U.S. patent application number 12/929719 was filed with the patent office on 2011-10-27 for surface shape measurement apparatus and method.
This patent application is currently assigned to QinetiQ Limited. Invention is credited to Andrew C. Lewin, Andrew M. Scott.
Application Number | 20110261369 12/929719 |
Document ID | / |
Family ID | 32011667 |
Filed Date | 2011-10-27 |
United States Patent
Application |
20110261369 |
Kind Code |
A1 |
Scott; Andrew M. ; et
al. |
October 27, 2011 |
Surface shape measurement apparatus and method
Abstract
Apparatus for indicating the departure of a shape of an object
(3;11;16;18;22;26) from a specified shape is described. The
apparatus comprises radiation means for directing an incident beam
of radiation (4) onto the object, and inspecting means (5) for
inspecting the final beam after transmission by or reflection from
said object. The apparatus is arranged so that the final beam will
have a substantially planar wavefront when said object has said
specified shape, and said inspecting means (5) is arranged to
determine any departure of the wavefront of the final beam from
planarity. In one embodiment, the inspecting means comprises
beamsplitting means, for example a diffraction grating (6) or
hologram, and detector means such as a CCD camera (8). The
beamsplitting means is then arranged to split the final beam into
two or more beams and to direct said two or more beams to laterally
displaced locations on the detector means.
Inventors: |
Scott; Andrew M.; (Malvern,
GB) ; Lewin; Andrew C.; (Malvern, GB) |
Assignee: |
QinetiQ Limited
London
GB
|
Family ID: |
32011667 |
Appl. No.: |
12/929719 |
Filed: |
February 10, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10589075 |
Aug 11, 2006 |
7907262 |
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PCT/GB2005/000455 |
Feb 10, 2005 |
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12929719 |
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Current U.S.
Class: |
356/612 ;
356/601 |
Current CPC
Class: |
G01B 11/16 20130101 |
Class at
Publication: |
356/612 ;
356/601 |
International
Class: |
G01B 11/24 20060101
G01B011/24 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 11, 2004 |
GB |
0402941.9 |
Claims
1-23. (canceled)
24. Apparatus for indicating the departure of a shape of an object
from a specified shape, the apparatus comprising: radiation means
for directing an incident beam of radiation onto the object,
inspecting means for inspecting a final beam, said object located
optically between said radiation means and said inspecting means at
least one wavefront shaping means, optically disposed between the
radiation means and the inspecting means for shaping the final beam
to have a substantially planar wavefront when said object has said
specified shape, and said final beam comprises a beam which has
been both transmitted by or reflected from said object and shaped
by said wavefront shaping means, said at least one wavefront
shaping means is arranged to compensate for non-planarity
introduced by said object having said specified shape, and said
inspecting means is arranged to determine any departure of the
wavefront of the final beam from planarity, wherein said inspecting
means comprises: beamsplitting means for splitting the final beam
into two or more beams and for directing said two or more beams to
laterally displaced locations; and detector means for detecting
radiation intensity of said two or more beams on the detector
means.
25. Apparatus according to claim 24 wherein said radiation means is
arranged to produce a collimated beam of radiation.
26. Apparatus according to claim 24 wherein said incident beam of
radiation is optical radiation.
27. Apparatus according to claim 24 wherein at least one said
wavefront shaping means is located between the radiation means and
the object.
28. Apparatus according to claim 24 wherein at least one said
wavefront shaping means is located between the object and the
inspecting means.
29. Apparatus according to claim 24 wherein at least one said
wavefront shaping means comprises a lens or curved reflector.
30. Apparatus according to claim 24 wherein at least one said
wavefront shaping means comprises a diffraction grating or
hologram.
31. Apparatus according to claim 24 wherein at least one said
wavefront shaping means is provided by a spatial light
modulator.
32. Apparatus according to claim 24 including means for adjusting
the relative position of the object and said wavefront shaping
means.
33. Apparatus according to claim 24 comprising a beam splitter
between said radiation means and said inspecting means.
34. Apparatus according to claim 24 wherein the beamsplitting means
of said inspecting means comprises at least one of a diffraction
grating and hologram.
35. Apparatus according to claim 24 wherein the beamsplitting means
of said inspecting means comprises non-diffractive beamsplitter
means for receiving light from two spaced object planes along a
common path for transmission to first and second image areas along
respective first and second optical paths, and focussing means
arranged to bring said first and second object planes into focus in
said first and second areas.
36. Apparatus according to claim 24 wherein the inspecting means is
arranged to provide an analysis of the shape, or components of the
shape, of the wavefront of the final beam.
37. Apparatus according to claim 24 wherein the detector means of
the inspecting means comprises a pixelated imaging photosensor.
38. Apparatus according to claim 37 wherein the pixelated imaging
photosensor is a charge coupled device (CCD) array.
39. A method of indicating the departure of a shape of an object
from a specified shape, the method including the steps of directing
an incident beam of radiation onto the object; shaping at least one
of said incident beam and a beam transmitted by or reflected from
said object to compensate for non-planarity introduced by said
object having said specified shape, to provide a final beam
comprising a beam which has been both transmitted by or reflected
from said object and shaped by said shaping step, said final beam
having a planar wavefront if the object has said specified shape;
and inspecting the final beam for any departure of its wavefront
from planarity, wherein the step of inspecting said final beam
comprises the steps of: splitting the final beam into two or more
beams; and directing said two or more beams to laterally displaced
locations on a detector.
40. A method according to claim 39 wherein said object is an
optical component.
41. A method according to claim 40 wherein said optical component
is a window or is of generally laminar form, or comprises a planar
reflective surface.
42. Apparatus for indicating the departure of a shape of an object
from a specified shape, the apparatus comprising: a radiation
source for directing an incident beam of radiation onto the object;
a beam inspecting device for inspecting a final beam, wherein said
final beam comprises a beam which has been both transmitted by or
reflected from said object and shaped by at least one wavefront
shaping device, said object located optically between said
radiation source and said inspecting device; said at least one
wavefront shaping device, optically disposed between the radiation
source and the inspecting device, for shaping the final beam to
have a substantially planar wavefront when said object has said
specified shape, said at least one wavefront shaping device is
arranged to compensate for non-planarity introduced by said object
having said specified shape, and said inspecting device is arranged
to determine any departure of the wavefront of the final beam from
planarity, wherein said inspecting device comprises: a device for
splitting the final beam into two or more beams and for directing
said two or more beams to laterally displaced locations; and a
photosensor for detecting radiation intensity of said two or more
beams at said laterally displaced locations.
Description
[0001] The present invention relates to an apparatus and method for
the determination of departures in a shape associated with an
object from a specified shape.
[0002] International Patent Application No. WO99/46768 (Secretary
of State for Defence) describes an imaging system which includes a
diffraction grating which is distorted substantially according to a
quadratic function to cause images to be formed under varying focus
conditions.
[0003] Our copending International Patent Application No.
WO03/074985 describes and claims measuring apparatus (and a related
method) for determining data relating to the local shape (or
distribution of local phase) of a radiation wavefront arriving at a
pupil plane, wherein said shape is defined by a set of
predetermined orthonormal functions, each function being provided
with a weighting coefficient for determining the shape, said data
comprising at least one said weighting coefficient, the apparatus
comprising a said input pupil, rate means responsive to said
radiation for determining a pixelwise distribution indicative of
rate of radiation intensity change as the radiation traverses the
input pupil, and converting means for converting said intensity
distribution to said data, wherein said converting means comprises
a store holding one or more matrices of predetermined values, each
said matrix corresponding to one said orthonormal function, and the
size of each said matrix corresponding to the number of pixels in
said pixelwise distribution, and calculating means for multiplying
said pixelwise distribution by a said matrix and adding the results
to provide said weighting coefficient for its said orthonormal
function.
[0004] Our copending International Patent Application No.
WO03/074984 describes and claims optical apparatus for use in
determining data relating to the wavefront of radiation arriving at
a main pupil plane of the apparatus, wherein said wavefront is
defined by a set of weighted predetermined orthonormal functions,
the weighting coefficients being a function of the shape of the
wavefront, the apparatus comprising first mask means for providing
a first grey-scale transmission mask determined by a said function
at a first location adjacent an incident side of a first pupil
plane, and second mask means for providing a second grey-scale
transmission mask determined by the same said function at a second
location adjacent the side of a second pupil plane opposed to the
incident side.
[0005] In the latter two copending applications, certain
embodiments of the apparatus comprise a distorted diffraction
grating of the type generally described in the aforesaid
International Patent Application No. WO99/46768, or a hologram
(with or without a supplementary lens) in lieu of the distorted
grating.
[0006] Our copending International Patent Application No.
WO04/068090 relates to related arrangements where a non-diffractive
arrangement may replace the distorted diffraction grating. This
provides optical apparatus for simultaneously focussing first and
second coaxially spaced object planes in respective separate first
and second areas of a common image plane, the apparatus comprising
non-diffractive beamsplitter means for receiving light from said
object planes along a common path for transmission to said first
and second image areas along respective first and second optical
paths, and focussing means arranged to bring said first and second
object planes into focus in said first and second areas.
[0007] The type of apparatus disclosed in these copending
applications is particularly useful where the wavefronts are
nominally flat or slightly curved, and so it can find applications
in (for example) detecting distortions in wavefronts from distant
sources or objects. However, as the wavefront exhibits an
increasingly significant degree of curvature the determination of
the wavefront shape and/or distortions from a nominal wavefront
shape (e.g. spherical or cylindrical) becomes increasingly
difficult or unreliable.
[0008] It is well known to examine an object by directing a
radiation beam thereon and inspecting the resulting radiation beam.
For example, U.S. Pat. No. 6,344,898 describes interferometric
apparatus in which radiation having a spherical wavefront is used
to analyse the shape of a reflective asperical object. However, in
a large number of cases (such as U.S. Pat. No. 6,344,898) the
wavefront of the resulting beam is significantly non-planar. It is
now appreciated that it is possible to provide where necessary
wavefront shaping means for distorting the beam wavefront before
and/or after it has encountered an object having a specified shape
so that the final beam should have a generally planar wavefront for
an object, making it possible to inspect the final beam wavefront
for planarity and to derive an indication of any departures in
shape from the specified shape.
[0009] While the invention encompasses the use of any known method
for inspecting wavefront planarity, in a series of preferred
embodiments of the invention this is performed by employing
apparatus of the type disclosed in our aforesaid copending patent
applications.
[0010] In a first aspect the present invention provides apparatus
for indicating the departure in a shape associated with an object
from a specified shape, the apparatus comprising radiation means
for directing an incident beam of radiation onto the object, and
inspecting means for inspecting a final beam following transmission
by or reflection from said object, wherein the apparatus is
arranged so that the final beam will have a substantially planar
wavefront when said object has said specified shape, and said
inspecting means is arranged to determine any departure of the
wavefront of the final beam from planarity, characterised in that
said inspecting means comprises beamsplitting means (e.g. a
diffraction grating or hologram) and detector means (e.g. a CCD
camera) wherein the beamsplitting means is arranged to split the
final beam into two or more beams and to direct said two or more
beams to laterally displaced locations on the detector means. The
inspecting means of this first aspect of the invention may thus
employ a wavefront sensor of the type described in our aforesaid
patent applications.
[0011] Where only transmission by the object is involved, the
associated shape may be the overall shape of the transmissive body
of the object, e.g. as determined by the shape of both curved
surfaces of a bi-convex or bi-concave optically transparent
lens.
[0012] Where only reflection by a surface of the object is
involved, e.g. the front reflective surface of a convex or concave
reflective element, the associated shape may be the shape of that
surface.
[0013] Where both transmission and reflection are involved, e.g. as
in a mirror with a rear reflective surface, the associated shape
may encompass not only the shape of the reflective surface but the
shape of the material lying over the reflective surface. Where the
reflective surface is known to be perfectly planar, the associated
shape will be only that of the overlying transmissive material.
[0014] It should be noted that the foregoing assumes that the
radiation transmissive material is homogeneous. Clearly any
variations in property of the material, for example due to strain
or uneven composition will also alter the shape of the final
wavefront.
[0015] A beam with a substantially planar wavefront is
substantially collimated. The intensity distribution across the
beam is not necessarily uniform however. Commonly, the radiation
means comprises means for forming a collimated radiation beam, so
that when the object is of the specified shape the collimated
illuminating radiation is converted to collimated reflected or
transmitted radiation.
[0016] In one form of apparatus according to the invention, where
the object has a specified shape which does not affect the
wavefront shape significantly, for example a plane mirror or a thin
transmissive sheet of material, then it is possible to employ a
substantially collimated incident radiation beam (i.e. having a
substantially planar wavefront) and to inspect the resulting
generally planar beam wavefront shape for departures from
non-planarity. Thus in this particular case no additional wavefront
shaping means, apart from the need to provide an incident
collimated beam, is required. Ideally the incident beam will fall
at normal incidence on the object, i.e. when the object has
parallel sides as in a sheet of material. A departure from normal
incidence will merely deflect the final beam slightly, while not
affecting its overall wavefront shape.
[0017] However, as the object shape increasingly affects the
wavefront shape, the correspondingly increasing non-planarity of
the beam wavefront after transmission or reflection by the object
will give rise to increasing difficulty in reliably determining its
shape and hence the shape of the object. In the invention this
difficulty is overcome or mitigated as explained above by employing
additional wavefront shaping means in the radiation path between
the radiation means and the inspecting means so that the wavefront
of the final beam is substantially planar, thereby enabling
accurate inspection or measurements of the final beam wavefront to
be made.
[0018] The form that the wavefront shaping means will take will
depend on its precise function in relation to the object to be
inspected and to the rest of the apparatus. In apparatus where the
requirements are relatively simple, for example when using a
collimated radiation source to inspect a transparent object with at
least one curved surface such as a simple optical transmissive lens
or a simple curved reflector such as a parabolic mirror, the
wavefront shaping means may be constituted by a (relatively simple)
single radiation wavefront shaping element, such as a simple
optical lens (or even a curved reflector). Similarly, where the
object shape departs from a laminar shape by only a relatively
small, but non-negligible amount, the wavefront shaping means may
be constituted by a single appropriate hologram or grating
wavefront shaping element. In other circumstances, it may be
necessary or desirable to use a wavefront shaping means comprising
at least two wavefront shaping elements. For example, an optical
lens under test may approximate a simple spherical lens, but with
non-negligible specified deviations from sphericity, in which case
it might be appropriate to use a relatively simple wavefront
shaping element (e.g. a second lens) to deal with the major
wavefront shaping aspect, and to additionally provide another
wavefront shaping element such as a hologram or grating to deal
with the deviations from sphericity.
[0019] Irrespective of how many elements it contains the wavefront
shaping means may lie between the radiation source and the object,
or between the object and the inspecting means. Furthermore, where
the wavefront shaping means comprises more than one radiation
element, it is also possible to have the elements distributed along
the radiation path between the radiation source and the inspecting
means so that at least one lies between the radiation source and
the object and the remainder between the object and the inspecting
means; all that is necessary is that the final beam incident on the
wavefront inspecting means is generally planar.
[0020] Preferably the wavefront shaping means is traversed once by
the radiation as it passes from the radiation source to the
inspecting means.
[0021] However it is equally possible for the wavefront shaping
means, or at least one element thereof to be arranged so as to be
traversed more than once (preferably no more than twice, and
preferably once as radiation passes from the source to the object
and once as the radiation passes from the object to the inspecting
means) as it passes from the radiation source to the inspecting
means. Plural traverses of the wavefront shaping means, or at least
one element thereof, may occur if the object itself acts as a
reflector. Alternatively, apparatus according to the invention may
additionally comprise at least one reflective element arranged so
as to obtain plural traverses of the wavefront shaping means and/or
the object to be inspected. If appropriate, a beamsplitter may be
employed to separate the reflected beam for transmission to the
inspecting means.
[0022] Where the radiation is collimated prior to incidence on the
wavefront shaping means, then on the grounds of reverse ray tracing
the shaping means may have the same form irrespective of whether it
is placed between the object and the radiation source or between
the object and the inspecting means. Nevertheless, in general, the
exact form of the wavefront shaping means will be dependent upon
its position in, and the overall geometry of, the apparatus.
[0023] Similarly, it is preferred that the object is arranged so
that a single reflection or transmission of the radiation beam
occurs. Of course, in some instances, for example a mirror with a
rear reflective surface, then it will be impossible to avoid a
double traverse of the overlying transmissive material, which may
contribute to the overall distortion of the final wavefront.
Nevertheless, arrangements of apparatus in which plural traverses
of transparent objects and/or plural reflections from reflective
surfaces also fall within the scope of the invention.
[0024] Where the apparatus is set up for testing one particular
type of object with a specified shape, the apparatus may provide a
predetermined location therefor. However, the apparatus may include
means for adjusting the position of the object to be inspected
relative to the rest of the apparatus. This may be useful for
example when a surface (e.g. of a lens or reflector) under test is
specified as having a part spherical surface but the radius is not
known, as will be described in more detail below.
[0025] The radiation beam may be of any known form, for example
acoustic, radio, microwave, x-ray, but in a preferred form of the
invention it is optical, i.e. in the IR to UV wavebands, and
preferably in the visible or IR band. For ease of reference visible
optical beams will be assumed from now on. However, the radiation
is selected to suit the task in hand, and for inspecting the
curvature of a single face of a optical lens transmissive in the
visible, for example, it may be possible to select a different
wavelength, e.g. in the infra-red, where the light is reflected.
Conversely, visible light may be used to inspect by reflection a
single face of an infra-red lens.
[0026] The detector means will preferably operate at the wavelength
of the radiation beam. Preferably, the detector means of the
inspecting means comprises a pixelated imaging photosensor. For
example, the detector means may comprises a charge coupled device
(CCD) array or a CCD camera. Alternatively, the detector means may
comprise a plurality of detector elements (e.g. two or more
discrete photosensors).
[0027] Any type of object may be so measured. However, one
important application of the invention is to the measurement of
optical components, such as windows, plane mirrors, and
transmissive and reflective lenses (e.g. optical lenses for use in
the IR, visible and/or UV ranges).
[0028] The inspecting means may provide a qualitative output, i.e.
provide an output which has one value when the wavefront planarity
or collimation of the final beam is within acceptable limits
(however determined), and a different output if not. This may be
regarded as indicative of whether or not the shape of the object
(surface) is acceptably close to the specified shape. In such
cases, the limits will be determined by the intended use of the
object.
[0029] Thus the inspecting means may be any known arrangement for
determining the degree of collimation. For example the final beam
could be split into two parts, and the beams inspected at
respective different distances from the object and compared. Such
comparison could be effected by allowing the beams to be incident
on respective like imaging photosensors, subtracting the resulting
intensity signals pixelwise to give the modulus of the local
intensity difference and integrating over the imaged area. The
degree of planarity or collimation may then be subjected to a
judging means (for example a simple thresholding circuit) for
indicating whether or not the object shape is sufficiently close to
the specified shape.
[0030] This type of approach falls within the broad scope of the
invention. However, while being useful and having the virtue of
simplicity, it fails to identify precisely how the actual shape
departs from the specified shape. There will be situations where
some modes of departure have a significant effect on the
performance of the component or object, so that tight tolerances in
respect thereof need to be adopted, whereas other modes of
departure have little effect and should therefore be associated
with rather less rigid tolerances. It follows that with the simple
approach there is a quandary in setting tolerance levels. Either
tight tolerances are set, leading to the possible rejection of some
objects which would perform satisfactorily, or tolerances are
slackened so that potentially some unsatisfactory objects are
accepted.
[0031] Accordingly in preferred embodiments of the present
invention the inspecting means includes means for analysing the
shape, or components of the shape, of the wavefront of the final
radiation beam. This analysis may provide a measurement of the
amplitude of one or more different wavefront modes contributing to
the wavefront shape, for example Zernike modes, and it may be
performed as in our previous aforesaid patent applications or in
any other known manner. By comparison of the amplitudes of the one
or more different wavefront modes with threshold values, again an
indication may be obtained of whether the object shape conforms
sufficiently to the specified shape. Where there is insufficient
conformity, the associated mode may be identified, to enable
corrective action (possibly automatic) to be taken if appropriate,
for example during the course of manufacture of a large mirror with
a complex surface shape.
[0032] Insofar as some embodiments of the invention are capable or
rendering a very precise quantitative measurement of how the
wavefront departs from planarity, and hence a very precise
indication of how the actual and specified shapes differ, the
invention is well suited to effecting measurements on precision
objects such as the aforesaid optical components.
[0033] According to a further aspect of the invention, apparatus is
provided for indicating the departure of a shape of an object from
a specified shape, the apparatus comprising radiation means for
directing an incident beam of radiation onto the object, and
inspecting means for inspecting the final beam after transmission
by or reflection from said object, wherein the apparatus is
arranged so that the final beam will have a substantially planar
wavefront when said object has said specified shape, and said
inspecting means is arranged to determine any departure of the
wavefront of the final beam from planarity characterised in that
said incident beam of radiation directed onto the object by the
radiation means has a non-spherical wavefront. Preferably, said
incident beam of radiation directed onto the object by the
radiation means has a substantially planar wavefront.
[0034] The invention extends to a method of indicating the
departure of a shape of an object from a specified shape, the
method including the steps of directing an incident beam of
radiation onto the object so that that a final beam following
transmission by or reflection from said object would have a planar
wavefront if the object has said specified shape, and inspecting
the final beam from the object for any departure of its wavefront
from planarity. The method is characterised by the step of
inspecting the final beam comprising the step of splitting the
final beam into two or more beams and directing said two or more
beams to laterally displaced locations on detector.
[0035] According to a further aspect of the invention, a method of
indicating the departure of a shape of an object from a specified
shape is provided, the method including the steps of directing an
incident beam of radiation onto the object so that that a final
beam following transmission by or reflection from said object would
have a planar wavefront if the object has said specified shape, and
inspecting the final beam for any departure of its wavefront from
planarity, characterised in that the step of directing an incident
beam of radiation onto the object comprises the step of directing a
beam of radiation having a non-spherical (e.g. substantially
planar) wavefront.
[0036] Further features and advantages of the invention will become
apparent upon a perusal of the appended claims, to which the reader
is referred, and upon a reading of the following more detailed
description of the invention, made principally with respect to
optical components and with reference to the accompanying drawings,
in which:
[0037] FIG. 1 schematically shows a first embodiment of apparatus
according to the invention as used for inspecting a transmissive
component or object in the form of a parallel sided sheet of
material, e.g. a window;
[0038] FIG. 2 schematically shows a second embodiment of apparatus
according to the invention as used for inspecting a transmissive
component or object having significant focussing power, such as a
surface of a simple bi-convex lens;
[0039] FIG. 3 schematically shows a third embodiment of apparatus
according to the invention as used for inspecting a reflective
component or object in the form of a planar sheet of material, such
as a simple mirror or a semiconductor wafer with a front reflecting
surface;
[0040] FIG. 4 schematically shows a fourth embodiment of apparatus
according to the invention as used for inspecting a simple curved
front reflective surface of an object such as a curved mirror;
[0041] FIG. 5 schematically shows a fifth embodiment of apparatus
according to the invention as used for inspecting an advanced lens
where the surface has a complex curvature;
[0042] FIG. 6 schematically shows a sixth embodiment of apparatus
according to the invention as used for inspecting a reflective
surface having a complex curvature;
[0043] FIG. 7 schematically shows a modification of the embodiment
of FIG. 4 as used for inspecting a curved mirror where the
curvature of the surface is initially unknown; and
[0044] Each of the embodiments of FIGS. 1 to 7 includes a wavefront
inspecting means 5 which could be in the form of a wavefront sensor
such as described in our copending International Patent Application
No. PCT/GB03/00964 or our copending International Patent
Application No. WO04/068090. However, as particularly shown in
outline, inspecting means 5 is of the form described in our
copending International Patent Application No. PCT/GB03/00979 and
comprises (inter alia) a quadratically distorted grating 6 for
directing light transmitted through the sheet 3 onto the lens 7 of
a CCD camera 8 providing an output signal 9 for further processing
as outlined in the aforesaid application. Ideally beam 4 falls at
normal incidence on the sheet 3.
[0045] As described in the aforesaid application, this combination
of elements produces laterally displaced spots on the pixelwise
imaging photosensor surface of camera 8 which correspond
respectively to the zero, +1 and -1 diffraction orders from grating
6. Since the photosensor is located at the focal point of lens 7,
the central zero order diffraction beam is brought to accurate
focus on the sensor surface when the beam incident on the grating 6
is generally collimated. In the latter case the spots for the +1
and -1 orders are equally and slightly out of focus before and
after the sensor surface (and so of equal size, which feature may
be utilised to ensure correct positioning of the optical components
of the arrangement, i.e. so that the beam incident of grating 6 is
generally collimated) and contain information on departures of the
received wavefront from perfect planarity. These departures may be
conveniently measured as coefficients of various Zernike modes (or
other orthogonal functions), which may then be translated into
shape components in the surface or shape of the object to be
inspected, e.g. the sheet 3 in the case of FIG. 1.
[0046] The output of the inspecting means 5 may be suitably
processed to provide an indication or measure of any departure of
the shape of the object to be inspected from its expected shape.
The processed output may provide an indication of both the presence
and magnitude of underlying basic contributions to distortions in
shape, for example as Zernike coefficients, and these may be used
to accept or reject the object, or to further process the object to
bring its shape within acceptable limits.
[0047] FIG. 1 illustrates apparatus according to the invention as
used for inspecting a transmissive parallel sided sheet 3 of
optical material such as a window. A light source 1 such as a laser
diode and a lens 2 serve to direct a substantially collimated beam
4 through the sheet 3 to the inspecting means 5. The processed
output of the inspecting means 5 will include an indication of any
departure from constant thickness, inter alia.
[0048] In this particular arrangement, the sheet of material is not
necessarily planar, particularly if it is relatively thin and the
curvature is relatively small, since the change in incident angle
across the sheet (for example a flexible sheet of material deformed
into a curve, or a rigid curved sheet) and the resulting change in
optical (radiation) thickness will be negligible and will not
contribute to any optical (focussing) power. This may not apply for
thicker curved sheets which would be expected to have some form of
negative focussing power insofar as the optical thickness at the
sheet edges will be greater, in which case it might be necessary to
employ a different or additional wavefront shaping means as
described in later embodiments.
[0049] One potential use of this arrangement is for measuring
significant deformations in plastic sheets such as are used in
mobile phone windows for example, or for measuring thickness
variations in glass sheets, in order to assess acceptability.
[0050] In the embodiment of FIG. 1, no additional wavefront shaping
means is required apart from the lens 2 for providing a collimated
beam.
[0051] Where the nominal sheet thickness has a known and
significant variation, it is possible to insert an additional
wavefront shaping means to compensate therefor to maintain
collimation of the beam incident on grating 6. This additional
collimating means could take the form of a hologram or grating, and
it may either be a passive means such as a predetermined grating,
or for example a spatial light modulating means which can be
controlled to give the required effect. The additional collimating
means may be located between the lens 2 and grating 6, either
before or after the sheet 3 so long as it introduces the requisite
compensation for the nominal thickness variation.
[0052] In the embodiment of FIG. 2 the collimated light beam 4 is
transmitted via a first lens 10 of known properties and a lens 11
under test to the inspecting means 5. The lens 10 is arranged so
that in conjunction with lens 11 it provides a collimated light
beam 12 for transmission to the grating 6 if lens 11 has its
nominal shape. As particularly shown, the lenses 10, 11 are
arranged as in a optical relay, and the correct relative
positioning of the two lens may be confirmed by equality of size in
the plus and minus one order diffracted spots at the camera sensor.
It should be clear that the order of lenses 10, 11 may be
reversed.
[0053] Any deviation of the lens 11 from its nominal shape will
produce non-planar components in the wavefront of beam 12 which can
be detected after processing of the output 9 of the camera 8. From
these components can be derived the type of deviation in the shape
of the lens 11. In this embodiment the lens provides a wavefront
shaping means in addition to the collimating lens 2. An additional
wavefront shaping means such as a hologram or diffraction grating
may be employed as in FIG. 1 to compensate for any known and
significant nominal departures in the shape of the lens from a
simple lens, and one arrangement thereof is shown in FIG. 5 to be
described later.
[0054] In FIG. 3, the collimated beam 4 is transmitted via a
polariser 13 to a polarising beam splitter where it is reflected
through a quarter wave plate 15 towards the reflective surface of a
nominally planar semiconductor wafer 16 under test. Light reflected
from the surface of the wafer 16 and transmitted again through the
plate 15 is now transmitted through the splitter 14 towards the
grating 6. Again the processed output 9 will give an indication of
non-planarity of the wavefront incident on grating 6 and hence an
indication of non-planarity of the surface of wafer 16.
[0055] FIG. 4 illustrates apparatus similar to that of FIG. 3 but
with substitution of the wafer 16 by a curved reflector 18, and
with a lens 17 of known properties between the quarter wave plate
15 and the reflector 18. The lens 17 is arranged so that in
conjunction with a reflector of nominal shape the wavefront
incident on grating 6 is planar. Any deviations in wavefront
planarity as indicated by suitable processing of the output 9 give
corresponding indications of deviations in the shape of the
reflector 18 from its nominal shape.
[0056] FIG. 5 shows an embodiment of apparatus adapted for use with
more complex surfaces or shapes. The collimated beam 4 is
transmitted via the polariser 13 for reflection by the splitter 14
through the quarter wave plate 15 to a reflective spatial light
modulator 19 acting as a hologram or diffraction grating for
modifying its originally planar wavefront. The modified reflected
beam is retransmitted through the plate 15 and splitter 14, a lens
20 of known properties and a pinhole aperture 21 to a lens 22 under
test, the lens 22 having a complex nominal shape. Together the lens
20 and the modulator 19 are arranged so that in conjunction with
lens 22 a collimated beam with planar wavefront should be incident
on the grating 6. Again, any deviation from planarity, as indicated
by processing of the output 9 indicates a departure of the shape of
lens 22 from its nominal shape. The pinhole serves to select one of
the plus and minus one orders of diffraction from the modulator
19.
[0057] FIG. 6 is an arrangement similar to that of FIG. 5, but
arranged for testing a curved reflector 26 having a complex shape.
Linearly polarised light after transmission through splitter 14
encounters a second quarter wave plate 23, and then a lens 24
having known properties. Light transmitted by the lens 24 and an
aperture 25 is incident on the reflector 26, reflected light
therefore being transmitted back via aperture 25, lens 24 and plate
23 for reflection by splitter 14 towards the grating 6. The lens 24
and modulator 19 are arranged so that together with a reflector 26
of nominal shape, the wavefront of the light incident on the
inspecting means 5 is planar, and deviations therefore as indicated
by processing of the output 9 being indicative of deviations from
nominal shape in the reflector 26.
[0058] The embodiment of FIG. 7 includes means for permitting
movement of the mirror 18 along the main optical axis of the
apparatus. In this way it is possible to deal with a mirror having
a surface with an unknown curvature. In use, the mirror 18 is moved
along the axis to first and second positions 18' and 18'' where the
wavefront impinging on the grating 6 is planar, as shown by the
fact that the sizes of the spots at the camera sensor from the plus
one and minus one diffraction orders are equal. The position 18'
corresponds to the position shown in FIG. 4, in which the area of
the beam incident on the mirror 18 is relatively large, preferably
covering substantially all of the area of the mirror, or at least
the area where the surface shape is of most concern. However, in
the position 18'' the beam from the lens 17 is brought to a focus
on the mirror 18 and hence covers the minimum area. While this is
of no practical use in detecting shape defects, the relation
between the two positions can be used to provide a measure of the
radius of curvature of the mirror surface, as is known in the prior
art. In a variation, the mirror 18 is held in a fixed position
while the lens 17 is moved.
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