Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
This invention relates generally to optical scanners
and more particularly to an optical scanner for creating a
flying spot linear trace of a beam of laser light. The inven-
tion finds particular use in t~ i~1d of laser beam scanners
as are used for r~ading info.rmation from a copy boand and
directly transferr mg the rea~ information for the exposure of
photosensitive surfaces as in the pr.oduction of printing plates.
In Canc~dian application Serial No. 243,825 fi.led
Janua~y 20, 1976, Richard E. Amtcwer, there is show.n an
apparatus for producin~ an exposed photo plate rom a copy
board paste-up~ A l.aser scanning system having a read laser
beam is focused to a spot scanned across the copy board in
a predeterm med pattern, such as a raster-like scan, the
reflection from the copy board being sensed, read and used
to control the intensity of a second laser beam via a
mcdulator~ The second laser beclm impinges upon and scans
a photosensi~ive sur~ace~ The read lase bec~m and the
write laser beam are cc~bined and passed through deflection
optics, and the tw~ beams are subsequ~ntly separated to
impinge ~on and be fo~used at the copy board and photo-
sensit:ive surface, respectively. Tn -this way there is a
resultc~nt exposure of the photosensitive surface in accord-
ance with the copy. As shown in Serial NoO 243,825, the
scannin~ optics employed u-tilizes a moving mirror galvano~
meter, with both the rec~d and write laser bec~ms being aligned
cl~l superimposed upon each other through suitable beam
comb ~ optics ~or being:passed th~ough the galvanometer
simul neously ~nd subssquently separated by suitable beam
~30 : deflecticn optics~to the respective planes. Ano~her optical
system shown:ther2in employs a polygonal scanning wheel . -
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hav~ng a plurality Oe surlsces psrsllel to ths aris of ~ ~ ~
rotation of the wheel, ~ith the surfaces serving to scan the read and write
beams through an angle, thereby creating a flying spot scan.
In United States Patent 4,081,842 March 28, 197B, Steven K.
Harbaugh et al there is disclosed a variation o~ laser read~write apparatus
in which a facsimile system is developed. As disclosed therein, a duplica-
tion of read and write equipment at separate locations can ke coordinated
to form a facsimile transmlssion system. At the read st~tion an optical
scc~nner scans the input copy ~ith the scanning spot and the reflected light
produces a video read da~a signal, a portion of which is directed through
a spatial mask to pr~vide a transmitter video reference which gates a video
read data before transmission. In the receiver, a second optical scanner
~f similar construction is controlled by a video writ~ data signal. The
-~ video write data si~nal gates a scanning spot of exposure laser keam light
on and off to expose the output photosensitive copy surface at the receiver.
Additianally, the scanning light is debected through a further spatial mask
to provide a receiver video reference signal utilized to form a video write
signal. The spatial masks in the transmitter and receiver have a kncwn
relationship, e.g., so that the scannLng of the output copy in the receiver
can be spatially synchronized with the scannm g of the inpu~ copy in the
2Q transmitter. As therein disclosed, each of the scanning op~ics includes a~galvanom~ter-operated mirror for scanning the incident laser keam kac~ and
forth,through a horizontal angl~.
l~e foregoing instruments as disclosed in the cross-reEerenced
~pplication employ a field-flatteniny lens for causing the beam provided
from the sca~ming device to be focused at the pl~le oE the copy boarl and
photosensitive surface res~ tively, and are known thereEore as flat bed
sc~nners. The scc~ning optics, hcwever, are subject to a number oE errors
which de~rade the perform3n~ of the system. In a polygonal drum scc~nLng
~ design, ~ery close~tolerano~s are requii0d during the manufacturing processes
~ so as to o~trol ~acet-to tacet tilt~ A~y error Ln facet ~Eacet orienta-
`~ tion, together
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with bearing run-out errors and the lik~, contribute to
produce an angular or positional error component normal
to the scan line. This error has come to be known as
"wobble" or vertical errox. In addition, the scan ef-
ficiency of a polygonal drum ~canning system is limited
to about 50 percent. Accordingly, the polygonal design
is expensive to produce due to ~he tolerances required,
and the facet-to-facet error h~s to be removed by ~ome
suitable means, termed a "dewobbler".
In a resonant or oscillating galvanom2ter scan-
ner, the mirror pivots in a sinusoidal manner, and only
the center portion of the scan is linear enough to be
utilized. This results in a scan efficiency of approxi-
mately 50~ with a 25% deviation in exposure or scan vel-
ocity. However, it is necessary to scan back and orth
in opposing directions in order to maintain this effici-
ency level. Such scanning requires lag compensation
which is accomplished by deviating the read beam from
its normal course as a function of system time delays
and scan velocity. Such compensation adds to the cost
and complexity of the system and in many instances is
only partially e~fective. In addition, if multiple ma-
chines are to communicate in a facsimile system, a great
deal of calibration of each machine is required to norm~
alize ~he amount of 12g produced in each machine. Lag
errors and other errors in the facsimile process when
scanning in both directions, result in left writing and
right writing images that are no longer superimposed, re-
sult.ing in severe image deyradatioll for even small errors.
Furthex, at the higher speeds particularly associated with
facsimile systems, the scanner requirements exceed the
capabilities of a galvanometer mirror system because of
the high torque to which the mirror and its support struc-
ture are subject~d.
Other existing systems utili~e cylindrically
curved fields but are also limited in scan efficiency.
For example, in one ~uch system using a spinner-type
scanner in a cylindrical configuration, one scan is ac-
complished for each rotation of the scanning device.
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With the exposure times commonly associated with a
standard printing format, extremely high rotational
speeds are required, and synchronization of facsimile
versions is difficult~ Furthermore, ~uch curved field
systems xequire that the exposure surface be adaptable
to a curved conformation which is often incompa~ible
with printing plate production.
Ideally, a 6canning system should provide a
high scan efficiency, a scanning operation in a single
direction 50 as to eliminate the problem of lag, and a
constant scanning velocity so as to reduce the cost ~f
the associated electronics. In addition, the system
should be ree of vertical error or wobble and should
be entirely reflective so as to above ab~rra~ion errors
caused by the read and write beam frequencies being at
different portions of the spectrum. Additionally, such
a scanning system should be compatible with flat field
optics so that the resulting flying spot scan can read
copy and expose plates lying on plane surfaces.
Objects and Summary of the Invention
In general it is an object of the invention
to provide a laser beam optical scanning apparatus which
will overcom~ the above limitations and disadvantages
and supply error-free scanning within the foregoing guide-
lines.
It is a further object of the invention to pro-
vide an optical scanning apparatus o the above character
which utilizes a rotating element and provides a resultant
scan which is free of vertical error or ~obble, which is
compatible with ~lat fie~d scanning, and which simultan-
eously has a substantially uniform scan velocity and high
scan efficiency while operatiny in a single dixection of
scan.
It is a further object of the invention to pro-
vide a laser beam optical scanning apparatus of the above
chaxacter which is inherently adaptable to extremely high
scanning ~peeds.
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Ano~her object of the invention is to provide
a~ optical scanner of the.abo~e character which is read-
ily adapted to synchronous ~acsimile ~peration.
A urthex object of the invention is to provide
a scanner of the above character which is designed for
multiple-beam read/write operation utilizing beams of dif~
ferent frequencies without introducing chromatic ~berra-
tion.
The.se and other objects are achieved in accord-
ance with the invention by providing a pyramidal mirror
having a plurality of reflective surfaces inclined at an
acute angle relative to a plane perpendicular to the axis
of the mirror. The mirror is rotated about its axis to
move the reflective surfaces successively through the
path of a beam to provide a varying deflection of the
beam from each successive surf~ce of the ~irror as that
surface moves through the path and presents a varying an-
gle of incidence to the beam, and optical means such as
a roof mirror doublet receives the deflected beam from
each successive ~urface and returns an inverted image of
the beam to the æame surface for further reflection by
that surface along an output path. As ea~h segment ro-
tates through the beam path, its varying angular orienta-
tion introduces horizontal and vertical components of an-
gular de~iation into the beam, with the horizontal angular
component being doubled upon the second (output) reflec-
tion from the rotating segment while the vertical compon-
ent i~ cancelled by the inversion provided by the roof
mirror.
In a system for scanning reading and writing
suraces wi~h laser beams, the beams are combined and the
combined beam .is deflected by the pyramidal mirror and
the roof mirror doublet to produce the desired scanning
action, following which the beams are separated and dir-
ected to the~respective readlng and writing surfaces.
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In accordance with ~he inv~ntion there is provided in a scanning
apparatus for causing a laser beam ~o scan a line at an output plane in space:
first mirror means forming a first planar reflective surface having a surface
vector lying in a plane common to said beam, second mirror means forming a
second planar reflective surface having its surface vector lying in the common
plane, said irst and second mirror means being disposed relative to each
other to form a reflective doublet about a line perpendicular to said common
plane, a pyramidal mirror having a plurality of reflective segments inclined
; at an acute angle relative to a plane perpendicular to the axis of the mirror,
and means Eor rotating said mirror about its axis to move said segments suc-
cessively through the path of the beam so that the beam is reflected from the
segment in the path to the doublet and back to the same segment or further
reflection by that segment along an output path with a varying component of
angular deviation perpendicular to the common plane and substantially no com-
ponent of angular deviation parallel to the common plane.
: In accordance with another aspect of the invention there is provided
in scanning apparatus: a pyramidal mirror having a plurality of reflective
surfaces inclined at an acute angle relative to a plane perpendicular to the
axis of the mirror, means for rotating the mirror about its axis to move the
: 20 re1ective surfaces successively through the path Gf a beam to provide a vary- `
~; ing deflection of the beam from each successive surface of the mirror as that
surface moves through the path and presents a vary:ing angle of inc.idcnce to
the beam, said deflection having components along first and second axes per-
~ pendicular to the mirror axis and to each other, and optical means for re-
ceiving the deflect;d beam from each successive surface and returning an image
of the beam inverted about the first axis to the same surface for further re
flection by that surface along an output path with a varying component of
angular deviation a.long the first axis and substantially no component of angu-
:. lar deviation ai~ong the second axis.
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Brief Descri~tion of_the Drawin~
Figure 1 is a diagrammatic perspective view of
one embodiment of laser beam optical sc~nniny apparatus
constructed .in accordance with the inven~ion for reading
a copy and exposing a photosensitive plate.
Figure 2 is an elevational view, partly in cross-
section, of the scanner assembly of the apparatus of Fig-
ure 1, taken generally along the line 2-2 thereof.
Figure 3 is a top plan view taken along the line
3-3 of Fiyure 2.
Figure 4 is a front face view of ~he scanning
wheel of the apparatus of Figure 1, taken along the line
4 4 of Figure 3.
Figure 5 is a ~op view of a roof doublet mirror
assembly of the scanner of Figure 1, taken along the line
5-5 of Figure 2.
Figure 6 is a cro~s-sPctional view of the upper
mirror of the doublet mirror asse~bly, taken along the line
6-6 of Figure 5.
Figure 7 is a cross sectional view taken along
the line 6 6 of Figure 5.
Figure 8 is a front, or input, view of the doub-
let mirror assembly taken along the line 8-8 of Figure 5.
: Figure g is a diagrammatic view illustrating the
principle by whi~h vertical angular deviation is removed
. from the beam as it passes through the roof mirror double~
assembly~
:~ Figure 10 is a perspective diagrammatic view il
~ lustrating the scanner portion of the invention and show-
; ing a ray trace of the principal beam path therethrough at
an intermediate angle of orientat:ion of the scanner whePl.
E'igura llA is a front view of the scanner wheel
of Figure 1 lllustratirlg a beam impinging on one segment of
~he wheel when that segment is at the mid-position of its
travel through the input beam path.
Figure llB shows a beam trace in a ver~ical plane
taken along the line llB-llB of Figure llA.
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Figure llC shows a ~op plan r or horizon~al plane,
ray trace of the beam of Figure llA taken along the line
llC llC thereof.
Figure 12A is a front view of the scanner wheel
of Figure l illuætrating a beam impinging ~n one segment
of the wheel when that segment is at an interm diate posi-
tion in its travel through the input beam path~
~ Figure 12B shows a beam trace in a vertical plane
taken along the line 12B 12B o~ Figure 12A.
Figure 12C shows a top plan, or hori20ntal plane,
ray trace of the beam of Figure 12A taken along the line
12C-l~C thereof.
Figure 13A i5 a front view of the scanner wheel
of Figure 1 illustxa~ing a beam impinging on one segment
of the wheel when that segment is near the end of its trav-
el through ~he input beam path.
Figure 13B shows a beam trace in-a vertical plane
taken along the line 13B-13B of ~igure 13A.
Figure 13C shows a top plan, or horizontal plane,
ray trace of the beam of Figure 13A taken along the line
13C-13C thereof~
Detailed Description of the Prefexred Embodiment
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,~ Referring now to Figures 1 and 2 there is shown
a laser read/write system constructed in accordance with
the invention which includes a~station 20 definin~ a sup-
port ~or an exposure or write platen 22 and another sta-
tion 24 defining a support for a read platen 26. The ex-
posure platen receives a pho~osensitive plate at 22 which
will be scanned by the apparatus to be described and there-
by exposed for ~ubsequent development into a printing plate.
The copy to b~ read i~ positioned on the read platen 26. A
laser beam station 30 is provided, the output of which is
directed through a scanning system 32 and redirected there-
by~to~cause~wrlte beam 34 and read beam 36 to ~can across
the respective platens. The scanning system 32 includes a
table 38 supported on a linear transport mechanism includ
ing~parallel guides 40 engaged in a prede~ermined direction,
~; ~ as indica~e~ at 48. The table is driven by a lead screw 44
and~ro~ary motor drive 46 which may conveniently be disposed
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on a suitable apparatus framework (not shownj so that
the laser beam station and platens remain subskantially
fixed in space while the acanni~g ~able moves along the
direction indicated at 48.
The table carrîes a horizontal sc~nning sub-
system 50 (Figure 2) constructed in accordance with the
invention which shifts the beam fxom side to side ~hori-
zontally) as the table is carried forward to thereby
develop raster scans 52,54 of the laser beams across
both the read and exposure platens.
Means i~ provided or generating the read laser
beam 36 and consists of a helium-neon ~He/Ne) laser 58
having an output at 6,328 angstroms i~ the red portion
of the visible spectrum which is then passed through a
beam expander and collimator 60 and turning mirror 62
for developing the same into a collimated beam along a
predetermined path 66 passing through a dichroic beam
co~biner 64 having surfaces selectively transmissive to
6,328 angstroms.
Means is provided for providing an exposure
laser beam which is actinic to the photosensitive sur-
face of the exposure plate carried at platen 22. One
typical system utilizes an argon ion laser 68 having an
output beam 34 at 4,880 angstroms in ~he blue portion
of the spectr~m at a power output of about 10 milliwatts.
This output beam is passed through an aoousto-optical
modulator 74 which controls the intensity of the beam
transmitted therethrough. Beam 34 i5 routed by a turn-
ing mirror 76 through a beam expander and collimator 78
to the dichroic beam combiner 67. The beam combiner re-
flects beam 34 along path 66 and thereby combines it with
read beam 36. The combined beams pass along the common
path to a turning mirror 80 cArried on the scan table and
then to the scanning apparatus 50. As indicated in Fig-
ure 1, the scanning apparatus serves to deflect the com-
bined beams through a horizontal angle to ultimately scan
the beams across the respective surfaces of read platen
26 and write platen 22. A flat field lens 82 serves to
: focus the scanning beams at the æurfaces of the respec-
tive platen~. After passing through lens 82, the combined
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beams pass to a dichroic beam splitter 84 which reflects
the blue actinic write beam 34 upwardly ~o a turning mir-
ror 86 and thence downwardly through an aperture 88 in
the table to exposure platen 22. The dichroic beam split-
ter 84 (similar to combiner 64) passes the red read beam
36 to a folding mirror 90 which directa the beam downward-
ly through a second aperture 93 in the scanning table to
impinge upon copy at read pla`ten 26.
An optical reader 94 is carried by ~he scanning
table for receiving read beam energy reflect~d by the copy
on platen 2~. ~rhe reader comprises a fiber optic bundle
96 which includes fibers arranged in elongated linear ar~
ray extending across the width of the copy to be scanned.
The output of the fiber ~ptic rea~er i5 directed to a pho-
tomultiplier tube (not shown) and converted to an electrical
signal which controls the intensity of the output of modu-
lator 74.
Referring now to Figures 2 - 8, the optical scan-
ner 50 will be described in greater detail. In general,
the scanner consists of a roof mirror assembly 100 to which
is optically coupled a generally pyramidal input/output
scanning wheel 102 having mirror segments 104,106,108 there-
on which progressively move through the path of the input
laser beam and cause the same to be deflected, as will be
described. The input turning mirror 80, which is moun~ed
on the underside of the scanning table 38, is positioned to
intercept the combined laser beam 66 from the laser table
30 and to deflect the same upwardly to the scanning wheel
102. The beam is then reflected by one of the mirror seg~
ments 104,106 or 108 toward a first mirror 110 of the roof
mirror assembly 100, then to a æecond mirror 112 of the
roof mirror assembly 100, and then back to the s~ne wheel
se~ment 104,106 or 108 from which it was reflected initial-
ly. After the second reflection from the wheel segment,
the beam passes to an output objective lens g2. The angles
of reflection of the reæpective wheel segment, roof mirrors
110~112 and turning mirror 80 define the vertical orienta-
tlon of the beam as it emerges from the scanner.
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Means i~ provided for mo~nting the xoof mirrors
110 and 112 in spaced relation ~o each other and includes
a framework 116 and a base 114 to which the lower mirror
112 is cemented. The upper mirror 110 is carried in a
support ring 118 which is adjusta~le in angulax orienta-
tion by a 3-point suspension consisting of suitable dif-
ferential screws 120 through an upper crosspiece 122 so
as to permit accurate alignment between the mirrors. As
shown, mirrors 110 and 112 are ~paced apart with a scan
output opening 124 between khem from which the emerging
scan beam is directed into the objective lens 82. Mirrors
1}0 and 112 are positioned with an i~cluded angle 126 of
about 5508 degrees. It can be shown that the total angle
through which the beam is turned is 360 degrees, including
the reflection by the tl-rning mirror 80, the two reflections
by the scanning wheel 102, and the reflections by the two
roof mixrors 110 and 112. These angles define a fixed ang-
ular relation in the vertical Airection between the input
and output beams. ~he angle of tilt of the scanning wheel
segment does not affect the vertical output angle but only
the vertical displacement of the beam, as will be described.
The scanning wheel 102 is mounted on a spindle
ox shaft 130 which is supported for rotation in bearings
132,134 mounted in a shaft housing 136. A drive motor 138
is mounted on the housing and coupled directly to the shaft.
The motor may for example be a DC motor having field wind-
ings 139 and being capable of output speeds up to 10,000
rpm. An encoder wheel 140 is sonnected to the shaft and
forms part of an optical ~ensor 142 for creating a chopped
electrical signal indicative of the scanner wheel speed and
orientation.
The scanning wheel and motor are supported by a
mount 144 on table 3B, with ~he axis of rotation of the
scan~ing wheel~in a plane common to the optical axis of
tha output objective lens and the axis of the input beam.
Roof mirrors 110,112 are adjusted so that their surface
vectors (i~eO,~ vectors perpendicular to the surfaces of
the mirrors) also lie in this pl~ne.
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As shown best in Figur s 2 - 4, the sca~ning
wheel front reflective surfaces conform to a pyramid in
shape. For convenience of manufacture the wheel is cut
and machined from a circular di~c. The scanning wheel
pyramid is preferably a regular triangulax pyramid having
an axis of symmetry and apex (imaginary) located along
the axis of rotation. As ~hown, the apex portion is flat-
ted at 120a so that the wheel is technically a frustrum
of a pyramid, but this trunca~ion is not material to the
invention. The pyramid thus defines a plurality of at
least three reflective side segments which are identical
and which are di~posed symme~rically about the axi~ of
rotation. Each of these segments is provided with a very
accuretely formed planar reflective surface. Typically,
the disc is fabricated of aluminum or beryllium and is
machined to form mounting surfaces for the reflective
elements. These elements are accurately formed optical
flats which are secuxed to the machined surfaces of the
disc by a suitable cement. It is important that each seg-
ment be optically flat to a high degree of accuracy, since
the input and output reflections from the segment will gen-
erally not be at the same position on each reflective segment
as the wheel r~tates.
Each segment defines a plane in ~pace which is
tilted at a small acute angle, e.g., 6 degrees, with respect
to a plane perpendicular to the axis of rotation of the wheel.
Since the segment passes throuyh the beam's path, the effect
is one of passing a plane through the path with the plane
varying in angle of orientatlon to the path. Since each se~-
ment of a triangular mirror is limited to 120 degrees, the
variation in the orientation of the plane passes from a min-
imum at one side through a maximun to a minimum on the other
side; that is to say, the normal vector of each segment starts
by making a maximum horizontal angle of deviation ~o the sym-
metry plane, passes through a null and proceeds to a maximwm
angle on the other side. Thereafter, the part line 146 be-
twee~ two adjacent 6egments passes through the beam path (dead
time), and the process is repeated. ~he trace produced by each
successive ~egment travels in ~he ~me direction from one side
of the 6ystem to the other.
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Thus, as illustra~ed in Fiyure 10, the beam 66 is
~eflected at lS0 by turning mirror 80 into a further series
of reflections:
(a) a first reflection from ~he wheel segment 104 at
152,
(b) a xeflection from the up~er mirror 110 of the roof
doublet at 154,
(c) a reflection from the lower mirror 112 of the roof
doublet at 156, and
~d) a second reflection from the wheel segment 104 at
158,
at which point the beam has been routed through vertical an-
gles totaling 360 degrees and has ~een vertically displaced
so as to emerge between the roof mirrors 110 a~d 112 and
through the objective lens 82 in a direction parallel to the
path of travel of beam 66 into turning mirror 80.
Each reflection by segment 104,106 or 108 actually
introduces four possible deviations of the beam: a horizon-
tal angular deviation, a vertical angular deviation, a hori-
zontal di~placement, and a vertical or height displacement.
Upon consideration it will be found that in order to produce
an accurate scan line tracing a straight path in the plane of
focus of the objective lens 82, the only requirement of these
deviations is that the vertical angular component be constant
and invariable while the ~orizontal angular compo~ent progres-
ses from slde to side in a repeating pattern. How this is
done is best understood by reference to Figure 9.
. Figure 9 illustrates that no change in the verti-
cal ang~e of a beam passing through a 90-degree roof mirror
do~blet M-l,M-2 is produced by a change in the angle of tilt
of a reflector R which ~exves botll as input and output to
the xoof mirror. It is a known property of the roof mirror
doublet itself that ~he input beam defines the angle of the
output beam unambiguously. For example, with a 90-degree
roof mixror doublet, the beam will be reflected out of the
doublet at exactly the same angle as it enters in a plane
perpendicular to the line of intersection of the roof mir-
rors. This is true regardless of the angle o~ tilt of the
reflector, provided the:~eflector is perfec~ly planar and
serves~bo~h as~an input reflector and an output reflector to
the~roof mirror system. Because of the inversion as the beam
passes~:through~the roof mIrrorsl the angular component of
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til~ of the input reflector i5 cancelled exactly, although
a displacement or height error _v will oc~ur. Since the
wheel segments 104,106,108 are flat, the vertical angle of
the output beam in the invention remains invariable with
respect to the input beam and precisely so even ~hough the
input/output reflecting ~egment 104,106 or 108 introduces
vertical height displacement as well as horizontal angular
and position displacements. ~owever, 6ince the beam is
J aligned v~r~ically with respect to the objective lens and
contains no change in vertical angular component, it traces
a s~raight line at each focal plane.
The oregoing i6 ~rue even under very loose tol-
erances fox segment-to-segment accuracy, bearing accuracy
of the spindle or haft mounting r vibration and other var-
iables to which the rotating wheel is subject. The sole
rigid and absolutely precise requirement is flatness of
each reflective segment of the scanner wheel.
The three~dimensional charactex of the motion of
the beam during scanning can be visualized by reference to
the perspective view of Figure 10. The beam segments are
- - labelled and charac,terized as follows:
160 stationary beam following reflection by turning
mirror 80,
162 hori~ontal and vertical deviation added by
first reflection from wheel,
164,166 - roof doublet reflection adding vertical
' and horizontal displacements,
168 - vextical angle removed, horizontal angle
doubled, vertical and horizontal displacement
increased.
Figure llA show~ the pyramidal mirror segment 104
at its mid-position, which is also the position of maximum
vertical deflection. Figure llB shows the beam being routed
by the roof mirrors back ~o segment 104 nearly on top of the
'input beam for its second reflection from that segment be-
ore being passed between the mirrors in and out of the sys-
tem. Figures 12A - 12C and 13A - 13C show the segment in
progressively moved positions, first turned slightly and
then progressing toward the limit of movement to one side.
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These igures also show the progression in the horizontal
angular deviation, the horizontal displacement and slight
vertical displacement of the beam as the mirror segment
moves, while also indicating that no vertical angular de~
viation is created. These figu_es also show an intere~t-
ing phenomenon in that the vertical displacement causes
the second reflection from ~he ~can wheel segment to fol-
low the moving reflective ~egment through its circular
path of rotation, thereby avoiding the possihility of the
beam walking off the reflective segment laterally.
Both the horizontal and vertical displacements
of the beam are controlled by the angle of tilt of pyra-
midal mirror segments 104,106 and 108. In fairly long
focal length system6, as generally described herein, the
horizontal sweep angle desired is about 13 degrees, and
the apex angle of the pyramidal mirror is such that each
surface of that mirror is inclined relative to a plane
normal to the axis of rotation or axis of symmetry by an
angle on the order of 6 degrees. The pyramid apex angle
is the angle between the side of a xegular pyramid having
an even number of sides, e.g., a square or regular pyra-
mid. For pyramids having an odd number of sides, the apex
angle i6 twice the angle between one of the sides and the
axis of symmetry of the pyramid. Should a greater throw
be desired, redesign of the component locations and an
increase in this angle will provide a ~reater horizontal
angular deflection. In this connection, it is also poss-
ible to built the ~canning wheel with means tnot shown~ for
adjusting the angle of tilt of the acets. At least for
small changes, this would have the effect of varying or
changing the hoxizontal scan width over a limited range,
which could be very useful in certain applications. I
changes in scan width greater than a certain amount were
required, the angles and positi.oning of the mirror doub-
let would also be changed.
By way of example, one scanner constructed in
accordance with the present invention had the following
dimensional and other characteristics:
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~a) angle of introduction to segment 104 and first
reflectign from segment 104 in plane of symme-
try ~ 30 ,
(b) mirror doublet angle = 55.8 t
(c) pyramidal mirror apex a~gle - 2(90 - 6~) = 168
for a segment tilt of 6 with respect to a plane
normal to the axis of rotation~
(d) diameter o~ wheel 102 = B inches,
(e) angle of ~otal reflection through scanning sys~
tem = 360 ,
(f~ rotational speed up to 10,000 rpm or 500 traces/
sec.
At high rotational speeds it is desirable to pro-
vide a wind shroud 170 ~urrounding all portions of the wheel
except for a small front-facing port 171 which permits the
beam to enter and exit on each reflection, as illustrated
in Figure 2.
It is a particular advantage of the invention that
the ~can wheel can be cut from a circular disc. It is evi-
dent tha~ a circular disc having an accurately machined and
aligned moun~ing to the shaft of its rotational support is
desirable for vibration-free operation. Achieving this re-
sult in circular configuration is relatively easy, and care-
ful manufacture of the wheel and rotating parts will result
in a substantially symmetrical mass distribution about the
axiq of rotation and permit high degxees of dymanic balance
of the rotational elements.
I~ carried through its entire circle o rotation,
each segment~actually txaces a sinusoidal angle of impinge-
ment with respect to the axis ~ the beam as delivered to
the wheel from mirror 80. Only a portion of this 360-degree
cycle is utilized, namelyl a 120-degree portion which repre-
sents a 6ubstantially linear change iII the angle of orienta-
tion relative ~o the beam and is generally symmetrical about
the maximum angle of tilt presented ^to the beam.
While operating speeds up to 7,000 rpm have been
~uggested, the inherent design o the scanner of the inven-
tion permits envisioned operating speeds which may reach
or even exceed 60,000 xpm. ~his would represent linear
; trace repetition time~ of up to 3,000 traces (scans) per
second, which~have here~ofore been impr~ctical in apparatus
o~this character. The trace times proYided by the invention
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e~sentially eliminate the scanning ~lement as the limiting
stxucture in apparatus for the production of printing plates
and the like. ~he scanning system of the invention has
achieved many of the desirable advantages which are essen-
tial to a good scanner. Effectively, vertical wobble has
been eliminated. While scan efficiencies of at least 75%
are easy to ohtain, the scan efficiency can be increased
by increasing the diameter ~f the ~canning wheel at least
up to reasonable dimensional limits. The scanning is of
single-direction character, and the velocity linearity for
- a ~hree-se~ment wheel has been held, in the embodiment shown,well within acceptable limits. Scan times for conventional
printing plates ~ith typical raster scan advance speeds and
the trace speeds provided by the invention are on the order
of one minutet which is a necessary objective for any system
for production of printing plates at high speed. As is ev-
ident, ~he cost of production of a system constructed in
accordance with the invention is reasonable, since the only
relatively critical tolerance is the tolerance of mirror
flats. The entire system i5 reflective in character, total-
ly eliminating refraction error in both single-beam and
multi-beam operation. This feature enables the use of the
system in multi-frequency operation where read and write
beams of different frequencies are superimposed along a
single beam path. In summary, by using the present inven-
tion, problems associated with lag errors, back-and-forth
scanning, vertical wobble, frequency dependency and other
~ disadvantages of prior systems are eliminated.
; ~ The system of the invention is also adaptable
to facsimile operations such as disclosed in the co-re~er-
enced application previously referred to, or in other scan-
ning systems, the angular position o~ the wheel being de-
termined either by the design o~ the encoder disc or by
spatial masking as may be required. In addition, the fore-
going scanner lends itself r~adily to incorporation into
flat-~ield scanning devices as shown in the present inven-
tion. The invention provides an output beam which is ver-
ticall~ pr~cise and stable, and no vertical wobble compen-
sation is requiredO
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:
:
To those skilled in the art to which this inven-
tion pertains, many modiications and adaptations thereof
will occur~ For example, while there has heen shown a
three-sided frustrum of a regular triangular pyramid having
a circularly cut disc-like base, changes in the pyramid
apex angle, the number of sides (for example, four, fiv~
or more sides), and many design details of the scanning
wheel may be made to adapt the invention to particular cir-
cumstances, format sizes or ~ructur~es. Wheels having ad-
justable tilt angles have alreadv been mentioned. A11 of
such changes and modifications are within the scope of the
invention. Additionally, while read/write laser plate pro-
duction systems have been disclosed and described specific-
ally and facsimil~ opera~ion has been mentioned, it should
be understood that this is for brevity of explanation. The
scanner of the invention is also applicable to one-, two-
or even multiple-beam systems such as may be used in multi-
ple-station facsimile operation. It should be understood,
however, that such modifications and adaptations are to be
included within the scope o the invention and by definition
in the scope of the subse~uent claims, the specific embodi-
ment disclosed and described herein being given for the pur-
pose of illustration and not limitation on the invention.
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