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Description  |
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The invention relates to an apparatus for the point-by-point scanning of an
object, which apparatus comprises a radiation source producing a scanning
beam, an objective system for focussing the scanning beam so as to form a
radiation spot on the object, and a radiation-sensitive detection system
for converting the scanning beam coming from the object into an electric
signal for an electronic processing circuit, which circuit renders the
signal suitable for reproduction, which detection system comprises at
least two radiation-sensitive detectors arranged after each other in the
scanning direction.
This apparatus may be an optical microscope, an acoustic microscope, an
electron-microscope or an X-ray microscope. Thus, said scanning beam may
be a beam of electromagnetic radiation, an acoustic-wave beam or a
particle beam. The term objective system should be taken in a broad sense
and is defined as a system which narrows a beam of one of the said types
of radiation to a very small scanning spot of the order of magnitude of
the wavelength of the radiation used and of the order of magnitude of the
object details to be detected. Obviously, the detection system should be
adapted to the radiation source used.
The Applicant's Netherlands Patent Application no. 78 03517 corresponding
to U.S. application Ser. No. 419,640, filed Sept. 17, 1982, which has been
laid open to public inspection, describes an apparatus for scanning an
optical record carrier having an information structure comprising
information areas arranged in tracks with a small read spot. The
information structure has the form of a phase structure and adjacent
information tracks differ from each other in that the information areas of
a first information track comprise comparatively deep pits and the
information areas of an adjacent track comprise pits of a smaller depth.
For correctly reading the two kinds of information areas two different
read methods must be used. The read apparatus comprises two
radiation-sensitive detectors which are disposed in the far field of the
information structure, namely after each other in the direction of
scanning. For reading the deeper phase structure the output signals of the
two detectors are added to each other, whilst for reading the shallower
phase structure said signals are subtracted from each other. The read
methods are known as the integral method and the differential method
respectively.
Since the two read methods have different optical transfer functions
(M.T.F.), the alternate use of the two read methods may affect the signal
ultimately supplied by the read apparatus. If a video signal is stored on
the record carrier, for example, the one transfer function will give rise
to different grey shades or a different colour saturation in the resulting
television picture than the other transfer function. Moreover, the picture
obtained by subtracting the signals from the two detectors is the first
derivative of the object, so that object structures of lower spatial
frequencies are not reproduced in an optimum manner. Hence, it is
desirable to read an optical record carrier with two different phase
structures by means of a single read method, the transfer function being
preferably variable as a function of the frequency.
The information pits read by the integral method may have such an optical
depth that they produce a phase difference of 180.degree. between the
zero-order beam and one of the first-order sub-beams, which are produced
when the read spot is projected onto such a pit. Such a phase difference
will also arise if the read spot scans an amplitude structure. A detector
arrangement employed in the read apparatus in accordance with the said
Netherlands Patent Application no. 78 03517 may therefore be used in an
optical microscope by means of which both phase and amplitude objects are
scanned.
It is an object of the present invention to provide a variable detection
function for an apparatus for the point-by-point scanning of an object, so
that such an apparatus becomes suitable for scanning objects of different
structures, that is, a phase structure, an amplitude structure or a
structure which is a combination of a phase structure and an amplitude
structure.
The scanning apparatus in accordance with the invention, which comprises at
least two detectors which are shifted relative to each other in the
scanning direction, is characterized in that a phase-shifting element with
a variable phase shift is arranged in at least one of the connections
between the detectors and the input terminals of an additive input stage
of the processing circuit.
The electronic phase shift provides a complex detection function, which can
simply be adapted by electronic means. The detection function is to be
understood to mean the transfer function of the system comprising the
radiation-sensitive detectors and the additive input stage of the
electronic processing circuit.
The principle of the invention may be applied to all kinds of scanning
devices, not only to optical scanning devices, but also to scanning
devices in which an acoustic beam, an electron beam or an X-ray beam is
employed as scanning beam.
It is to be noted that in the article: "A detection method for producing
phase and amplitude images simultaneously in a scanning transmission
electron microscope" in "Philips Technical Review" Vol. 37, No. 1, pages
1-9, a scanning electron microscope is described comprising two detectors
which are shifted relative to each other in the scanning direction, by
means of which both a phase image and an amplitude image of an object can
be obtained. A phase image is obtained by subtracting the detector signals
from each other and an amplitude image by adding said signals to each
other. In the known electron microscope the detectors are not connected to
an electronic phase shifter, so that said microscope is not as versatile
as the scanning apparatus in accordance with the invention.
In the scanning apparatus a phase-shifting element may be arranged between
only one detector and one input terminal of the processing circuit. For
reasons of symmetry, a preferred embodiment of the apparatus in accordance
with the invention is further characterized in that a variable phase
shifter is arranged in each of the connections between the detectors and
the associated input terminals of the input stage, the phase shifts
introduced by said phase shifters being equal but of opposite sign.
The modulus of the scanning signal, supplied by the additive input stage,
it optimized by said phase shift. If the phase of only one of the detector
signals is shifted, this also affects the phase of the scanning signal.
The phase of the scanning signal can be restored if the scanning device is
further characterized in that the output of the additive input stage is
connected to a phase shifter which produces a phase shift-.phi.e/2, where
.phi..sub.e is the phase shift produced by a single phase shifter arranged
in one of the connections between the detectors and the input terminals of
the input stage.
If the scanning apparatus is further characterized in that the additive
input stage is connected to a phase shifter which introduces a phase shift
which depends on a phase asymmetry in the scanning spot, said asymmetry
which may be caused by coma of the optical system, can be compensated for.
The phase-shifting elements may be of different construction, depending on
the desired field of application of the scanning apparatus. A first and
simple embodiment of the scanning apparatus in accordance with the
invention is characterized in that the phase-shifting elements are
switchable between two substantially fixed values which correspond to a
phase image and an amplitude image of the object.
A second embodiment of the scanning apparatus, which offers more
possibilities, is characterized in that the phase-shifting elements are
continuously variable. By means of this scanning apparatus it is possible
to obtain an optimum reproduction of both shallow phase objects and
amplitude objects, but also of objects whose structure is a combination of
a phase structure and an amplitude structure. Moreover, this apparatus has
the possibility of suppressing object details having a specific phase
depth, that is the possibility of applying a phase-filtration.
In accordance with a further characteristic feature of the scanning
apparatus, the phase shifts of the phase-shifting elements are a function
of the spatial frequency in the object. Then it is possible to compensate
for scanning-spot asymmetry as a result of errors in the scanning-beam
sphericity.
If, in accordance with a further characteristic feature of the scanning
apparatus, the gain of the input stage of the processing circuit is a
function of the spatial frequency in the object, details of a specific
spatial frequency may be suppressed or may be enhanced during
reproduction.
When a complex detection function as obtained in the foregoing is used,
whose amplitude and phase are electronically variable as a function of the
frequency, spatial filtering of the object or image enhancement can be
obtained without the use of complex optical filters.
The scanning apparatus may be further characterized in that the input stage
of the processing circuit comprises separate amplifiers for each of the
detector signals and an adder circuit, the gain factor of each of the
amplifiers being adjustable. Then it is possible to compensate for an
amplitude asymmetry in the scanning beam or to adopt a so-called single
side-band principle.
The invention will now be described in more detail, by way of example, on
the basis of an optical scanning apparatus, which is, for example, used
for reading an optical record carrier. For this reference is made to the
drawing. In the drawing:
FIG. 1 shows a first embodiment of a scanning apparatus in accordance with
the invention,
FIG. 2 shows a part of the information structure of an optical record
carrier,
FIG. 3 shows sectional views, in the plane of the detectors, of the
difracted beams produced when scanning the record carrier,
FIG. 4 represents a method of processing the detector signals,
FIG. 5 illustrates the principle of an electron microscope in accordance
with the invention,
FIG. 6 illustrates the principle of an X-ray microscope in accordance with
the invention, and
FIG. 7 represents the principle of an acoustic microscope in accordance
with the invention.
FIG. 1 illustrates the principle of the scanning apparatus. By means of an
objective system, which is schematically represented by a single lens
L.sub.1, the beam b emitted by a radiation source S is focussed to form a
radiation spot V on the object O to be scanned. Said object splits the
incident beam b into a plurality of subbeams of different diffraction
orders, of which for the present description the non-deflected zero order
subbeam and the first-order subbeams are most important.
In FIG. 1 the object O is represented very schematically. Said object may
for example be an optical record carrier having an information structure
comprising information areas arranged in information tracks. FIG. 2 is a
plan view of a small part of such a record carrier 1. In the case of a
round disk-shaped record carrier the information tracks 2, which are
represented as straight tracks, are in reality concentric tracks or quasi
concentric tracks which together constitute one spiral track. The
information tracks comprise very small information areas 3, whose lengths
are of the order of 1 .mu.m and whose widths are smaller than 1 .mu.m,
which areas alternate with intermediate areas 4 in the track direction t.
It may be assumed that the information structure is periodic in two
mutually orthogonal directions, the period in the track direction, or
tangential direction, t being p and that in a direction transverse to the
track direction, or radial direction, r being q. In the case of a record
carrier in which a frequency-modulated video signal is stored the period p
is determined by the video signal. The period q is equal to the distance
between the tracks.
The information structure may be a pure phase structure, the information
areas comprising pits pressed into the record-carrier surface or hills
which project from said surface. Alternatively, the information structure
may be an amplitude structure. Then the information areas, for example,
comprise non-reflecting areas in a reflecting surface or
radiation-absorbing or reflecting areas in an otherwise transparent record
carrier.
In FIG. 2 the read spot is designated V. The width of said spot is of the
order of magnitude of the dimensions of the information areas 2. The
information structure behaves as a two-dimensional diffraction grating.
Said grating splits the read beam b into a zero-order sub-beam, a number
of first-order sub-beams, and a plurality of higher-order sub-beams. For
reading the information areas 3 mainly the zero-order subbeam b(0,0) and
the two first-order subbeams b(+1,0) and b(-1,0), which are diffracted in
the tangential direction t, are of interest. In FIG. 1 the record carrier
should then be thought to occupy the position of the object O, the
tangential direction t being the vertical direction. During reading the
record carrier is rotated about an axis 7.
FIG. 3 represents cross-sections of the beams b(0,0), b(+1,0) and b(-1,0)
in the plane of the detector D. The x and y axes in FIG. 3 respectively
correspond to the tangential direction t and the radial direction r in
FIG. 2. The beams returning from the record carrier b(0,0), b(+1,0) and
b(-1,0) have complex amplitudes, which may be represented by:
B(0,0)
B(+1,0) exp (-i.omega.t)
B(-1,0) exp (+i.omega.t)
It is assumed that the record carrier moves with a constant angular
velocity, which gives rise to the time-dependent phase factor exp
(.+-.i.omega.t). Here .omega. is a time frequency, which is determined by
the angular velocity and the spatial frequency, in the tangential
direction, of the information areas. Furthermore, it is assumed that no
tracking errors occur. If the information areas are symmetrical in the
radial direction and the tangential direction, then B(+1,0) is equal to
B(-1,0).
There is a specific phase difference .psi..sub.10 between the complex
amplitudes B(+1,0), B(-1,0) and the amplitude B(0,0). Said phase
difference mainly depends on the phase retardation of radiation from the
bottom of an information pit relative to radiation from the surface of the
information layer, that is, on the optical depth of the information pits
or the optical height of the information hills.
As is described in Netherlands Patent Application no. 78 03517, the
information areas may have an optical depth such that said phase
difference .psi..sub.10 =180.degree., or an optical depth such that
.psi..sub.10 =90.degree.. For .psi..sub.10 =90.degree. the pits are very
shallow, so that the amplitudes of the diffracted beams are very small.
Suitably, the optical depth is selected to be slightly greater, so that
.psi..sub.10 is between 110.degree. and 120.degree.. As already described
in the article "Position-sensing in video-disk read-out" in: "Applied
Optics", Vol. 17, No. 13, pages 2013-2021, an amplitude structure behaves
in the same way as a phase structure whose information areas introduce a
phase difference of .psi..sub.10 =180.degree..
As will be explained hereinafter, the scanning apparatus in accordance with
the invention is suitable for reading both an amplitude information
structure, or a deep phase information structure (.psi..sub.10
=180.degree.), and a shallow phase information structure (.psi..sub.10
=90.degree.). For this purpose, as is shown in FIG. 1, the detector D is
divided into two subdetectors D.sub.1 and D.sub.2, whose outputs are
connected to the input terminals of a summing device 9, which constitutes
the input stage of an electronic processing circuit 10, which is known per
se. Such a circuit is, for example, described in the article: "Signal
Processing in the Philips' VLP system" in: "Philips' Technical Review",
Vol. 33, No. 7, pages 178-180. In FIG. 3 the subdetectors D.sub.1 and
D.sub.2 are represented by the dashed circle halves. In accordance with
the invention an element 8 is arranged between the detector D.sub.2 and
the summing device 9, which element shifts the phase of the signal from
the detector D.sub.2 through an angle .phi..sub.e.
The phase differences .phi.(+1,0) and .phi.(-1,0) between the first-order
beams (b(+1,0), b(-1,0) and the zero order beam may be represented by;
.phi.(+1,0)=.psi..sub.10 +.omega.t
.phi.(-1,0)=.psi..sub.10 -.omega.t
and the complex amplitudes may be written as
B(0,0)=.vertline.B(0,0).vertline.
B(+1,0)=.vertline.B(+1,0).vertline. exp i (.psi..sub.10 +.omega.t)
B(-1,0)=.vertline.B(-1,0).vertline. exp i (.psi..sub.10 -.omega.t)
The intensity variations as a result of interference between the
first-order subbeams and the zero-order subbeam at the location of the
detectors D.sub.1 and D.sub.2 are converted into electric signals S.sub.1
and S.sub.2 by said detectors.
Within the detector area different areas may be distinguished, namely two
areas d, represented by simple hatching, in which a first-order subbeam
interferes with the zero-order subbeam, and two areas c, represented by
cross-hatching, in which in addition to interference between a first-order
subbeam and the zero order subbeam interference occurs between the two
first-order subbeams. The areas c and d may be related to the well-known
modulation transfer function (MTF) of an optical system without
aberrations. The modulation transfer function, hereinafter designated M,
may be assumed to correspond to the area of overlapping of the two
relevant orders.
At the location where a first-order subbeam interferes with the zero-order
subbeam, that is, in an area the size of 2c+d, the transfer function
M(.upsilon.) is valid, where .upsilon. is the spatial frequency of the
information areas. At the location where the two first-order subbeams
interfere, that is, in an area the size of 2c in FIG. 3, the transfer
function M(2.upsilon.) is valid. Hence
2c+d=M(.upsilon.)
2c=M(2.upsilon.), so that
d=M(.upsilon.)-M(2.upsilon.), and
c=1/2M(.upsilon.).
For determining the signals S.sub.1 and S.sub.2 the contributions provided
by the areas c and d should be added to each other. Within the area c
parts of the zero-order subbeam and parts of the two first-order subbeams
are located. Within the area d a part of the zero-order subbeam and a part
of one of the first-order subbeams are located. The signal S.sub.1 may
therefore be represented by:
S.sub.1 -.vertline.B(0,0)+B(+1,0)+B(-1,0).vertline..sub.c.sup.2
+.vertline.B(0,0)+B(+1,0).vertline..sub.d.sup.2
where the indices c and d denote that the relevant contributions should be
weighted with the magnitudes of the areas c and d. For the information
signal itself the d.c. components of the expression for S.sub.1 are of
less significance, so that said components may be ignored. The signal
S.sub.1 then becomes:
S.sub.1 =2Re{B(0,0).multidot.B.sup.x (+1,0)}.sub.c
+2Re{B(0,0).multidot.B.sup.x (-1,0)}.sub.c +2Re{B(0,0)B.sup.x
(+1,0)}.sub.d
Here, Re represents the real part of the relevant component. If the areas c
and d are replaced by the MTF's, then S.sub.1 becomes:
S.sub.1
=M(2.upsilon.).multidot..vertline.B(0,0).vertline..vertline.B(+1,0).vertli
ne..multidot.{ cos (.psi..sub.10 +.omega.t)+cos (.psi..sub.10
-.omega.t)+2.vertline.B(0,0).vertline..vertline.B(1,0).vertline.{M(.upsilo
n.)-M(2.upsilon.)}.multidot.cos (.psi..sub.10 +.omega.t).
Here it is assumed that the information areas are symmetrical, so that
.vertline.B(-1,0).vertline.=.vertline.B(+1,0).vertline.. The signal
S.sub.1 is proportional to: (indicated by the sign: .alpha.)
S.sub.1 .alpha.2[M(.upsilon.)-M(2.upsilon.)].multidot.cos (.psi..sub.10
+.omega.t)+2M(2.upsilon.) cos .psi..sub.10 cos .omega.t.
Similarly, the signal S.sub.2 from the detector D.sub.2 may be written as:
S.sub.2 .alpha.2[M(.upsilon.)-M(2.upsilon.)] cos (.psi..sub.10
-.omega.t)+2M(2.upsilon.) cos .psi..sub.10 cos .omega.t.
The signal S.sub.2 is subject to a phase shift .phi..sub.e, which yields
the signal S.sub.2 '.
S.sub.2 '.alpha.2[M(.upsilon.)-M(2.upsilon.)].multidot.cos (.psi..sub.10
-.omega.t-.phi..sub.e)+2M(2.upsilon.) cos .psi..sub.10 .multidot.cos
(.omega.t+.phi..sub.e).
The sum signal S.sub.s is given by:
##EQU1##
For reading an amplitude structure or a deep phase structure, in which
.psi..sub.10 =180.degree., .phi..sub.e is selected 0.degree.. Then, the
sum signal is:
S.sub.s .phi.-4M(.upsilon.) cos .omega.t.
For a phase difference .psi..sub.10 which is constant over the entire
record carrier and for a constant angular velocity of the record carrier,
the signal S.sub.s only depends on the spatial frequency .upsilon. of the
information areas in the track direction, that is on the information
signal stored.
For reading a shallower phase structure where .psi..sub.10 =90.degree.,
.phi..sub.e =180.degree. is selected. Then;
S.sub.s .alpha.-4[M(.upsilon.)+M(2.upsilon.)] sin .omega.t.
At a constant phase difference and a constant angular velocity this signal
only depends on the spatial frequency .upsilon.. The image of the shallow
phase structure is the first derivative of the structure itself, whilst
the image of the amplitude structure appears in the normal
non-differentiated manner.
Instead of shifting the phase of one detector signal through .phi..sub.e,
preferably the phase of S.sub.2 is shifted through +.phi.e/2 and that of
S.sub.1 through -.phi.e/2 for reasons of symmetry, as is shown in FIG. 4.
The signals S.sub.1 ' and S.sub.2 ' are then given by:
##EQU2##
and the sum signal S.sub.s by
##EQU3##
for .psi..sub.10 =180.degree. and .phi..sub.e =0.degree.: S.sub.2
.alpha.-4M(.upsilon.) cos .omega.t is valid, whilst for
.psi..sub.10 =90.degree. and .phi..sub.e =180.degree.
S.sub.s .alpha.[4M(.upsilon.)+4M(2.upsilon.)] cos .omega.t is valid.
In this case the image of the shallow phase structure also appears in the
normal, undifferentiated manner.
In the description of the principle in accordance with the invention the
higher-order subbeams have been left out of consideration. The
higher-order beams are largely diffracted outside the detector and the
amplitudes of said beams are substantially smaller than those of the
first-order beams, so that the influence of higher-order beams is
negligible in a first-order approximation.
An information structure whose information areas introduce in a phase
difference .psi..sub.10 =90.degree. is a theoretical structure. As already
stated, the diffracted beams produced by such a structure have a low
amplitude, so that the signal S.sub.s is very weak. Therefore, in practice
an optical depth is selected which is such that the phase angle is
slightly greater than 90.degree., for example
110.degree..ltoreq..phi..sub.10 .ltoreq.120.degree.. For .psi..sub.10
=120.degree. the term with M(.upsilon.) in the expression for S.sub.s is a
maximum for .phi..sub.e =120.degree.. If the phase of only one of the
detector signals is shifted through 120.degree., the phase of the signal
S.sub.s is shifted through 30.degree. relative to the signal S.sub.s for
.psi..sub.10 =90.degree..
The apparatus described in the foregoing, which comprises one or two phase
shifters which can be switched between two positions, which apparatus may
be regarded as a scanning apparatus with a detection function which can be
switched to two states, is a special embodiment of the general inventive
concept of providing a scanning apparatus with a complex and variable
detection function which, within the detector area, may be represented by:
g(x,y)=a for x>0
g(x,y)=a, exp (i.phi..sub.e) for x<0
or by:
g(x,y)=a.exp (-i.phi..sub.e /2) for x>0
g(x,y)=a.exp (+i.phi..sub.e /2) for x<0
Outside the detector area g(x,y)=0.
The imaging function of the scanning apparatus in accordance with the
invention is the product of the so-called optical transfer function (OTF)
of the optical system and a further transfer function F, whose modulus and
phase are:
##EQU4##
In the asymmetrical situation the phase of only one of the detector signals
is shifted, namely through .phi..sub.e, whilst in the symmetrical
situation the phase of each of the two detector signals is shifted through
+.phi.e/2 and -.phi.e/2 respectively.
The electronic phase shift .phi..sub.e is utilized in order to optimize the
modulus of the signal S.sub.s :
##EQU5##
In the symmetrical situation the argument will not change owing to the
modulus optimization. However, such a change does occur in the
asymmetrical situation. The change of the argument can be eliminated by
arranging an additional phase shifter 11 after the summing device 9, which
phase shifter introduces a phase shift of -.phi.e/2, as is shown in FIG.
1.
Both in the asymmetrical situation and in the symmetrical situation the
phase of the signal S.sub.s may be influenced by a phase asymmetry in the
scanning spot V. The principal cause of such an asymmetry is coma of the
optical system. Compensation for this phase error is possible by shifting
the phase of the signal S.sub.s through .theta., where .theta. is a
function of the said phase asymmetry. In the preferred embodiment of FIG.
4 an additional phase shifter 12 is then arranged after the summing device
9. In the asymmetrical situation of FIG. 1 the phase shifter 11 is then
adapted so that it produces a phase shift through .theta.-.phi.e/2.
The phase shift .phi..sub.e is generally variable between 0 and
360.degree.. For a read apparatus which should be capable of reading both
record carriers having a phase structure and record carriers having an
amplitude structure, or record carriers with phase structures of different
depths, such a continuous electronic phase shift over a wide range is not
needed. However, this will be different for an optical microscope, which
serves for visualizing not only shallow phase structures or amplitude
structures but also all kinds of intermediate structures, that is,
structures which are neither pure phase structures nor pure amplitude
structures.
The phase depth of the objects to be observed by means of such a
microscope, for example, biological tissues or organisms, need not be
known in advance. The object may be scanned a few times in succession,
each time using a different value for .phi..sub.e until a satisfactory
image quality is obtained.
The objects to be observed with the microscope proposed here need not have
such a constant phase depth as the optical record carriers mentioned in
the foregoing. Said objects may comprise parts which each have a different
phase depth. The object may then be scanned several times, each time using
a different value for the electronic phase shift .phi..sub.e. During each
scan a specific phase depth is reproduced with maximum contrast. The
original object can then be reconstructed from all the individual images.
The phase shifters 8 in FIG. 1 and 8' and 8" in FIG. 4, may be devices
whose phase shift is a function of the time frequency. At a constant
scanning velocity a specific spatial frequency (.upsilon.) in the object
corresponds to a specific time frequency (.omega.). Frequency-dependent
phase shifters in the form of transversal digital filters are known per
se, for other purposes, for example from the book: "Theory and application
of digital Signal processing", Rabiner and Gold, Prentice-Hall Inc. 1975,
inter alia page 40. When using such phase shifters it is possible to
ensure that only for specific spatial frequencies the electronic phase
shift has the value which is optimum for scanning. A result of this is
that only structures of parts of the object with a specific spatial
frequency are correctly reproduced, whilst parts of structures with a
different spatial frequency are imaged in an attenuated form.
Furthermore, the gain of the adding circuit 9 can be made
frequency-dependent, so that the desired spatial frequency can be further
boosted and the undesired frequencies can be further attenuated.
By the use of frequency-dependent phase shifters and a frequency-dependent
amplifier a spatial filtering of the object or an image enhancement can be
achieved without the use of optical filters. The hitherto difficult
problem in optical filtering, of manufacturing suitable optical filters,
has been overcome by the use of electronic filters with the desired phase
and amplitude characteristics which are easier to design.
The scanning apparatus may comprise two separate amplifiers for the
detector signals S.sub.1 and S.sub.2. In FIG. 4 these amplifiers are
designated 13' and 13". The detection function of a scanning apparatus
comprising two separate frequency-dependent amplifiers and
frequency-dependent phase shifters may be written as follows:
##EQU6##
where .upsilon. is the spatial frequency in the object. The two separate
amplifiers may be employed in order to compensate for an amplitude
asymmetry in the scanning beam. When two separate amplifiers are used, it
is possible to amplify one detector signal and to suppress the other
signal, so that a so-called single side band principle is employed.
For scanning, in accordance with the invention, an object in two, for
example, mutually perpendicular directions, the scanning spot can be made
to describe a number of lines in a first direction. The data thus obtained
may be stored in an image storage device. Subsequently, the scanning spot
can be made to describe a number of lines in the second direction.
Finally, the data of the scans in the two directions may be combined.
For scanning in two directions two detectors may be employed, the detectors
and the object being rotated through 90.degree. relative to each other
upon the transition from the one scanning direction to the other.
Alternatively, it is possible to employ four detectors, with one set being
is used for the the other scanning direction and one set for the other
scanning direction.
The present invention relates to the division of a detector into two
subdetectors and to the method of electronically processing the signals
supplied by the subdetectors. The invention is not limited to a specific
type of scanning radiation, such as light. It is only of importance that
the scanning radiation can be concentrated to form a small scanning spot.
Apart from an optical microscope, the invention may be used in an electron
microscope, an X-ray microscope or in an acoustic microscope, provided
that said microscopes are aberration-free to a satisfactory extent. By
means of these microscopes details are observed whose magnitude is at the
limit of resolution of the relevant microscope.
FIG. 5 schematically represents an electron microscope. The electron source
ES emits an electron beam b.sub.e. This beam is focussed by an electron
lens EL in the plane of the object O, which is for example a weak phase
object. The object splits the beam b.sub.e into a zero-order subbeam
b.sub.e (0,0) and into, inter alia, two first-order subbeams b.sub.e
(+1,0) and b.sub.e (-1,0). The zero-order beam and parts of the
first-order beams are received by two detectors DE.sub.1 and DE.sub.2,
which convert the electron radiation into an electric signal. The angle
.beta..sub.e through which the first-order subbeams are diffracted is of
the same order of magnitude as the numerical aperture, which is equal to
sin .alpha..sub.e, of the electron lens, in the same way as in the light
microscope. The signals S.sub.1 and S.sub.2 from the detectors DE.sub.1
and DE.sub.2 are processed in the same way as described with reference to
FIGS. 1 and 4.
FIG. 6, very schematically, represents an embodiment of an X-ray
microscope. XS is the X-ray source, which, because it should be a bright
source, suitably comprises a synchrotron. THe X-ray beam b.sub.x is
focussed at the object, for example a biological specimen or a crystal
structure. The focussing system XF may comprise a zone plate as is shown
in FIG. 6, or a plurality of mirrors. The X-ray beam returning from the
object is received by two X-ray detectors DX.sub.1 and DX.sub.2. The
signals S.sub.1 and S.sub.2 from these detectors can be processed as
described with reference to FIGS. 1 and 4. For further details on the
X-ray source XS, the focussing system XF and the X-ray detectors DX.sub.1
and DX.sub.2, which components do not form part of the present invention,
reference is made to the article: "The scanning X-ray microscope", pages
365-391 of the book: "Scanned image microscopy" E. A. Ash., Academic
Press, 1980.
FIG. 7 represents the principle of an acoustic microscope in accordance
with the invention. Such a microscope comprises a piezo-electric
transducer PEC, which has a uniform response over its entire surface area.
By means of this transducer a sound wave is produced, which is aimed at
the object to be examined, for example a reflecting layer. If the
transducer is flat and the sound wave is a plane wave, an acoustic lens
may be arranged between the object and the transducer, which lens converts
the acoustic wave into a spherical, convergent wave. As is shown in FIG.
7, the transducer itself may be curved, so that the emitted sound wave is
already convergent. The sound wave is reflected by the object and returns
to the transducer, which converts the sound wave into voltage. Then,
integration is effected over the entire surface area of the transducer.
The transducer PEC thus functions as source and as detector. The input
voltage and the output voltage are distinguished from each other in that
short pulses are used.
The output voltage depends on the phases of the individual beam components.
If a reflector is arranged in the plane of focussing, all beam components
traverse the same pathlength and the beam components in points 15 and 16
are in phase. However, if the reflector is moved in a vertical direction,
that is, if the surface O to be observed which exhibits vertical
irregularities, moves in the x-direction, the various beam components will
traverse different path-lengths and the beam components will exhibit a
specific phase shift in 15 and 16, causing the output voltage to change.
For further details on the acoustic microscope, which in itself does not
form part of the present invention, reference is made to the article:
"Scanning acoustic microscopy", pages 24-55 of the said book: "Scanned
image microscopy". In accordance with the invention the transducer is
divided into two sections DA.sub.1 and DA.sub.2, DA.sub.1 being connected
direct to a summing device 9 and DA.sub.2 via a phase-shifting element 8.
The signal processing is identical to that in accordance with FIG. 1 or
that in accordance with FIG. 4.
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