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Description  |
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The invention relates to a measuring system employing a measuring method
based on the triangulation principle for the non-contact measurement of a
distance from the surface of a contoured object to a reference level,
which system comprises: a transmission section including a source for
emitting measuring radiation and means for guiding the measuring radiation
so that a beam of measuring radiation is formed, which measuring radiation
guide means define a transmission path along which the beam of measuring
radiation is projected onto the object's surface under inspection; and a
reception section including means for guiding reflected radiation and a
detector sensitive to reflected radiation and operative to convert
reflected radiation received thereby into a corresponding signal of a
different type, which reflected radiation guide means form reflected
radiation originating from a radiation spot caused by the projection 2of
the beam of measuring radiation onto the object's surface into a beam of
reflected radiation and define a reception path along which the beam of
reflected radiation is projected onto the radiation-sensitive detector.
A measuring system of the above type is known from, for example, DE-OS No.
3,122,712 A1. This publication further describes the general principle of
triangulation used as a basis for the non-contact measurement of the
distance between a given point of a surface contour of an object's surface
under inspection and a reference plane. FIG. 1 is illustrative of this
principle. The beam of measuring radiation is projected onto a given point
of an object's surface under inspection, which surface has been
schematically indicated by hatching.
The distance .DELTA.z to be measured between this point of the surface
contour and a reference plane indicated by Z=0 is then given by the
equation .delta.=.DELTA.z.M.sin .theta., where .theta. is the viewing
angle at which a radiation spot projected onto the reference plane is
viewed from a detector D sensitive to reflected radiation; M is the
measure of optical magnification; and .delta. is the distance between the
projection of the reference level on the detector and the projection of
the respective point of the object's surface on this detector.
The object of the proposals disclosed in the aforesaid publication is to
increase the measuring rate by employing a so-called wavelength marking
technique, without variations in the reflectivity of an object's surface
under inspection introducing errors into the result of the measurement and
without the results being ambiguous. To this end, use is made of a beam of
measuring radiation of at least two different wavelengths; a raster
pattern characterized by these wavelengths is projected onto the object's
surface under inspection and the reflected radiation originating from this
pattern is passed through separate wavelength-sensitive receiving channels
to discrete detectors responsive to the respective wavelengths. This prior
art technique entails the following drawbacks: the measurements are highly
responsive to background radiation, which results in an adverse
signal-to-noise ratio in respect of the output signal derived from the
detectors; it is necessary to use at least two separate detector units
each having a sensitivity matching one of the different wavelengths
selected for the measurement; and the measurements are responsive to
changes in colour of the object's surface under inspection.
The DE-OS No. 3,110,644 A1 likewise describes the general principle of
triangulation as a basis for the non-contact measurement of the distance
between a reference plane and a given point of an object's surface. The
object contemplated by the proposals disclosed in this publication is to
provide a solution to the problem arising when the optical axis of the
beam of measuring radiation is shifted as a result of drift, causing the
point where this beam impinges upon the object's surface to be shifted
too. Such a shift results in the introduction of measuring errors as in
this prior art technique the only measuring information available is the
distance between a first spot projected on the detector as corresponding
to the zero or reference level and a second spot projected on this
detector as corresponding to the point of the object's surface under
inspection. The crux of the technique disclosed in this publication
resides in the derivation of an error signal representative of the
magnitude of the shift experienced by the optical axis of the beam of
measuring radiation, which error signal is used for the correction of the
measuring signal derived from the detector. However, this prior
publication fails to disclose measures resulting in an adequate
elimination of the effect of false reflections on the result of the
measurement.
The Selcom firm has developed a measuring system for the non-contact
measurement of data relating to an object's surface. This known measuring
system is likewise based on the triangulation principle. Moreover, this
measuring system likewise entails the drawback of being highly responsive
to background radiation, which results in an adverse signal-to-noise ratio
for the detector output signal. An additional drawback is the relatively
poor lateral resolution, i.e. the resolution in planes normal to the beam
of measuring radiation.
It is an object of the invention to eliminate the above-mentioned
drawbacks. In particular, it is an object of the invention to provide a
measuring system of the type described above, in which the adverse effect
of background radiation on the results of the measurement can be
considerably reduced, so that the signal-to-noise ratio of the detector
output signal is enhanced accordingly.
A measuring system according to the invention is based on the idea of
substantially restricting the sensitivity to radiation of the reception
section to a spatial volume having the form of an optimally slim beam
extending substantially normal to an object's surface under inspection.
A basic embodiment of the measuring system according to the invention is
characterized in that the detector is sensitive to radiation only in a
substantially linear area; and that only elements of the object's surface
which are directly illuminated by the beam of measuring radiation, at any
rate a longitudinal section thereof constituting the measuring zone, are
imaged as corresponding, focussed spots on the linear radiation-sensitive
area of the detector. In this manner, consequently, a thus-illuminated
point of the object's surface is imaged as a sharply defined point on this
area of the detector.
In such a measuring system, in principle any radiation originating from a
region of the object's surface located outside the beam of measuring
radiation will not result in the detector producing an, in that case
spurious, output signal. In fact, when radiation originates from a region
of the object's surface located substantially within the "plan" defined by
the substantially linear beam of measuring radiation and the substantially
linear radiation-sensitive area of the detector though outside this beam
of measuring radiation, this radiation will be imaged on the detector or
an imaginary plane containing this detector as an out-of-focus spot, which
does not result in the production of a significant signal at the detector
output. The spot will be more out of focus as the region from which the
radiation originates is farther away from the beam of measuring radiation.
Furthermore, when radiation originates from a region of the object's
surface located outside the aforesaid "plan", the resultant spot will be
imaged, either in or out of focus, on the aforesaid imaginary detector
plane though outside the linear radiation-sensitive detector area, so that
such a spot does not result either in the production of a spurious
detector output signal.
An enhanced version of the above basic embodiment of the measuring system
according to the invention is characterized in that a focussing lens
included in the measuring radiation guide means is operative to so focus
the beam of measuring radiation that, in the measuring zone, the
cross-sectional area of this beam varies as a function of the distance as
seen in the direction of the beam, so that all object spots formed within
the measuring zone are imaged on the radiation-sensitive detector as spots
of substantially equal size. In particular, the power and the position of
the focussing lens are such that the size of each object spot formed by
the beam of measuring radiation within the measuring zone increases as the
respective object spot is farther remote from the focussing lens. In this
manner, the essentially linear variation of the measure of reduction
introduced by the objective lens system of the detector, i.e. the extent
to which an object spot is reduced when imaged on the detector, is
compensated for. This means that with a given position of the beam of
measuring radiation, a succession of object spots defined thereby is
imaged on the detector as a succession of focussed spots of substantially
equal size. The measure of reduction introduced by the detector objective
lens system is preferably selected so that the size of a detector spot
corresponding to an object spot is substantially equal to the area of a
few detector elements In this manner, the image definition and the lateral
resolution are optimized throughout the entire measuring zone.
Furthermore, as radiation originating from a surface region located
outside the beam of measuring radiation will not cause a significant
detector output signal, as explained above, a highly satisfactory
signal-to-noise ratio is achieved.
A measuring system according to the invention is especially suited for use
in situations involving the presence of substantial sources of background
radiation, such as high-intensity light sources. An example of such a
situation is the use of the measuring system in an automated welding
process, in which case the measuring system should be immune to the
high-intensity light radiation caused by the welding arc. In a measuring
system to be used to advantage in such situations, use is preferably made
of a radiation source, such as a laser, emitting monochromatic radiation.
In accordance with the invention, the wavelength of this radiation is
selected to be in a portion of the spectrum where the presence of
wavelength components of spurious background radiation to be expected, is
small; while an optical bandpass filter is mounted within the spatial
viewing angle of the detector, a wavelength of the passband, preferably a
medium wavelength thereof, corresponding to the wavelength of the
monochromatic radiation. In practice, a He-Ne laser appears to provide
good results.
The invention can further be used to advantage when one or more surface
contours or surface profiles of an object's surface under inspection are
to be measured. In accordance with the invention, a measuring system
suiting this purpose is characterized in that the measuring radiation
guide means are coupled to a drive means operative to have the beam of
measuring radiation performascanning movement in a plane of measurement;
and that the reflected radiation guide means are adapted to image object
spots illuminated by the beam of measuring radiation during a scanning
movement on a substantially linear radiation-sensitive area of the
detector as focussed spots moving isochronally with the illuminated object
spots.
A further object of the invention is to provide a measuring system of the
type described above, in which the adverse effect of false reflections can
be eliminated by means of a simple structure. In general, false
reflections are formed by reflected radiation imaged on the respective
detector as a result of the beam of measuring radiation incident on a
given point of the object's surface being reflected to the detector by
other surface elements located within or just outside the volume of the
beam of measuring radiation.
In accordance with the invention, an embodiment of a measuring system
suited to achieve the aforesaid object is characterized in that the
reflected radiation guide means define, in addition to the first-named
reception path, a second reception path forming with the first reception
path a mirror-symmetrical configuration relative to a plane containing the
beam of measuring radiation, this second reception path projecting
reflected radiation originating from the respective object spot and guided
along this path onto a second, separate radiation-sensitive detector; and
that the outputs of the two radiation-sensitive detectors are coupled to a
comparator for so comparing the output signals produced by these detectors
that reflected radiation resulting from singular reflection of the beam of
measuring radiation from the object's surface provides a useful output
signal.
By correlating the two signals produced at the outputs of the separate
detectors with each other, for example by multiplication, it is pcssible
to distinguish a spot projected onto the detectors by a false reflection
from a spot projected onto the detector by a "good" reflection.
An improved embodiment of a measuring system according to the invention is
characterized in that the reflected radiation guide means define a second
reception path in addition to the first-named reception path, an initial
portion of this second reception path as extending from a respective
object spot forming with a corresponding initial portion of the first
reception path a mirror-symmetrical configuration relative to a plane
containing the beam of measuring radiation, this second reception path
likewise projecting reflected radiation originating from the respective
object spot and guided along this path onto the aforesaid
radiation-sensitive detector; and that the output of this detector is
coupled to a signal processing means operative to so process the detector
output signals that a useful signal is provided in response to reflected
radiation resulting from singular reflection of the beam of measuring
radiation from the object's surface.
In such an embodiment, the use of a single detector suffices, which saves
considerable cost and results in an arrangement of simpler structure.
In accordance with the invention, a preferred embodiment of such a
measuring system is characterized in that a portion of the second
reception path is oriented to so deviate from a corresponding portion of
the first reception path that reflected radiation originating from one and
the same object spot as defined by the intersection of the plane
containing the beam of measuring radiation and the respective object's
surface is imaged on the detector via one reception path as a detector
spot spaced a fixed distance from the detector spot imaged on the detector
via the other reception path as a projection of that object spot; and that
the signal processing means are arranged to perform an auto-correlating
function in respect of the detector output signal containing two discrete
signal components spaced apart an interval corresponding to the aforesaid
fixed distance.
In accordance with the invention, a first realization of such a measuring
system is characterized in that the reflected radiation guide means
include two prisms secured to each other to have one prism face in common,
so that reflected radiation traversing one reception path is passed by
this common prism face in partially undiffracted fashion into the
direction of the detector and reflected radiation traversing the other
reception path is partially reflected into the direction of this detector.
In accordance with the invention, a second realization of such a measuring
system is characterized in that the aforesaid common prism face is tilted
relative to the plane containing the beam of measuring radiation, the
tilting angle being determinative of the aforesaid fixed distance.
For additional protection against false reflections, if any, satisfying the
discrimination criterion established by the embodiments described above, a
measuring system according to the invention is further characterized in
that both reception paths include a reflector surface operative to direct
reflected radiation originating from the object's surface onto the
aforesaid common prism face; that each of these reflector surfaces is
tilted through one and the same angle about a respective one of two
intersecting axes extending mirror-symmetrically relative to the plane
containing the beam of measuring radiation; and that the beam of measuring
radiation is directed along the line of intersection of two planes via
which this beam is imaged on the detector along the first and the second
reception path.
In accordance with the invention, a structurally simple embodiment of a
measuring system is characterized in that both reception paths include a
reflector surface operative to direct reflected radiation originating from
the object's surface onto the aforesaid prism face, and that each of these
reflector surfaces forms together with the common prism face one of the
two prisms.
An alternative embodiment of a measuring system according to the invention
is characterized in that the reflected radiation guide means define a
second reception path in addition to the first-named reception path, an
initial portion of the second reception path as extending from a
respective object spot forming with a corresponding initial portion of the
first reception path a mirror-symmetrical configuration relative to a
plane containing the beam of measuring radiation, both initial portions of
this mirror-symmetrical configuration including a periodically active
light radiation switch causing the respective optical path to successively
be in blocked and transmissive state, so that reflected radiation is
presented to the radiation-sensitive detector alternately via the one and
the other reception path in accordance with a succession of time slots
defining a first and a second time channel; and that the output of this
detector is coupled to a signal processing means including a demodulator
operative to convert the time-divided signals of the first time channel
and of the second time channel into two spatially separated signals, and a
comparator operative to so compare these two spatially separated signals
with each other that reflected radiation resulting from singular
reflection of the beam of measuring radiation from the object's surface
provides a useful output signal.
In accordance with the invention, an additional criterion for
distinguishing between "good" and "false" reflections can be obtained by
introducing a predetermined characteristic into the beam of measuring
radiation. By means of, for example, a planoparallel plate it is possible
to give the beam of measuring radiation an asymmetric intensity
distribution. In accordance with the invention, an embodiment based on
this principle is characterized in that the transmission section includes
beam-distorting means for irradiating a respective object's surface with a
measuring beam of asymmetric intensity distribution; and that the
reception section includes a signal processor operative to only pass those
signal portions of a detector output signal applied thereto that
correspond to image spots being a projection on the detector of the object
spot located in the plane of the measuring beam that has an asymmetric
intensity distribution as determined by the measuring beam. In this
embodiment, use is made of the fact that "good" reflections provide an
asymmetric waveform in the detector output signal, which waveform is the
mirror image of a waveform provided in this detector output signal by a
"false" reflection, such a "false" reflection actually signifying that the
measuring beam has been reflected twice from the object's surface.
In illustration of the invention, a number of embodiments thereof will now
be described in greater detail with reference to the accompanying
drawings. Self-evidently, the invention is not restricted to the
embodiments described. In the drawings:
FIG. 1 schematically illustrates the use of the principle of triangulation
for the measurement of a distance between a point of an object's surface
and a reference level;
FIGS. 2a and 2b schematically show an embodiment of a measuring system
according to the invention in which the response to background radiation
has been greatly reduced;
FIG. 3 schematically shows a longitudinal cross-section of the measuring
zone of a focussed beam of measuring radiation, the shape shown being
particularly suited for forming a substantially uniform projection of
object spots on the detector;
FIG. 4 schematically shows an embodiment of a measuring system according to
the invention employing two separate optical channels for the measurement;
FIG. 5 schematically shows the manner in which false reflections can be
eliminated in accordance with the invention by means of an embodiment as
shown in FIG. 4;
FIG. 6 schematically shows a simplified embodiment of a measuring system
according to the invention; and
FIG. 7 schematically shows a variant of the embodiment shown in FIG. 6.
FIG. 2a schematically shows a basic embodiment of a measuring system
according to the invention, which embodiment is characterized by the fact
that the detector is irresponsive to spurious background radiation and
only radiation originating from an object spot illuminated by the beam of
measuring radiation is imaged in optimally focussed fashion on a linear
radiation-sensitive area of the detector. In FIG. 2a, the measuring beam
MB is schematically shown as a straight line extending in the Z direction.
The measuring zone MG is a zone bounded in the YZ plane in which the
"depth" of surface regions of an object can be measured when "illuminated"
by the measuring beam. By way of example, FIG. 2a shows two of such object
spots 01 and 02 produced when illuminating two surface elements of the
object at different "depths" by means of a measuring beam incident along
the Z axis. In a measuring system according to the invention, only object
spots such as 01 and 02, i.e. object spots defined as surface regions
directly illuminated by the measuring beam, are imaged via a schematically
shown objective lens system OL on a detector surface DV as focussed spots
01' and 02'. This implies that, in principle, all of such object spots
located within the measuring zone, which are defined as surface regions
that can be illuminated by the measuring beam, are imaged on the detector
surface DV in a substantially linear area schematically indicated by DG.
In accordance with an aspect of the present invention, the detector
comprises a substantially linear configuration of detector elements, a
substantially linear radiation-sensitive area of detector elements being
used for preference. In a detector restricted to such a substantially
linear radiation-sensitive area, all radiation originating from a surface
region located (a) within the "plane" defined by the substantially linear
measuring beam and the linear radiation-sensitive detector area, and (b)
outside the measuring beam, will be imaged on this radiation-sensitive
detector area or on the extension thereof. The objective lens system OL is
so constructed that such radiation, in sofar as originating from surface
elements located within the viewing angle defined by the length of the
radiation-sensitive detector area and the lens aperture though outside the
measuring beam, is imaged on the radiation-sensitive detector area DG as
out-of-focus spots, e.g. B1 is imaged as B1', whereas radiation
originating from places located in the aforesaid "plane" though outside
the aforesaid viewing angle is projected onto the extension of and outside
the radiation-sensitive area, such as e.g. B2 as B2'. In neither of these
instances, the detector will produce an output signal which, if produced
indeed, should be regarded as a spurious signal. This will also apply to
radiation originating from surface regions located outside the aforesaid
"plane". For example, the surface region A1 or A2 located outside the
aforesaid plane, i.e. the XZ plane in FIG. 2a, will be projected via the
objective lens system OL as in-focus or out-of-focus spot A1' or A2',
consequently outside and aside of the radiation-sensitive detector area
DG. All surface regions located outside the aforesaid XZ plane, such as
A1, will always be projected outside and aside of the radiation-sensitive
detector area DG and be imaged on the plane DV, either in or out of focus
as depending upon the spatial position. As a result, the actual detector
DG will be irresponsive to such sources of spurious radiation.
As the surface regions illuminated by the measuring beam are viewed from
the detector at different angles, such as .theta., in dependence upon the
"depth" in the Z direction to be measured, and the measure of imaging,
i.e. the extent to which an object spot illuminated by the measuring beam
is reduced when imaged on the detector, likewise varies as a function of
the Z coordinate over the measuring zone MG, the detector plane DG and
hence the actual detector DG is mounted at a given angle relative to the
optical axis 01-P thereof.
For the sake of clarity, this is shown in greater detail in FIG. 2b
illustrating the situation obtaining in the XZ plane of FIG. 2a. By such
an arrangement, it is achieved that the focussing effect of the objective
lens system OL on all object spots resulting from illumination by the
measuring beam will be substantially uniform, so that irrespective of
their position in the Z direction such object spots are imaged on the
linear radiation-sensitive detector area in equally, in fact highly,
focussed fashion.
In a measuring system according to the invention, it is further
contemplated to optimize the lateral resolution throughout the measuring
zone, i.e. the resolution in planes transverse to the measuring beam, to
the effect that irrespective of the position at which an object spot is
located within the measuring beam, this object spot is imaged on the
detector as an image spot having a cross-sectional area corresponding to
that of a few, e.g. two to four, detector cells (a detector cell has
dimensions of e.g. 25.times.26 .mu.m). By means of an appropriately
dimensioned focussing lens mounted in the optical path of the measuring
beam it is possible to achieve a satisfactory compromise between the two
requirements of, on the first hand, an optimally slim measuring beam and,
on the other hand, a substantially uniform projection of the object spots
on the detector. By way of example, FIG. 3 shows a satisfactory shape of
the measuring beam in longitudinal section. As appears from FIG. 3, the
measuring beam will be essentially divergent in the direction of
radiation. In a measuring system according to the invention it is thus
achieved that the substantially linear variation of the measure of imaging
as a function of the distance between the focussing lens for the measuring
beam and a respective object spot caused by the illumination of a surface
element by this measuring beam, which distance varies in the Z direction,
is compensated for by the variation in the size of the object spot as a
function of this distance as achieved as a result of the particular beam
shape chosen. Consequently, the choice of the particular shape of the
measuring beam results in the size of the object spot being increased in
proportion as the measure of magnification is decreased as a function of
the distance in the Z direction. In this manner, surface elements
illuminated by the measuring beam throughout the entire measuring zone
thereof can be imaged on the radiation-sensitive detector area as spots of
substantially equal size.
The above concerned the performance of a .DELTA.z measurement by means of a
measuring beam having only a single position, thus permitting only the
depth of surface elements illuminated by this beam to be measured.
However, it is often desired to perform such measurements for different
positions of the measuring beam so as to allow the measurement of surface
profiles or surface contours of an object. Furthermore, it is then often
desired to perform measurements to a large number of different surface
elements within a brief period of time, for example more than 1000
measurements per second. In accordance with a further aspect of the
invention, the transmission section of the measuring system is arranged to
realize movement of the measuring beam over the object's surface,
preferably in accordance with a succession of sweeping movements in
mutually parallel planes, for example planes parallel to the YZ plane of
FIG. 2a. To this end, after a sweep the measuring beam is so shifted in,
for example, the X direction of FIG. 2a that a next sweep is performed in
a plane parallel to the plane of the previous sweep, etc. In this manner,
an object's surface can be systematically scanned, for example at a
measuring rate of 115 .DELTA.z measurements per sweep and a scanning rate
of 10 sweeps per second. The reception section of the measuring system is
so arranged that the reflected radiation guide means are operative to
image the surface elements successively illuminated during a sweeping
movement of the measuring beam on the radiation-sensitive detector area as
spots isochronously moving and corresponding with these elements. In other
words, at each angular position of the measuring beam during a sweep the
illuminated object spot is sharply imaged on the radiation-sensitive
detector area as a corresponding, well-defined spot.
In an embodiment suiting the above purposes, the desired sweeping movement
of the measuring beam is achieved by means of a measuring beam mirror
mounted for pivotal movement about a shaft, the measuring beam produced by
a fixedly mounted focussing lens and radiation source being directed onto
this mirror. The shaft is coupled to a drive mechanism arranged for
imparting a reciprocatory sweeping movement through a desired sweep angle
to this shaft and hence the measuring beam mirror. In this manner, the
measuring beam can be swept through a desired angular distance, for
example in the YZ plane in the arrangement of FIG. 2a. A preferred
embodiment additionally includes a mirror for receiving reflected
radiation, which receiving mirror is likewise mounted for pivotal movement
about the aforesaid shaft and is so mounted thereon relative to the
measuring beam mirror that, when the drive mechanism is operative the
measuring beam mirror and the receiving mirror perform isochronous,
reciprocatory sweeping movements. In this manner, the surface elements
illuminated by the measuring beam mirror are viewed by the receiving
mirror, and the reflected radiation originating from these surface
elements is passed via this receiving mirror and via a fixedly mounted
objective lens to the likewise fixedly mounted detector. The arrangement
further includes a measuring beam position sensor producing position
signals representative of the instantaneous measuring beam position. Data
necessary for the measurement of the surface profile are derived from such
position signals. In this preferred embodiment, the object spots
illuminated by the measuring beam during the sweeping movement are
isochronously followed by the reception section, so that spots
corresponding to the respective object spots are imaged on the detector.
In this manner, at any point of time during the scanning procedure it is
known from which spatial volume area a response is to be expected, it
being ensured that the reception section is substantially only responsive
to reflected radiation originating from the spatial volume bounded by the
measuring beam. In other words, sources of spurious radiation located
outside this volume area have practically no effect on the measurement.
This implies that the measuring system will have a highly satisfactory
signal-to-noise ratio.
It is not necessary to have one or more components of the reception section
move along with the measuring beam during a scanning procedure, as this is
the case with the receiving mirror of the embodiment described above. In
principle, the same result can be achieved by using a reception section
fixedly mounted in space and lacking moving components. In order to
achieve a proper signal-to-noise ratio, however, it is then necessary to
employ a two-dimensional detector with an associated control means
operative to move a linear radiation-sensitive detector area in
isochronism with the measuring beam in this two-dimensional detector
plane.
If the measuring system is to be used in the vicinity of substantial
sources of spurious radiation, for example when the measuring system
according to the invention is employed in an automated welding process,
additional steps may advantageously be taken to eliminate the effect of
such spurious radiation on the detector. In accordance with a further
aspect of the invention, use is made of the fact that the spectrum of
spurious light radiation will exhibit "valley zones" and that sources of
monochromatic radiation are available emitting radiation of a wavelength
corresponding to such a zone. In accordance with the invention, a He-Ne
laser is used as a source of monochromatic radiation, an optical bandpass
filter, for example an interference filter, being mounted within the
spatial viewing angle of the detector. The passband of this filter
includes the wavelength corresponding to that of the radiation emitted by
the laser. The effect of spurious radiation can efficiently be eliminated
in this manner.
In the embodiments discussed above, the reflected radiation is passed to
the detector via a single optical transmission path. This renders it
possible that so-called false refle | | |
