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Description  |
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Technical Field
The present invention relates generally to analysis of the microstructure
present in large transparent or semi-transparent volumes or over large
object surface areas and, more particularly, to the performance of such
analysis through the use of optical processing techniques.
BACKGROUND ART
As is well known in the art, an optical Fourier transformation can be
performed on a subject by illuminating the subject with a coherent light
source and using a lens to collect the reflected, and/or diffracted and/or
transmitted light containing optical information corresponding to the
subject. The lens will define a Fourier transform of the orignal subject
information in a plane located one focal length away from the lens at its
focus. With this process, subject information is redistributed in the
Fourier transform plane to correspond to spatial frequency content. A
second lens, located two focal lengths from the first, images the
information onto a display screen, recording medium, or the like.
The optical information coming from the subject may be manipulated for the
purpose of enhancing detail, removing unwanted subject information,
isolating defects, or making precise topographic and optical path
measurements. This is accomplished by blocking a portion of the subject
information or changing its phase within the transform plane. These
techniques are particularly useful where the subject consists of
repetitive spatial frequency content such as is present in photomasks or
wafers used in the production of microelectronic circuits, since the
optical Fourier transform will consist of an array of regularly spaced
points of light whose distance from the optical axis is proportional to
the spatial frequency.
One example of such a technique is disclosed in U.S. Pat. No 4,000,949
issued Jan. 4, 1977 to Watkins. Using the basic processing scheme outlined
above, a photomask is used as the subject. An optical spatial filter is
placed within the Fourier transform plane for blocking all subject
information other than that corresponding to nonperiodic defects in the
mask. Since only defect information will pass through to the display
screen, the number of defects and their locations can be determined.
It can be easily seen, however, that aberrations within the optical
components of an optical processor, particularly within lenses focusing
the subject information and/or the processed information, will be a
significant detriment to use of the otpical processor with subjects having
microscopic detail. On axis, for example, a focus error aberration can
cause loss of spatial frequency information, as illustrated by way of
example for a typical optical telescope system in FIG. 1.
A three-lens system 24 is shown, wherein lenses 1.sub.1 and 1.sub.2 are
F/1.50, 50 mm diameter, 75 mm focal length lenses, and lens 1.sub.3 is an
F/1.50, 150 mm diameter, 225 mm focal length lens. Assuming the sum of the
spherical aberration from lenses 1.sub.2 and 1.sub.3 to be 2 mm, the focus
of the rays 26 entering the system 24 is moved from a point 28 to a point
30, 77 mm away from lens 1.sub.1. This results in a smaller, F/1.54
collection angle at lens 1.sub.1.
Therefore, with a 225 mm focal length for lens 1.sub.3, which would give an
F/4.5 collection angle for lens 1.sub.3 and the system 24 as a whole if
lens 1.sub.1 collected F/1.50, the lens can only collect:
##EQU1##
Similarly, for 1 mm of special aberration, the focal point is moved to 76
mm away from lens 1.sub.1 and:
##EQU2##
for the system 24.
To consider these effects directly, the optical transfer function (OTF) of
the system may be looked to.
For a diffraction limited system:
##EQU3##
where a (f.sub.x,f.sub.y) is the area of overlap of the pupil collecting
spatial frequencies with the restricting pupil function. The OTF with
aberrations for two pupils given by:
##EQU4##
where W is the aberration function.
The Schwarz inequality,
.vertline..intg..intg.X Y d.xi.d.eta..vertline..sup.2
.ltoreq.(.intg..intg..vertline.X.vertline..sup.2 d.xi.d.eta.)
(.intg..intg..vertline.Y.vertline..sup.2 d.xi.d.eta.)
can be used to show directly that
H'(f.sub.x, f.sub.y).sub.aber. .vertline..sup.2 .ltoreq..vertline.H
(f.sub.x,f.sub.y).sub.no abberrations .vertline..sup.2
This means that aberrations never increase the MTF (the modulus of the
OTF), but rather lower the contrast of each spatial frequency component.
Thus, the cutoff to the spatial frequency passed by the system will be
effectively decreased and the effective F-number thereof will increase.
This is especially important if there is a random background to further
decrease the contrast.
Off-axis, spatial frequency information can be lost even in an ideal
aberration-free system directly due to vignetting, as shown in FIGS. 2 and
3. Again, considering the sample three-lens optical system 24, the limit
of the system 24 is found by tracing rays from the edge of lens 1.sub.1.
The angle of incidence of these rays at a point on the subject plane 34
gives the cone angle of spatial frequency information which can be
collected from that point on the subject.
In FIG. 2, using a thin lens ray trace, it can be seen that only an F/9
cone can be collected by system 24 from a point 36 located 37.5 mm off the
axis 37. In FIG. 3, a point 38 located 50 mm off the axis 37 is
considered. Again using a thin lens ray trace, it can be seen that only an
F/13.5 cone can be collected due to vignetting.
In order to utilize optical processing techniques on objects having
microscopic detail with an optical system such as is used in Watkins, it
is necessary to employ within the system opitical components of very high
quality. Such components will then minimize aberrations within the system.
Additionally, to avoid loss of information due to vignetting, system
apertures must be made as large as possible. Unfortunately, large aperture
optical components of very high quality, particularly lenses, can be
obtained only at extremely high cost, making optical processing for such
objects far less practical.
Use of high quality lenses creates additional complications other than cost
where high-resolution records are desired, since as the resolution of an
imaging optic is increased, the depth of focus thereof decreases. Thus,
the depth of focus available in the image plane is limited.
Full-field documentation with relatively high resolution can be
accomplished through holography. By holographically recording a subject,
and then reconstructing the holographic image with the conjugate to the
reference beam, a three-dimensional real image of the object is created in
space. This real image can then be examined microscopically as if it were
the original illuminated object. An added advantage is that a holographic
system can document not only reflecting surfaces but also the
microstructure inside a thick, transparent object such as optical
components. Since the holographic image has no substance, a microscope can
focus through the image, even to the opposite side if desired, without the
need for a long focus and high quality objective required when a solid
object is in the way.
Examples of previous work in holographic microscopy are disclosed in Leith
and Upatnieks, "Microscopy by Wavefront Reconstruction," 55 J. Opt. Soc.
Amer. 569 (1965); Toth and Collins, "Reconstruction of a Three-Dimensional
Microscope Sample Using Holographic Techniques," 13 Appl. Phys. Letters 7
(1968); and Briones, Heflinger and Wuerker, "Holographic Microscopy," 17
Appl. Optics 1944 (1978). The records produced are large aperture wide
field of view, large depth of field, three-dimensional images of both
transmitting volumes and specularly or diffuse reflecting subjects. Such
work, however, has not as yet obtained both the necessary resolution and
field of view for viewing and/or analyzing the microstructure of large
object surface areas.
One technique available in holography, described in Toth and Collins, is
known as reverse ray-tracing. When a hologram 10 is made of a subject 12
through a lens 14 or other optical components, the image information 16
from the subject may be aberrated by the lens 14 (FIG. 4a). If the
hologram 10 is repositioned accurately with respect to the lens 14, and
the conjugate to the reference beam 18, i.e., the same wavefront as the
reference beam but travelling in the opposite direction, is used to
reconstruct the holographic image, the image rays 20 will exactly retrace
the path of the original subject rays back through the optical system
(Fig. 4b). This is not the same as merely turning the lens 14 around,
since the information about the lens aberration is stored in the hologram
10. Therefore, the aberrations of the lens 14 will be completely
compensated for upon reconstruction and the holographic image 22 will be
diffraction limited.
It can be seen that to produce a high resolution holographic image 22, it
is not necessary to use a high quality lens 14. Thus, it would appear
promising to attempt to incorporate the holographic reverse ray-trace
technique into an optical processing system.
What is needed, therefore, is a method and apparatus for optical processing
that utilizes holography, and specifically the reverse ray-tracing
technique. Such a method and apparatus would enable optical processing to
be performed on both detailed three-dimension microscopic and macroscopic
subjects with high resolution.
Disclosure of Invention
The present invention provides a method and apparatus for performing
optical processing on optical information corresponding to a subject that
combines known optical processing techniques with holography. By utilizing
a reverse ray-trace holographic system, aberrations introduced by the
system optics are completely compensated for, producing
diffraction-limited three dimensional images of a subject. A single large
aperture lens is used to define a Fourier transform of the optical subject
information as the hologram is recorded or the image reconstructed, and
the subject information is manipulated within the transform plane.
A source for generating a coherent beam of light, preferably an argon ion
laser operating single frequency, provides a beam to a beamsplitter that
divides the beam into a subject beam and a reference beam. The subject
beam is directed to the subject where it is reflected and/or diffracted
thereby. The beam, which now contains optical information regarding the
subject, is passed through a lens and focused to define a Fourier
transform, imaging the information in the vicinity of a photo-sensitive
material, such as a photographic plate. The reference beam is
simultaneously directed onto the plate at a predetermined angle of
incidence.
After processing, the hologram is replaced and illuminated with the
reference beam which is redirected onto the plate in a direction conjugate
to the original reference beam. Subject information from the hologram will
then exactly reverse ray trace back through the optical system following
the same path but in the reversed direction as original subject rays
during recording. A three-dimensional real image of the original subject
will be formed at the original subject location. Because of the exact
reverse ray-trace feature, optical distortions or, aberrations introduced
by optical elements in the system will be completely eliminated thus
providing a diffraction limited three-dimensional real image.
The subject information is manipulated at the Fourier transform either
during recording of the hologram or during image reconstruction. This may
be performed, for example, by placing a blocking filter within the
transform plane for blocking at least a portion of the subject
information. Alternatively, a filter may be placed within the transform
plane for altering the phase of at least one of the beams passing
therethrough. Other processing techniques are also possible.
This invention offers several important advantages over known optical
processing systems and methods.
First, the reverse ray-trace of the image information through the optical
system provides an image that is free of aberrations introduced by optical
elements.
Second, since such aberrations are eliminated, the optical quality of the
imaging optic is not as critical as in known systems. This permits use of
components of lesser quality, with very significant reductions in costs.
Third, the use of a single, large-aperture lens provides for a wide
field-of-view, with little information loss due to vignetting.
Fourth, the high resolution obtained with this invention provides for
microscopic evaluation of recorded images, and enhances results obtained
through optical processing and interferrometric techniques.
Accordingly, it is an object of the present invention to provide a method
and apparatus for the performance of optical processing to provide such a
method and apparatus for utilizing holographic images free from
aberrations due to components of the system; to provide full-field,
high-resolution holograms capable of microscopic examination; and to
provide such a method and apparatus that can perform a variety of optical
processing and interferrometric techniques.
Other objects and advantages of the invention will be apparent from the
following description, the accompanying drawings, and the appended claims.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a diagram of a hypothetical telescope system, showing the effect
of focus error aberration;
FIG. 2 illustrates the hypothetical telescope system of FIG. 1, showing the
effect of vignetting;
FIG. 3 illustrates the hypothetical telescope system of FIG. 1, again
showing the effect of vignetting;
FIG. 4a is a diagram illustrating generally the recording of a hologram,
showing introduction of aberration into the image information;
FIG. 4b is a diagram illustrating generally the reconstruction of a
holographic image, wherein reverse ray tracing eliminates aberration from
the image information;
FIG. 5 is a diagram of the apparatus of the present invention;
FIG. 6 is a diagram of an alternative embodiment of a portion of the
present invention for use when the subject is a transparent or
semi-transparent volume;
FIG. 7 is a diagram of a further alternative embodiment of a portion of the
present invention for use when the subject is a transparent or
semi-transparent volume;
FIG. 8 is a diagram of a still further alternative embodiment of a portion
of the present invention for use when the subject is a transparent or
semi-transparent volume;
FIG. 9 is a diagram of an alternative embodiment of a portion of the
present invention for recording an image plane hologram; and
FIG. 10 is a diagram showing image reconstruction with the embodiment of
FIG. 9.
MODES FOR CARRYING OUT THE INVENTION
Referring now to the drawings of the preferred embodiments of the
invention, and particularly to FIG. 5, the optical processing apparatus
includes a laser 40 that provides a source for generating a coherent beam
of light 42.
In the preferred embodiment, an argon ion laser operating single frequency
at a wavelength of 0.5145 micrometers is used, in particular, a Spectra
Physics Model 166-09 Argon Laser. Any laser source, however, whose output
wavelength is compatible with the recording medium will work, including
but not restricted to helium-neon, krypton, ruby, neodymium YAG or glass,
and metal vapor lasers.
Beam 42 is directed by mirrors M1 and M2 into the system and passes through
a half-wave plate 46 which allows the angle of polarization to be
continuously adjusted before entering the polarizing beamsplitter 48. It
should be noted, however, that although polarizing beamsplitting
techniques are used in the preferred embodiment to conserve energy and
allow maximum flexibility, any beamsplitting technique that divides the
incoming beam into two components will work, such as metal films, grating,
dielectric films and holographic elements.
Beamsplitter 48 divides the input beam 42 into a first or subject beam 50
and a second or reference beam 52 having an intensity ratio determined by
the angle of polarization of the incoming beam.
The subject beam 50 is then directed to a means 54 for expanding, filtering
and collimating beam 50 including lenses 56 and 58 and a mask having a
pinhole 60. It then passes through a second polarizing beamsplitter 62
which serves as a selective beam director. This is accomplished in the
following manner: light entering this director (beamsplitter 62) passes
through because of the polarization orientation established by
beamsplitter 48. Beam 50 then passes through a quarter-wave plate 64 where
it is converted from linear to circular polarization. It then strikes a
subject 66 and is reflected thereby back through the quarter-wave plate 64
where it is converted back to linear polarization but orthogonally
oriented. This causes the subject beam 50 to be reflected by the
beamsplitter 62 towards a large aperture lens 68 which forms an image in
the vicinity of a means for recording the hologram. In the preferred
embodiment, lens 68 is a symmetric doublet pair, imaging subject 66 on or
near the recording means at one-to-one magnification to maximize the
information collected by the lens 68. Other means for directing the
subject information to the recording means may be used, including placing
the recording means close to the subject and eliminating the imaging
system.
While the recording means is shown in the preferred embodiment as a
photographic plate 70, it will be recognized that it may be any
photo-sensitive material such as photographic films or plates,
thermoplastic films, crystalline materials, or any other material capable
of recording a hologram.
FIG. 6 shows the preferred arrangement if the subject is a transmitting
volume 72. Here a mirror M3 is placed behind the subject 72 which causes
the subject beam 50 to be reflected back through the quarter wave plate
64. This double pass scheme offers the advantage that anomalies of
interest present in a subject volume 72 are illuminated from both the
front and the back thus enhancing visibility. When using the invention to
make interferometric measurements of optical path changes or differences,
double passing the volume doubles the measurement sensitivity.
The variation shown in FIG. 7, or the more preferred FIG. 8, is used if the
double passing or front illumination of a subject volume 72 is undesirable
(e.g., in recording or optical processing of transparencies). Referring to
FIG. 7, a half-wave plate 74 is placed in the subject beam 50 ahead of
polarizing beamsplitter 62. This allows for adjustment of the subject beam
polarization angle and is set to provide vertical, or polarization.
Beamsplitter 62 now reflects the subject beam 50 toward mirror M4. This
mirror directs the beam to mirror M5, where it is directed through the
subject or subject volume 72 and back into beamsplitter 62 and thus into
lens 68 as described above.
Similarly, in the preferred variation shown in FIG. 8, the subject beam 50
is directed around the beamsplitter 62 by mirrors M6 and M7 where it is
directed through the subject 72 and half-wave plate 76 by mirror M8. It
then passes through the system to the photographic plate 70 as previously
described.
Referring back to FIG. 5, the reference beam 52 generated from beamsplitter
48 is directed by mirror M9 towards a means 78 for expanding, filtering
and collimating beam 52 consisting of lenses 80 and 82, and a mask having
a pinhole 84. Beam 52 is expanded and collimated and directed towards
mirror M10. In the recording mode, mirrors M10 and M11 direct the
reference beam 52 along reference path 86 to the photographic plate 70
where beam 52 combines with the subject beam 50 to expose plate 70,
whereupon a hologram 88 is formed in plate 70.
After processing the hologram 88 and returning it to position, mirror M10
is removed and mirror M12 directs the reference beam 52 towards the
hologram 88 from the conjugate direction along conjugate path 90. Subject
information from hologram 88 will then exactly reverse ray-trace back
through lens 68 and beamsplitter 62 following the same path but in the
reversed direction as the original subject beam 50 during recording. A
three-dimensional image of the subject 66 will be formed at its original
location. Because of the exact reverse ray-trace feature, optical
distortions or aberrations introduced by optical elements such as lens 68,
beamsplitter 62, or the like will be eliminated, providing a diffraction
limited three-dimensional real image.
In the variation shown in FIG. 9, when the subject 66 or a portion thereof
is flat (for example, an integrated circuit mask), the lens used is color
corrected (for example, achromat 92), and if care is taken to image the
subject 66 on the surface of the plate 70, an image plane hologram is
recorded. This image plane hologram can then be illuminated for
reconstruction with a broadband source such as white light. This technique
offers the advantage that incoherent source illumination will eliminate
coherent noise in the reconstructed image. Reconstruction is illustrated
in FIG. 10 where the hologram 88 is illuminated with a broadband beam 94
along the conjugate path 90. Reconstruction of the image point will occur
at the original subject position 96.
There may be background noise in the holographic image which is not located
at the subject plane. If the reference beam's angle of incidence is
changed slightly during image reconstruction, this dislocated background
noise moves across the field of view while the image is stationary.
Therefore, much of this background noise originates from other optical
components in the system. Dithering the reference beam 52 while viewing or
photographing the holographic image smoothes out the dislocated background
and increases the effective contrast. However, this is effective only for
a single plane in the reconstructed image, because only a single-image
plane of the subject can be exactly imaged in the plane of the hologram.
Points in the reconstruction corresponding to points off the hologram are
displaced with dithering of the reference beam 52 and will smear out.
In the preferred embodiment, hologram 88 is formed in an Agfa 8E56HD 10.2
cm.times.12.7 cm (4".times.5") plate. The preferred developing processes,
as experimenta11y determined, are either PAAP or HRP/Bromine Methanol.
These were found to give substantially better results than standard D-19
processing. The lens 68 described herein collects approximately an F/3.5
cone of light. Thus, the reconstructed real holographic image is
diffraction limited for the F/3.5 optical system:
diffraction limited diameter=2.44 .lambda.(F-number).
=2.44 (0.5145 .mu.m)(3.5)=4.4 microns.
In experimental testing of the system, however, it appears that resolution
of the holographic image better than four microns is attainable.
Because of the high resolution obtained with the presently disclosed
system, documentation of the microstructure of a subject is readily
facilitated. The reconstructed image may be studied microscopically at the
original subject plane through an appropriate means therefor, such as
microscope 98 shown in FIG. 5. Moreover, the observed information may be
recorded, for example, by a camera (not shown) or other means, used in
conjunction with microscope 98.
Because collimated coherent light is used to illuminate the subject in
conjunction with a focusing element, an optical Fourier transform of
subject information is formed in each of the embodiments of FIGS. 5-9
between the hologram 88 and the lens 68, one focal length from the
focusing element, as seen for example at 91 in FIG. 6. This transform
plane 91 can be used to manipulate subject information such as spatial
frequency content, distribution, and phase, by placing appropriate phase
and blocking filters at this location. Such filters may be of any of the
presently known optical filters, examples of which are disclosed in U.S.
Pat. No. 4,000,949, issued Jan. 4, 1977, to Watkins, and U.S. Pat. No.
4,299,443, issued Nov. 10, 1981, to Minami et al. This serves to optically
process subject information and can be performed prior to initial
recording of the hologram or after the hologram has been recorded. In
either case, the present invention offers several advantages over standard
optical processors.
First, the inverse Fourier transform is performed by the same optics that
produced the original transform because of the reverse ray-trace feature
of this process. Second, the optical quality of the imaging optic need
only be sufficient to separate spatial frequencies at the Fourier
transform plane relating to the desired image information. This
significantly reduces the cost and broadens the applications for optical
processing. In general, the Fourier transform is less sensitive to lens
aberrations than the image. Third, the use of a single, large aperture
lens provides for a wide field-of-view, with little information loss due
to vignetting. Finally, the same apparatus may be used to produce
effective blocking filters on photosensitive material such as photographic
plates, which are accurately matched to the subject's spatial frequency
locations in the Fourier transform plane.
The optical processing may be performed through a number of techniques for
a variety of reasons. For example, the blocking filters may be placed
within the transform plane in order to partially or completely block all
or a part of the subject information within the Fourier transform. This
will enable the reconstructed image to have certain details enhanced or
suppressed, or a portion of the image to be removed entirely. Particular
information may be isolated, for instance where the locations of defects
within a particular subject are sought. While these advantages are
available from optical processing generally, the high resolution achieved
with the present invention enables such techniques to be carried out at
microscopic levels and at reasonable cost. Additionally, since the
filtering may be performed during image reconstruction, the subject itself
is required only long enough to record a hologram, allowing optical
processing to be performed at a later time.
The optical processing may also be performed through a technique known
generally as matched filtering. Rather than completely blocking all or a
portion of the subject information as with blocking filters, all or a
portion of the information may be diffracted (and/or reflected) out of the
optical system at the transform plane. This may serve as a means of
effectively blocking information, but can also be used to direct the
deflected portion of the information into one or more other systems where
the information can be detected, observed, recorded, and/or further
manipulated. Matched filtering may have any one of a number of practical
applications, such as the separate isolation or detection of specific
information pertaining to selected objects or features of a single object.
This technique is accomplished using a selective refractive and/or
diffractive medium, such as a hologram, as the matched filter.
The subject can also be compared to an unperturbed reference wavefront in
the following manner. A hologram 88 is made of the subject, processed,
repositioned at its original location, and reilluminated with the
reference beam 52 from the conjugate direction 90 via mirror M12. A second
reference beam is then combined with the real holographic image at the
original subject location via the path of original subject beam 50 and
beam director 62. If the intensities are then made equal by adjusting
half-wave plate 46, a fringe pattern can be observed. This fringe pattern
will map the difference in optical path between the surface figure or
optical density of the subject and the wavefront shape of the reference
wavefront. Thus, the subject may easily be compared to an ideal flat or
spherical surface.
If a plane reference wavefront is combined with the subject image wavefront
but is shifted in phase by a quarter-wave relative to the beam used to
reconstruct the holographic image, small changes in the subject's surface
topography will be highlighted by interference fringes. This is
accomplished using the technique outlined above for comparison to an
unperturbed reference wavefront, but with a filter placed at the Fourier
transform 91 of lens 68 so as to pass only the zero order component
therethrough. Either the reference beam 52 or the subject beam 50 is
shifted in phase by means of a quarter-wave plate. The phase contrast
fringe pattern is then viewed at the subject location.
A permanent record of the phase contrast image of the subject can be
recorded by means of a double exposure hologram. A hologram of the subject
is first recorded, and the filter is placed at the transform plane 91.
Either the reference or subject beam 52 or 50 is shifted in phase by a
quarter-wave, and a second hologram is recorded on the same holographic
plate. After processing and replacement to its original position, the
hologram is then illuminated from the back or conjugate reference beam
direction via mirror M12. Both the holographic image of the subject and
the reference wavefront will then be reconstructed at the original subject
position and can be viewed, for instance, with microscope 98.
Real time holographic interferometric analysis of the subject may be
accomplished by first recording a hologram with the apparatus arranged as
in any of FIGS. 5-9, depending on the subject type. The developed hologram
88 is then replaced in its original position, with the aid of a kinematic
mount, and illuminated with the recording reference beam 52 with mirror
M10 in place. Viewing would occur from the position indicated by arrow 100
(Fig. 5).
If either the original subject 66 or 72, or some new subject to be
compared, is simultaneously illuminated with light from subject beam 50,
and made equal in intensity to the holographic image intensity by
adjusting half-wave plate 46, then differences in subject optical paths
will be observed as fringes appearing on or near the subject. These
optical path differences are referenced to the subject as originally
recorded.
A diffraction limited interferogram can be reconstructed in the following
manner. A double exposed hologram is recorded by first recording a
hologram of the subject in its reference state. Changes to the subject are
then made or a different subject is inserted, and a second hologram is
recorded on the same holographic plate. After processing and replacement
to its original position, it is then illuminated by reference beam 52 from
the back or conjugate direction 90 via mirror M12. Both holographic images
will then reconstruct at the original subject position and can be viewed
therefrom. Optical path differences due to changes in the subject
occurring between exposures or differences between the two subjects will
be observed as fringes on or near the reconstructed subject's image. Since
this interferogram is inherently diffraction limited, due to the reverse
ray-trace feature, extremely small fringe spacings can be accurately
measured through a magnifying device, such as microscope 98.
While the method herein described, and the form of apparatus for carrying
this method into effect, constitute preferred embodiments of this
invention, it is to be understood that the invention is not limited to
this precise method and form of apparatus, and that changes may be made in
either without departing from the scope of the invention, which is defined
in the appended claims.
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