DEVICE FOR CHARACTERIZING A SAMPLE

Abstract

The present invention relates to a device for optical characterisation of a sample and/or of the material(s) of the same having an illumination unit that can be orientated to illuminate with incident light a sample spatial portion into which the sample can be introduced, a detection unit which is orientated or can be orientated to image the sample introduced into the sample spatial portion by receiving light reflected by the sample, and which is configured to detect at least two different, preferably orthogonal, polarization components in the reflected light, and an evaluation unit with which, in the imaging data recorded by the detection unit, those imaged surface elements (reflection elements) of the sample can be identified, and with which the detected different polarization components for these reflection elements can be evaluated for optical characterisation.

Claims

1. A device for characterizing a sample, the device comprising: a light source configured to illuminate the sample; a detector configured to receive light from the light source reflected by the sample, capture a multi-pixel image of the sample from the received reflected light, and detect at least two different polarization components from the received reflected light; and a processor configured to: determine a subset of image pixels of the captured image, such that for each image pixel in the subset of image pixels, the reflected light contributing to the image pixel is specularly reflected from the sample, and for each image pixel of the captured image not included in the subset of image pixels, the reflected light contributing to the image pixel is diffusely reflected from the sample, and for each image pixel in the subset of image pixels, output the detected values of the at least two different polarization components.

2. The device of claim 1, wherein: the sample defines a surface normal as being orthogonal to the sample at a center of the sample; the light source defines an incident axis as extending from a center of the light source to the center of the sample, the incident axis forming an incident angle with the surface normal; the detector defines an exiting axis as extending from a center of the detector to the center of the sample, the exiting axis forming an exiting angle with the surface normal; and the processor determines the subset of image pixels, such that for each image pixel in the subset of image pixels, the incident angle equals the exiting angle.

3. The device of claim 2, wherein the incident angle equals a Brewster angle of the sample.

4. The device of claim 2, wherein the light source includes a plurality of individual illumination elements positioned to illuminate the sample with incident light from different directions.

5. The device of claim 4, wherein the plurality of individual illumination elements includes two individual illumination elements.

6. The device of claim 4, wherein the plurality of individual illumination elements includes four individual illumination elements.

7. The device of claim 4, wherein the plurality of individual illumination elements are positioned in a plane that is orthogonal to the incident axis.

8. The device of claim 7, wherein the individual illumination elements are spaced apart equally along a circle, wherein the incident axis intersects the plane at the center of the circle.

9. The device of claim 7, wherein at least one illumination element of the plurality of illumination elements is positioned at an intersection of the incident axis and the plane.

10. The device of claim 1, wherein the light source comprises an individual illumination element.

11. The device of claim 1, wherein the detector includes two cameras and a polarizing beam splitter, the polarizing beam splitter configured to direct light having a first polarization state to one of the two cameras, the polarizing beam splitter configured to direct light having a second polarization state, orthogonal to the first polarization state, to the other of the two cameras.

12. The device of claim 1, wherein two of the at least two different polarization components in the received reflected light are orthogonal to each other.

13. The device of claim 1, wherein: the light source includes a laser configured to linearly scan the sample; and the detector includes a receiver configured to receive laser light reflected by the sample, the detector further including a plurality of polarization-sensitive elements for separating the received laser light according to the different polarization states, the detector further including a plurality of detecting elements configured to receive the separated light in a one-to-one correspondence from the plurality of polarization-sensitive elements.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The present invention is described subsequently in detail with reference to several embodiments.

[0048] There are thereby shown:

[0049] FIG. 1 a first embodiment of a device according to the invention using an individual illumination element as illumination unit.

[0050] FIG. 2 a further embodiment of the invention in which the illumination unit consists of two separate illumination elements.

[0051] FIG. 3a to 3d examples of identification of a defined material in a bulk material flow of different materials.

[0052] FIG. 4 an example of a device according to the invention configured as testing system for coatings.

[0053] FIG. 5 a further embodiment of the invention which is configured for complete characterisation of the polarisation state of the reflected light.

[0054] FIG. 6 a diagram of a device with a retroreflector and a transmitting and receiving unit.

DETAILED DESCRIPTION OF THE INVENTION

[0055] FIG. 1a) shows a device according to the invention which is configured for characterisation of individual sample elements or test pieces P in the form of a bulk material sorting system. The individual objects of the bulk material flow or of the sample P are transported on a planar conveyer belt 30, the outer surface of which, on which the elements of the sample P come to be situated, is white. This serves for better identification of the individual sample elements in the image (see subsequently). The conveyer belt 30 is actuated by two rollers 31, 32; transport of the sample elements P is effected here to the right in the image (arrows); further elements of the bulk material sorting device (e.g. blowing units or collection containers for the sample elements of different material) are not shown here.

[0056] The illumination unit 2 of the illustrated device comprises a monochromatic light source 21 which is configured here as LED strip and emits in the green range (550 nm). A diffuser 22 which reduces the modelling of the LED structure 21 is disposed in the beam path after the light source 21. In the beam path behind light source 21 and diffuser 22, the illumination unit 2 has in addition also a polariser 23. The optical axis of the illumination unit 2 consisting of the elements 21, 22 and 23 is characterised here with the reference number 2o. The light E incident on the sample P along the optical axis 2o of the illumination unit 2 is incident at an angle θ.sub.B (relative to the normal N to the surface of the conveyer belt 30 covered with the individual sample elements) onto the surface of the sample spatial portion 1 which here comprises a defined surface segment parallel to the longitudinal direction of the conveyer belt 30. The corresponding conveyer belt portion is provided here with the reference number 7.

[0057] The detection unit 3 of the illustrated system is disposed, relative to the conveyer belt 30, in the same half-space as the illumination unit 2 (i.e. in the half-space situated above the conveyer belt 30) but, viewed relative to the conveyer belt portion 7 or to the sample spatial portion 1 illuminated by the illumination unit 2, is disposed in this half-space on the side situated opposite the illumination unit 2. The optical axis of the detection unit 3 configured as polarisation camera is described here with the reference number 3o.

[0058] The illumination unit 2 or the optical axis 2o thereof, the centre of the sample spatial portion 1 or of the illuminated conveyer belt portion 7 and the detection unit 3 or the optical axis 3o of the same, form an isosceles triangle, the longitudinal side of which is formed by the connection line light source 2—detection unit 3 and the cathetus of which is formed by the connection lines light source 2—sample spatial portion 1, 7 and sample spatial portion 1, 7—detection unit 3 (reflection arrangement). The normal N of the longitudinal side of this triangle or the normal to the conveyer belt surface hence bisects the angle between the two optical axes 2o and 3o into two angles θ.sub.B of equal size, here θ.sub.B=63° applying.

[0059] An evaluation unit 4 in the form of a personal computer with suitably configured evaluation programs is connected to the detection unit 3 via a bidirectional data line.

[0060] The mode of operation of the device illustrated in FIG. 1a) is described subsequently.

[0061] The device is adjusted to differentiate sample elements made of zirconium from sample elements made of glass. For this purpose, the angle θ.sub.B=63° was adjusted to the Brewster angle of the material zirconium. The evaluation or the optical characterisation is now based on the idea that the surface portions of the individual sample elements, which are orientated towards the illumination unit-detection unit half-space, can constantly be differentiated, that there is hence (cf. FIG. 1b) at least one surface element for each sample element P, the normal of which surface element is orientated parallel to the normal N or to the angle bisector of the two optical axes 2o, 3o. For such a surface element of a sample element P, the incident radiation E hence impinges on the surface of the sample element P precisely at the Brewster angle θ.sub.B of zirconium.

[0062] FIG. 1b) illustrates how those surface elements, for which this reflection condition is fulfilled and which are therefore reflection elements 5 of the sample elements P, can be differentiated from other imaged surface elements of the sample or from imaged surface elements of the background or of the conveyer belt surface (these surface elements are subsequently described in summary as scattered elements 6 although the physical process underlying their imaging can also be a process other than a scattering process): if in fact (relative to the overall reflected light radiation Z) not only does a reflected beam component Z1 arrive at the detection unit 3 from reflection elements 5 of the sample P and lead there to imaging of the corresponding surface element by the detection unit 3, but also light Z2 which is scattered for example on a scattering element 6 likewise arrives at the detector 3 (in FIG. 1b), this is for example light which has been reflected already once on the surface of the conveyer belt 30 and is therefore incident, from a direction of incidence E2 which does not coincide with the optical axis 2o, onto the scattering element 6 of the surface of the sample P, which light is then scattered in the direction Z2=Z1 into the polarisation camera 3). However scattered light from scattering elements 6 can be differentiated from reflected light from reflection elements 5 by evaluation of the intensity of a polarisation component recorded by the polarisation camera 3 (see subsequently) or also by evaluation of the incident overall intensities of all detected polarisation components. Thus the reflection elements 5 effect for example a significantly higher overall intensity which impinges on the corresponding image element of the polarisation camera 3 than the scattering elements 6. The two types 5, 6 of surface elements can therefore be differentiated by setting a predetermined threshold value (which can be determined for example from an average intensity over the entire image). Surface elements 5 which fulfil the reflection condition are therefore particularly bright in the imaging. These surface elements 5 alone are then evaluated further for characterisation of the sample P or the individual sample elements thereof.

[0063] In order to ensure that the specific reflection elements 5 in fact also concern imaged surface elements of sample elements P (and not for example light components reflected on the white background or on the surface of the conveyer belt 30), in addition the position of the potential candidates for reflection elements 5 can be evaluated in the total recorded image: by means of image processing algorithms for edge detection, known to the person skilled in the art (search for closed curves in the image which is differentiated once or twice and threshold value-treated), the position, the size and the shape of the individual sample elements of the sample P can be established for example. Reflection elements R can then be merely those surface elements or points in the image which come to be situated inside the image of a sample element or inside such closed curves. In order to determine the reflection elements 5, a combination of intensity- and position evaluations can therefore be used (only particularly bright surface elements in the central region of the imaging of a bulk material object P can hence be reflection elements 5 in the system of FIG. 1).

[0064] Further evaluation of the identified reflection elements 5 in the image of the camera 3 and the sample material characterisation based thereon then takes place as follows: the polarisation camera 3 is configured for separation of two orthogonal polarisation components, namely of the polarisation component of light E which is incident parallel to the plane of incidence of the reflection elements 5 (plane parallel to the conveyer belt surface) and of the polarisation component incident perpendicular thereto. If an imaged sample element P concerns an element made of zirconium, then, since here the Brewster condition is fulfilled, merely light polarised parallel to the above-described plane is reflected. Only this polarisation component can hence be detected for zirconium sample elements P with one channel of the camera 3, whilst the other channel of the camera 3 (which is configured for detecting light polarised perpendicular thereto) can detect no reflected light. If the observed sample element P concerns an element made of a material other than zirconium, then light of both polarisation components is detected by the polarisation camera 3 (i.e. both channels of the camera are affected). If the ratio of the intensities of both polarisation components in both channels or images of the sample spatial portion 1, recorded by the polarisation camera 3, is hence formed for all those surface elements which are reflection elements 5, then this ratio varies significantly for reflection elements of zirconium surfaces and for reflection elements of surfaces of other materials. By setting a suitable threshold value, zirconium sample elements can hence be differentiated from other sample elements.

[0065] If light which is polarised for example parallel to the plane of incidence is displayed by the polarisation camera in blue and light polarised perpendicular thereto in red, then this means that, in the images recorded and superimposed by the polarisation camera, the reflection elements of zirconium sample elements P appear to be purely blue.

[0066] FIGS. 3a to 3d show examples of the differentiation of diamond and quartz glass (FIG. 3a to 3c) and of zirconium crystals in a bulk material flow P of such crystals, of glass shards and of metal rings (FIG. 3d). The polariser 23 was adjusted for FIG. 3d such that the conveyer belt surface (background) reflects both polarisation directions with the same intensities in the direction of the polarisation camera 3. θ.sub.B is thereby 63° (Brewster angle for zirconium). Even if the sample elements P conveyed through the sample spatial portion 1 have an irregular geometry and their precise position is unknown, nevertheless an object characterisation is possible since the individual sample elements have differentiatable surfaces, i.e. each bulk material particle has at least one surface element which fulfils the reflection condition (angle of incidence=θ.sub.B). Since reflection provides very much stronger signals than scattering, these surface elements can be identified. The underlying physical principles of this reflectometry (reflection of polarised light on the medium, Fresnel formulae for perpendicular and for parallel polarisation and also the law of refraction) are known to the person skilled in the art.

[0067] FIG. 3a shows how a sorting criterion can be developed from the Fresnel formulae by calculation of curves for the relative reflection capacity of two different materials: FIG. 3a shows the reflection capacity as a function of the angle of incidence for the materials quartz (refractive index=1.46) and diamond (refractive index=2.41), i.e. in the case where differentiation of diamond from quartz glass is desired. The Figure clearly shows the different Brewster angles for the two materials; for separation of the two materials, an arrangement at the Brewster angle θ.sub.B of the sought material can hence be effected (i.e. for diamond at θ.sub.B=67.5°). Those reflection elements 5 in which merely one polarisation component remains after reflection can then be determined. R.sub.S is the reflection capacity for light polarised perpendicular to the plane of incidence and R.sub.P is the reflection capacity for light polarised parallel to the plane of incidence.

[0068] An increase in sensitivity for separation of the two materials can be effected by adaptation of the illumination. For example in an arrangement for differentiating zirconium (θ.sub.B=63°), the illumination can be adjusted by means of the polariser 23 such that the two reflected intensities are the same for the extraneous material (for example glass or metal). This adjustment can be effected by means of the polariser 23 such that then an extraneous material sample is brought into the measuring field and subsequently the position of the polariser is changed thus until both intensities are the same.

[0069] FIG. 3b shows again, for the example of diamond/quartz glass, the degree of reflection, likewise (cf. FIG. 3a) as a function of the angle of incidence θ.

[0070] In the case of an arrangement in which the Brewster angle θ.sub.B of diamond is chosen as angle of incidence, the characteristic line shown in FIG. 3c is finally produced for the optical differentiation of diamond and of materials with a deviating refractive index (e.g. quartz glass). For the illustrated example, the polarisation of the illumination was adjusted such that not reflecting, but scattering particles or surface elements with R.sub.P=R.sub.S have a quotient of 5. Hence a sorting criterion which falls monotonically within a wide range with the refractive index n is produced.

[0071] When adjusting the system for identification of zirconium, the ratio between blue and red channel is then highest on the surface elements 5 of the zirconium crystals which are orientated parallel to the conveyer belt surface and fulfil the reflection condition. The ratio can hence be used for the purpose of identifying the zirconium crystals in the individual sample elements of the bulk material flow.

[0072] FIG. 3d shows a corresponding result, in which, for formation of the ratio, the blue channel B has been divided by the sum of both channels R+B (R=red channel intensity). After setting the threshold value (FIG. 3d on the right), it can be readily detected that zirconium crystals are marked in image 3. The glass shards (further irregular elements in FIG. 3d on the left) and a metal ring present in the bulk material flow (FIG. 3d on the left at the top) remain dark, i.e. are not identified.

[0073] FIG. 4 illustrates a further test task which can be achieved with the device according to the invention shown in FIG. 1: paper/gauze is coated with Vaseline during production. What is sought is a test system which automatically tests the entire coating during production. The approach for the solution according to FIG. 1 is based here on reflectometry, the laminate sample P which is flat here being disposed at the Brewster angle θ.sub.B of 55.5° for Vaseline. The curves calculated from the Fresnel formulae for the reflection capacity of Vaseline are shown in FIG. 4. By way of comparison, the expected course of a paper scattering homogeneously with 20% is plotted. Here also, the separation of the two mentioned materials can be implemented again by evaluation of the two channels of the polarisation camera 3 (test on R.sub.S=0, i.e. for the presence only of reflected light polarised parallel to the interface). The result of the test is the information as to whether paper is coated with Vaseline or not. Paper surfaces coated with Vaseline are hence distinguished by an intensely blue colour, merely the blue channel of the polarisation camera 3 responds. The degree of polarisation of the imaged surface elements can hence be calculated from the intensities B of the blue channel and from the intensities R of the red channel as follows: B/(B+R). Vaseline-coated surface components hence produce the value B/(B+R)=1. For production control, for example the coated surface component on the total surface component can be evaluated.

[0074] FIG. 2 shows a further device according to the invention in which a plurality of individual illumination elements 2a, 2b, as illumination unit 2, are used in the form of monochromatic light sources with emission wavelengths of respectively λ=550 nm. Viewed in the direction of incidence, a diffuser 22a, 22b and a polariser 23a, 23b are disposed behind each illumination element 2a, 2b, similarly as shown in FIG. 1. The detection unit 3 and the evaluation unit 4 (not shown here) are configured similarly to the case described in FIG. 1 (differences see below). The illustrated device is configured as bulk material sorting device in which the bulk material (of which only a single sample element P is shown here) traverses a sample spatial portion 1 in the form of a free falling stretch part 6f below a vibrator (not shown). The optical axis 3o of the polarisation camera 3 is situated here in a horizontal plane perpendicular to the falling direction F of the sample elements P. In each of the half-spaces configured on both sides of this horizontal plane, an illumination element 2a, 2b (together with associated diffuser 22a, 22b and polariser 23a, 23b) is disposed respectively. The angle between the two optical axes 2oa and 2ob of the two illumination elements 2a, 2b and of the above-described horizontal plane is respectively the same, the illumination elements 2a, 2b and the camera 3 are thereby disposed such that their optical axes 2oa, 2ob and 3o are situated in a plane perpendicular to the above-described horizontal plane.

[0075] Due to this arrangement, the reflection condition for the two illumination elements respectively is hence the same: the angle bisector N.sub.a divides the angle spanned by the two optical axes 2oa and 3o or the angle spanned by the direction of incidence E.sub.a of the upper illumination element 2a and of the reflection direction Z1 into two angles θ.sub.aB of equal size, which are configured corresponding to the Brewster angle θ.sub.B of a material to be identified in the sample flow P. The angle bisector N.sub.b likewise divides the angle spanned by the optical axis 2ob of the lower illumination element 2b (i.e. the incident light E.sub.b) and the optical axis 3o of the polarisation camera 3 (or the corresponding reflected imaged light component Z1) into two angle portions θ.sub.bB of equal size. Due to the above-described arrangement, there applies here θ.sub.aB=θ.sub.bB. Both illumination elements 2a and 2b are hence adjusted to one and the same angle, the Brewster angle of the material to be identified.

[0076] Identification of the reflection elements 5 and the subsequent evaluation of the polarisation components for these reflection elements for optical characterisation of the sample elements P is now effected analogously to the case described for FIG. 1. However the reflection condition for the partial system consisting of the illumination unit 2a and the camera 3 is fulfilled at a different, later point in time than for the further partial system consisting of the illumination element 2b and the camera 3: if a sample element P falls through the illustrated falling stretch F, then surface elements situated on the rear-side thereof (in the image: side situated at the top) fulfil the reflection condition, i.e. are reflection elements 5 when the observed sample element P is disposed, with its rear-side, exactly at the height of the horizontal plane of the optical axis 3o, this horizontal plane is therefore precisely the tangential plane to the rear-side of the sample element P. For the system 2b, 3, the reflection condition is in contrast already fulfilled at a point in time preceding this point in time, namely when the front-side of the falling sample element P (the side situated at the bottom in the image) impacts precisely from above on the horizontal plane of the optical axis 3o, this horizontal plane abuts therefore tangentially at the front-side of the sample element P.

[0077] The significant surface elements of the sample P which are potentially possible as reflection elements 5 must hence be situated in the images recorded currently by the camera 3o initially on the front-side and then on the rear-side of the imaged object (which can be identified again by for example gradient-based image processing mechanisms with the aid of its outline). In this respect, the conditions for identification of the reflection elements 5 differ from those of the system shown in FIG. 1 in which the reflection elements 5 must be situated for instance in the centre of the images of the individual identified sample elements. Apart from the above-described differences during identification of the reflection elements 5, evaluation of the different polarisation components for the identified reflection elements can however be effected for optical characterisation of the sample P entirely analogously to the case described for FIG. 1.

[0078] Analogously to the case shown in FIG. 2, an illumination unit which comprises, instead of two illumination elements 2a, 2b, in total four illumination elements which are disposed in a plane perpendicular to the optical axis 3o and equidistantly on a circle about this optical axis 3o (angle spacings of the individual illumination elements 90°) can also be used. Similarly to the case shown in FIG. 2, illumination is then effected such that surface elements on the edge of the objects P are examined from four directions on the basis of intensity as to whether they have matching surface normals N. Hence characterisation of the falling sample elements P of the bulk material flow with up to four points is possible.

[0079] For the objects in FIG. 2, it can hence be tested whether there are surface elements or points with a pure colour, e.g. blue (cf. description for FIG. 1: then merely one of the two polarisation components is present) and whether these points are situated on the front- or rear-side of the respective sample elements P (relative to the direction of movement F).

[0080] With corresponding adjustment to the Brewster angle and in the case of four individual illumination elements at a 90° spacing (not shown), objects made of the material to be identified are characterised according to the Brewster angle θ.sub.aB=θ.sub.bB, for example by blue image elements at a second, later point in time (scanning of the front), red image elements at a first later point in time (scanning of the left and of the right side) and by further blue image elements at a third, still later point in time (scanning of the rear).

[0081] FIG. 5 shows finally a further device according to the invention for the optical characterisation of a planar, laminate sample P in a sample spatial portion 1 on the basis of a laser scanner system. The laser 2 as illumination unit, which scans, one-dimensionally, the sample spatial portion 1 in the direction SR perpendicular to the direction of incidence E of the light, beams light at the angle of incidence θ (angle between the sample normal N and the direction of incidence E of the laser light) onto the sample surface of the sample P. Cf. in this respect FIG. 5 on the right at the top which shows a section perpendicularly through the irradiated sample surface and FIG. 5 in the centre at the right which shows a plan view on the irradiated sample surface, i.e. a view in the direction of the normal N. The light Z reflected at the corresponding angle of reflection θ (reflection law) is conducted for evaluation to the receiver 3 shown on the left and centre in FIG. 5.

[0082] The device shown in FIG. 5 is based on the observation that the emission laser 2 of the illustrated scanner emits monochromatic, coherent radiation so that the radiation E received by the sample is already completely polarised. In this case, the detection of three Stokes' parameters from the reflected laser light components Z hence suffices for complete characterisation of the polarisation state of the reflected or detected light radiation Z.

[0083] Viewed in the irradiation direction of the reflected light component Z, the illustrated receiver 3 now comprises in succession in the beam path the following components: [0084] A hollow mirror 40 configured for focusing the light component Z reflected on the sample surface P towards a beam splitter plate 8. [0085] The polarisation-obtaining beam splitter plate 8 with which respectively 50% of the incident, reflected radiation Z is divided into a first partial beam path T1 and into a second partial beam path T2. [0086] In the first partial beam path T1: firstly a delay plate (λ/4 plate) 9 which directs the light of the first partial beam path T1 towards a first polarisation beam splitter 10a which is configured for differentiating two polarisation components of the incident light which are orthogonal relative to each other. The first of these two polarisation components is detected with a first receiving element 11a, the other of these two polarisation components with a further receiving element 11b (intensity detectors). [0087] The second partial beam path T2 is basically constructed just like the first partial beam path T1, however the delay plate 9 is omitted here so that, in this partial beam path, merely a second polarisation beam splitter 10b and two further receiving elements 11c and 11d are disposed, with which the two polarisation components which are orthogonal relative to each other can be detected in the second partial beam path T2. [0088] The four receiving elements 11a to 11d are then connected respectively via bidirectional signal lines to an evaluation unit 4 (not shown).

[0089] With the illustrated receiver 3, the polarisation state of the reflected radiation Z can hence be characterised completely as follows:

[0090] With the help of the receiving elements 11c and 11d of the partial beam path T2, the intensities I.sub.0 and I.sub.90 for two linear polarisation components which are orthogonal relative to each other are determined. The combination of the delay plate 9 and of the splitter 10a produces a beam splitter for splitting the incident light into right-circular and left-circular polarised light. (Intensities I.sub.RZ and I.sub.LZ for right-circular and for left-circular polarised light). Hence four different polarisation components can be determined.

[0091] The four sought Stokes' parameters I, S, U and V can hence be determined from the linear polarisation components (intensities I.sub.0 and I.sub.90) which are detected by the receiving elements 11a to 11d, i.e. orthogonal to each other, and from the circular polarisation components (right-circular polarised component with the intensity I.sub.RZ and left-circular polarised component with the intensity I.sub.LZ) as follows

I=I.sub.0+I.sub.90

S=I.sub.0−I.sub.90

V=I.sub.RZ−I.sub.LZ,

then with the secondary condition for monochromatic coherent laser radiation of

S+U+V=1

the fourth Stokes' parameter U=I.sub.45−I.sub.135 being able to be calculated.

[0092] The illustrated device for optical characterisation of FIG. 5 hence enables calculation of the complete polarisation state of the reflected light component Z from the received signal intensities of the four receiving elements 11a to 11d. Since the polarisation state of the reflected light Z depends upon the respectively examined sample material of the sample P, the device shown in FIG. 5 can be used for material characterisation of the sample P.

[0093] If receiver beam path and transmitter beam path are produced in the same housing (integrated transmitting and receiving unit), a corresponding characterisation of the material can be effected provided that light reflected on the sample (reflective) impinges on a retroreflector which reflects the beams per se back to the combined transmitting and receiving unit. In contrast to the arrangement with separate transmitter and receiver, the light is however reflected twice on the sample. The polarisation effects on the sample hence influence the received intensities quadratically.