U.S. patent application number 15/670297 was filed with the patent office on 2017-11-23 for device for characterizing a sample.
The applicant listed for this patent is Fraunhofer-Gesellschaft zur Forderung der angewandten Forschung e.V.. Invention is credited to Matthias Hartrumpf.
Application Number | 20170336316 15/670297 |
Document ID | / |
Family ID | 44675527 |
Filed Date | 2017-11-23 |
United States Patent
Application |
20170336316 |
Kind Code |
A1 |
Hartrumpf; Matthias |
November 23, 2017 |
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.
Inventors: |
Hartrumpf; Matthias;
(Karlsruhe, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fraunhofer-Gesellschaft zur Forderung der angewandten Forschung
e.V. |
Munchen |
|
DE |
|
|
Family ID: |
44675527 |
Appl. No.: |
15/670297 |
Filed: |
August 7, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14837012 |
Aug 27, 2015 |
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15670297 |
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13818188 |
May 6, 2013 |
9222879 |
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PCT/EP2011/004553 |
Sep 9, 2011 |
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14837012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 2021/845 20130101;
G01N 21/84 20130101; G01N 21/21 20130101; G01N 15/1475 20130101;
G01N 15/1463 20130101; G01N 2201/0438 20130101; G01N 2201/06113
20130101; G01N 2015/144 20130101; G01N 15/1434 20130101 |
International
Class: |
G01N 21/21 20060101
G01N021/21; G01N 15/14 20060101 G01N015/14 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 24, 2010 |
DE |
10 2010 046 438.4 |
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
PRIORITY CLAIM TO RELATED APPLICATIONS
[0001] This application is a divisional application and claims the
benefit of priority of U.S. patent application Ser. No. 14/837,012,
filed Aug. 27, 2015, which is a divisional application and claims
the benefit of priority of U.S. patent application Ser. No.
13/818,188, filed May 6, 2013, which is a national stage
application under 35 U.S.C. .sctn.371 of PCT/EP2011/004553, filed
Sep. 9, 2011, and published as WO 2012/038036 A1 on Mar. 29, 2012,
which claims priority to German Application No. 10 2010 046 438.4,
filed Sep. 24, 2010, which applications and publication are
incorporated by reference as if reproduced herein and made a part
hereof in their entirety, and the benefit of priority of each of
which is claimed herein.
TECHNICAL FIELD
[0002] The present invention relates to a device and a method for
optical characterisation of a sample and/or of the material (or the
materials) of the same. The characterisation is thereby effected on
the basis of evaluation of the polarisation of light which is
radiated onto the sample and reflected by the sample. The device
and the method can be used in particular for surface inspection or
also for sorting bulk material by evaluation of the polarisation of
the reflected light.
BACKGROUND
[0003] Methods for optical characterisation of samples based on
reflectometry or ellipsometry are already known from the state of
the art. See for example Thomas Geiler "Polarisationsbildgebung in
der industriellen Qualitatskontrolle" (Polarisation imaging in
industrial quality control), VDM Press, August 2008. Devices for
classification of samples in the form of bulk material are also
known from the state of the art (WO 2009/049594 A) which operate on
the basis of light which is reflected on a retroreflector, which
can be polarisation-selective, and detected with respect to its
various polarisation components.
[0004] However, all these implementations demand either a planar
surface of the test pieces or a time-consuming, simultaneous
variation of the angle of incidence and of reflection is
implemented. In addition, also a variation in the polarisation of
the illumination is often implemented.
SUMMARY
[0005] It is the object of the present invention to develop devices
and methods for optical characterisation of samples, in particular
devices (and corresponding methods) based on the technology of
reflectometry or ellipsometry and polarimetry respectively such
that the samples (in particular also non-planar samples or test
pieces, e.g. in the form of bulk materials) can be characterised
easily and reliably with respect to their material/materials. It is
also the object in particular to configure the devices and the
corresponding methods such that this characterisation can be
effected rapidly (in the range of a few 1/10 seconds up to a few
seconds), i.e. in step with the production of samples (in-line) or
with a bulk material flow.
[0006] This object is achieved by a device according to claim 1, by
a device according to claim 12 and by a method according to claim
14. Advantageous embodiments of the devices according to the
invention and of the method according to the invention can be
deduced respectively from the dependent patent claims.
[0007] In the following, the present invention is described firstly
in general, then in detail with reference to various embodiments.
The individual features of the invention which are described in the
embodiments and produced in combination with each other need not
thereby be produced precisely in the feature combination shown in
the respective embodiment within the scope of the invention, but
rather can also be produced in different combinations with each
other. In particular, some of the illustrated features can also be
omitted or combined in different ways with further illustrated
individual features of the embodiments.
[0008] The present invention, as described subsequently, uses as
basis the technologies of reflectometry or of ellipsometry which
are known to the person skilled in the art. The corresponding bases
are known for example in H. G. Tompkins. W. A. McGahan
"Spectroscopic Ellipsometry and Reflectometry", Wiley Interscience,
1999 or in M. Faupel "Abbildende Ellipsometrie und ihre Anwendung"
(Imaging ellipsometry and application thereof), VDI reports no.
1996: Optische Messung technischer Oberflachen in der Praxis
(Optical measurement of technical surfaces in practice); 2007 and
are therefore not described in detail in the following.
[0009] A basic idea of the present invention is based on
identifying, for those surface elements of the sample which reflect
light radiated onto the sample into a detection unit configured for
detecting the light, and on detecting and evaluating, for these
surface elements of the sample (subsequently also termed reflection
elements), different polarisation components of the reflected
light. The sample is illuminated for this purpose with preferably
monochromatic light. (Monochromatic light is not absolutely
necessary but in general is better suited for example in the case
of samples with dispersion). Alternatively hereto or in combination
therewith, it is likewise possible according to the invention to
use monochromatic, coherent radiation (laser light) for
illumination of the sample and, by means of suitable configuration
of the detection unit which receives the beam components reflected
on the sample, to calculate all four Stokes' parameters and to use
them for characterisation of the light reflected on the sample (and
hence for characterisation of the sample itself). In every case, at
least two different (preferably: orthogonal) polarisation
components in the reflected light are hence detected.
(Alternatively to the term of polarisation component, also the term
of "polarisation state" is used subsequently within the scope of
the invention although, strictly speaking, light can have merely
one polarisation state; however, it is clear to the person skilled
in the art with the help of the description respectively, what is
intended.)
[0010] Within the scope of the subsequently described invention,
there is thereby understood by the term of light reflected by the
sample, all that light emanating from the sample, which is received
finally by means of the device according to the invention and can
be used for evaluation. The reflected light hence generally
concerns the sum of different light components, namely in
particular light components scattered on the sample, light
components reflected diffusely on the sample and light components
reflected reflectively on the sample. (Reflected light therefore
relates precisely to those light components which reach the
detection unit and not to those light components which arrive back
at the illumination unit.) Subsequently, the light reaching the
detection unit due to a reflective reflection is termed also
reflected light in short: this light hence relates to light of
those surface elements of the sample which reflect the light
radiated onto the sample into the detection unit whilst fulfilling
the reflection condition.
[0011] A device according to the invention for optical
characterisation of a sample (or of one or more materials of the
same) can hence comprise the following elements: firstly an
illumination unit which is orientated to illuminate the sample (the
illumination unit can also be directed towards a spatial portion of
the sample or a spatial volume into which the sample is introduced
for illumination). This device comprises in addition a detection
unit which is configured to detect a plurality of different
(preferably: orthogonal) polarisation components. The detection
unit is orientated such that light components, reflected by the
sample, of the light radiated onto the sample can be detected.
Finally, this device according to the invention comprises an
evaluation unit. This can be produced for example as a computer
program in a personal computer. However, it is likewise conceivable
to configure the evaluation unit as part of the detection unit
(e.g. as evaluation program integrated in a camera). In the imaging
data recorded by the detection unit (this device according to the
invention is hence configured for planar optical imaging of the
sample or of a sample portion), those imaged surface elements of
the sample, the reflected light of which, received in the detection
unit, is based on a reflection of the incident light on the sample,
can be identified with this evaluation unit. These surface elements
of the sample are subsequently also termed reflection elements, in
contrast to those surface elements of the sample which, because of
physical effects other than a reflection (e.g. i.e. by light
scattering) reflect light into the aperture of the detection unit.
The evaluation unit of this device according to the invention is
finally configured such that the detected different polarisation
components can be evaluated precisely for the reflection elements
in order to obtain the desired optical characterisation of the
sample. On the basis of this evaluation, it is then possible for
example to separate, with this device according to the invention,
objects or object regions which have a narrowly tolerated range of
optical material constants from objects with deviating optical
material constants (sorting device) or to examine the maintenance
of optical material constants automatically, e.g. in one production
process. Sorting of bulk material is possible in particular with
the device according to the invention.
[0012] An essential feature of the above-described solution
according to the invention (device or also method for optical
characterisation using such a device) is hence that, provided it is
desired, e.g. by using an evaluation unit with a corresponding
computing capacity, also in step with a production or even with a
bulk material flow, a plurality of different (e.g. two orthogonal)
polarisation components in the light reflected by the sample can be
determined for precisely those surface elements of the sample, the
normal of which bisects the angle between illumination unit and
detection unit, and which therefore represent the reflection
elements of the surface of the sample. Automatic detection, as to
whether a specific surface element (i.e. an observed object point)
is a reflection element, i.e. fulfils the above-mentioned
reflection condition for orientation of its surface normal, can be
effected, as described subsequently in more detail, for example by
an intensity test of the intensity which is reflected or detected
in total by the imaged surface element.
[0013] Within the scope of the present invention, an individual
physical object is not necessarily understood by a sample, a sample
can quite generally also concern a flow of many individual, moving
objects of different materials (i.e. a sample flow within the scope
of a bulk material to be sorted or to be characterised, i.e. a bulk
material flow). As described subsequently in more detail with
reference to concrete examples, an illumination unit used within
the scope of the invention need not necessarily concern an
individual light source, rather also in parallel a plurality of
suitably disposed light sources which illuminate one and the same
sample can be used. In general, the reflection condition is then
fulfilled for each of the light sources used. Within the scope of
the present invention, there is understood by reflected light, all
those light components which originate in light components,
radiated by the illumination unit and incident on the sample, and
which are not absorbed by the sample, but which again leave the
sample or the surface thereof--generally in a different direction
from the direction of incidence--by means of any processes
(reflection, scattering, . . . ) and hence can be detected outside
the sample.
[0014] An essential idea of the above-described device according to
the invention is hence to detect and evaluate only the beam
components reflected on the sample (reflectively), i.e. to evaluate
only different polarisation states for the thus reflected beam
components for characterisation of the sample, but not for the
other reflected, e.g. scattered, beam components. As described
subsequently in more detail, it is thereby particularly
advantageous to dispose the illumination unit and the detection
unit at the Brewster angle provided that the characterisation task,
when using the device according to the invention, resides precisely
in detecting the presence of a defined material or in identifying
individual elements of this defined material in a sample comprising
a large number of objects of different materials (the Brewster
angle adjusted in the device is then the Brewster angle of this
material). This arrangement at the Brewster angle is advantageous
in particular for the reason that the beam components reflected on
samples or sample elements of this material have merely one
polarisation direction so that the material characterisation is
possible in a particularly simple manner. An arrangement at the
Brewster angle is however not absolutely necessary since the
different polarisation components for the reflection elements on
the surface of the sample can also be evaluated without maintaining
this special reflection condition (e.g. a differentiation can be
made with respect to the incident intensities thereof).
[0015] In a first advantageous embodiment of the above-described
device according to the invention, the evaluation unit is
configured such that firstly the reflection elements can be
identified in the recorded imaging data before the detected
different polarisation components can be evaluated for these (or
based on these) identified reflection elements. For example, the
reflection elements can be established with reference to an
evaluation of the total detected intensities of the individual
surface elements (or of the image pixels of the images detected by
the detection unit) by those surface elements being identified as
reflection elements, the total intensity of which (sum of the
intensities of all the detected polarisation components) lies above
a fixed threshold value (e.g. that intensity value, above which the
intensity of 20% of all the imaged surface elements lies, can be
defined as threshold value). Then the different (e.g. orthogonal)
polarisation components are evaluated merely for the thus
identified reflection elements, e.g. viewed separately or viewed
with respect to the intensity ratios thereof.
[0016] As an alternative thereto, it is also possible, in the
recorded imaging data for all imaged surface elements (these
comprising both imaged surface elements of the sample and imaged
surface elements of structures which do not belong to the sample
but are nevertheless imaged), to consider firstly the detected
different polarisation components, e.g. separately or according to
the ratios thereof, and (for example by setting a threshold value)
to evaluate them in order to determine those surface elements of
the sample which are reflection elements. Thus for example all
surface elements, the intensity of which exceeds a predetermined
threshold value for a defined polarisation component in the imaging
data, can be defined as reflection elements. For the reflection
elements thus identified by means of the different polarisation
components, the different polarisation components are then
evaluated further (for example by forming a ratio of the
intensities in imaging data or polarisation partial images which
correspond to different polarisation components and are recorded by
the detection unit) in order to implement the optical
characterisation of the sample.
[0017] In an advantageous embodiment, the device according to the
invention is configured such that the differentiation, required for
identifying the reflection elements, of reflection elements and of
surface elements, the reflected, received light of which is not
based on a reflection of the incident light on the sample
(scattered elements), is effected on the basis of the intensity or
of the intensities of imaging data of one, of a plurality or of all
of the detected polarisation components. In particular, intensity
differences or also intensity ratios of the different detected
polarisation components can be used for determination of the
reflection elements. It is also possible to use the total intensity
of all polarisation components, detected by the detection unit, of
the light arriving for imaging for identification of the reflection
elements. Thus for example, those surface elements, the associated
imaging values of which, in the images detected by the detection
unit, are in total above a predefined threshold value, can be
identified as reflection elements. Such a threshold value can be
defined for example as 90%/10% threshold value, i.e. it can by
means of this be established that 90% of the total intensity values
of all imaged surface elements are below this threshold value and
10% above.
[0018] As an alternative thereto or also in combination with the
intensity-based identification of the reflection elements, the
reflection elements can also by defined on the basis of the
position thereof in images of the sample produced corresponding to
the different polarisation components (or also corresponding to the
received total intensity): for this purpose, the position of the
surface elements (e.g. taking into account the intensities thereof)
can be evaluated relative to each other and/or relative to one or
more reference point(s) in the images of the sample. In particular
centres or edge points of images of the sample can serve as
reference points. For example, by suitable configuration of the
sample background or by additional illumination (which ensures a
constant, low background intensity), individual elements of the
sample (e.g. bulk material particles) can be differentiated from
the background by for example the change in intensity at the edge
of these sample elements being detected (evaluation of gradients in
the image). As an additional condition, that an observed surface
element is a reflection element of the sample, it can then be
established--in addition to the above-described threshold value
setting--that the reflection elements must be located within the
thus detected outlines of individual sample elements.
[0019] In contrast thereto, those surface elements, the intensity
or brightness of which lies below the above-described, adjustable
threshold, are then not reflection elements but scattered elements.
Also surface elements which are situated outside the sample element
limits, which can be identified as described above, are not
reflection elements of the sample. Further evaluation of these
surface elements is therefore not sensible.
[0020] In a further advantageous embodiment of the above-described
device according to the invention, evaluation of the identified
reflection elements is effected for the purpose of optical
characterisation of the sample as follows: a ratio is formed from
different polarisation components, which are recorded by the
detection unit for the identified reflection elements (e.g. from
two linear polarisation components which are orthogonal to each
other). This can take place for example by the intensity value, for
all the surface elements which were identified as reflection
elements, in the image recorded for a first polarisation component,
being divided by the intensity value of the corresponding
reflection element in the image recorded for a second, different
(e.g. orthogonal) polarisation component.
[0021] If the thus formed ratio then exceeds or falls below a
specific value for a specific minimum number of reflection elements
(relative to the total number of surface elements and/or of
reflection elements), then information about the presence or
absence of a defined material in the sample can hence be obtained:
if for example, as reflection condition for the reflection
elements, the angle between the optical axis of the detection unit,
on the one hand, and the optical axis of the illumination unit, on
the other hand, in the triangle which is spanned by the detection
unit, the illumination unit and the sample, is adjusted to twice
the Brewster angle of a sought material (the reflection elements
are then those surface elements of the sample, the normal of which
bisects the angle between the two above-mentioned axes), then all
those reflection elements which can be assigned to the sought
material reflect light components which are polarised merely
parallel to the surface of the sample but not light components with
a polarisation direction perpendicular thereto. However, this can
be detected via setting a corresponding threshold value for the
ratio calculated as described above, so that a differentiation of
sample elements of the sought material from sample elements made of
a different material is possible.
[0022] According to the invention, it is hence possible to test
whether the ratio of intensities of different polarisation
components is within a certain range in order to differentiate
defined materials from other materials. In particular in the case
of bulk material flows as samples, also the absolute number of
those image points or surface elements in the imaging data, which
are recorded by the detection unit and for which the ratio
calculated as described above exceeds or fall below a threshold
value, can thereby be used as sorting criterion. Alternatively
thereto, it is possible to evaluate not the absolute number of such
surface elements but the relative number of these surface elements
in comparison with those surface elements which do not fulfil the
threshold value criterion.
[0023] In the above-described embodiment variants of the invention,
the consideration is crucial that, even with irregular surfaces of
samples (e.g. of bulk materials), there is at least one point in
the case of each object or element of the sample, i.e. a surface
element, which fulfils the reflection condition and with which the
object can hence be characterised.
[0024] It is particularly advantageous, within the scope of the
invention, to make use of surface elements or reflection elements
from different directions at the same time or also in succession
for the evaluation. Thus of course it basically suffices that the
illumination unit used has merely a single illumination element
(e.g. a single monochromatic light source, see subsequent
embodiment 1).
[0025] However, the illumination unit can also comprise a plurality
of individual illumination elements which are configured to
illuminate the sample with incident light from different
directions. The angle between the detection unit or the optical
axis thereof, on the one hand, and the respective illumination
element or the optical axis thereof, on the other hand, in the
triangle which is spanned by the detection unit, the corresponding
illumination element and the sample can thereby be identical in all
illumination elements. Advantageously, two or four individual
illumination elements can be used. The illumination elements can be
disposed on the side of the sample situated opposite the detection
unit and in a plane orientated preferably perpendicular to the
optical axis of the detection unit.
[0026] In particular, the individual illumination elements can be
disposed at equidistant angle spacings on a circle about the
optical axis of the detection unit in this plane. For example, when
using four illumination elements, these can thus be disposed at
angle spacings of 90.degree. on a circle about the optical axis of
the detection unit. For all these illumination elements, the angle
ratios described above for the illumination unit (e.g. adjustment
to a Brewster angle for a defined material) can then be
maintained.
[0027] All the devices for optical characterisation described
within the scope of the present invention can be configured by
suitable provision of further components (e.g. sample storage units
etc.) for surface testing of planar coatings as sample.
[0028] However it is likewise possible to develop the devices for
characterisation, differentiation and/or separation of individual
elements of a sample comprising a large number of elements (in
particular: bulk material flow). This can take place for example by
the illumination unit and the detection unit being disposed for
illumination and imaging of a free falling stretch part, e.g. below
a vibrator for bulk material. As an alternative thereto, of course,
also conveyer belt portions on which bulk material is transported
can be illuminated by the illumination unit and scanned by the
detection unit. The last-mentioned devices can then be configured
in particular also for sorting sample elements which deviate from
one or more predefined material parameter(s) (which is/are
determined for optical characterisation by evaluation of the
reflection elements).
[0029] The above-described devices according to the invention can
be configured as a laser scanner system with an illumination unit
which scans one- or two-dimensionally the sample or sample spatial
portion in which this sample is disposed, on the basis of one or
more laser(s) and with one or more suitable receiving unit(s) as
detection unit.
[0030] As a alternative thereto, it is however also possible to use
one or more monochromatic light sources as illumination unit or
illumination element(s). As detection unit, one or more camera(s),
in particular polarisation camera(s) and/or CCD-based camera(s),
can then be used.
[0031] Illumination of the sample is effected advantageously with
one or more defined wavelength(s) in the visible range; it is
however basically also conceivable to use for example infrared
radiation for the illumination provided that the receiving units
are then correspondingly adapted.
[0032] Subsequently, a few concrete embodiments of the illumination
unit-detection unit system of the present invention are now
described:
[0033] Thus, a camera which is used in a reflection arrangement, as
described above, and with which for example two orthogonal
polarisation components can be detected for example for each
scanned surface element, can be a camera consisting of two
individual cameras. In the beam path in front of the individual
cameras, a polarising optical element (e.g. prism or beam splitter)
is disposed, with which the light reflected by the sample can be
split into two different polarisation components. The light of the
one polarisation component is then directed by the polarising
optical element towards the one individual camera, the light of the
other polarisation component towards the other of the two
individual cameras. It is thereby advantageous to provide a pixel
adaptation in order to coordinate the position of the detected
reflection elements in the images of both individual cameras.
[0034] As an alternative thereto, a multiline camera can be
provided, a number of polarisers which corresponds to the number of
lines of the camera being provided in the beam path in front of
this camera. Two (or more, e.g. six) different types of polarisers,
for example two polarisers orientated orthogonally relative to each
other (in the case of six polarisers, e.g. 0.degree., 45.degree.,
90.degree., 135.degree., left-circular and right-circular
polarising polarisations) thereby exist. The two or more
polarisation directions are detected by the camera: in front of the
individual lines of the camera, polarisers of the one type and of
the other type, in the case of two different polarisers, are
disposed alternately so that light of a first polarisation
component and of a second polarisation component, e.g. orthogonal
to the first polarisation component, is imaged respectively
alternately onto the camera lines. In the case of for example six
different polarisers, accordingly six adjacent lines for the six
different types are required.
[0035] As an alternative thereto, a camera which, in the beam path
in front of its sensor chip, comprises a polarisation strip filter
or a polarisation mosaic filter can be used. Such a filter splits
the light reflected by the sample into the different polarisation
components which then are directed, in lines or corresponding to
the mosaic arrangement of the filter, towards the respective sensor
cells of the camera chip (which sensor cells of the chip receive
light of which polarisation component is known on the basis of the
already known filter shape so that the evaluation can be
correspondingly effected).
[0036] It is likewise conceivable to provide a plurality of
differently polarised illumination sub-units (e.g. individual
lamps) which are all orientated to illuminate the sample with
incident light. The individual illumination sub-units are switched
on and off again temporally in succession, therefore illuminate the
sample respectively in succession for a predefined time duration.
The individual polarisation components are then detected during
different, precisely defined time intervals respectively by the
total camera surface of a non-polarisation-sensitive camera
(preferably a multiline camera is used), hence the differently
polarised illuminations are quasi-flashed. If the device is used
with moving samples (bulk material flow), the flashing frequency or
the frequency of the switch-over between the individual
illumination sub-units must be synchronised advantageously with the
sample speed.
[0037] This synchronisation has the following advantage: in
general, the sample is in motion during the scanning (e.g. on a
corresponding falling stretch or also flight stretch in the case of
a for example parabolic discharge from a conveyer belt, i.e. the
image of the sample is moved, in the case of the individual camera
images taken temporally in succession for evaluation, within these
camera images, i.e. changes its position in the individual camera
images. Since now however generally both (or the more than two)
polarisation states are required from one and the same point or
surface element of the sample, it must in general be known how far
the sample has moved between two adjacent camera images in order
also to detect and evaluate in fact the polarisation states of one
and the same surface element of the sample (it is thus established
for example that the sample is moved between two temporally
adjacent camera images by 5 pixels with respect to its image so
that a corresponding displacement of the temporally adjacent camera
images can be effected in order to compensate for the sample
movement).
[0038] A further camera-based system construction according to the
invention for the illumination unit and the detection unit uses an
LED illumination in which the individual LED illumination elements
which emit light of one wavelength are disposed such that, for each
image point or for each imaged surface element, the same reflection
condition is given over the entire sample surface to be scanned
(total camera image). This can be achieved by different angles of
inclination of the individual LED elements in a strip, by means of
an arrangement of these elements on a bent plate or in the case of
a planar arrangement of the LED elements by means of an attachment
lens system.
[0039] By means of the bent plate or the attachment lens system, in
particular effects of the e.g. funnel-shaped camera opening can be
compensated for (which would otherwise prevent exact detection and
evaluation): because of the corresponding bent plate or attachment
lens system, the camera then no longer has a telecentric beam path
but rather a fan-shaped one which leads to the fact that actually
the same reflection conditions are given for each image point over
the entire sample surface to be scanned.
[0040] Within the scope of the scope of the present invention, it
is also possible to provide the illumination unit which is used
with means for changing and/or adjusting the polarisation of the
illumination (for example LC element, as used in LC displays).
[0041] The illumination of the sample can then be effected, as in
the case of the "flashing" described earlier, in succession with
different polarisation states. For each illumination-polarisation
state, different polarisation components for the reflection
elements can then, as described earlier, be detected and
evaluated.
[0042] As an alternative to switching on and off or illumination of
the sample by "flashing", also suitable polarisers which produce
the desired polarisation states can hence be placed in front of the
illumination unit (or in the beam path between illumination unit
and sample).
[0043] It is also possible to use the illumination unit and the
detection unit together with a polarisation-obtaining
retroreflector. A flow diagram of such an illustration is provided
in FIG. 6. In the case of a laser used in the form of a laser
scanner as illumination unit and in the case of a receiver
configured suitably as associated detection unit (which can have in
particular a polarisation-obtaining beam splitter for dividing the
incident laser light into two partial beam paths and also
polarising optical elements in these partial beam paths), the
illumination unit and the detection unit can also be configured as
integrated transmitting and receiving unit 11. A retroreflector 13
must then be provided, the combined transmitting and receiving unit
and the retroreflector being configured and disposed as follows: in
the combined transmitting and receiving unit 11, transmitting and
receiving beam path are coupled via a beam splitter 8 on the same
axis. The transmitter illuminates the sample P, the beam components
which are reflected by the sample P (i.e. scattered, diffusely
reflected or reflectively reflected) are reflected back on the
retroreflector 13 per se and arrive at the combined transmitting
and receiving unit 12, preferably via the sample P, and there, via
the beam splitter 8, at the receiver beam path of the receiver of
this combined transmitting and receiving unit. FIG. 6 is a flow
diagram and does not illustrate angles of transmitted or reflected
light.
[0044] With a corresponding arrangement of a laser, which has a
non-integrated configuration, and of a receiver which is suitably
configured and orientated to receive the reflected light of the
laser, the retroreflector is disposed in the above-described case
wherever the receiver stands. This has the advantage that if the
beams components arrive at the combined transmitting and receiving
unit via the object, the radiation is reflected twice on the sample
or on the object. Hence improved imaging of the sample or of the
object with an improved intensity ratio is effected (if for example
in the case of a single reflection on the sample, an intensity
ratio of 1:1,000 is present, then the intensity ratio with this
construction is 1:1,000.sup.2). By means of suitable
retroreflectors or adjustment of the back-reflection, in addition a
very compact construction can be produced. Finally, within the
scope of the devices for optical characterisation as described
above and according to the invention, it is also possible (and in
particular in the above-described retroreflector variant) to use a
laser which scans the sample in lines or in a grid shape as
illumination unit (laser scanner system). The detection unit then
comprises a receiver, which is suitable for receiving the laser
light reflected by the sample, with one or more optical element(s)
for separation of the received laser light according to the
different polarisation components. In the receiver, a plurality of
receiving elements are configured, the number of which corresponds
to the number of partial beams resulting from the separation. The
evaluation unit can establish, for example by testing the received
total intensity, whether surface elements are reflection elements
of the sample, i.e. fulfil the reflection condition. The evaluation
unit of the above-described devices for optical characterisation
according to the invention is configured such that, with it, the
reflection elements of the sample can be identified (for example by
evaluation of the total intensity received by each object point as
a criterion for whether the corresponding object point fulfils the
reflection condition, i.e. has the orientation of its surface which
is required for further evaluation). The reflection elements are
then evaluated further by the evaluation unit for optical
characterisation of the sample, i.e. one or more processing step(s)
is/are provided in order to implement an overall characterisation
of the sample by evaluation of the significant surface elements or
of the reflection elements. In order to detect further sample-
and/or material parameters of the sample within the scope of this
overall characterisation, in addition, delay elements (e.g.
.lamda./4 plates) can be provided in front of the illumination. It
is also possible to use further beam splitters or filters in front
of the detection unit or the light-sensitive surface thereof. The
orientation, adjustment and arrangement of such delay elements
and/or beam splitters or filters can be effected such that
determination of further Stokes' parameters is possible. In
particular also a monochromatic coherent illumination unit (laser)
can be provided so that, due to physical secondary conditions known
to the person skilled in the art, merely three Stokes' parameters
are required instead of four Stokes' parameters for complete
characterisation of the polarisation state of the light reflected
by the sample.
[0045] According to the invention, systems with angles of incidence
and reflection between the two special cases 0.degree. and
180.degree. are possible: in the boundary case of 90.degree., the
sample should hereby be placed between illumination unit and
detector. In the special case of 90.degree. and when using a
combined illumination- and detection unit or transmitting and
receiving unit, the retroreflector is situated behind the sample.
In the special case of a 90.degree. arrangement, this involves a
polarimetry construction for the optical characterisation of
transparent objects. In this case also, in comparison with
arrangements according to the state of the art, the advantage is
gained that a significantly faster evaluation can be effected: by
means of a complete characterisation of the polarisation state, the
variation in the illumination conditions required with devices
according to the state of the art can be dispensed with. As a
result, a test in step with the production of goods is possible for
the first time. Just as in the case of the reflection evaluation, a
test of the overall intensity is sensible in this case in order to
find the object points or surface elements which are relevant for
characterisation of the sample. However, in this case, these points
are not reflection elements but transmission elements. In this
case, the light components reflected by the sample can therefore
also concern components transmitted through the sample
(transmission instead of reflection or also both components:
transmission and reflection). The device can therefore also be
configured as transmission system or transmission-reflection
system. In a further embodiment, the device according to the
invention for optical characterisation comprises the following
elements: a laser orientated for one- or two-dimensional scanning
with incident light of a sample spatial portion in which the sample
can be introduced (illumination unit). For receiving the light
reflected through the sample, a receiver which is suitable for
receiving laser light is provided as detection unit. This receiver
comprises a first, preferably polarisation-obtaining beam splitter
for dividing the laser light incident on the receiver into a first
and a second partial beam path. In each of these two partial beam
paths, a polarising optical element (e.g. polarising prism or
polarising beam splitter) is provided, with which the light of the
respective partial beam path can be split into two different
polarisation components. In the beam path of each of the two thus
separated polarisation components, respectively one receiving
element is provided, with which the respective polarisation
component can be detected (hence in total four receiving elements
are provided, two in each of the above-described partial beam
paths). A polarisation-changing element is provided merely in one
of the two partial beam paths in addition after the beam splitter
and in front of the polarising optical element, with which
polarisation-changing element the polarisation of this partial beam
path can be changed. This changing element can concern in
particular a delay plate which is configured preferably as
.lamda./4 plate. The evaluation unit of the device is configured
such that, with it, on the basis of the different polarisation
components detected by the plurality of receiving elements of the
receiver, the polarisation state of the light reflected by the
sample can be determined completely for the optical
characterisation of the same.
[0046] For this purpose, in particular the beam splitter, the
polarising optical elements, the changing element and the four
receiving elements can be configured, disposed and adjusted such
that three or four Stokes' parameters of the reflected light can be
calculated from the detected different polarisation components.
Since the incident laser light is completely polarised,
determination of three of the four Stokes' parameters suffices to
calculate the fourth Stokes' parameter (using a secondary condition
for monochromatic, coherent light) and hence to characterise
completely the polarisation state of the reflected light. With the
help of the polarisation state of the reflected light, which is
thus determined completely, for example different materials of
different sample elements of a bulk material sample can then be
identified and differentiated.
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 .theta..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
.theta..sub.B of equal size, here .theta..sub.B=63.degree.
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 .theta..sub.B=63.degree. 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 .theta..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. .theta..sub.B is thereby 63.degree.
(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=.theta..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 .theta..sub.B of
the sought material can hence be effected (i.e. for diamond at
.theta..sub.B=67.5.degree.). 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
(.theta..sub.B=63.degree.), 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 .theta..
[0070] In the case of an arrangement in which the Brewster angle
.theta..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
.theta..sub.B of 55.5.degree. 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 .lamda.=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 .theta..sub.aB of equal size, which
are configured corresponding to the Brewster angle .theta..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 .theta..sub.bB of equal size. Due to the
above-described arrangement, there applies here
.theta..sub.aB=.theta..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.degree.) 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.degree.
spacing (not shown), objects made of the material to be identified
are characterised according to the Brewster angle
.theta..sub.aB=.theta..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 .theta. (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 .theta. (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 (.lamda./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.
* * * * *