Abstract:
Systems and methods are disclosed for the detection and identification of objects, wherein an illumination device emits polychromatic light in the infrared range, creating a light curtain, or an essentially two-dimensional area of light in the X and Z axis. The light from the light curtain and light reflected or transmitted by an object in the light curtain is imaged, via aperture-imaging optics, onto an aperture that is in the optical path and behind the aperture-imaging optics. The aperture is an elongated opening extending along the Z axis. A wavelength-dispersive device, such as a grating, diffracts light admitted by the aperture wavelength-dispersively in a diffraction direction along the Y axis. An image sensor detects the diffraction image and generates image signals which are analyzed to identify the materials comprising the object. An output signal may be generated in response to the material identified.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a device for the detection and identification of objects by means of a light curtain, meaning an essentially two-dimensional light range. 
       BACKGROUND 
       [0002]    The purpose of light curtains is to detect objects that enter a monitoring area. These objects can be various materials, including inanimate items, or also persons. For this purpose, a relevant area is commonly illuminated by one or several light sources, and the radiation striking one or several recipients is analyzed, wherein for example intensity differences of the radiation striking the recipients are determined, and from this inferences are made regarding the changes due to the objects that were introduced into the light path. If applicable, additional optical components can be provided, for example reflectors, lenses, filters, polarizers, as well as control and regulation electronics in general. 
         [0003]    Consequently the presence or the entry of an object into a light barrier or a light curtain can be determined. More detailed qualitative statements regarding the objects acquired by a light barrier or a light curtain are, however, only possible with great difficulty. To some extent the external shape of an object moving through a light curtain can be identified. 
         [0004]    An essentially point-shaped detection by means of a light ray is commonly referred to as a light beam. If an essentially line-shaped light barrier is used, a two-dimensional area therefore arises that is referred to as a light curtain, and by means of which the intrusion into an area can be verified over an extensive surface. 
         [0005]    Light curtains are particularly used in security technology for automation applications. 
         [0006]    In spectroscopy the wavelength-dependent absorption and emission signals of materials are investigated and analyzed. Spectroscopy in the near infrared range (NIR spectroscopy) facilitates the very exact characterization of organic chemical materials, based on rotational and vibrational bands in the absorption spectrum of the material. By these means plastics can, for example, be distinguished precisely, the contents of liquids can be identified, or also human tissue can be distinctly identified. For that purpose, the object to be investigated is commonly placed in a measurement chamber and examined using a spectrometer. WO 02/44673 A1, WO 99/40414 A1, U.S. Pat. No. 6,433,338 B1 and DE 196 01 923 C1 present methods for the examination of objects by means of spectral analysis. 
         [0007]    US 2004 0 218 172 A1 describes the application of spatial light modulations for spectroscopy and imaging. For that purpose, a device is operated in the visible and near infrared light range. A sample can be illuminated with suitable, energy-weighted spectral bands in order to only identify relevant components. In so doing, the energy of the applied spectrally resolving elements can be modulated digitally in order to generate a tunable weighted spectral light. 
         [0008]    U.S. Pat. No. 6,993,176 B2 describes a method and a device for the imaging of a container that is filled with a liquid, wherein light from a light-emitting device illuminates the container, and light that has penetrated the container is detected and analyzed. In this case light in the near infrared range can be emitted in addition to the visible light. 
         [0009]    EP 0 562 726 B1 describes a method for the operation of a light curtain system with at least three light curtain segments that respectively have one transmission device for emitting a plurality of light ray bundles along channels toward receiving devices for the purpose of scanning. In this case temporal scanning is performed, wherein the channels extend at specified angles relative to one another and in a common acquisition plane in such a manner that they intersect one another. Selected ray bundles in each transmission device can be deactivated. 
         [0010]    DE 20 2004 020 863 U1 describes a device for securing a work area that has a hazardous as well as a less hazardous or non-hazardous section. If, by means of a first security system, an intrusion of an object in the hazardous section is determined, and, by means of an additional security system, no object is determined in the less or non-hazardous section, a signal trigger level for the generation of a signal is activated. 
       SUMMARY 
       [0011]    The apparatus and method disclosed herein advantageously provides for the detection of objects by means of a light curtain that permits the examination of detected objects. 
         [0012]    Furthermore, a system may be created which includes a machine with a device as disclosed herein. 
         [0013]    The apparatus disclosed is based on the idea of using the light curtain, meaning an essentially two-dimension light range, for the spectroscopic examination of the detected objects. For that purpose, light or radiation in the infrared range is used, in particular in the near infrared range, such as the VNIR range or from 750 to 1000 nm, and, using an elongated aperture, an essentially elongated or one-dimensional image is initially generated, which can be wavelength-dispersively fanned out, in particular diffracted, in a direction other than that of the image, preferably a direction perpendicular to that of the image. 
         [0014]    It has been recognized that using such measures a two-dimensional image can be generated using a surprisingly simple method with relatively simple means that provides wavelength-resolved information about objects acquired via the light curtain. Consequently, the use of NIR radiation, for example, can facilitate a differentiation of hydrocarbons and/or organic compounds. Since the radiation is detected by an image sensor or an image converter, a subsequent analysis is made possible so that the materials comprised in the object, meaning the materials that the object features, can be determined. The identification of the materials comprising the objects facilitates identifying the composition of the acquired object, in particular its material or chemical composition. Furthermore, output signals may be generated which provide the classification of the object or also for the initiation of measures, such as a selection of the objects or the halt of an operation. 
         [0015]    Consequently, in accordance with the disclosure, the functionality of a light barrier or a light curtain can be combined with a spectroscopic examination and analysis. The objects can be detected in the reflection direction, to which end the illumination device can also implement the light curtain via the first imaging optics, and illuminates for example a diffuse background. The objects can also be detected in the transmission direction. 
         [0016]    In accordance with the disclosure, the slot can correspond essentially to the direction of the line of the light curtain. The diffraction direction or dispersion direction can run perpendicular to this slot direction, so that the lines and columns of a two-dimensional pixel array of the image sensor can correspond to these directions. Consequently, this results in an image with a one-dimensional spatial component, corresponding, for example, to the vertical direction of the light curtain, and the orthogonal diffraction direction (dispersion direction) thereof for the determination of a diffraction image (dispersion image) and the relevant absorption spectra. 
         [0017]    The lens system is implemented advantageously with a telecentric lens as the first imaging optics in order to image the area of the light curtain onto the plane of the aperture or a slot of the aperture. The use of a telecentric lens facilitates a high depth of field, in particular across the entire relevant area of the light curtain, for example, the illuminated diffuse background and an area located in front of it where the objects are detected by the light curtain. By means of the first imaging optics, the imaging of the area to be monitored onto the slot of the aperture can be achieved, so that the aperture effectively blanks areas outside of the light curtain. Consequently the use of the aperture also makes it possible to illuminate a somewhat larger area with the light curtain than the area that is to be subsequently spectroscopically examined and which is delimited by the aperture. 
         [0018]    The wavelength-dispersive device comprises a dispersive optical element, usually an optical grating, in particular a holographic grating, which is a blazed grating in an advantageous embodiment, in order to facilitate a high light-yield in the diffraction order that is detected by the camera or the image sensor and in the wavelength range from 750 to 1000 nm. In principle the use of a prism is also possible, but usually a prism is not as advantageous as a grating. 
         [0019]    The diffraction images acquired by the image sensor can subsequently be analyzed directly by an evaluation device, for example, by a comparison with relevant reference spectra. In so doing the device according to the disclosure can also be calibrated beforehand by introducing relevant objects into the acquisition area or the light curtain in order to record corresponding reference spectra of different materials. In accordance with the disclosure, a comparison with theoretical reference spectra is also possible and sufficient. 
         [0020]    In the evaluation device, deviations of the spectral composition from, for example, the diffuse white background can be determined, wherein, for example, a multivariate statistical analysis can be performed in order to determine the characteristic spectral reflections or absorption components from the acquired spectrum. Different multivariate statistical analysis methods can be used, for example, correlation, regression, variant analysis, discriminant analysis, as well as principal components and factor analysis. 
         [0021]    In particular, in the range from 750 nm to 1000 nm it is possible to determine the characteristic properties of organic materials, in particular overtone and combination vibrational absorption bands. In this wavelength range, fully developed and cost-effective optical sensors with good signal to noise ratios are available. In particular, it is possible to use image sensors with CCD or CMOS technology. In particular, CMOS sensors offer the possibility of configuring a setting for a required image range, and the electronic circuit for the amplification of the analog signal and digitization of the analog image signals can be integrated in CMOS technology. 
         [0022]    Consequently, in accordance with the disclosure, the image sensor can be monolithically integrated in a semiconductor component together with the evaluation device and a control device, if applicable, as well as a storage device for reference data, if applicable, so that a compact and cost-effective embodiment is possible and elaborate additional wiring is omitted or can be kept to a minimum. 
         [0023]    In accordance with the disclosure, all optical means are also understood to be an aperture that blank out an elongated, strip-shaped range of the area of the light curtain that is imaged via the first imaging optics (lens). In so doing, the strip-shaped range does not have to necessarily be continuous, but can, for example, be assembled from a sequence of individual image elements. 
         [0024]    The device in accordance with the invention can have, for example, three imaging optics or lenses. Of these, the first imaging optics generates a two-dimensional, in particular, telecentric, image of the illuminated area on the aperture that is elongated or slot-shaped and preferably disposed in the image plane of this first imaging optics. The second imaging optics then images the slot-shaped aperture, for example, at infinity, for example, again as a telecentric arrangement on the image side, so that it facilitates the collimation of the light strip that penetrates the slot. A wavelength-dispersive device, which facilitates the wavelength-dispersive splitting of the light in the second direction, is disposed behind this second imaging optics. 
         [0025]    In accordance with the disclosure, the image sensor can be positioned in the optimized wavelength range, and will cover, for example, only a relatively small solid angle. The maximum diffraction efficiency can be selected such that it falls into the wavelength range in which the sensor being used has the lowest sensitivity. The blazed grating can, for example, be a reflection grating with an asymmetric, sawtooth-shaped grating profile, wherein the sawtooth flanks are implemented each as individual mirrors in such a manner that they reflect the light in the direction of the desired diffraction order. Furthermore holographic gratings can also be used. For example, can VPH gratings (volume phase holographic gratings) be used as blazed or holographic gratings. These VPH gratings are transmission gratings wherein a transparent, photo-sensitive material is enclosed between two glass or plastic plates, in which a desired pattern of a varying diffraction index was generated, for example, using holographic illumination and the resulting structural modification of the material. In accordance with the disclosure, the use of such blazed gratings makes it possible for high efficiencies of over 60% of the diffraction intensity to be achieved in a small specified wavelength range. 
         [0026]    The spectral composition of the light or the radiation emitted by the illumination devices is preferably distributed in a spectrally homogeneous manner across the wavelength range to be measured. The illumination device transmits preferably collimated light so that the distance of the object from the illumination device does not affect detection. For example, halogen lamps, meaning thermal radiators, or also different broad-band LEDs can be used as an illumination device, wherein also a light source with LEDs of other wavelengths and an additional fluorescence dye can be used that generates a broad-band emission in the spectral range from 750 to 1000 nm. 
         [0027]    The illumination device or light source can be operated continuously or also pulsed in time. Pulsed operation in this context has the advantage that the light curtain is on the one hand less dependent on changing external light influences, on the other only very short instances in time are recorded, which minimizes the movement effect of the objects. Furthermore higher currents can be used during pulsed operation. 
         [0028]    In accordance with the disclosure, the spectra can be evaluated as part of the analysis, in particular, in regard to their second derivatives. As a result, the method can be implemented more independently of device-dependent influences, such as variations in illumination or also broad-band parasitic absorptions that can overlay the characteristic rotational vibration spectra due to different surfaces or color pigments, etc. 
         [0029]    The device in accordance with the disclosure can be deployed, in particular, as a light curtain for the protection of security areas, in particular danger zones of systems or machines. In addition to just the detection of an object, it can, in this case, be possible to additionally perform a chemical analysis or a determination of the material composition in order to, for example, distinguish human skin or a human body from synthetic materials. Consequently, it is possible to detect, for example, when the hand of an operator or another body part without protective cover reaches into a hazardous area. 
         [0030]    A further application is the detection and sorting of materials, in particular recyclable materials or refuse. It is therefore possible that goods to be examined, for example, for refuse separation or redemption of recyclable goods, are transported through a light barrier in accordance with the invention or a light curtain in accordance with the invention, and examined with regard to the respective material composition during the traversal of the light barrier or the light curtain in order to, if applicable, subsequently select the materials and/or to prevent the intrusion of impermissible substances. Depending on the analysis, certain different subsequent output signals can therefore be set that can be used directly for subsequent interventions or to halt a device. 
         [0031]    It is to be understood that both the foregoing description and the following description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    In what follows, the invention is explained based on the enclosed drawings, using several exemplary embodiments. In the exemplary figures: 
           [0033]      FIG. 1  shows an exemplarly embodiment of a device in accordance with the disclosure in a) a top view and b) a front view; 
           [0034]      FIG. 2  shows a flow chart of the exemplary embodiment of the device in accordance with the disclosure; 
           [0035]      FIG. 3  shows an exemplary spectra of different organic materials or plastics; 
           [0036]      FIG. 4  shows an alternative embodiment to  FIG. 1   b ); and 
           [0037]      FIG. 5  shows a diffraction image recorded by the image sensor. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    Reference will now be made in detail to the present exemplary embodiments consistent with the disclosure, examples of which are illustrated in the accompanying drawings. Wherever convenient, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
         [0039]    A device  1  according to the disclosure has one or several illumination devices  2   a ,  2   b ,  2   c . According to the exemplary embodiment shown in  FIG. 1   a ),  b ), illumination devices  2   a ,  2   b  are provided for illumination in reflection. According to the exemplary embodiment shown in  FIG. 4 , an illumination device  2   c  is provided for illumination in transmission; combinations of these embodiments are fundamentally also possible. The illumination devices  2   a ,  2   b , or  2   c  serve the purpose thereby of implementing a light curtain  3  as a primarily two-dimensional area or an area with relatively small extent in the Y direction. The light curtain  3  is correspondingly evident in the top view of  FIG. 1   a ) as an essentially dotted line, and likewise in its two-dimensional extent in the front view of  FIGS. 1   b ) and  4 . 
         [0040]    In  FIGS. 1   a ) and  b ), the illumination devices  2   a ,  2   b  can emit light  4  for the formation of the light curtain  3  via an optical device  5 , for example a large biconvex lens, and illuminate a diffuse background  9 , which in turn reflects light. In  FIG. 1   a ), the illumination devices  2   a ,  2   b  are located slightly off the optical axis A for improved illustration purposes. Consequently the light curtain  3  is formed in  FIG. 1   a ) and  b ) between the optical device  5  and the background  9 . The illumination device  2   c  that is shown in  FIG. 4  for illumination in transmission can be, for example, a linear spatially and spectrally homogeneous light source that therefore implements the light curtain  3  directly. 
         [0041]    An object  6  that is detected by the light curtain  3  in the exemplary embodiments is therefore illuminated in each case by the light curtain  3 , and gives off object light  8  corresponding to its transmission or reflection response in the direction of the optical axis, meaning the x direction. 
         [0042]    In so doing, the light  4  or the light curtain  3  is broad-band in the near infrared range (NIR), for example in the wavelength range between 750 and 1000 nm, in particular with a spectrally homogeneous distribution in this wavelength range, which is correspondingly also displayed in  FIG. 3 . The object light  8  given off by the object  6  has a spectral distribution corresponding to its material composition, as is further explained below in reference to  FIG. 3 . Consequently the object light  8  contains spectra in the relevant wavelength range for the identification of its material composition. 
         [0043]    The light  4  of the light curtain  3 , along with the object light  8  given off by object  6 , reach an aperture  14  via the imaging optics  11 , which is formed by the optical device  5  and an additional optical device  10 . The imaging optics  11  therefore serve as a lens, in particular a telecentric lens for the imaging of the background  9  or the illumination device  2   c  onto the aperture  14 . In the case of such a telecentric lens, the depth of field can hereby be designed to be so large that the area of the light curtain  3 , and detected objects  6 , are within the depth of field, and are therefore imaged in focus. The imaging optics  5  is preferably correspondingly large, so that it collects light essentially parallel to the optical axis, or in the x direction. 
         [0044]    The aperture  14  is elongated, preferably as a slot or a slit, for example with a width of 30 μm, and extends in the vertical direction or Z direction, meaning in the plane of the light curtain  3 . If optical elements that change the Y and Z direction, such as for example mirrors, are placed in the optical path, this fact has to be taken into account correspondingly; it is only relevant, in order to be in accordance with the disclosure, that the light curtain  3  be imaged onto the slot in such a manner that its extent in the Z direction corresponds to the slot direction. In accordance with the disclosure, it is in principle also possible to use a wider light curtain  3 , since the aperture  14  limits the width of the utilized image. 
         [0045]    The strip of the image blanked by the aperture  14  of the light curtain  3  or the background  9 , including, if applicable, detected object  6 , is projected as light  15  via the second imaging optics  16  onto a grating  17 . The grating  17  is built and positioned in such a manner that the wavelength-dispersive fan-out of the diffracted light  19  takes place perpendicularly to the slot direction, meaning again in the transverse direction or Y direction; modifications would be apparent to those skilled in the art. The diffracted light  19  is imaged via a third imaging optics  18  as a diffraction image  30  onto a sensor area  20   a  of an image sensor (image converter)  20 . A diffraction image  30  of the aperture  14  or its slot  14   a  is therefore imaged on the sensor area  20   a , with the longitudinal extent of the slot  14   a  (the Z direction) in one direction or one axis, and the wavelength-dispersive fan-out of the diffraction image along the other axis. 
         [0046]    The image sensor  20  is preferably a two-dimensional CMOS digital camera sensor; it has, as indicated in the flow chart of  FIG. 2 , a pixel array  21  comprising individual pixels  22  that are sensitive in the NIR spectral range and that are positioned in a matrix arrangement. The pixel array  21  can, for example, be arranged in such a manner that the Z direction, meaning the longitudinal extent of the aperture  14 , corresponds to the abscissa or the horizontal axis of the pixel array  21 , and the Y direction, or the diffraction direction, corresponds to the ordinate or the direction of the columns, meaning the columns of the pixel array. 
         [0047]    Such a CMOS image sensor  20  makes it possible to record, for example with a single image recording, simultaneously up to a thousand or more spectra, meaning a spectrum per column, at a data resolution of, for example, 12 bit. Each of the spectra, therefore, corresponds to the spectrum of an image element of the aperture. That is, each spectrum corresponds to a subdivision of the slit-shaped aperture  14  into image elements that are arrayed next to each other in the Z direction and correspond to the pixel number of the dimension of the sensor. The light of each of these light elements is then imaged via the grating  17  onto the column of the image sensor  20 . 
         [0048]    The exemplary embodiment shown in  FIG. 5  displays such a diffraction image  30  recorded by image sensor  20 , wherein the abscissa is labeled with the spatial direction Z and the ordinate with the diffraction direction, in this case already converted into the wavelength λ that corresponds to the respective diffraction angle. In the image area  30 - 2 , an object  6  is detected between Z 1  and Z 2 . In this example, object  6  is a polyethylene beaker imprinted with a material that contains, for example, carbon black. In the areas  30 - 1  and  30 - 3  to the left and right hereof, meaning Z&lt;Z 1  and Z&gt;Z 2 , no object is present in the slot of aperture  14 . Thus, essentially the white light  4 , homogeneously and evenly distributed across the wavelength λ, of the light curtain  3  is directly recorded, and white light  4  therefore appears white as a result of the white balance of the diffraction image  30 . In the image area  30 - 2 , there is initially partial absorption in a lower section  30 - 2 - 1  and an upper section  30 - 2 - 3 , which is displayed as a gray value, represented here with shading. In section  30 - 2 - 2  around wavelength λ PE  there is strong absorption, so that this area appears black here, wherein depending on the thickness of the object there can be stronger or weaker absorption at the respective Z location. This has the corresponding effect that the λ absorption bands are broader in the case of higher absorption, meaning they are evident in a larger λ range. The top side of the object, which in this exemplarly embodiment is imprinted with carbon black or another broad-band absorbing material, is located at approximately Z 2 ; it absorbs across the entire wavelength range and, therefore, forms the range  30 - 2 - 4 . 
         [0049]    The image sensor  20  can repeat the image recording, for example with image repetition rate of, for example, 50 per second. Since in accordance with the disclosure, for example, only a small spectral range is relevant in the near infrared from 750 nm to 1000 nm, the partial image recording that is possible with such image sensors  20  can be used, so that partial images are configured as a so-called “region of interest” (ROI), which make it possible to only read out the configured, interesting image area of the image sensor  20 , while simultaneously maintaining the base data rate, which increases the number of transmitted frames, meaning images or partial images per second. 
         [0050]    According to the flow chart in  FIG. 2 , a control device  24 , which, for example, is disposed on the chip of the image sensor  20 , controls the illumination devices  2   a ,  2   b  (or corresponding to  2   c ) with control signals  51  in order to implement the light curtain  3 . Furthermore the control device  24  controls the image sensor  20  by means of control signals S 2  directly or initially via a sensor control device that is not shown here. The image sensor  20  passes on imaging signals S 3  for analysis to an analysis device  28 , which may transmit its analysis signals S 4  to the control device  24 . Alternatively, analysis device  28  may be implemented as part of the control device  24 . The elements  20 ,  24 ,  28  can also be integrated monolithically or be implemented on a semiconductor element. 
         [0051]    In  FIG. 3 , exemplary relevant spectra of materials are shown, as a second derivative subject to maximum-minimum normalization, meaning essentially equal amplitude of the respective spectra on the ordinate, wherein the ordinate of  FIG. 4  is labeled with arbitrary units, and on the abscissa the wavelength λ is plotted from 750-1000 nm. In accordance with the disclosure, what is assessed to be relevant in this context is the material, not its quantitative composition. In this context spectra are shown of polyethylene terephthalate (PET), polyethylene high density (PEND), polystyrene (PS), paper (Pa), and, as an example of organic material, human skin (O) that corresponds to the hand of an operator. 
         [0052]    Consequently different plastics can be identified, for example in the context of the sorting of deposit and return goods, such as bottles and containers, or also in the context of refuse separation. Various signals can be identified and differentiated from one another materials can be detected based on the output signals S 5 . The output signals S 5  can be used to identify objects depending on the different allocations of materials. Alternatively, output signal S 5  may also issue as an error signal. 
         [0053]    Furthermore, corresponding to the solid line O, parts of a human body, for example, the hand, can also be identified as organic material. In accordance with the invention, the device  1  may be part of a machine  35  or another device to deploy the light curtain  3  to protect an area  36  of the machine  35 , for example, to prevent injuries of an operator in the area  36 . 
         [0054]    For the evaluation, a multivariate statistical analysis method may be used. Advantageously, individual spectra of all relevant materials to be identified are measured in advance, and stored, for example, in a memory  29  indicated in  FIG. 2 . To the extent that, according to  FIG. 3 , the second derivatives of the spectra are considered, only such substances should be considered whose second derivatives are non-zero in the wavelength range at issue. 
         [0055]    Consequently, an output signal S 5  may issue if, depending, for example, a hand is detected as material O. Output signal S 5  may be sent, for example, to a control device  38  for the control of a function of the machine  35 , in particular a tool, for example a saw, milling cutter or other. This output signal S 5  may, for example, be issued as an emergency stop signal in order to stop this tool or the monitored machine. On the other hand, if a metal or a plastic box is detected as material 0, no such output signal S 5  is issued. If applicable, a separate signal can be issued as an output signal S 5  in the case of the detection of an unidentified substance, for example, in the case of a hand with glove, wherein in this case also, if applicable, an output signal S 5  can be issued as an emergency stop signal. 
         [0056]    The interfaces of the device  1  to the outside may include the power supply and the signal outputs, wherein, for example, only the signal output for S 5  can be provided. 
         [0057]    Other embodiments of the disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the disclosure disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the disclosure being indicated by the following claims.