an illumination light source, for example an LED arrangement with differently colored LEDs, with which the test objects are illuminated in the control area, can be changed. Alternatively or additionally, the light falling from the test object through the objective onto the image sensor can pass through a color filter whose transmission spectrum is changed. Finally, it is also possible to perform a color change by varying spectrally different sensitive sensors; In the present case, this also includes the use of a so-called color sensor whose "pixels" are each subdivided into a plurality of "subpixels" of different spectral sensitivity, as is the case with typical color CCD sensors. The achievable image sharpness of a given lens depends strongly on the wavelength of the light used. In this respect, the depth of field depends on the wavelength, ie on the selected color channel. Is there now a requirement for each color channel to have a minimum Depth of field is usually followed by the approach to correct the lens so high that the sharpness and depth of field requirement for all wavelengths are met simultaneously. This requires a tremendous amount of correction, making the lens expensive and difficult to assemble and susceptible to the high number of lenses commonly associated therewith.

From the EP 1 791 075 B1 For a barcode reader, it is known to improve the depth of field by imaging the barcode in duplicate on different subregions of an image sensor, wherein the light for the different images passes through different aperture stops with differently sized aperture apertures. This approach, which is not suitable for imaging filigree objects because of its asymmetric beam guidance, also offers no approach to solving the above-explained color channel problem.

From the WO 2006/103663 A1 The approach is known to exploit a lack of chromatic correction of a lens targeted to improve the depth of field. Due to the lack of chromatic correction, different focal lengths of the objective result for different color channels, so that an object positioned anywhere within a comparatively large axial area is sharply imaged in any color channel. The corresponding color channel image is then used as representative of a sharp image of the overall object. This approach works only if the special color channel is irrelevant, since it can not be said in advance which of the (absolutely required) white light illumination channels will be sharply displayed under the specific circumstances, while all other color channels will be due to the lack of chromatic Correction just can not be focused. In particular, the method is not suitable for ensuring the sufficiently sharp imaging of test objects in multiple color channels.

From the DE 10 2004 028 616 A1 a camera lens, in particular for use in a visual enhancement device in a motor vehicle is known, the aperture diaphragm is formed by a transparent in the visible and infrared spectral range disc with an annular disc-shaped, radially outwardly disposed, visible light impermeable filter coating. This filter coating causes a larger f-number for visible than for infrared light, so that the depth of field is optimized both for day trips, in which the camera is essentially to detect visible light, as well as for night driving, in which the camera is essentially to detect infrared light. This system is suitable for coarse, rigid adjustment of the depth of field to very different lighting situations. For a quality assurance system that can handle a number of narrow color channels in the visible (and possibly infrared) spectral range and must be adaptable to different requirements of the individual case quickly and cost-effectively, this approach is unsuitable.

From the DE 60 2006 000 559 T2 For example, a quality control device for steel rolling mills is known which simultaneously emits visible and infrared light. It is proposed as an alternative to an achromatic objective to direct the light behind the camera lens to color channel-specific optical paths and to position the corresponding sensors in wavelength-dependent different focal planes.

task

It is the object of the present invention to develop generic quality control methods and devices such that high demands on the depth of field in different color channels with reduced effort in the lens design can be achieved.

Presentation of the invention

This object is achieved in conjunction with the features of the preamble of claim 1, characterized in that with each color channel change automatically resizes the Aperturblendenöffnung, wherein each set size of Aperturblendenöffnung is determined according to predetermined rules depending on the respective current target color channel.

The object is further achieved in conjunction with the features of the preamble of claim 8, characterized in that the control unit is connected to an aperture diaphragm adjustment device and is set, each time a color channel change a respective size of the Aperturblendenöffnung according to predetermined rules depending on the current target To determine color channel and to control the aperture diaphragm adjustment, adjust the aperture aperture accordingly.

Preferred embodiments of the invention are the subject of the dependent claims.

The invention is based on the finding that the limit spatial frequency which can be transmitted by a lens depends on the quotient of the numerical aperture and the wavelength of the transmitted light. In order to set the same transmittable limit spatial frequency for different color channels, one would have with an ideal lens the change the numerical aperture color channel-specific and indeed set the larger, the longer the wavelength of the respective color channel. With a real objective, this finding can be exploited in such a way that an aperture diaphragm with adjustable aperture aperture is used and the size of the aperture aperture is set color channel-specifically. In particular, as a general rule, the larger the aperture size to be set, the greater the wavelength or, more precisely, a specific wavelength value of the target color channel. This in turn means that, in particular for color channels of short wavelength, a complex correction in the outer regions of the objective is unnecessary. On the other hand, the correction required for long wavelengths also in the outer areas of the lens can be tailored specifically to these long wavelengths. The overall correction of the objective can thus be made simpler and more precise in terms of avoiding wavelength-overlapping design compromises. This not only leads to cost savings, but also to improved overall performance, so that particularly high requirements, as explained above, partly due to the invention for the first time be fulfilled.

Of course, such a lens for general applications, such as general photography, in which many color channels are recorded simultaneously and / or within individual color channels due to special lighting circumstances, the most different aperture settings must be feasible, little suitable. However, the invention relates specifically to the explained quality control sector with its special requirements.

As a rough rule for aperture diaphragm adjustment, it has already been explained above that, preferably, the larger a specific wavelength value of the target color channel, the larger the size of the aperture diaphragm aperture to be set is determined to be. Specifically, what is used as the specific wavelength value can be selected by those skilled in the art in consideration of the requirements of the case. For example, in one embodiment of the invention it may be provided that the value of the central wavelength of the target color channel is used as the specific wavelength value. The central wavelength, i. the wavelength lying centrally between the cut-off wavelengths of the color channel represents a particularly easily determinable value for characterizing the color channel.

Alternatively it can be provided that a wavelength mean value of the target color channel is used as the specific wavelength value. The term mean value is to be understood broadly and includes in particular also weighted or otherwise determined wavelength average values. Such an average is more difficult to determine; but may more accurately characterize the color channel, e.g. if there is a clear wavelength inequality distribution within the color channel.

It is advantageously provided that the size of the aperture diaphragm opening to be set depends linearly on the specific wavelength value of the target color channel. Of course, depending on the requirements of the case, it is possible to deviate from this general rule. As a rule of thumb for a possibly to be refined basic attitude, this rule of thumb is quite suitable.

In general, it can be specified as default of the aperture diaphragm setting that the size of the aperture diaphragm aperture to be set is ideally determined in each case in such a way that the same depth of field results for the target color channel as for the output color channel. This depth of field can correspond to the required depth of field (as exactly as possible). A smaller depth of field is out of the question due to the requirements of the system; However, if significantly higher depth of field is realized in a color channel, the potential for savings opened up by the invention in the context of lens design may have been reduced. only partially exploited.

Further features and advantages of the invention will become apparent from the following specific description and the drawings.

list of figures

• 1: Linsenschnitt und Strahlengang eines beispielhaften Objektivs zur Realisierung des erfindungsgemäßen Verfahrens und der erfindungsgemäßen Vorrichtung,
• 2: ein Schärfentiefen-Diagramm des Objektivs von 1 im Weiß-Kanal,
• 3: ein Schärfentiefen-Diagramm des Objektivs von 1 im Rot-Kanal,
• 4: ein Schärfentiefen-Diagramm des Objektivs von 1 im Grün-Kanal,
• 5: ein Schärfentiefen-Diagramm des Objektivs von 1 im Blau-Kanal.

Show it:

• 1 : Lens section and beam path of an exemplary objective for realizing the method according to the invention and the device according to the invention,
• 2 : a depth of field diagram of the lens of 1 in the white channel,
• 3 : a depth of field diagram of the lens of 1 in the red channel,
• 4 : a depth of field diagram of the lens of 1 in the green channel,
• 5 : a depth of field diagram of the lens of 1 in the blue channel.

Description of preferred embodiments

1 shows a lens designed as a simple double Gaussian lens 10 comprising an object-side lens group 101 and a mirror-symmetrically constructed and arranged image-side lens group 102 , Midway between the lens groups 101 . 102 is an aperture stop 103 arranged, which, as by the adjustment arrow 104 indicated, automatically adjustable. The objective 10 is suitable for an object 20 in the scale β '= - 1 on an image sensor 30 map.

In 1 not shown are means for changing the color channel, which are known to those skilled in various configurations. For example, by filtering the object 20 illuminating light or by filtering the object 20 on the image sensor 30 falling light a color channel defined and changed accordingly. In the latter case, it is possible for the filter on the image side or object outside the lens 10 or within the lens 10 , For example, in the same or closely adjacent position to the aperture 103 to arrange. Also in the image sensor 30 Integrated filtering to define or change the color channel is conceivable. As an alternative to filtering, it is also possible to generate spectrally narrow-channel light, for example by means of an LED arrangement, and to illuminate the object 10 be used.

As discussed in detail in the general part of the description, it is an essential feature of the invention that the aperture size of the aperture stop 103 is changed automatically with the change of the color channel. In order to meet a given depth of field condition in all color channels, a lens with fixed aperture size would have to be extremely highly corrected, as far as the requirements on the required color channels can be fulfilled. By contrast, the color channel-specific adaptation of the aperture diaphragm size according to the invention makes it possible, on the other hand, to have high depth of field requirements even with a comparatively simply constructed objective, such as a lens 10 according to 1 , fulfill.

The 2 to 5 show example of depth-of-field diagrams of the lens 10 from 1 in different color channels, each with adapted size of the aperture aperture. The diagrams are to be understood as follows: on the ordinate is in each case the modulation transfer function ( MTF ) plots the quotient of image contrast to object contrast for a given spatial frequency, for example given as a particular number of line pairs per centimeter. The MTF can be in percent or, as here, as a normalized function between the maximum value 1 (same contrast in picture and object) and the minimum value 0 (complete loss of contrast in the picture). On the abscissa of the diagrams is the defocusing D in millimeters, for example realized by a variation of the distance of the image sensor 30 from the lens 10 where "0.0" represents the optimum position for a sharp image. The curves show the behavior of the MTF depending on the defocusing D at selected measuring positions distributed over the sensor surface. For the sake of clarity, the graphs show curves of only 4 Measuring points shown. Typically, significantly more measuring points are used, although for each measuring point, a distinction is usually made between the contrasts in the sagittal and meridional planes.

As in the example shown, the default is that over a defocus distance of at least 100 microns a MTF For a given spatial frequency of at least 0.4 over the entire sensor surface, the curves for all measurement points in a defocus region of at least 100 microns must be above the dashed line boundary of MTF = 0.4. In the illustrated embodiment, this is achieved by the white channel ( 2 ) the aperture stop is adjusted so that a numerical aperture ( N / A ) of 0.04. This results in a Defokussierungsbereich of 120 microns, in which the MTF for all measuring points is above the limit of 0.4. In the red channel ( 3 On the other hand, the aperture stop needs to be increased until a numerical aperture of NA = 0.048 is established in order to achieve a defocusing range of 101 micrometers, in which the MTF for all measuring points is above the limit of 0.4. A smaller aperture aperture setting would result in contrast degradation due to diffraction effects.

In the green channel ( 4 ) the same aperture setting can be used as in the white channel. However, this results in a larger depth of field of 149 micrometers due to the narrower color channel. Again, a reduction in aperture aperture due to diffraction effects would lead to a decrease in contrast. An enlargement of the N / A would due to lack of correction of the lens 10 lead to a contrast deterioration.

In the blue channel ( 5 Finally, a numerical aperture of 0.034 is optimal and results in a depth of field of 176 microns.

The implementation of the idea according to the invention in a method according to the invention for quality control or in a quality control device according to the invention is easy for the person skilled in the art to carry out. In particular, he has for a given lens only by measuring the MTF determine at different measuring points of the image sensor which numerical aperture or Aperturblendenöffnungs setting for a given color channel the optimum depth of field. The determined values are then stored in a control unit which is set up to control color channel changes, and the control unit is to be coupled to an aperture diaphragm adjustment device such that an aperture diaphragm adjustment to the corresponding, stored opening value takes place with each color channel change. For the optics designer, who has to design lenses for such methods and devices, this means that he can leave it compared to over the entire spectral corrected lenses for a much simpler correction and the concrete depth of field requirements just as well or even better ,

Of course, the embodiments discussed in the specific description and shown in the figures represent only illustrative embodiments of the present invention. A broad range of possible variations will be apparent to those skilled in the art in light of the disclosure herein.

LIST OF REFERENCE NUMBERS

10

lens

101

first lens group of 10

102

second lens group of 10

103

Aperture aperture of 10

104

Verstellpfeil

20

object

30

image sensor

MTF

Modulation Transfer Function

D

defocusing

N / A

Numeric Appertur

W

White channel

R

the red channel

G

green channel

B

blue channel

Claims (8)

A method of optical quality control by taking, by means of a lens (10), comprising a lens arrangement (101, 102) and an aperture stop (103) with an aperture aperture, from test objects (20) imaged on an image sensor (30) and comparing the recordings and / or values derived therefrom with stored reference data, a color channel change being performed between individual recordings from an output color channel in which the immediately preceding recording was made to a target color channel at which the immediately following recording is to take place, characterized in that With each color channel change, a change in size of the aperture diaphragm opening takes place automatically, wherein the respective size of the aperture diaphragm opening to be set is determined according to predetermined rules as a function of the respectively current target color channel. Method according to Claim 1 , characterized in that the larger a specific wavelength value of the target color channel is, the larger the size of the aperture diaphragm aperture to be set is determined. Method according to Claim 2 , characterized in that the value of the central wavelength of the target color channel is used as the specific wavelength value. Method according to Claim 2 , characterized in that a wavelength mean value of the target color channel is used as the specific wavelength value. Method according to Claim 4 , characterized in that the wavelength average is a weighted wavelength average. Method according to one of Claims 2 to 5 , characterized in that the size of aperture aperture to be adjusted depends linearly on the specific wavelength value of the target color channel. Method according to one of the preceding claims, characterized in that the size of the aperture diaphragm opening to be set is respectively determined in such a way that the same depth of field results for the target color channel as for the output color channel. A quality control apparatus, comprising - a conveying device for conveying test objects (20) in a control area, - an image pickup device with a lens (10) and an image sensor (36) for picking up the respectively positioned in the control area and by means of the lens (10), comprising a lens arrangement (101, 102) and an aperture diaphragm (103) with an aperture aperture, test objects (20) imaged onto the image sensor (30), - an evaluation unit for comparing the recordings of the test objects and / or values derived therefrom with stored reference data, and - a control unit for controlling color channel changes between individual recordings, in each case from an output color channel in which the respectively immediately preceding recording took place, to a target color channel, in which the immediately following recording is to take place, characterized in that the control unit is provided with an aperture diaphragm Adjustment connected and egg is prepared, with each color channel change one to be set in each case size of the Aperturblendenöffnung according to predetermined rules depending on the respective current target color channel and to control the aperture diaphragm adjustment, adjust the aperture aperture accordingly.

Images