Spectrophotometer with impurity detection

Abstract

Spectrophotometer for non-contact color measurement of a sample surface (P), comprising a light source (3) which is designed to illuminate the sample surface (P) with incident light (E); a detector (4) arranged such that light (S) diffusely reflected from the sample surface along a detection direction can be detected by the detector (4), wherein the detector (4) is designed to output a detection signal that correlates with a light intensity of the diffusely reflected light (S) along the detection direction; a calibration device (9) having a first reference surface (111), wherein the first reference surface (111) can be illuminated with the incident light (E) instead of the sample surface (P) for calibrating the spectrophotometer, wherein the detection signal upon illumination of the first reference surface (111) corresponds to a calibration signal; wherein the calibration device (9) further comprises a second reference surface (112), wherein the second reference surface (112) can be illuminated with the incident light (E) instead of the sample surface (P) for contamination detection, wherein the second reference surface has a lower diffuse reflectance than the first reference surface, wherein preferably the diffuse reflectance of the second reference surface is at most 1% of the diffuse reflectance of the first reference surface, characterized in that the spectrophotometer has a signal processing unit which is designed to output a contamination signal if the detection signal is above a threshold value when the second reference surface (112) is illuminated.

Description

TECHNICAL FIELDThe present invention relates to a spectrophotometer for contactless color measurement with a calibration device, wherein the calibration device is designed such that impurities of the calibration device can be detected.PRIOR ARTSpectrophotometer is used, for example, in printing rooms, laboratories or production facilities in order to acquire color data of sample surfaces and thus to ensure a constant color quality of the products.In particular in industrial series production, so-called "in-line" spectrophotometer is used, which is mounted permanently in a production line and can detect the color data of passing products, such as printed products, in a contactless manner. Particularly suitable for smooth or matt sample surfaces are spectrophotometer which determine spectral information only for a single narrow range of illumination angles, for example at 45° to the normal of the sample surface, and a single narrow range of observation angles, for example at 0° to the normal. An example of a spectrophotometer with such a "45° / 0°" geometry is the ERX50 from X-Rite.Spectrophotometer includes integrated light sources, the light intensity of which may vary, in particular decrease, over the course of their lifetime. In order to obtain repeatable color data, reliable calibration of the spectrophotometer is therefore essential. It is known from the prior art to use white calibration tiles, preferably made of ceramic. These white calibration tiles have a high diffuse reflectance known for the respective calibration tile and are therefore particularly well suited for measuring a reference value of the light intensity diffusely reflected below 0° on the basis of a detector arranged accordingly in the spectrophotometer, this reference value then serving for calibrating the subsequent measurements.However, if the calibration tile is contaminated, the diffuse reflectance decreases, which can be measured as a reduced light intensity at the detector. If this reduced light intensity is now used as a reference value for the calibration, subsequent sample surfaces are incorrectly measured brighter than they are actually.In particular in a series production environment such as the paper and textile industry, in which dust and other contaminants are hardly to be prevented, there is therefore a need to quickly identify contaminated calibration tiles.SUMMARY OF THE INVENTIONIt is an object of the present invention to provide a spectrophotometer that can quickly and reliably detect contaminants and thus prevent calibration errors.This object is achieved by a spectrophotometer according to claim 1. Further embodiments are set forth in the dependent claims.A spectrophotometer for contactless color measurement of a sample surface is thus disclosed, comprising:a light source configured to illuminate the sample surface with incident light;a detector which is arranged such that light diffusely reflected from the sample surface along a detection direction is detectable by the detector, wherein the detector is configured to output a detection signal which correlates with a light intensity of the diffusely reflected light along the detection direction;a calibration device having a first reference surface, wherein the first reference surface is illuminatable with the incident light instead of the sample surface for calibration of the spectrophotometer, wherein the detection signal corresponds to a calibration signal when the first reference surface is illuminated;wherein the calibration device additionally has a second reference surface, wherein the second reference surface can be illuminated with the incident light instead of the sample surface for impurity detection, wherein the second reference surface has a lower diffuse reflectance than the first reference surface, wherein the diffuse reflectance of the second reference surface is preferably at most 1% of the diffuse reflectance of the first reference surface, andwherein the spectrophotometer comprises a signal processing unit configured to output a contamination signal when the detection signal is above a threshold value when illuminating the second reference surface.Contaminants of the second reference surface are manifested in that the detection signal is higher when the second reference surface is illuminated than in the absence of the contaminants. On the basis of the detection signal which occurs when illuminating the second reference surface, it is thus possible to determine whether contaminants are present in the calibration device. The threshold value can indicate a limit from which cleaning of the calibration device should take place. The threshold value can be chosen as desired, in particular also zero. If the second reference surface is contaminated, there is a high probability that the first reference surface is also contaminated and calibration errors can thus arise. Thus, the detection signal obtained from the second reference surface represents an indirect contamination indicator for the first reference surface.In the present context, a distinction is made between specular reflection and diffuse reflection, wherein specular reflection is understood to mean a "mirror-like" reflection of the incident light beam at a reflection angle which corresponds to the illumination angle with inverted sign. Diffuse reflection refers to reflections which do not occur spicularly.In the present context, diffuse reflectance refers to the ratio of the light intensity of the diffusely reflected light to the light intensity of the incident light.The first reference surface and the second reference surface are preferably arranged adjacently in a common plane, as a result of which it can be assumed with a higher probability that contaminants are also actually located on the first reference surface used for calibration if the detection signal obtained from the second reference surface is above the threshold value.The first reference surface preferably consists of white ceramic, which represents a good approximation to a perfect light-scattering body and is therefore particularly well suited for calibrating a spectrophotometer, since it can lead to a maximum detection signal, which can then be stored as a white standard in the spectrophotometer and used for calibrating the color measurements.The second reference surface preferably consists of black glass, wherein the black glass ideally reflects the incident light purely spécularly. This ideally produces diffusely reflected light which is detected by the detector of the spectrophotometer only when contaminants occur on the second reference surface, which lead to diffuse reflection of the incident light.The light source can be configured to illuminate the sample surface with the incident light from at least one illumination direction at a fixed illumination angle relative to a normal to the sample surface if the spectrophotometer is arranged as intended at a predefined measurement distance relative to the sample surface, in particular above the sample surface, wherein the detection direction defines a detection angle relative to the normal to the sample surface, and wherein the detection angle differs in terms of amount from the illumination angle.The fixed illumination angle can be in particular 45° with respect to a normal to the sample surface, and the detection angle can be 0° with respect to the normal to the sample surface. Such a measurement geometry is particularly well suited for color measurements of smooth or matt sample surfaces, such as paper or textiles.In particular, the light source can be an annular light source which emits the incident light conically, wherein, in a sectional plane which comprises the normal to the sample surface, the incident light is incident on both sides of the normal at an illumination angle of 45° with respect to the normal to the sample surface. The spectrophotometer can also have at least one further light source which is designed to illuminate the sample surface with further incident light at at least one further illumination angle. Xenon flash lamps with UV components and / or light sources without UV components can be used in particular as light sources. In a preferred embodiment, the light source is a xenon flash lamp with standard light type D65 and the further light source is a xenon flash lamp with a UV filter.The spectrophotometer may further comprise a housing in which the at least one light source and the detector are disposed. The housing can have an aperture through which the incident light from the at least one light source can pass to the sample surface and through which the reflected light from the sample surface can pass to the detector. The calibration device preferably comprises a shutter which selectively releases the aperture in order to allow the incident light to pass through or at least partially covers it in order to block the incident light, wherein the first reference surface and the second reference surface are preferably arranged in the shutter.In particular, the first reference surface and the second reference surface are arranged on an inner side of the shutter, which inner side faces the incident light of the at least one light source when the shutter covers the aperture.Such a shutter with an integrated calibration device performs several tasks simultaneously, in particular the shutter protects the components arranged in the interior of the housing during operating pauses, serves for calibration and contamination detection and thus enables a compact construction of the spectrophotometer.The shutter can be connected to the housing in a displaceable and / or rotatable manner, wherein the shutter can preferably be brought into a calibration position in which the incident light impinges on the first reference surface, wherein the shutter can preferably be brought into an impurity detection position in which the incident light impinges on the second reference surface, and wherein the shutter can preferably be brought into a retraction position in which the aperture is completely released and thus a color measurement can be carried out on the sample surface. The shutter can be located outside or inside the housing. The normal to the sample surface is preferably identical to the normal to the first and second reference surfaces when these are illuminated by the incident light, i.e. in the calibration position or the contamination detection position.The shutter can preferably be brought into a cleaning position in which the first reference surface and the second reference surface are accessible from the outside and can be cleaned. In particular, the first reference surface and the second reference surface are accessible to a cleaning tool in the cleaning position. In order to be able to be brought into the cleaning position, the shutter can be tiltable and / or pivotable about an axis, wherein the axis can be arranged parallel or perpendicular to the sample surface when the spectrophotometer is positioned as intended with respect to the sample surface.In order to be able to be thoroughly cleaned or completely exchanged, the shutter can be removable from the housing. If the shutter is located within the housing in the operating state, the housing can be screwed on and / or can be folded open at a corresponding point in order to be able to remove the shutter from the housing.In order to enable automated handling, which is advantageous in particular in the case of a spectrophotometer installed in a fixed manner in a production line, the shutter can be electrically controllable. The calibration device then preferably has a control device which is designed to bring the shutter into the calibration position, the contamination detection position or the retraction position and / or into the cleaning position via an electrical signal. The control device can be coupled to the detector in this case in order to trigger a calibration measurement along the detection direction when the shutter is in the calibration position, and / or in order to trigger a contamination measurement along the detection direction when the shutter is in the contamination detection position, and / or in order to trigger a measurement of the sample surface when the shutter is in the retraction position.BRIEF DESCRIPTION OF THE DRAWINGSPreferred embodiments of the invention are described below with reference to the drawings, which are used merely for explanation and are not to be interpreted as restrictive. In the drawings, there are shown: FIG. 1 shows a highly schematic sectional view of a spectrophotometer according to an embodiment of the invention with a shutter which is in a retracted position; FIG. 2 shows the sectional view of the spectrophotometer of FIG. 1 with the shutter in a calibration position; FIG. 3 shows the sectional view of the spectrophotometer of FIG. 1 with the shutter in a contamination detection position without contaminants; FIG. 4 shows the sectional view of the spectrophotometer of FIG. 1 with the shutter in a contamination detection position with contaminants; FIG. 5 shows the sectional view of the spectrophotometer of FIG. 1 with the shutter in a cleaning position according to a first embodiment; FIG. 6 shows the sectional view of the spectrophotometer of FIG. 1 with the shutter in a cleaning position according to a second embodiment, and FIG. 7 shows the sectional view of the spectrophotometer of FIG. 1 with a removable shutter.DESCRIPTION OF PREFERRED EMBODIMENTSFIGS. 1-5 show in a highly schematic manner sectional views of a spectrophotometer 1 according to an embodiment of the present invention, which is arranged at a measurement distance d above a sample surface P. The spectrophotometer 1 shown here has a housing 2. In the present embodiment, an annular light source 3 is arranged inside the housing 2, which light source conically illuminates the sample surface P with incident light E at an illumination angle α of 45° to a normal of the sample surface. The incident light E leaves the housing 2 through an aperture 20 in the housing 2, and in FIGS. 1-5 two sectional surfaces of the annular light source 3 are shown schematically on the left and right above the aperture. The spectrophotometer further comprises a detector 4 which detects light S diffusely reflected from the sample surface at a fixed detection angle of 0° with respect to the normal of the sample surface. In order to conduct the diffusely reflected light S onto the detector, a beam optics 5, here a lens, is arranged between the aperture 20 and the detector 4. The beam optics 5 can also have a plurality of lenses and / or beam splitters. The detector 4 is preferably a spectrometer which outputs a detection signal which corresponds to a light intensity integrated over a specific spectral band. For processing the detection signal, the spectrophotometer has a signal processing unit 6, which can comprise, for example, an electrical amplifier and / or an analog-to-digital converter. The spectrophotometer additionally comprises a processor 7 and a power supply source 8, The spectrophotometer 1 is additionally preferably connected to an external computer C, for example via a USB interface, for the purpose of data exchange.The spectrophotometer 1 shown here has a calibration device 9 which comprises a shutter 10 in which a first, white reference surface 111 and a second, black reference surface 112 are arranged. In the embodiment shown here, the shutter 10 is located outside the housing 2, but it is likewise conceivable for the shutter 10 to be arranged inside the housing 2 and to be accessible from the outside, for example, via a foldable and / or screwable closure in the housing 2 (not shown here). The two reference surfaces are arranged directly adjacent to one another in the shutter 10 in a plane which runs parallel to a light plane in which the ring-shaped light source lies and which here also runs parallel to the sample surface P when the spectrophotometer is arranged as intended.In FIG. 1, the shutter 10 is in a retracted position, in which the aperture 20 is completely released, i.e. in which the incident light E impinges on the sample surface P. The spectrophotometer 1 has a control device 12 which is designed to bring the shutter 10 optionally also into a calibration position and / or a contamination detection position and / or a cleaning position instead of into the retraction position.In FIG. 2, the shutter 10 can be seen in a calibration position in which the incident light E impinges on the first reference surface 111. The situation illustrated in FIG. 1 corresponds to an ideal calibration situation in which the first reference surface 111, here a white ceramic tile, is clean, wherein the first reference surface 111 has been brought into the beam path of the incident light E in order to obtain a calibration signal on the basis of the light S diffusely reflected at 0°.In FIG. 3, the shutter 10 can be seen in an impurity detection position in which the incident light E impinges on the second reference surface 112, wherein the second reference surface 112 in this exemplary embodiment consists of black glass. The situation illustrated in FIG. 2 corresponds to a contamination detection situation in which the second reference surface 112 has been brought into the beam path of the incident light E in order to detect whether contaminants are located on the second reference surface 112, and therefore also with high probability on the first reference surface 111 arranged directly next to it. If the black glass is clean, as shown in FIG. 2, a purely specular reflection ideally arises, i.e. a reflection with a reflection angle which corresponds to the illumination angle α with the reversed sign. Since ideally no diffuse reflection occurs, the detection signal measured by the detector arranged at the detection angle of 0° is equal to zero.In contrast to FIG. 2, FIG. 4 shows a situation in which contaminants 30 are located on the second reference surface. These contaminants 30 lead to a diffuse reflection which generates a detection signal in the detector 4 arranged at 0° which is greater than zero and in particular lies above a fixed threshold value which can be selected as desired as an indicator for unacceptable contamination. According to the invention, the signal processing unit 6 is designed to output a contamination signal if the detection signal is above the threshold value when the second reference surface ( 112) is illuminated.In FIG. 5, the shutter 10 can be seen in a cleaning position in which the first reference surface 111 and the second reference surface 112 are accessible for cleaning, for example with compressed air or another suitable cleaning tool such as, for example, a microfiber cloth or lens cleaning paper. In the present example, the cleaning position is a position in which the shutter 10 is tilted about a tilting axis A, wherein the tilting axis A runs parallel to the sample surface P when the spectrophotometer is arranged as intended with respect to the sample surface P. If the spectrophotometer 1 for the color measurement is to be located close to the sample surface P to be measured, for example at a measurement distance d of 0.5-5 cm, it may be necessary to first position the spectrophotometer 1 at a cleaning distance s from the sample surface P which is greater than the measurement distance d, before the shutter 10 can be tilted and cleaned.Alternatively, the cleaning position, as shown in FIG. 6, can be a position in which the shutter 10 is pivoted about the pivot axis B, wherein the pivot axis B runs perpendicular to the sample surface P when the spectrophotometer is arranged as intended with respect to the sample surface P. The advantage of the cleaning position shown in FIG. 6 is that the spectrophotometer 1 can remain positioned at the measurement distance d for cleaning the shutter 10, even if the measurement distance d is small.Alternatively or additionally, the shutter 10, as shown in FIG. 7, can be completely removable from the housing 2 in order to be able to be subjected to a thorough cleaning far from the sample surface P to be measured or in order to be exchanged for a new shutter 10.LIST OF REFERENCE CHARACTERS1 Spectrophotometer 2 Housing 3 Light source 4 Detector 5 Beam optics 6 Signal processing unit 7 Processor 8 Energy supply source 9 Calibration device 10 Shutter 111 First reference surface 112 Second reference surface 12 Control device 20 Aperture 30 Contaminants d Measurement distance s Cleaning distance P Sample surface E Incident light S Diffusely reflected light A Axis B Axis C Computer α Illumination angle

Claims

Spectrophotometer for contactless color measurement of a sample surface (P), comprising a light source (3) which is designed to illuminate the sample surface (P) with incident light (E); a detector (4) which is arranged such that light (S) diffusely reflected from the sample surface along a detection direction is detectable by the detector (4), wherein the detector (4) is designed to output a detection signal which correlates with a light intensity of the diffusely reflected light (S) along the detection direction; a calibration device (9) which has a first reference surface (111), wherein the first reference surface (111) can be illuminated with the incident light (E) instead of the sample surface (P) for calibration of the spectrophotometer, wherein the detection signal corresponds to a calibration signal when the first reference surface (111) is illuminated; wherein the calibration device (9) additionally has a second reference surface (112), wherein the second reference surface (112) can be illuminated with the incident light (E) instead of the sample surface (P) for impurity detection, wherein the second reference surface has a lower diffuse reflectance than the first reference surface, wherein the diffuse reflectance of the second reference surface is preferably at most 1% of the diffuse reflectance of the first reference surface, characterized in that the spectrophotometer has a signal processing unit which is designed to output an impurity signal if the detection signal is above a threshold value when the second reference surface (112) is illuminated.The spectrophotometer according to claim 1, wherein the first reference surface (111) and the second reference surface (112) are adjacently arranged in a common plane.Spectrophotometer according to claim 1 or 2, further comprising: a housing (2) in which the light source (3) and the detector (4) are arranged, wherein the housing (2) has an aperture (20) through which the incident light (E) can pass from the light source (3) to the sample surface (P) and through which the diffusely reflected light (S) can pass from the sample surface (P) to the detector (4), wherein the calibration device comprises a shutter (10) which selectively releases the aperture (20) to allow the incident light (E) to pass or at least partially covers it to block the incident light (E), and wherein the first reference surface (111) and the second reference surface (112) are arranged in the shutter.Spectrophotometer according to claim 3, wherein the shutter (10) is slidably and / or rotatably connected to the housing (2), wherein the shutter (10) can be brought into a calibration position in which the incident light (E) impinges on the first reference surface (111), wherein the shutter (10) can be brought into a contamination detection position in which the incident light (E) impinges on the second reference surface (112), and wherein the shutter (10) can be brought into a retraction position in which the aperture (20) is completely released.Spectrophotometer according to either of Claims 3 and 4, wherein the shutter (10) can be brought into a cleaning position in which the first reference surface (111) and the second reference surface (112) are accessible from the outside and can be cleaned.Spectrophotometer according to claim 5, wherein the shutter (10) is electrically controllable and wherein the calibration device (9) further comprises: a control device (12), which is configured to bring the shutter (10) into the calibration position, the contamination detection position or retraction position and / or into the cleaning position via an electrical signal.The spectrophotometer according to any one of claims 3-6, wherein the shutter (10) is removable from the housing.The spectrophotometer according to any preceding claim, wherein the first reference surface (111) is made of white ceramic.The spectrophotometer of any preceding claim, wherein the second reference surface (112) is made of black glass.Spectrophotometer according to one of the preceding claims, wherein the light source (3) is configured to illuminate the sample surface (P) with the incident light from at least one illumination direction at a fixed illumination angle (α) relative to a normal to the sample surface (P), wherein the detection direction defines a detection angle relative to the normal to the sample surface, and wherein the detection angle differs in terms of amount from the illumination angle (α).The spectrophotometer according to claim 10, wherein the illumination angle (α) is 45° with respect to a normal to the sample surface, and wherein the detection angle is 0° with respect to the normal to the sample surface.

Images