U.S. patent application number 15/363520 was filed with the patent office on 2017-03-16 for imaging device.
The applicant listed for this patent is Landa Labs (2012) Ltd.. Invention is credited to Ofer Aknin, Benzion LANDA, Michael NAGLER, Nir RUBIN BEN HAIM.
Application Number | 20170075226 15/363520 |
Document ID | / |
Family ID | 56296864 |
Filed Date | 2017-03-16 |
United States Patent
Application |
20170075226 |
Kind Code |
A1 |
NAGLER; Michael ; et
al. |
March 16, 2017 |
Imaging Device
Abstract
An imaging device is disclosed for projecting individually
controllable laser beams onto an imaging surface that is movable
relative thereto in X-direction. The device includes a plurality of
semiconductor chips comprising a plurality of individually
controllable laser emitting elements arranged in a two dimensional
array of M rows and N columns. The chips are mounted on a support
in at least one pair of rows, such that each pair of adjacent chips
in Y-direction are offset from one another in the X-direction, and
the laser beams are substantially uniformly spaced in the
Y-direction. The chips are arranged such that corresponding
elements in any group of three adjacent chips in the X and
Y-directions lie at the apices of congruent equilateral triangles.
A plurality of GRIN rod based lens systems focuses the beams for
each of the chips onto the imaging surface.
Inventors: |
NAGLER; Michael; (Tel Aviv,
IL) ; RUBIN BEN HAIM; Nir; (Hod HaSharon, IL)
; Aknin; Ofer; (Petach Tikva, IL) ; LANDA;
Benzion; (Nes Ziona, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Landa Labs (2012) Ltd. |
Rehovot |
|
IL |
|
|
Family ID: |
56296864 |
Appl. No.: |
15/363520 |
Filed: |
November 29, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/IB2016/053137 |
May 27, 2016 |
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15363520 |
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PCT/IB2016/053138 |
May 27, 2016 |
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PCT/IB2016/053137 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B41J 2/45 20130101; G03G
15/04072 20130101; B41J 2/451 20130101; B41J 2/447 20130101; G03G
15/342 20130101; B41J 2/455 20130101; G03G 15/043 20130101; G03F
7/70025 20130101 |
International
Class: |
G03F 7/20 20060101
G03F007/20 |
Foreign Application Data
Date |
Code |
Application Number |
May 27, 2015 |
GB |
1509073.1 |
May 27, 2015 |
GB |
1509077.2 |
Claims
1. An imaging device for projecting individually controllable laser
beams onto an imaging surface that is movable relative thereto in a
reference X-direction, the device comprising: a support; a
plurality of semiconductor chips coupled to the support, each of
the chips comprising a plurality of individually controllable laser
beam emitting elements arranged in a two dimensional main array of
M rows and N columns, the emitting elements in each row having a
uniform spacing A.sub.r and the emitting elements in each column
having a uniform spacing a.sub.c; the chips being arranged in at
least one pair of rows such that the main array of each pair of
chips that are adjacent one another in a reference Y-direction,
transverse to the X-direction, are offset from one another in the
X-direction, and such that the center of each two Y-direction
adjacent laser beam emitting elements in the main arrays of both
chips in the pair be uniformly spaced from one another in the
Y-direction by a nominal distance A.sub.r/M, and wherein the
centers of laser beams emitting elements of both chips do not
overlap in the Y-direction; the chips within the two rows of the
pair being aligned such that corresponding elements in any group of
three adjacent chips in the X and Y-directions lie at the apices of
congruent equilateral triangles and the distance in the Y-direction
between corresponding elements equals nominally to NA.sub.r, where
A.sub.r is the nominal distance between the centers of any two
adjacent laser beam emitting elements in each row; and, a plurality
of lens systems, each serving to focus the laser beams of all the
laser elements of a respective one of the chips onto the imaging
surface, each lens system comprising at least one gradient index
(GRIN) rod.
2. An imaging device as claimed in claim 1, wherein each lens
system comprises a plurality of GRIN rods arranged in series with
one another.
3. An imaging device as claimed in claim 2, wherein the GRIN rods
of each lens system are inclined relative to one another to form a
folded light path, light from each GRIN rod being directed to the
next GRIN rod in the series by a reflecting element.
4. An imaging device as claimed in claim 2, wherein the GRIN rods
have a cylindrical surface and corresponding GRIN rods of different
lens systems associated with different chips are arranged in an
array of at least one pair of rows, and wherein the cylindrical
surfaces of the GRIN rods in each row of any pair of rods contact
one another and the cylindrical surface of each rod in each row
additionally contacts the cylindrical surfaces of the two adjacent
GRIN rods in the other row, the nominal distance between any two
adjacent GRIN rods centers being 2NA.sub.r.
5. An imaging device as claimed in claim 1, wherein each chip has
an equal number of rows and columns of laser beam emitting elements
in the main array.
6. An imaging device as claimed in claim 1, wherein the support is
fluid cooled.
7. An imaging device as claimed in claim 1, wherein the support is
constructed of a rigid metallic or ceramic structure.
8. An imaging device as claimed in claim 1, wherein the surface of
the support is formed of, or coated with, and electrical insulator
and thin film conductors are formed on the electrically insulating
surface to supply electrical signals and power to the chips.
9. An imaging device as claimed in claim 1, wherein each of the
plurality of lens systems has a magnification of absolute value of
1.
10. An imaging device as claimed in claim 1, wherein each of the
plurality of lens systems has a magnification of +1.
11. An imaging device as claimed in claim 1, wherein each of the
plurality of lens systems has a magnification of absolute value
greater than 1.
12. An imaging device as claimed in claim 1, wherein in addition to
the N columns of elements of the main array, each chip further
comprises at least one additional column, arranged at a side of the
array in the Y-direction, the additional column containing at least
one selectively operable laser beam emitting element having a
center that lies between the respective two sets of MN lines of the
pair of chips, for compensating for misalignment in the Y-direction
in the relative positioning of the respective adjacent pair of
chips on the support.
13. An imaging device as claimed in claim 1, wherein each
individually controllable laser beam element can emit a laser beam
having a plurality of selectable energy levels.
14. An imaging device as claimed in claim 1, wherein the laser beam
emitting elements are vertical cavity surface emitting laser
(VCSEL) elements.
15. An imaging device as claimed in claim 1, wherein the GRIN rods
have a cylindrical surface and corresponding GRIN rods of different
lens systems associated with different chips are arranged in an
array of at least one pair of rows, the cylindrical surfaces of the
GRIN rods in each row of any pair of rods contact one another and
the cylindrical surface of each rod in each row additionally
contacts the cylindrical surfaces of the two adjacent GRIN rods in
the other row, the GRIN rods having a diameter equal to
2NA.sub.r.
16. An imaging device as claimed in claim 15, wherein the GRIN rods
of each lens system are inclined relative to one another to form a
folded light path, light from each GRIN rod being directed to the
next GRIN rod of the same lens system by a reflecting element.
17. An imaging device as claimed in claim 1, wherein each
individually controllable laser beam element can emit a laser beam
having a controllable plurality of energy levels.
18. An imaging device for projecting individually controllable
laser beams onto an imaging surface that is movable relative
thereto in a reference X-direction, the device comprising: a
support; a plurality of semiconductor chips coupled to the support,
each of the chips comprises a plurality of individually
controllable laser beam emitting elements arranged in a two
dimensional main array of M rows and N columns, the elements in
each row having a uniform spacing A.sub.r and the elements in each
column having a uniform spacing a.sub.c, each individually
controllable laser beam element can emit a laser beam of a
controllable plurality of energy levels; wherein the chips are
mounted on the support such that the main arrays of each pair of
chips that are adjacent one another in a reference Y-direction,
transverse to the X-direction, are offset from one another in the
X-direction, wherein when the chips are nominally placed, were all
the laser emitting elements to be activated continuously and the
imaging surface and the imaging device to be relatively moved in
the X-direction, the emitted laser beams of the respective main
arrays of the two chips of the pair would trace on the imaging
surface 2MN parallel lines that extend in the X-direction and are
uniformly spaced from one another in the Y-direction by a nominal
distance A.sub.r/M, whereby the laser beams of each chip trace a
set of MN lines without overlapping the set of lines of the other
chip; the alignment of the chips within the at least one pair of
rows is such that corresponding elements in any group of three
adjacent chips in the X and Y-directions lie nominally at the
apices of congruent equilateral triangles; and, a plurality of lens
systems, each serving to focus the laser beams of all the laser
elements of a respective one of the chips onto the imaging surface,
each lens system comprising at least one gradient index (GRIN) rod;
wherein corresponding GRIN rods of different lens systems
associated with different chips are arranged in an array of at
least one pair of rows, the GRIN rods having cylindrical surface
and the cylindrical surfaces of the GRIN rods in each row of any
pair of rods contact one another and the cylindrical surface of
each rod in each row additionally contacts the cylindrical surfaces
of the two adjacent GRIN rods in the other row, the GRIN rods
having a diameter equal to 2NA.sub.r.
19. An imaging device as claimed in claim 18, wherein each lens
system comprises a plurality of GRIN rods arranged in series with
one another.
20. An imaging device as claimed in claim 18, wherein in addition
to the N columns of elements of the main array, each chip comprises
at least two additional columns, arranged at least one at each side
of the array, each additional column containing at least one
selectively operable laser beam emitting element capable of tracing
at least one additional line that lies between the two sets of MN
lines, for compensating for misalignment in the Y-direction in the
relative positioning of the respective adjacent pair of chips on
the support.
Description
RELATED APPLICATIONS
[0001] This Patent Application incorporates by reference in their
entirety International Patent Applications Nos. PCT/IB2016/053138
and PCT/IB2016/053137, filed on May 27, 2016, and GB Patent
Applications Nos. 1509073.1 and 1509077.2, filed on May 27,
2015.
FIELD
[0002] The present disclosure relates to an imaging device for
projecting a plurality of individually controllable laser beams
onto a surface that is movable relative to the imaging device.
BACKGROUND
[0003] U.S. Pat. No. 7,002,613 describes a digital printing system
to which the imaging device of the present disclosure is
applicable, by way of example. In particular, in FIG. 8 of the
latter patent specification, there is shown an imaging device
designated 84 that is believed to represent the closest prior art
to the present disclosure. The imaging device serves to project a
plurality of individually controllable laser beams onto a surface,
herein termed an imaging surface, to generate an energy image onto
that surface. The laser image can be used for a variety of
purposes, just a few examples being to produce a two dimensional
printed image on a substrate, as taught for instance in U.S. Pat.
No. 7,002,613, in 3D printing and in etching of an image onto any
surface.
[0004] For high throughput applications, such as commercial
printing or 3D lithography, the number of pixels to be imaged every
second is very high, demanding parallelism in the imaging device.
The laser imaging device of the present disclosure is intended for
applications that require energy beams of high power where the
total power required can be of tens or hundreds of milliwatt (mW).
For instance, in the field of printing, depending on the desired
printing speed, the energy beams can provide powers of up to 10 mW,
100 mW and even 250 mW or higher. One cannot therefore merely scan
the imaging surface with a single laser beam, so as to expose the
pixels sequentially. Instead, the imaging device is required to
have a plurality of laser emitting elements for various pixels
(picture elements) each laser capable of tracing a line of pixels
in the image area of an imaging surface in relative motion.
[0005] To achieve acceptable print quality, it is important to have
as high a pixel density as possible. A high resolution image, for
example one having 1200 dpi (dots per inch), requires a density of
laser emitting elements that is not achievable if the laser
emitting elements all lie in a straight line, due to the amount of
overlap necessary between the laser sources to achieve a uniform
printing quality. Aside from the fact that it is not physically
possible to achieve such a high packing density, adjacent elements
would interfere thermally with one another.
[0006] Semiconductor chips are known that emit beams of laser light
in an array of M rows and N columns. In U.S. Pat. No. 7,002,613 the
rows and columns are exactly perpendicular to each other but the
chips are mounted askew, in the manner shown in FIG. 1 of the
latter patent, so that each row can fill in the missing pixels of
the preceding row(s). In this way, such an array can achieve a high
resolution image but only over the width of the chip and such chips
cannot simply be mounted side by side if one is to achieve a
printed image without stripes along its length, because the chips
cannot have laser emitting elements positioned sufficiently close
to their lateral edges.
[0007] U.S. Pat. No. 7,002,613 avoids this problem by arranging
such chips in two rows, in the manner shown in FIG. 8 of the latter
patent. The chips in each row are staggered relative to the chips
in the other row of the pair so that each chip in one row scans the
gap left unscanned by the two adjacent chips in the other row.
[0008] U.S. Pat. No. 7,002,613 recognizes the requirement for beam
shaping of the laser beams emitted by the elements on the chips and
proposes the use of micro-optical components (acting on only one or
more laser beams of the VCSEL [Vertical Cavity Surface Emitting
Laser] bar) and/or macro-optical components (acting on all laser
beams of the VCSEL bar). In particular, arrays of micro-optical
components, such as microlens arrays, are proposed where the
spacing between the individual components corresponds to the
spacing of two laser emitters or a multiple thereof.
SUMMARY
[0009] In the present disclosure, there is disclosed an imaging
device for projecting individually controllable laser beams onto an
imaging surface that is movable relative thereto in a reference
X-direction, the device including a plurality of semiconductor
chips each of which comprises a plurality of individually
controllable laser beam emitting elements arranged in a two
dimensional main array of M rows and N columns (MN), the elements
in each row having a uniform spacing A.sub.r and the elements in
each column having a uniform spacing a.sub.c, wherein the chips are
mounted on a support in such a manner that when nominally placed,
each pair of chips that are adjacent one another in a reference
Y-direction, transverse to the X-direction, are offset from one
another in the X-direction, and such that the center of laser beam
emitting elements of the main MN emitting elements arrays of both
chips in the pair are uniformly spaced in the Y-direction by a
nominal distance A.sub.r/M, i.e. without overlap in the Y-direction
between the beam emitting elements of the adjacent chips. The
alignment of the chips within the pair of rows is such that the
respective centers of corresponding elements in any group of three
adjacent chips in the X and Y-directions lie nominally at the
apices of congruent equilateral triangles. The imaging device
further comprises a plurality of lens systems each serving to focus
the laser beams of all the laser elements of a respective one of
the chips onto the imaging surface without altering the separation
between the laser beams, each lens system comprising at least one
gradient index (GRIN) rod.
[0010] Stated differently, were all the laser emitting elements of
the pair of nominally placed adjacent chips to be activated
continuously, and the chips and imaging surface be in relative
motion in the X-direction, the emitted laser beams of the
respective main arrays of the two chips of the pair would trace on
the imaging surface a set of parallel lines that extend in the
X-direction and that are nominally uniformly spaced in the
Y-direction. The lines traceable by emitting elements of the first
chip would not interlace with the lines traceable by emitting
elements of the second chip.
[0011] It is convenient for the chips to be arranged in at least
one pair of rows on the support, with corresponding laser emitting
elements of all the chips in each of the two rows lying in line
with one another in the Y-direction. By "corresponding elements" it
is meant that the individual laser emitting elements of the MN main
array should occupy the same row and column positions within their
respective chips. It is advantageous for corresponding elements in
any group of three chips in the pair of rows that are adjacent one
another in the X and Y-directions to lie at the apices of congruent
equilateral triangles as described above. This arrangement
simplifies the construction of the lens system to focus the laser
beams onto the imaging surface.
[0012] As absolute alignment accuracy is expensive and often
impractical, it is important to realize that placement terms relate
to the desired positioning within certain tolerances that enables
satisfactory results from the imaging device. Therefore, the term
"nominally", should be construed to denote the desired spatial
relationship when the chips or other relevant elements are disposed
at their intended placing. However, different aspects of the
invention allow for displacements that diverge from that nominal
position within such tolerance, and for compensating for such
displacement. Similarly, when used to indicate spatial relationship
the term "beam" should be considered as relating primarily to the
center of the beam, unless otherwise indicated or clear from the
context. Thus by way of example the uniform spacing A.sub.r and
a.sub.c relate to the distance between the centers of the laser
beam emitting elements.
[0013] Assuming that the M rows and N columns of laser emitting
elements of the chip array do not include any elements that are
normally redundant, the spacing between the centers of adjacent
laser beams along the Y-direction, or equivalently adjacent lines
in the set of traced lines (assuming a nominal magnification of
|1|), will equal A.sub.r/M, namely the quotient of the spacing of
the adjacent elements in each row divided by the number of rows.
Furthermore, as no intentional overlap is provided between the
lines traced by any two adjacent chips, the total number of lines
traced by the two chips will equal 2MN, namely twice the product of
the number of rows and the number of columns in each chip, if the
chips have equal numbers of rows and columns respectively.
[0014] It is understood that for high throughput applications, such
imaging devices would require a relatively high number of chips,
each having multiple laser beam emitting elements arranged in
columns and rows. This creates challenges for the optic systems to
be associated with such multitude of laser elements, in particular
when precise and accurate transmission of the laser signal to the
imaging surface is desired (e.g., to achieve quality print in
printing systems).
[0015] Neither the micro-optical nor the macro-optical solution
proposed in U.S. Pat. No. 7,002,613 is practicable. In a lens
system comprising one lens per beam, achieving acceptable lens
quality and uniformity is problematic and correctly aligning the
micro-lenses with the laser emitting elements presents serious
difficulty. In any system using the same lens to focus multiple
laser beams, be they beams from the same chip of different chips,
because of the manner of emission of the beams, a single
conventional lens cannot focus all the beams onto a flat imaging
plane without introducing distortion, because beams located off
axis tend to be displaced laterally. The use of complex
multi-element lenses is also clearly not practicable. By contrast,
the use of GRIN rods as herein disclosed provides a practical
solution to the design of a suitable lens system.
[0016] The alignment of the chips within each pair(s) of rows in
the present disclosure is such that corresponding elements in any
group of three adjacent chips in the X and Y-directions nominally
lie at the apices of congruent equilateral triangles. In this case,
if the GRIN rods have a diameter equal to 2NA.sub.r, being the
distance between corresponding elements in adjacent chips in the
same row, the GRIN rods may more conveniently be arranged in at
least one pair of rows in such a manner that cylindrical surfaces
of the GRIN rods in each row of the pair contact one another and
the cylindrical surface of each lens in each row additionally
contacts the cylindrical surfaces of the two adjacent GRIN rods in
the other row of the pair. In such a configuration, construction of
the lens system is particularly simplified because simply stacking
the rods in their most compact configuration will automatically
ensure their correct alignment between the chips, thus a correct
alignment of each GRIN rod with their respective chips.
[0017] Notably, any arrangement where the GRIN rods of the lens
system adjacent the chips are arranged in a pair of rows such that
corresponding the centers of corresponding rods in any group of
three rods in the pair of rows that are adjacent one another in the
X and Y-directions lie at the apices of congruent equilateral
triangles would provide adequate arrangement according to this
aspect of the invention. Therefore, certain embodiments utilize
GRIN rods with a circular cross-section of diameter D, where
D=2A.sub.rN.
[0018] While the lens system may comprise a single GRIN rod
associated with each chip, it may alternatively comprise a
plurality of GRIN rods arranged in series with one another and
forming a folded light path where the fold is in the space where a
beam emitted by the laser elements is substantially individually
collimated. In folded light path embodiments, a reflecting member
such as a prism or mirror which is optionally common to all the
chips may serve to direct the laser beams from one GRIN rod element
to the next in each series. In such a folded light path
configuration, it is desirable for the reflecting member to be on a
facet of a folding prism made of a material, typically a glass,
having a higher refractive index than the highest refractive index
in the GRIN rods. The higher index of refraction of the prism will
limit the angular divergence of the collimated beams and allow
larger separation between the sequential GRIN rod segments. A
suitable light path folding prism can be for example a right angle
prism, the folding face of the prism being a reflecting surface.
Other types of reflecting members and folding angles can be used
depending on the geometry of the system and the direction to be
given to beams in the series.
[0019] It has been found particularly advantageous for all the
laser beams emitted by one chip to be focused on the imaging
surface by a common single lens, or a common set of lenses arranged
in series, having a magnification M.sub.o whose absolute value is
greater than or equal to one (1), however magnification lower than
one (1) is also explicitly considered. It was found to be even more
advantageous if the magnification M.sub.o was substantially equal
to +1, as that would ensure that the laser elements can be spaced
adequately on the chip even for high resolution systems. Stated
differently, the image of the array of laser elements on the
imaging surface (i.e. an array of dots) would have the same size as
the array on the chip, though it may be inverted with a
magnification of -1. Notably, even if a slight misalignment of the
lenses exists, such as GRIN rod (Gradient-Index) lenses, in the XY
plane perpendicular to the optical axis of the lens, the position
of the illuminated laser spot on the imaging surface will remain
unchanged, as it only depends on the position of the laser emitting
element on the laser array chip. The former elements can be
positioned with very high accuracy on every laser array chip using
standard semiconductor manufacturing techniques.
[0020] It should be noted that optical magnifications of -1 may
require more precise positioning and alignment of the GRIN rod
lenses.
[0021] It is convenient for the main array of each chip to have an
equal number of rows and columns of laser beam emitting elements
(i.e., M=N), as this minimizes the size of the lens system.
[0022] Within each chip, the separation between the laser elements
is desirably sufficiently great to minimize thermal interference
between adjacent laser emitting elements.
[0023] The support for the chip arrays may be fluid cooled to help
dissipate the heat that may be generated by the chips.
[0024] In certain embodiments, the support may be a rigid metallic
or ceramic structure and it may be formed of, or coated with, an
electrically insulating surface bearing film conductors to supply
electrical signals and power to the chips.
[0025] The chips in some embodiments are vertical cavity surface
emitting laser (VCSEL) chip arrays. Equivalently other types of
laser sources may be utilized, and the term VCSEL should be
construed as encompassing such laser sources.
[0026] In some embodiments, the intensity of the laser beam emitted
by each element may be adjustable either continuously (in an
analogue manner) or in discrete steps (digitally). In one
embodiment, the chips may include D/A converters so as to receive
digital control signals. In this way, the laser beam intensity may
be controllably adjusted in a plurality of discrete steps, such as
2, 4, 8, 16, 32, . . . 4096 and the like.
[0027] Clearly in operation the laser emitting elements are
switched on and off as needed to provide the required image on the
imaging surface, as continuous operation of all laser beams would
result in a substantially uniformly irradiated surface.
[0028] In a further aspect of the present disclosure, there is
provided a method of projecting individually controllable laser
beams onto an imaging surface that is movable relative to an
imaging device utilizing any of the embodiments of the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] Some embodiments of the imaging device are described herein
with reference to the accompanying drawings. The description,
together with the figures, makes apparent to a person having
ordinary skill in the art how the teachings of the disclosure may
be practiced, by way of non-limiting examples. The figures are for
the purpose of illustrative discussion and no attempt is made to
show structural details of an embodiment in more detail than is
necessary for a fundamental and enabling understanding of the
disclosure. For the sake of clarity and simplicity, some objects
depicted in the figures are not to scale.
[0030] In the Figures:
[0031] FIG. 1 is a schematic diagram of a digital printing system
utilizing an imaging device according to an embodiment of the
present disclosure;
[0032] FIG. 2 shows part of an imaging device comprising a set of
VCSEL chips mounted on a support;
[0033] FIG. 3 is a schematic representation of the laser emitting
elements of two VCSEL chips and the lines that they can trace on a
relatively moving imaging surface;
[0034] FIG. 4 is a schematic representation that demonstrates in
one pair of rows the alignment between the VCSEL chips and the GRIN
rods used as lenses to focus the emitted laser beams onto the
imaging surface;
[0035] FIG. 5A shows prior art proposals for correction of chip
misalignment;
[0036] FIG. 5B shows the manner in which an embodiment of the
invention compensates for chip misalignment;
[0037] FIG. 6 shows the energy profiles produced by the laser
elements at the ends of two adjacent arrays, to illustrate how a
single line can be traced using two laterally positioned laser
elements, there being shown for each array three elements of the
main array and one of the additional elements;
[0038] FIG. 7A is a similar energy diagram to FIG. 6 to show how
the energies of two adjacent laser elements of the main array can
be combined on the imaging surface to produce an additional dot
that does not fall on the center line of either of the laser
elements;
[0039] FIG. 7B shows the dot pattern on the imaging surface
produced by activating four laser elements of the main array in the
manner shown in FIG. 7A;
[0040] FIG. 8A shows how the dot pattern of FIG. 7B assists in
anti-aliasing;
[0041] FIG. 8B shows for comparison with FIG. 8A the jagged edge
that normally occurs when printing an oblique line; and
[0042] FIG. 9 shows an alternative lens system to that shown in
FIG. 1 that has a folded light path to permit more compact
packaging in a printing system.
DETAILED DESCRIPTION
[0043] The imaging device will be described herein mainly by
reference to its application in digital printing systems however
its use is not limited to this application, and different aspects
of the invention may be implemented to controllably project image
forming light beams onto any surface with relative motion between
the surface and the chips.
Overall Description of an Exemplary Printing System
[0044] FIG. 1 shows a drum 10 having an outer surface 12 that
serves as an imaging surface. As the drum rotates clockwise, as
represented by an arrow, it passes beneath a coating station 14
where it acquires a monolayer coating of fine particles. After
exiting the coating station 14, the imaging surface 12 passes
beneath an imaging device 15 of the present disclosure where
selected regions of the imaging surface 12 are exposed to laser
radiation which renders the particle coating on the selected
regions of the surface 12 tacky. Next, the imaging surface passes
through an impression station 19 where a substrate 20 is compressed
between the drum 10 and an impression cylinder 22. The pressure
applied at the impression station causes the selected regions of
the coating on the imaging surface 12 that have been rendered tacky
by exposure to laser radiation by the imaging device 15 in the
correspondingly termed imaging station to transfer from the imaging
surface 12 to the substrate 20.
[0045] The term "tacky" as used herein is intended to mean that the
irradiated particle coating is not necessarily tacky to the touch
but only that it is softened sufficiently to be able to adhere to
the surface of a substrate when pressed against it in the
impression station 19.
[0046] The regions on the imaging surface 12 corresponding to the
selected tacky areas transferred to the substrate 20 consequently
become exposed, being depleted by the transfer of particles. The
imaging surface 12 can then complete its cycle by returning to the
coating station 14 where a fresh monolayer particle coating is
applied only to the exposed regions from which the previously
applied particles were transferred to the substrate 20 in the
impression station 19.
[0047] Advantageously, a monolayer of particles facilitates the
targeted delivery of radiation as emitted by the laser elements of
an imaging device according to present teachings. This may ease the
control of the imaging device and process, as the selectively
irradiated particles reside on a single defined layer. When
considered for use in a printing system, an imaging device
targeting a monolayer can preferably focus the laser radiation to
form upon transfer to a substrate a dot of approximately even
thickness and/or relatively defined contour.
[0048] Reverting to the coating station 14, it may comprise a
plurality of spray heads 1401 that are aligned with each other
along the axis of the drum 10 and only one is therefore seen in the
section of FIG. 1. The sprays 1402 of the spray heads are confined
within a bell housing 1403, of which the lower rim 1404 is shaped
to conform closely to the imaging surface leaving only a narrow gap
between the bell housing 1403 and the drum 10. The spray heads 1401
are connected to a common supply rail 1405 which supplies to the
spray heads 1401 a pressurized fluid carrier (gaseous or liquid)
having suspended within it the fine particles to be used in coating
the imaging surface 12.
[0049] The imaging device 15 in FIG. 1 is composed of a support 16
carrying an array of chips each having an arrangement of
individually controlled laser sources capable of emitting laser
beams. In some embodiments, the laser beam emitting elements can
coherently emit light in a range of wavelengths from about 400 nm
to about 12 .mu.m, or up to about 10 .mu.m, or up to about 8 .mu.m,
or up to about 3 .mu.m, or up to about 1.4 .mu.m. Such ranges
includes regions generally known as Near Infra Red (NIR,
.about.0.75-1.4 .mu.m), Short-Wavelength Infra Red (SWIR,
.about.1.4-3 .mu.m), Mid-Wavelength Infra Red (MWIR), also called
Intermediate Infra Red (IIR, 3-8 .mu.m), and Long-Wavelength Infra
Red (LWIR, 8-15 .mu.m), also known as Thermal Infra Red (TIR). In a
particular embodiment, the laser beam emitting elements are NIR
lasers. The laser sources may by way of example, be of VCSEL
(Vertical Cavity Surface Emitting Laser) type, however other types
may be utilized. By way of example, semiconductor lasers
commercially available as laser diodes are capable of emitting at
wavelengths from 375 nm to 3,500 nm, covering most of NIR and SWIR
regions of the spectrum. Gas lasers can emit over various area of
the spectrum, depending on the elected gas and some optical design.
Commercial carbon dioxide (CO.sub.2) lasers, for instance, can emit
hundreds of watts in the thermal infrared region at 10.6 .mu.m.
While for brevity the term VCSEL is predominantly used herein, it
should be construed as encompassing any such laser sources which
may be better suited for certain embodiments. Each chip has
individually controllable laser beam emitting elements arranged in
a two dimensional main array of M rows and N columns (MN), the
elements in each row having a uniform spacing A.sub.r and the
elements in each column having a uniform spacing a.sub.c. As
disclosed below, at least one additional column may be provided.
Preferably, the chips can be individually or collectively
associated with an array of corresponding lenses 18 that focus the
laser beams on the imaging surface 12 is also used. FIGS. 2 to 4
provide more details of the chips 30 according to some embodiments
of the invention and on the manner in which they can be mounted on
the support and aligned with the lenses 18.
[0050] FIG. 2 shows a support 16 on which are mounted a plurality
of VCSEL chips 30 arranged in two rows in accurately predetermined
positions relative to one another, as will be described in more
detail by reference to FIGS. 3 and 4.
[0051] The support 16 is a rigid, and in some embodiments at least
partially hollow elongate body fitted with connectors 34 to allow a
cooling fluid to flow through its internal cavity. In some
embodiments, the body of the support may be made of an electrically
insulating material, such as a suitable ceramic, or it may be made
of a metal and at least its surface 36 on which the chips 30 are
mounted may be coated with an electrical insulator. This enables a
circuit board made of thin film conductors (partial and symbolic
depiction of the conductors is schematically shown to the
lower-right chip at FIG. 2) to be formed on the surface 36. The
chips 30 are soldered to contact pads on this circuit board and a
connector 32 projecting from the lower edge of the support 16
allows control and power signals to be applied to the chips 30. The
laser emitting elements 40 of each chip 30 are individually
addressable and are spaced apart sufficiently widely to minimize
thermal interference with one another.
[0052] In some embodiments, the individually controllable laser
elements of a chip can emit laser beams having variable energy that
is preferably digitally controllable in discrete steps, allowing
the laser intensity to be set at discrete levels such as 2, 4, 8,
16. . . and the like, and in some embodiments individual laser beam
sources may be controllably set to emit up to 4096 levels or more.
The lowermost level of energy is defined as 0, where the individual
laser element is not activated, the uppermost level of energy can
be defined as 1. The distinct intermediate levels therebetween may
be considered analogous in the field of printing to "grey levels",
each level providing for a gradually distinct intensity (e.g.,
shade when considering a colored output). Taking for instance, a
laser beam emitting element having 16 levels of activation, level 0
would result in lack of impression (e.g., leaving a substrate bare
or white if originally so) and level 1 would result in transfer of
a tacky film formed by a particle irradiated at maximum energy
(e.g., forming a full black dot in the event the particles are so
colored). In previous illustrative example, levels 1/16, 2/16, 3/16
and so on would correspond to increasingly stronger shades of grey,
comprised between white (0) and black (1). Typically, the energy
levels are evenly spaced.
[0053] In an alternative embodiment, the individually controllable
laser elements of a chip can emit laser beams having variable
energy that can be modulated in a continuous analogue manner.
[0054] Once a region of the imaging surface has reached a
temperature at which the particles become tacky, any further
increase in temperature will not have any effect on the transfer to
the substrate. However, it should also be noted that as the
intensity of the laser is increased the size of the dot that is
rendered tacky also increases.
[0055] The energy profile of each dot resembles the plots shown in
FIG. 6, that is to say that it is symmetrical with tapering sides.
The exact profile is not important as the distribution may be
Gaussian, sinusoidal or even an inverted V. In any such profile, as
the peak intensity increases, the base widens and the area of
intersection of the profile with a threshold at which the particle
coating is rendered tacky also increases in diameter. A consequence
of this energy distribution is that points of the imaging surface
that are not in alignment with the centerline of any one laser
emitting element will receive energy from adjacent elements. It is
possible for two nearby elements to be energized to below the level
needed to render coating particles on the centerline of the
elements tacky, yet for the cumulative energy in the region of
overlap between the two centerlines to rise above the level
necessary to render the coating particles tacky. In this way, it is
possible to create potential raster lines between the centerlines
of the laser lines in addition to, or as an alternative to, the
raster lines coinciding with the centerlines of the laser elements.
This ability to combine the energies from adjacent elements is used
to achieve different effects. These effects are dependent upon the
ability of the imaging surface to combine energies received from
different laser elements, even if there is a slight difference
between the times of irradiation.
[0056] FIG. 3 shows schematically, and to a much enlarged scale,
the relative positioning of two laser emitting element arrays 130a
and 130b of chips 30 that are adjacent one another in the
Y-direction but are located in different rows. Each of the chips
has a main array of M by N laser emitting elements 40, as
previously described, which are represented by circular dots. In
the example illustrated, M and N are equal, there being nine rows
and nine columns. The spacing between the elements in a row,
designated A.sub.r, and the spacing between the elements in a
column, designate a.sub.c, are shown as being different from one
another but they may be the same. The array is shown as being
slightly skewed so that the columns and rows are not perpendicular
to one another. Instead, the rows lie parallel to the Y-direction
while the columns are at a slight angle to the X-direction. This
enables lines, such as the lines 44, traced by the elements 40 on
the imaging surface, if energized continuously, to be sufficiently
close together to allow high resolution images to be printed. FIG.
3 shows that the element at the end of each row traces a line that
is a distance A.sub.r/M away from the line traced by the
corresponding element of each adjacent row, the separation between
these lines being the image resolution I.sub.r. Thus, assuming a
magnification of |1|, A.sub.r and M are selected in dependence upon
the desired image resolution, based on the equation
A.sub.r=MI.sub.r.
[0057] It should be mentioned that it is possible for the elements
to lie in a square array where the columns are perpendicular to the
rows. In this case, the chips would need to be mounted askew on
their support and compensation would need to be applied to the
timing of the control signals used to energize the individual
elements.
[0058] As is clear from FIG. 3, and also FIG. 5B which shows the
traced lines to a larger scale, the positioning of the array 130b
is such that the line traced by its bottom left element 40 should
ideally also be spaced from the line traced by the top right
element of the array 130a by a distance equal to A.sub.r/M.
Therefore when all the elements 40 of both arrays 130a and 130b are
energized, they will trace 2MN lines that will all be evenly spaced
apart by a distance A.sub.r/M between adjacent lines, without any
gaps.
[0059] If one wishes to provide compensation for defective
elements, the array could include additional rows of laser emitting
elements 40, but it is alternatively possible to compensate for a
defective element by increasing the intensity of the laser beams
generated by the laser emitting elements that trace the two
adjacent parallel lines.
[0060] Optionally, in addition to the M by N array of elements 40,
each chip has at least one additional column that is arranged along
the Y-direction on the side of the main array, the additional
column containing at least one laser beam emitting element 42.
These further elements 42 are represented in FIG. 3 by stars, to
distinguish them from the main array elements 40. As seen in FIG.
4, in some embodiments at least two such additional columns each of
one element 42 are provided, at least one column disposed in
Y-direction on each side of the main N by M array. The additional
laser elements of the additional columns on one or both sides of
each main array can be respectively positioned at a distance of 1/2
or 1/3 the spacing between traced lines that can be imaged by the
lenses onto the imaging surface. Furthermore additional elements
could be placed in the gap between two arrays that nominally spans
a distance of A.sub.r/M so that higher sensitivity is achieved in
correcting the spacing errors between adjacent arrays.
[0061] Any additional element 42 of an additional column can be
positioned in the column at any desired distance from the edge
element of the main array, the distance in the Y-direction
depending on the total numbers of additional elements/additional
columns each two sets of main arrays of a pair of chips to be
aligned would bound. Assuming n additional elements 42 between a
first and second main array, n being a positive integer number,
each additional element can be spaced from the edge element of the
main arrays or from one another in the Y-direction by a distance
equal to A.sub.r/(n+1), namely the spacing of the adjacent elements
in each row divided by one more than the number of additional
elements in the gap. Considering now the X-direction, the
additional elements can either be aligned with a row of elements of
their respective main arrays or positioned at any desired
intermediate position above or below such rows. Preferably the
positioning of an additional element 42 with respect to adjacent
elements of the main array shall minimize thermal interference.
Notably, the additional element or elements may be disposed at any
position along the X-direction of the chip.
[0062] In practice n elements 42 positioned in any of the
additional columns on one or both sides of the main array, can
correct for alignment errors of up to about a 1/(n+1) of the
nominal spacing between the edge elements of two adjacent chips.
If, by way of example, the edge elements of the two chips are at a
distance of 20 um (micrometers) in the Y-direction, and there is a
single additional laser emitting element on adjacent sides of each
array, such elements may correct a spacing error of up to about one
third of the nominal spacing, in the exemplified case approximately
7 .mu.m. Any positional deviation from the desired position on the
chip (e.g., with respect to its edges) or nominal distance between
elements not exceeding 10%, is considered within tolerances,
however in most cases due to the high precision of the
semiconductor manufacturing methods, such errors are unlikely.
[0063] As can be seen from FIG. 3 and FIG. 5B, when activated,
these elements 42 trace two additional lines 46 between the two
sets of evenly spaces parallel lines 44a and 44b traced by the
elements 40 of the two arrays 130a and 130b, respectively.
[0064] While the two additional elements 42 in the present
embodiment are shown in FIG. 3 and FIG. 5B as tracing two separate
lines 46, the energies of these two elements can be combined on the
imaging surface, as earlier described, to form a single line of
which the position is controllable by appropriate setting of the
energies emitted by each of the additional elements 42. This is
shown in FIG. 6 in which the energy profiles of the lines 44a and
44b are designated 94a and 94b, respectively and the energy
profiles of the additional lines 46 are designated 96a and 96b. In
FIG. 6, neither of the profiles 96a and 96b (shown in dotted lines)
has sufficient energy to render the coating particles tacky but at
the centerline between the two arrays the cumulative energy, shown
as a solid dark line 96, is sufficient to soften the particles
coating and to create a trace line filling the gap between the
trace lines 44a and 44b of the two main arrays. While in FIG. 6 the
energy profiles of the two additional elements are matched, it is
possible by varying the relative intensity of the two beams emitted
by the additional laser sources to position the centerline of the
combined energy at a different distance from the traces of the main
arrays.
[0065] FIG. 7A shows how the ability to create dots that do not
fall on the centerlines of the energy profiles of the laser
elements can be used to advantage to achieve anti-aliasing. FIG. 7A
shows the energy profiles of four adjacent elements of the main
array. The first two profiles a and b are set at a desired level,
say 8 (out of sixteen), corresponding to mid-grey. The energy
profiles c and d, on the other hand are set to say 12 and 4,
respectively. The resulting dot pattern produced on the imaging
surface is shown in FIG. 7B. This can be seen to comprise two
regular sized dots A and B aligned with the line of symmetry of the
profiles a and b in FIG. 7A, a larger sized dot C aligned with the
centerline of energy profile c, and a smaller dot D that lies
somewhere between the centerlines of the profiles c and d.
[0066] The result of repeating such a dot pattern diagonally is
shown in FIG. 8A. When this image is compared with FIG. 8B, where
no anti-aliasing steps have been taken, it will be seen that the
small dots in between regular raster line yield oblique edges that
have reduced jaggedness and produce an image that is comparable
with one achievable by a printing system having a greater image
resolution.
[0067] The interaction of energies from nearby laser elements can
also be used to compensate for missing or inoperative elements in
that the elements producing the two adjacent raster lines can be
used to combined in the same manner as previously explained to fill
in a gap between them.
[0068] For the arrays 130a and 130b in FIG. 3 to function correctly
as described above, their relative position in the Y-direction is
very important. In order to simplify the construction of the lens
system serving to focus the emitted laser beams on the imaging
surface it is advantageous to adopt a configuration shown in FIG. 4
which enables the two rows of lenses corresponding to a pair of
chip rows to be self-aligning.
[0069] FIG. 4 shows arrays of seven adjacent chips 130 each shown
lined up with a respective lens 18. Additional laser elements 42,
on each side of the main array of each chip, are also schematically
illustrated in the figure. Each lens 18 is constructed as a GRIN
(Gradient-Index) rod, this being a known type of lens that is
shaped as a cylinder having a radially graduated refractive index.
In the case of the geometry shown in FIG. 4, the respective centers
of corresponding elements of any three bi-directionally adjacent
chip arrays 130 lie nominally on the apices of an equilateral
triangle, three such triangles designated 50 being shown in the
drawing. It will be noted that all the triangles 50 are congruent.
As a result, if the diameter of the GRIN rods is now selected to be
equal to 2NA.sub.r, which is the length of the sides of the
equilateral triangles 50, or the distance between corresponding
laser emitting elements of adjacent VCSEL chips 30 in the same row,
then when stacked in their most compact configurations, after
aligning the lens array to the Y-direction over the chips, the
lenses 18 will automatically align correctly with their respective
chip. For such construction, the relationship between the rod lens
diameter D, the image resolution I, and the size of the matrix of
laser elements is: D=2I.sub.rMN where I, is the spacing in the
Y-direction between adjacent lines traceable in the X-direction and
M is the number of rows and N the number of columns in the main MN
array, assuming absolute magnification value of |1|.
[0070] Though the lens 18 has been schematically illustrated in
FIG. 1 (side view) and FIG. 4 (cross section view) as being an
individual GRIN rod, in alternative embodiments the laser beams of
each chip can be transmitted by a series of lenses. In the
simplified embodiment shown in FIG. 9, the single GRIN rod 18 is
replaced by two mutually inclined GRIN rods 18a and 18b and the
light from one is directed to the other by a reflecting member
which in the example of FIG. 9 is embodied by a prism 87 of high
refractive index glass, so that the light follows a folded path. It
is noted that other reflecting members such as mirrors and the like
may be utilized. Such a configuration enables coating stations in a
colour printing system to be arranged closer to one another in a
more compact configuration. Such a folded light path can adopt
different configurations while fulfilling all the requirements of
magnification and light transmission. To enable the light path to
be split in this manner, the length of the GRIN rods is preferably
selected such that light beams are individually collimated on
leaving the rods 18a and entering the rods 18b as shown by the
light rays drawn in FIG. 9.
[0071] The radiation guided by GRIN rod 18a, the proximal end of
which is arranged at a distance WD.sub.o from the chip, may be
captured by the corresponding GRIN rod 18b which can collect the
collimated light emerging from rod 18a on the same light path and
focus it at a distance WD.sub.r from the distal end of the second
GRIN rod 18b. When the two GRIN rods are made of the same material
and the same radial gradient profile and WD.sub.o=WD.sub.i a
magnification of M.sub.o=+1 or -1 can be obtained.
[0072] Laser elements that are away from the longitudinal axis of
the GRIN rod 18a will leave the distal end of the GRIN lens
collimated but at an angle to the axis. In certain cases, it is
necessary for the distance between the two rods 18a and 18b to be
large, causing the off axis collimated beams exiting the first rod
segment to miss partially or entirely the second segment. Some
embodiments the invention take advantage of Snell's law by causing
the beam exiting the first rod to travel through a material with a
high refractive index, thus causing the angle the collimated beam
makes with the optical axis to decrease and enabling a larger
separation between the rods 18a and 18b before the collimated beams
leaving the first rod miss the entrance to the second rod.
[0073] Notably, with straight or folded path light paths, the
magnification should be considered substantially equal to its
nominal value if within .+-.0.5% or even 1% or 2%.
[0074] Laser elements that are away from the longitudinal axis of
the GRIN rod 18a will leave the distal end of the GRIN lens
collimated but at an angle to the axis. In certain cases, it is
necessary for the distance between the two rods 18a and 18b to be
large, causing the off axis collimated beams exiting the first rod
segment to miss partially or entirely the second segment. It is
possible to take advantage of Snell's law and cause the beam
exiting the first rod to travel through a glass with a high
refractive index, thus causing the angle the collimated beam makes
with the optical axis to decrease and enabling a larger separation
between the rods before the collimated beams leaving the first rod
miss the entrance to the second rod.
[0075] In the description and claims of the present disclosure,
each of the verbs, "comprise" "include" and "have", and conjugates
thereof, are used to indicate that the object or objects of the
verb are not necessarily a complete listing of members, components,
elements, steps or parts of the subject or subjects of the
verb.
[0076] As used herein, the singular form "a", "an" and "the"
include plural references and mean "at least one" or "one or more"
unless the context clearly dictates otherwise.
[0077] Positional or motional terms such as "upper", "lower",
"right", "left", "bottom", "below", "lowered", "low", "top",
"above", "elevated", "high", "vertical", "horizontal", "backward",
"forward", "upstream" and "downstream", as well as grammatical
variations thereof, may be used herein for exemplary purposes only,
to illustrate the relative positioning, placement or displacement
of certain components, to indicate a first and a second component
in present illustrations or to do both. Such terms do not
necessarily indicate that, for example, a "bottom" component is
below a "top" component, as such directions, components or both may
be flipped, rotated, moved in space, placed in a diagonal
orientation or position, placed horizontally or vertically, or
similarly modified.
[0078] Unless otherwise stated, the use of the expression "and/or"
between the last two members of a list of options for selection
indicates that a selection of one or more of the listed options is
appropriate and may be made.
[0079] In the disclosure, unless otherwise stated, adjectives such
as "substantially" and "about" that modify a condition or
relationship characteristic of a feature or features of an
embodiment of the present technology, are to be understood to mean
that the condition or characteristic is defined to within
tolerances that are acceptable for operation of the embodiment for
an application for which it is intended. For instance, each two
adjacent elements of the group of elements under consideration
(such as by way of example of a chip row, of a chip column, or of
adjacent chip arrays, when applicable) are considered
"substantially uniformly spaced" if the deviation of each pair of
adjacent elements from a desired nominal distance does not exceed
10% of this predetermined spacing. Pairs of adjacent elements
deviating from the nominal distance by less than 5%, 4%, 3%, 2% or
1% are further considered "substantially uniformly spaced" or
"having a substantially uniform spacing". By way of example,
assuming a desired A.sub.r=20 micrometers, and the desired nominal
spacing in the Y-direction between corresponding main array laser
emitting elements in two adjacent chips equals A.sub.rN, spacing
deviations resulting from manufacturing tolerance of no more than 2
.mu.m, are considered to fall within the nominal spacing. Clearly,
smaller or no deviations are desired.
[0080] While this disclosure has been described in terms of certain
embodiments and generally associated methods, alterations and
permutations of the embodiments and methods will be apparent to
those skilled in the art. The present disclosure is to be
understood as not limited by the specific embodiments described
herein.
* * * * *