Exposure method for reducing distortion in models produced through solid imaging by forming a non-continuous image of a pattern which is then imaged to form a continuous hardened image of the pattern

A method for reducing curl in three dimensional computer generated models, created by the sequential exposure of adjacent layers of a photoformable composition, comprising exposing each layer twice, the first exposure being with an image modulated exposure further modulated to produce a series of isolated, anchored islets along the imaged areas, and the second exposure also being image modulated but without the additional modulation, so as to fuse the islets into a continuous solid image.

BACKGROUND OF THE INVENTION 
1. Field Of the Invention 
This invention pertains to a method and apparatus for three dimensional 
modeling and, more particularly, to a method for controlling the exposure 
of photoformable layers to produce models exhibiting reduced distortion 
due to internal stresses. 
2. Description of Related Art 
Many systems for production of three-dimensional (solid) modeling by 
photoforming are in existence. European Patent Application No. 250,121 
filed by Scitex Corporation Ltd., on Jun. 6, 1987, discloses a 
three-dimensional modeling apparatus using a solidifiable liquid, and 
provides a good summary of documents pertinent to this art. U.S. Pat. No. 
4,575,330 issued Mar. 11, 1986, to C. W. Hull, describes a system for 
generating three-dimensional objects by creating a cross-sectional pattern 
of the object to be formed at a selected surface of a fluid medium capable 
of altering its physical state in response to appropriate stimulation by 
impinging radiation, particle bombardment or chemical reaction. Successive 
adjacent laminas adhering to each other, representing corresponding 
successive adjacent cross-sections of the object, are formed resulting in 
a three-dimensional object. U.S. Pat. No. 4,752,498 issued Jun. 21, 1988, 
to E. V. Fudim, describes an improved method of forming three-dimensional 
objects, which comprises irradiating an uncured photopolymer by 
transmitting an effective amount of photopolymer solidifying radiation 
through a radiation transmitting material in contact with the uncured 
photopolymer. The transmitting material is one that leaves the irradiated 
surface capable of further cross-linking, so that a subsequently formed 
layer will adhere thereto. 
In generating a three-dimensional object using successive layer hardening 
by exposure to an imagewise modulated hardening radiation to form 
successive cross-sections of the object, it is important to assure that 
each layer accurately represents the desired cross-section of the object, 
so as to generate a three-dimensional object which is an accurate 
representation of the desired object. Therefore, it is important that 
there be no distortion introduced in the creation of each layer which will 
result in a distorted object. Unfortunately, distortions in the individual 
layers do occur as the photoformable composition changes during the 
irradiation process from a substantially freely flowing state to a 
substantially hardened solid state, due to stresses in the layers. Such 
stresses are believed to be the result of substantial forces generated 
during molecular shrinkage as a result of the hardening irradiation 
process. Numerous solutions to this problem have been proposed. 
One solution involves exposing the layers to a WEAVE pattern, discussed in 
Chapter 8, Advanced Part Building, pages 195-219, "RAPID PHOTOTYPING & 
MANUFACTURING, Fundamentals Of Stereo Lithography" First Edition, by Paul 
F. Jacobs, Published by The Society of Manufacturing Engineers, One SME 
drive, P.O. Box 930, Dearborn, Mich. 48121-0930. U.S. Pat. No. 4,945,032 
issued Jul. 31, 1990, to Murphy et al., proposes a process for the 
reduction of distortions whereby the exposure of a layer is stopped and 
then repeated at least once. Japanese Patent application 63[1988]-172685, 
published Jan. 26, 1990, also uses two exposures, a first imagewise 
exposure which leaves only partially cured resin in the exposed areas and, 
after the model is completed, a second overall exposure which hardens the 
partially cured resin to produce a solid model. While these methods may 
reduce distortion, there is still need for a method that will result in an 
undistorted solid model, having minimal uncured material trapped in its 
body and maximum hardness. 
SUMMARY OF THE INVENTION 
The present invention comprises a method for fabricating a three 
dimensional object from a multiplicity of cross sectional portions of the 
object. The cross-sectional portions correspond to solidified portions of 
contiguous layers of a photoformable composition created by the sequential 
imagewise exposure of the layers. Each layer is exposed in accordance with 
a predetermined pattern representing a desired cross section of the three 
dimensional object. Using terminology defined herein below, the present 
invention comprises: 
(1) imagewise exposing at least one layer to the predetermined pattern for 
the one layer using an exposure source supermodulated to produce on the 
one layer a noncontinuous image of the pattern having discreet, anchored, 
hardened image areas separated by image areas of partially hardened 
photoformable composition; and 
(2) repeating the imagewise exposure of the one layer to the same 
predetermined pattern for the one layer using the exposure source 
modulated to produce on the one layer a continuous hardened image of the 
pattern. 
It is also within the scope of the present invention to effect the 
imagewise exposure in step (1) above in a manner to produce dash-shaped 
hardened areas, and wherein the dash-shaped areas along one image line are 
laterally displaced from the dash-shaped areas in adjacent image lines. 
The exposure source preferably comprises a scanned laser beam submodulated 
with a pulse signal to control the exposure of the photoformable 
composition; both the modulation and the supermodulation of the beam for 
the two exposures are accomplished by selectively varying the intensity of 
the beam impinging on the one layer from an "on" state to an "off" state 
at predetermined time intervals. In generating a solid model, steps (1) 
and (2) are repeated a preselected number of times to expose a plurality 
of layers to the corresponding predetermined patterns.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In describing the creation of a three-dimensional model and improvements to 
the process in accordance with the present invention, wherein models 
exhibiting reduced distortion are obtained, certain terms will be used 
which for clarity are defined herein as follows: 
Photoformable composition: A material which is originally in an unhardened, 
readily deformable state, such as a liquid state, and which becomes 
hardened or partially hardened upon exposure to a sufficient amount of an 
appropriate radiation. 
Unhardened: The state of the photoformable composition prior to any 
substantial exposure to radiation, where such composition is in a readily 
deformable state. 
Partially hardened: The state of the photoformable composition after it has 
been exposed to hardening radiation at a level of intensity insufficient 
to harden the composition beyond a point where it remains deformable. A 
flexible, compliant, rubbery state. 
Hardened: The state of the photoformable composition after it has been 
exposed to hardening radiation at a level of intensity sufficient to 
render the composition solid. (Some unhardened or partially hardened 
material may be trapped within lattices in the hardened composition, 
however such material is not present in sufficient quantities to render 
the material compliant.) 
"On" and "Off" states: "On" state is a condition where the intensity and 
velocity of an exposing radiation beam are such that the Exposure 
(Intensity of incident radiation * duration of incidence), measured on the 
image plane, is above a threshold level sufficient to harden the 
photoformable material. "Off" state is a condition where the beam 
intensity and velocity are such that the Exposure, similarly measured on 
the image plane, is below the threshold level sufficient to harden the 
photoformable material. 
Submodulation: A process whereby a laser beam intensity and/or velocity is 
varied to produce "on" and "off" states with a pulse train of variable 
pulse width and repetition rate. The pulse width and repetition rate 
(frequency) are a function of the instantaneous scanning speed of the beam 
on an image plane. The pulse width and repetition rate are selected to 
provide substantially uniform exposure levels on the image plane 
throughout the full scanning path of the beam. 
Modulation: A process whereby a laser beam impinging on an image plane is 
imagewise modulated by varying the beam intensity on the image plane 
between "on" or "off" states, based on a signal representing image 
information, to expose areas on the image plane to reproduce an image 
corresponding to the image information in the signal. The beam may be a 
submodulated laser beam. 
Supermodulation: A process whereby the exposure level generated by a light 
beam on an image plane is further varied between "on" or "off" states in 
accordance with a preselected control signal selected to create an 
interrupted series of exposures which produce a pattern of discreet, 
disconnected, exposed regions within an exposed portion of an image area 
on an image plane. This may be accomplished by varying either the 
intensity of the exposing radiation incident on the image plane or by 
varying the time the radiation impinges on a point on the image plane or 
both. 
Throughout the following detailed description, similar reference characters 
refer to similar elements in all figures of the drawings. 
FIG. 1 shows an apparatus for practicing this invention. The apparatus 
comprises a source of exposing radiation, such as a laser 10 which 
produces an exposing beam 12 of radiation. The laser 10 is preferably a 
high power laser which may have major bands in the visible, infrared, or 
ultraviolet regions of the radiation spectrum, the particular band choice 
being a function of the spectral sensitivity of the composition to be 
hardened from exposure to the radiation. The term "high power" is a 
relative term and depends to a great degree on the amount of radiant 
energy needed to harden a particular photoformable composition (i.e., the 
photospeed of the composition). With presently available photospeeds, 
"high power" is considered to be a power greater than 20 mW, and 
preferably over 100 mW as measured from the intensity of the beam 12. 
Focussing means, which focus the beam 12 onto an image plane 13, are 
available but omitted from FIG. 1 for the sake of clarity. 
In addition to a laser generated exposure beam 12, other sources of 
radiation may be used, such as Electron beams, X-rays, etc. again 
dependent on the spectral sensitivity of the hardenable composition that 
is exposed to such radiation. The beam 12 cross section typically has a 
generally circular shape with a Gaussian intensity distribution, as shown 
in FIG. 3, where "D" is the 1/e2 irradiance level point. 
The beam 12 passes through a modulator 14 which is preferably an 
acousto-optical modulator. The acousto-optical modulator varies the 
emerging beam intensity between an "on" state and an "off" state as herein 
above defined in response to an electronic control signal applied thereto. 
The beam is next directed through a beam deflection system which preferably 
comprises two orthogonal beam deflection devices 16 and 18. Devices 16 and 
18 are controlled by a beam deflection control means included in a control 
module 28, over lines 15 and 15'. The beam deflection system may comprise 
two mirrors rotating around two orthogonal axes mounted on two 
servo-controlled motors. The servo motors may be controlled with an 
electronic signal, such as a series of stepping pulses, to rotate the 
mirrors to position the beam at any point on the image plane 13, and may 
scan any shape line on such plane 13. A beam deflection system employing 
servo motors, useful for practicing this invention, was developed and sold 
by Greyhawk Systems Inc. of Milpitas, Calif. In the alternative, the 
mirrors may be mounted on galvanometers such that, by appropriate 
selection of an applied voltage to each of the two galvanometers, the beam 
may again be positioned at any point on the image plane 13, and may scan 
any shape line on such plane 13. 
The image plane 13 is located in a vat 20 which contains a radiation 
hardenable composition 22 and a movable support platform 26. The platform 
26 is positioned in the vat 20, relative to the laser beam deflection and 
modulation arrangement, so as to selectively present to the beam 12 at the 
image plane 13 photoformable composition 22 supported thereon. Means 24 
for raising or lowering the platform 26, in response to a control signal 
applied to the means 24 over line 25, are provided. The control module 28 
includes appropriate raising and lowering control means. 
The control module 28 may comprise a computer appropriately programmed to 
perform the functions of modulating and placing the laser beam on the 
image plane 13 as well as raising the platform to bring photoformable 
composition to the image plane 13. The computer must perform these 
functions using appropriate transducers, however, this is well known 
technology, not of significance to the present invention. 
Coating means, also not shown in FIG. 1 for clarity, are provided to permit 
formation on the platform 26 (or over an object on the platform) at the 
image plane 13, a layer of photoformable composition of uniform thickness 
and smooth surface. Again, such coating means are known in the art and not 
the subject of this invention. 
U.S. Pat. No. 5,006,364 issued Apr. 9, 1991, and U.S. Pat. No. 5,094,935 
issued Mar. 10, 1992, both assigned to E. I. du Pont de Nemours and 
Company, disclose photoformable compositions useful in producing 
multi-layer models using the solid imaging process and equipment similar 
to the equipment schematically shown in FIG. 1. Preferably, the 
photoformable composition is a liquid composition comprising: 
(a) 45 to 55% of a difunctional acrylated polyurethane oligomer, or a 
mixture of said oligomers; 
(b) 25 to 40% of a polyglycol ester, or a mixture of such esters, of the 
following formula: 
EQU H.sub.2 C.dbd.CH(CO)--O--(CH.sub.2 --CH.sub.2 --O).sub.4 
--(CO)HC.dbd.CH.sub.2 
(c) 4 to 6% of a photoinitiator system (or a mixture of photoinitiator 
systems) sensitive to actinic radiation such as 2,2-dimethoxy-2-phenyl 
acetophenone; and 
(d) 10 to 20% of a polyfunctional reactive diluent also such as an 
acrylate, or a mixture of reactive diluents, and wherein the liquid 
photoformable composition has a viscosity of 300 to 3000 cP at 25.degree. 
C. 
The beam 12 is modulated in the modulator 14. In the preferred embodiment 
the beam 12 is modulated three different ways. First, the beam 12 is 
submodulated by a variable repetition rate pulse train. The pulse 
occurrence is a function of the angular position of the beam-deflecting, 
stepper-motor-mounted mirrors, and calculated such that the exposure of 
the photoformable composition, that is the product of the radiation 
intensity and the time of application of the radiation at any point along 
a scanned line on the exposure plane 13, is constant. This is obtained by 
controlling the pulse repetition rate so that the distribution of exposing 
pulses on the image plane 13 as a function of distance from start to end, 
along a scanned line, is regular, as shown in FIG. 2. 
Control of the beam 12 in this manner is used to obtain constant exposure 
along the full length of an exposed image line. Because the beam 
deflection means do not have zero mass, acceleration and deceleration are 
not instantaneous, and will affect the exposure if left uncompensated. 
Thus, a non uniform exposure will result, if no correction is applied, 
because the scanning speed of the beam 12 is different at the start of 
each scan, as compared to the steady state along an exposed image line, 
reached after initial acceleration of the deflecting mirror is completed 
and operating scanning speed has been reached. The same problem appears at 
the end of a scanned line, where the mirror decelerates prior to stopping. 
In the preferred embodiment, an encoder associated with the 
stepper-motor-mounted mirrors is used to control the pulse rate of the 
submodulation signal. The encoder resolution is 1 arc second, and a signal 
adapted to submodulate the beam 12 on, for a given pulse time width, is 
generated every 2 arc seconds of mirror rotation. Since the beam 12 is a 
reflected beam, 2 arc seconds of mirror rotation rotates the beam 4 arc 
seconds. The radius of scan of the beam 12 from the mirror to the image 
plane 13 is nominally 51 inches, therefore, a pulse of light will be 
generated in the image regions approximately every 0.001" (one mil). 
Because the 1/e2 beam diameter "D" in the current system is approximately 
137 .mu.m (5.4 mils), there is substantial overlap of the substantially 
gaussian submodulated exposures. This overlap is enough to harden a line 
being exposed such that the line has substantially uniform thickness along 
its length, as shown in FIG. 2 by the second graph depicting depth of 
hardening as a function of surface spacing. 
A solid object is created by the sequential exposure of a plurality of 
contiguous overlapping layers, each having been imagewise exposed. The 
imagewise exposure of each layer is obtained by imagewise exposing 
photoformable composition at the image plane. This, in turn, is done by 
imagewise modulating the laser beam. FIG. 4 represents the modulating 
voltage amplitude "V" as a function of time "t" and illustrates the 
imagewise modulation of the beam. As shown in FIG. 4, the submodulated 
beam is further modulated by turning the beam on and off to generate two 
lines, line A-B (Beam "on") and line C-D, separated by a space B-C (Beam 
"off"). U.S. Pat. No. 5,014,207 issued May 7, 1991, assigned to E. I. du 
Pont de Nemours and Company, discusses in more detail this dual modulation 
exposure process. 
In accordance with the present invention, the layers are exposed twice. 
Preferably, all layers are created by exposing each layer twice. The first 
exposure is an imagewise exposure with the exposing submodulated radiation 
beam further modulated with image information and supermodulated with a 
preselected pulse signal, to produce a noncontinuous image of the pattern 
having a plurality of anchored, hardened image areas separated by image 
areas of partially hardened composition, resembling a series of dashes. In 
the present invention, it is important not only that the image areas 
between the hardened image areas be partially hardened, but also that the 
hardened image areas be anchored to the underlying contiguous layer, 
thereby prevent a "floating" effect during subsequent exposure. We have 
discovered that distortions in such layers, due to internal stresses 
during subsequent irradiation, are significantly reduced when the hardened 
image areas are anchored by this first exposure and, also, when such 
hardened image areas are separated by partially hardened areas, 
particularly in overhang regions where "anchoring" is not feasible. 
When this first imagewise exposure of the layer is completed, the layer is 
subjected to a second imagewise exposure, again using an imagewise 
modulated, submodulated radiation beam, but without the additional 
supermodulation which created the dashes in the first exposure. The second 
exposure is calculated to produce a hardened, continuous image. Thus, the 
image in each layer is produced in two steps, first as a dashed line 
image, then as a solid image. 
This process is explained in more detail with reference to FIGS. 4, 5, 6, 
and 7. The first exposure of a layer is with a beam modulated with a 
signal as shown in FIG. 5. This signal is the result of three different 
modulations of the beam. First, there is the submodulation of the signal 
which generates a series of short pulses P. Superimposed to these pulses, 
there is a signal representing the image information, i.e., a solid line 
A-B followed by an unexposed portion B-C, which is in turn followed by 
another line C-D. Submodulation pulses appear only in the solid line 
portions. 
An additional series of supermodulating pulses, e-f, g-h, i-j, etc. is 
superimposed resulting in an exposing beam comprising the sets S1, S2, S3, 
S4, S5, and S6, of submodulation pulses P, shown in FIG. 5. The result of 
the exposure of the hardenable composition to this set of pulses is to 
generate in the exposed layer at the image plane a series of disconnected 
hardened areas or islets 32, shaped like dashes along the exposure line 
30, separated by areas 34 of partially hardened composition, as shown in 
FIGS. 6 and 8. In FIG. 8, the hardened islets 32 are shown shaded by a 
uniform shade. However, it is understood that the degree of hardening is 
not uniform throughout the islets and, because of the gaussian shape of 
the exposing beam, there is a fall off on the exposure level near the 
edges of the islets, which is not shown in the figure. The frequency of 
the pulse sets must be calculated to assure that there is no hardened 
material bridging the space between the dashes, the areas 34 between the 
dashes containing at most only partially hardened composition. 
Preferably, the exposure level is calculated to harden the exposed material 
to a depth extending beyond the thickness of the layer to provide 
anchoring of the islets in the layer below, by a depth "d" as illustrated 
in FIG. 6. The size of "d" is not critical, and so long as the exposure 
will harden material to a depth greater than the layer thickness, such 
exposure will suffice. The aforementioned U.S. Pat. No. 5,014,207 teaches 
how to calculate the depth of photohardening as a function of beam 
irradiance. 
Following this first exposure, a second exposure of the same layer to the 
same imaging information is undertaken. The beam intensity is modulated as 
shown in FIG. 4, resulting in a set of submodulated pulses for the full 
length of all solid lines, A-B, C-D, etc. The result of this second 
exposure is to completely harden all exposed areas, bridging the portions 
between the previously created discontinuous islets or dashes, as shown in 
FIGS. 7 and 9. FIG. 9 shows a top view of the result of this second 
exposure where all material along scan lines 36 has been hardened. As 
shown, the hardened portion includes the islets 32 and the intermediate 
spaces 34. 
FIG. 10 shows a slight variation in this process, whereby the position of 
the islets 32 in adjacent scan lines 30 is offset so that islets 32 and 
32' are not exactly side by side. 
It has been observed that in some polymers shrinkage may continue more than 
a minute after exposure. In cases where such delay is observed, the second 
exposure, i.e., the exposure with the modulated beam, may be delayed 
following the exposure with the supermodulated beam by a period of time 
sufficient to allow for polymer post exposure shrinkage to occur. 
After the two imagewise exposures are completed, a new layer of unexposed 
material is coated over the exposed layer, and the process repeated for 
this new layer, and so on until a three dimensional object is completed 
using a plurality of contiguous imaged layers, each imaged layer 
representing a cross section of the object. 
The object is removed from the vat, the unexposed material removed from the 
object and, if so desired, the object is further hardened by exposing the 
object to flooding hardening radiation. 
In order to save overall exposure time, the scan line spacing for the first 
exposure scans may be different, i.e., larger, than that of the second, 
covering the same area in fewer scans. Thus for the first scan, the scan 
line spacing may be 0.005 inches, while for the second, it may be 0.002 
inches resulting in more uniform hardening of the image areas. 
As an example, using a liquid photoformable composition such as described 
hereinabove, and setting each layer to a thickness of 0.005 inches, during 
the first, supermodulated exposure, the submodulation pulse "P" time width 
is set at 1.56 sec. A coherent Argon Ion Model 326 laser producing a beam 
of radiation in the UV spectrum with primary lines of 351.1 nm and 363.8 
nm is used as the exposure source to generate an imaging beam having a 
generally circular cross section generating a circular spot with a 
Gaussian power distribution at the image plane. The power of the beam in 
the image plane is 225 mW, and the 1/e2 spot radius is 0.00685 cm. The 
length of the dash exposure (e-f, g-h, i-k, etc.) is set at 0.004 inches, 
the space between the dash exposures (f-g, h-i, j-k, etc. ) is 0.002 
inches, and the dash scan spacing (that is the distance from center to 
center, between adjacent beam sweeps) is 0.005 inches. 
For a 0.002 inch scan spacing and only submodulated exposure, this material 
has a depth of polymerization to exposure polynomial curve fit of 
approximately: 
EQU Depth (cm)=A+B[In(.SIGMA.E)]+C[In(.SIGMA.E)].sup.2 
where: 
A=-0.00314 
B=0.004243 
C=0.001325, and 
E=Exposure level in (mJ/cm.sup.2). 
By summing the adjacent exposures about each point in the image region for 
a 0.005 inch radius, we can calculate the maximum exposure (at the center 
of the dash) to be approximately 13.68 mJ/cm.sup.2. The maximum thickness 
(at the center of the dash) is 0.0067 inches. The anchoring or engagement 
of each dash is therefore 0.0017 inches. The spacing of the dashes is 
selected so that there is no bridging of hardened material between dashes 
but only unhardened or partially hardened material exists between dashes. 
This first exposure is followed by a second. In the second exposure the 
beam spot at the image plane is scanned with a 2 mil scan line spacing. 
The submodulation pulse time width is 0.88 sec. The laser power and laser 
spot size are unchanged. By summing the exposure, from the first dash scan 
and the second non-dashed scan, about a point, taking into account 
contributions of exposure from points within a 5 mil radius, the maximum, 
minimum, and average exposure is calculated to be 29.04 mJ/cm.sup.2, 18.62 
mJ/cm.sup.2, and 22.71 mJ/cm.sup.2, respectively. The resulting maximum, 
minimum, and average hardened thickness is calculated to be 0.010 inches, 
0.008 inches, and 0.009 inches. Since the nominal thickness of the coated 
layer is 0.005 inches, the material was hardened through its full 
thickness. 
In the above examples and discussion, the invention is described in an 
environment where a single object is produced by the exposing equipment at 
a time. It is often the practice in the industry to produce more than one 
object side by side on the platform in the vat using different imaging 
information and at times different scan spacings for each. It is also 
possible to produce multiple objects simultaneously while using different 
layer thicknesses for the various objects. In all of the above instances 
the present invention may still be used, and for implementing the 
invention each object will be considered as existing independently of the 
others, each being subjected to the two exposures in accordance with the 
teachings herein. 
An illustrative software embodiment used by the present invention is 
included in an Appendix A to this specification. The software program is 
written as a UNIX shell script for use in computer 28 or similar signal 
processor, for generating the first and second exposures, as well as the 
supermodulation signal which appears immediately before the claims. 
The effect obtained by the supermodulated beam was described in terms of 
supermodulation of the beam using radiation intensity (amplitude) 
modulation. However, the supermodulation effect can also be achieved by 
keeping the beam intensity constant and varying the scanning speed of the 
beam on the image plane. This can be achieved by for instance 
superimposing a sinusoidal type signal on the scanning mirror driving 
system, such that the beam oscillates as it moves along a scanning line. 
This oscillation results in portions on the scanning line receiving longer 
exposure while others receive shorter exposures. By properly selecting the 
oscillation frequency, the longer and shorter exposures can be calculated 
to be above and below an exposure threshold level for the photoformable 
material used, resulting in hardened and unhardened areas along the scan 
line. 
The system as described above is substantially a digital implementation of 
the process of this invention. However, the same first and second 
exposures can be implemented in an analog environment without difficulty. 
Those skilled in the art having the benefit of the teachings of the 
present invention as hereinabove set forth, can effect numerous 
modifications thereto. For instance, one may choose to eliminate the 
submodulation exposure and use a constant intensity beam modulated only 
with the imagewise modulation and the supermodulation signals in 
accordance with the present invention. One may also choose to expose, with 
the two imagewise exposures as taught herein, selected ones of the layers 
comprising the solid object rather than each and every layer. These and 
similar modifications are to be construed as being encompassed within the 
scope of the present invention as set forth in the appended claims.