Note: Descriptions are shown in the official language in which they were submitted.
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HOLOGRAPHIC OPTICAL ELEMENT
BACKGROUND OF THE INVENTION
The present invention relates to a holographic optical element for
measuring the dimension and position of an object with the aid of a deflected
light beam that sweeps across a specific angular range, the element having
an interference pattern in one section which can be created in a manner
known per se through simultaneous exposure to a fan-shaped reference wave
front, generated by a monochromatic and coherent laser light source, and a
parallel wave front that is also generated by the same monochromatic,
coherent laser light source, but hits the element at a different angle than
the
reference wave front, as well as through the subsequent development.
For the purpose of this document, a holographic optical element of this
type is henceforth referred to as an HOE.
A special holographic laboratory is generally required in order to
produce an HOE. The equipment by and large corresponds to that of a photo
laboratory, with the exception that only monochromatic, coherent laser light
is
used. Film plates (coated glass plates) are used to produce holograms and,
in particular an HOE. These plates are exposed to selected wave fronts and
are subsequently developed, depending on the film base that is used.
An HOE of this type is described, among other things, in European
Patent Application No. EP-A 0 245 19g. This reference also contains detailed
instructions for producing an HOE used in a device and with a method for
generating light beams for measuring the dimension andlor position of an
object in the deflection displacement region of this light beam:
If the HOE, produced as described in the above, is exposed to a
suitable laser reference wave front, then the other wave front used during the
picture taking is correspondingly reconstructed.
With the above-mentioned device, the object to be measured, in
particular a cable or the like, can be measured only in one direction.
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SUMMARY OF THE INVENTION
1t is an object of the present invention to provide a holographic optical
element which can be used to determine several parameters of the object to
be measured.
The above and other objects are accomplished according to the
invention by the provision of a holographic optical element for measuring at
least one of the dimension and position of an object with aid of a deflected
laser beam generated by a .monochromatic and coherent laser light source
that sweeps across an angular range to produce a fan-shaped reference
wave front, the element comprising: at least two interference patterns,
wherein each interference pattern is created through simultaneous exposure
of the element to the fan-shaped reference wave front generated by the
monochromatic and coherent laser light source and a parallel partial wave
front generated by the same monochromatic and coherent laser light source
and hitting the element at a different angle than the reference wave front,
wherein the number of parallel partial wave fronts used for the exposure of
the
element corresponds to the number of interference patterns, and wherein if
the parallel partial wave fronts are virtually extended through the
holographic
optical element, they intersect behind the element in a center of a measuring
field.
The HOE according to the invention thus comprises at least two
different interference patterns which are present in a specific region of the
HOE. According to the invention, the interference patterns may be allocated
respectively to spatially separate sections, or may at least partially overlap
one another in one section. The type and design of these sections will be
explained in further detail in the following.
To produce the HOE according to our invention, the number of partial
wave fronts used during the exposure corresponds to the number of
interference patterns. The partial wave fronts are generated by the same
laser light source and their course is such that when virtually extended
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through the holographic optical element, they intersect in one point andlor
one
region behind the element.
According to one preferred embodiment, the HOE according to the
invention has 3 or more (meaning 4, 5, 6, ...) interference patterns.
If the HOE created in this way is exposed to a suitable laser reference
wave front, then the other wave fronts used during the picture taking are
correspondingly reconstructed. Thus, with a suitable selection and
arrangement it is possible to generate an almost optional number of wave
fronts with a defined reference wave front.
In contrast to conventional optical elements, for example lenses,,
prisms and mirrors, which can reproduce only a single image through
refraction or reflection of light, the HOE is based on the diffraction
principle,
thus making it possible to generate several independent images with a
suitable film structure. A precondition for this, however, is the use of
monochromatic laser light which should have the same wave length as the
wave length for the laser light used during the picture taking.
The HOE according to our invention makes it possible to measure the
object to be measured in a device in several directions and thus be able to
determine not only the thickness in one direction when measuring cables, for
example, as is the case with the known device. By making it possible to take
measurements in several directions, it is also possible to measure other
parameters than the diameter, wherein these other parameters include, for
example, the non-roundness of a cable.
According to one preferred embodiment, the parallel partial wave fronts
used for exposing the HOE according to the invention are all located in one
plane. For the exposure, the angle between the reference wave front and the
joint plane for the parallel partial wave fronts is preferably 40° to
50° and, in
particular, approximately 45°, wherein the bisector of this angle in
particular is
positioned perpendicular on the plane of the holographic optical element.
According to another preferred embodiment, the HOE according to the
invention comprises separate andlor spatially separated sections with
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respectively one interference pattern, wherein these interference patterns are
different. In other words, the first section comprises the first interference
pattern, the second section the second interference pattern and the third
section comprises the third interference pattern and so forth.
The sections with the different interference patterns, however, can also
spatially overlap on the HOE, at least in some regions, or can coincide
completely. Thus, an HOE according to the invention can have a single
section which comprises three superimposed interference patterns. In other
words, the aforementioned section represents a super-imposition of three
sections with separate interference patterns. Also possible are mixed forms
where the aforementioned sections overlap only in part.
The HOE according to the invention can be a component of a device
for detecting a dimension andlor position of an object, wherein this object
can
be a cable, a profile or a pipe leaving an extruder. A device of this type is
known and normally comprises a transmitter part and a receiver part. A light
beam is generated in the transmitter part, which is deflected such that it
sweeps over a specific angular range. The HOE according to the invention in
this case can be inserted into the transmitting part as well as the receiving
part or into both, depending on the problem definition. Of course, these HOEs
must be matched to each other. It is furthermore possible to install the HOE
according to the invention in either the transmitter part or the receiver part
and
to use an HOE of the known type in the other part. An HOE of this type
preferably is a holographic film plate.
The HOE according to the invention not only can be used in a device
as described in the above, but for all purposes where wave fronts are
generated as a result of diffraction on the HOE. However, the HOE according
to the invention is preferably used for measuring the dimension and position
of an object, in particular a cable or a pipe, with the aid of a deflected
laser
beam that sweeps over a specific angular range.
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BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in further detail in the following with the aid
of Figs., which show in:
Fig. 1 A schematic view of the front of a known device according to
EP-B 0 245 198 for determining the dimension and position of an object.
Fig. 2 A view from above of the known device shown in Fig. 1.
Fig. 3 A perspective basic view, showing the creationJexposure of an
HOE according to the invention.
Fig. 4 A perspective view of the exposure of the HOE according to
Fig. 3 with a deflected laser beam.
Fig. 5a A schematic view from above of a complete measuring system
for measuring a cable with an HOE having three separate sections with
respectively different interference patterns.
Fig. 5b A schematic view from the side of the system shown in Fig. 5a.
Fig. 6a A view from the top that approximately corresponds to Fig. 5a,
wherein the regions v~rith interference patterns of the HOE according to the
invention are not spatially separated and/or arranged separately.
Fig. 6b A side view of Fig. 6a.
Fig. 7 A schematic view from the tap of a measuring system where
two HOEs according to the invention are used, which are arranged at an
angle of approximately 90° to each other.
DETAILED DESCRIPTION OF THE INVENTION
Figs. 1 and 2 show a device for detecting a dimension andlor the
position of an object S1, indicated with dash-dot line in these Figs., such as
a
cable or pipe leaving an extruder. Features known from prior art are
otherwise provided with reference numbers having a capital S in front. For
example, the Figs. 1 and 2 are taken from European Patent Application No.
EP-B1 0 245 198, wherein the reference numbers are supplemented with the
aforementioned letter S. These two Figs. are provided for an easier
understanding of the options for using the HOE according to the invention.
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The measuring device shown in Fig. 1 comprises a transmitting part
S2, which is used to generate a telecentric light beam in the measuring range.
A laser source S3 is provided which guides a continuous, monochromatic light
beam over a mirror S4 to a spherical expansion optic S5 from which the
expanded beam enters a cylindrical expansion optic S6. A flat light beam is
thus generated for which the plane extends parallel to the longitudinal axis
of
the object to be measured. This is indicated in Figs. 1 and 2 in that the
light
beam S7 has a very narrow width in the region of object S1 in the projection
according to Fig. 1, but has a certain width in the projection according to
Fig.
2. An expansion of this type, however, is not absolutely necessary. The
desired dimension can also be detected with a non-expanded light beam.
The light beam S7 is then transmitted via additional deflection mirrors
S8 and S9 onto an octagonal rotating mirror S10. When this mirror turns in a
clockwise direction, the entering light beam is periodically deflected from
the
top toward the bottom over an angular range indicated in Fig. 1 with dashed
lines. In the process, the beam hits a holographic optical element (HOE) S11.
This HOE, which has a very thin optically effective layer and is located on an
optically transparent carrier, is connected to a prism body S12 and is thus
mechanically stabilized. A partial radiation share of the Ot" order S7o
penetrates the HOE S11 without being diffracted and hits the front wall S13 of
the transmitter S2 housing from the inside. However, the main share of the
entering beam S7 is diffracted and leaves the HOE as beam of the 1S' order
S7~ under a specific angle. This beam is reflected on a totally reflecting or
mirrored surface S14 of the prism body S12 and is projected through a
window S15 into the measuring range. One or several optoelectronic
converters S16 can be arranged at the point where the exiting beam of the Ot"
order S7o impinges. On the opposite side of the measuring region, the beam
S7~ passes through a window S15 and enters the housing for a receiver S17,
which contains a prism body S18 that is designed to correspond to the prism
body S12 with an HOE S19 that corresponds to the HOE S11. The entering
light beam is projected by the reflecting surface S20 of the prism body S18
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onto the HOE S19, which always projects the beam onto an optoefectronic
converter S21, for example a photoelectric cell.
With the periodic deflection of the telecentric beam around the
deflection displacement, delimited by the dashed lines in Fig. 1, the beam
travels to the converter S21 as long as it is not blanked out by the object
S1.
The dimension and the position of object S1 can be determined based on the
duration of the fadeout and the starting and ending point of the fadeout.
Additional instructions for calculating the required values can be found in
the
aforementioned European Patent Application No. EP-B1 0 245 198.
The HOE according to the invention is used to replace the HOE S12
shown in Fig. 1. Of course, the HOE according to the invention can also be
used in differently conFig.d devices of the same type.
The production and/or exposure of a first embodiment of an HOE
according to the invention is shown in Fig. 3 in a perspective and schematic
view that is not true to scale. A coherent wave front is generated with the
laser andlor the laser light source 1. To obtain a fan-shaped wave front, the
laser beam is focused with a lens 2 onto a pinhole mask 3. A point source is
thus created, which determines the geometric source of the reference wave
front 14.
The remaining wave fronts must be generated with the same laser
beam to meet the coherence conditions. In the process, a first deflection
occurs at the beam divider 4, which guides the deflected light beam onto the
parabolic mirror 5. The wave front reflected there is on the whole "divided"
into 3 parallel wave fronts by the beam dividers 6 and 7 that are arranged in
the beam path of the wave front reflected by the parabolic mirror 5. These
wave fronts consequently are parallel partial wave fronts.
The parallel partial wave front 16 in the center travels to a section 12 of
the HOE 10 where it generates on the HOE 10 the necessary interference
pattern and/or diffraction pattern 12' with the aid of the fan-shaped
reference
wave front 14. If the HOE 10 had only this one interference pattern 12', it
would represent an HOE as described in the prior art.
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As previously explained in the above, two partial wave fronts 16 and 17
are deflected from the wave front reflected by the parabolic mirror 5 with the
aid of the beam dividers 6 and 7.
The two parallel partial wave fronts 15 and 17 on the side are beamed
with the aid of deflection mirrors 8 and 9 into the HOE sections 11 and 13 on
the side where they generate the corresponding interference patterns 11' and
13' together with the fan-shaped reference wave front 14. The optical length
of all partial wave fronts 15, 16, 17 and the reference front 14 in this case
must be the same. The holographic film plate 10 is thus exposed with the aid
of the interference patterns, generated as explained in the above, and is
subsequently developed.
The parallel partial wave fronts 15, 16 and 17 are selected andlor
deflected to the HOE 10, such that when they are virtually extended through
the HOE 10, they intersect behind this element 10 in the region/point 18 which
is positioned in the center of the future measuring field 18 of the measuring
device.
Otherwise, all three partial wave fronts 15, 16 and 17 are located in
one plane. The angle enclosed between the reference wave front 14 and this
plane is approximately 45°. The bisector of this angle is positioned
perpendicular on the plane for the HOE 10 and is thus located in the paper
plane for Fig. 3 if the HOE 10 is in this paper plane.
If a reference wave front 14 is beamed onto the HOE 10 that is
completed as described in the above, parallel wave fronts 15', 16' and 17'
that
intersect in the measuring field 18 are generated as a result of diffraction
on
the interference patterns of the corresponding HOE sections 11, 12 and 13;
as shown in Fig. 4. These wave fronts 15', 16' and 17' therefore extend in the
direction and in the plane corresponding to the previously mentioned virtual
extension of the partial wave fronts 15, 16, and 17 used for the exposure.
The HOE 10 behaves in the same way as a fan-shaped wave front if a laser
beam 21 is deflected fan-shaped by a rotating polygonal mirror 23 at the
source 3 for the reference wave front 14. If the deflected beam 14' hits the
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sections 11, 12 and 13 of HOE 10, it is diffracted by the local, associated
interference pattern 25, 26, 27 in such a way that it is deflected parallel to
the
side in the measuring field after it leaves the HOE 10. The diameter of a
cable 20 can thus be determined from three different directions. The time
during which the parallel laser beam coming from one measuring direction is
interrupted therefore represents a measure for the respective diameter.
For the described HOE 10, the sections 11, 12 and 13 with the
associated interference patterns 25, 26 and 27 are spatially separated. In
other words, the HOE 10 has three separate andlor discrete sections 11, 12
and 13, wherein the measuring also occurs in three discrete axes.
Fig. 5a schematically shows a view from above of a complete
measuring system, not true to scale, while Fig. 5b shows a view from the side.
An HOE 10 according to Fig. 4 is integrated into the transmitting part of this
measuring system. The arrangement of the polygonal mirror 23 etc. also
corresponds to the one in Fig. 4, so that the same reference numbers are
used for the same parts and/or elements. Additionally shown in Fig. 5a is a
deflection mirror 19, which does not have a critical function:
However, HOE 10 is used only in the transmitting part, but not the
receiving part. An HOE 30 is used there which comprises only one section 31
with only one interference pattern 29. This HOE 30 consequently only
functions in the manner of a normal lens. If a parallel beam hits a lens, and
in
the present case the HOE 30 with the interference pattern 29, the parallel
rays
are focused in the focal point of the lens. This focal point normally lies on
the
optical axis if the parallel beam of rays also extends parallel to the optical
axis. These conditions exist for the central measuring beam 16' and the
following focusing beam path 34 up to the receiving element 35.
If the parallel beam is beamed at an angle into the HOE 30 andlor the
lens, the focal point is also displaced to the side, meaning to the axis
extending through the center of the lens or the HOE 30 and parallel to the
beam of rays. These conditions exist with the two measuring beams 15' and
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17' on the side, so that the focusing beams 32 .and 36 correspondingly hit the
receivers 33 and 37 in the displaced focal points.
Since the HOE 10 in Fig. 5a has three separate and discrete sections
11, 12 and 13, the cable 20 is also measured in three discrete axes andlor
zones. With the view in Fig. 6a, which corresponds to the view in Fig. 5, the
HOE 10 of Fig. 5 is replaced with an HOE 40. This HOE 40 does not contain
separate sections and associated, separately arranged interference patterns.
Rather, this HOE 40 only contains one section with one interference pattern
28, consisting of three different interference patterns 43, 44 and 45 that
overlap. In order to create an HOE 40 of this type, the sections irradiated by
the parallel wave fronts must overlap. The HOE 40 in that case optically
behaves as if three different lens systems were nestled into each other, which
is not possible with normal lenses.
The HOE 30 in the receiver part for the embodiment shown in Fig. 6a
corresponds to the HOE 30 for the embodiment shown in Fig. 5a
The elements andlor parts shown in Figs. 6a and 6b are also given the
same reference numbers or reference characters as in Figs. 5a and 5b, but
are additionally provided with one or two apostrophes (' or ")
The use of extremely flat measuring angles, additionally shown in Fig.
6a, has the advantage that with an irregular profile a possible maximum
dimension can be clearly detected at the same time. The maximum can also
be interpolated for higher requirements.
Fig. 7 contains an additional embodiment in a view from above, shown
in a schematic representation that is not true to scale, wherein two HOEs 10,
10' are used which correspond to the HOE 10 for the embodiment shown in
Fig. 5a. However, these two HOES 10, 10' are arranged perpendicular to
each other, so that the cable 20 can be measured from two main directions
that are perpendicular to each other. For the embodiment shown in Fig. 7, the
same elements and/or parts are also given the same reference numbers or
reference characters and are provided additionally with one or two
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apostrophes (' or "). Additional deflection mirrors 24, 24', 38 and 38' are
also
provided for practical and economic reasons.
With this embodiment, the HOEs 10, 10' have separate sections 11, 12
and 13, and 11', 12' and 13', respectively, in the transmitting part and thus
have separate interference patterns 25, 26 and 27, and 25', 26' and 27',
respectively. The HOES 30, 30', respectively have only one section and thus
one interference pattern 29. With this embodiment, cable 20 measurements
are possible for a total of 6 discrete directions andlor axes. In both HOE 10,
10' of the transmitter part, 2 x 3 separate sections are provided.
The invention has been described in detail with respect to referred
embodiments, and it will now be apparent from the foregoing to those skilled
in
the art, that changes and modifications may be made without departing from the
invention in its broader aspects, and the invention, therefore, as defined in
the
appended claims, is intended to cover all such changes and modifications that
fall within the true spirit of the invention.
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