Note: Descriptions are shown in the official language in which they were submitted.
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UNITARY, MODULAR, DEMOUNTABLE
OPTICAL SYSTEM FOR LASER DIODE/
2RINTING COPYING APPARATUS
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to photo-optical
scanning apparatus utilizing a laser ~iode light
generating device and a polygonal mirror assembly in
conjunction with suitable light beam collimating and
focusing apparatus. More specifically, the invention
relates to a system for precisely shapin~ the laser
generating light beam into an efficient spot scanning
size for electrophotographic printing and/or copying.
2. Descriptlon of the Prior Art
Many problems are associated with scanning
systems wherein a modulated/or unmodulated light beam
is caused to scan by means of a rotatable polygonal
mirror. For example, the position of each scanning
line becomes dificult to control. This problem is a
result of the angular relationship between adjacent
facets of the polygon as well as between the facet
planes and the rotational axis of the polygon.
Another problem is associated with the
location of the laser light generating apparatus and
its angular relationship to the operably associated
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hardware. A further problem is that as the polygon
mirror asse~bly is rotated at a constant rotational rate
the speed of the generated spot will be constant along
an arc but will not be constant with respect to a
straight line scan. ~n fact, the laser beam or spot
speeds up at the periphery of the scan line which in
- turn has the effect of changing the dimension of the
output data being developed. These and other ~imilarly
associated problems have caused many of the prior art
devices to be less than commercially satisfactory.
2. Description of the Prior Art
A number of techniques and apparatus have been
suggested for use in laser printing copying. Each has
certain characteristics which recommend it for a
specific application. Obviously, the goal is to provide
a type of printing/copyiny device which accommodates a
variety of purposes and performs these functions
effectively and efficiently.
The following patents are considered to be
pertinent to the present invention which iq considered
to be an improvement thereover, as well as an
improvement over the earlier filed application `USSN
27~,2~0, as will be described later on herein.
~ Fleischer, 3,750,189 shows and describes a
helium-neon laser scanner whose light output is coupled
through a lens system to a rotating polygon mirror from
which the light is reflected through a lens system to a
rotating drum. In the Fleischer structure a cylindrical
lens is employed to focus the collimated beam of light
to a line on a flat facet of the polygon mirror while a
second cylindrical or toric lens after the polygon is
used to recollimate the light reflected from the
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polygon. A spherical lens then focuses the recollimated
light beam to a small spot on the scan line of the
photocopy drum.
For appreciable scan an~les it is necessary to
use a toroid in order to maintain the spot size over the
scan line. However, toroidal lenses are relatively
difficult to manufacture and are therefore usually not
economically or commercially feasible due to their high
cost.
Grafton, 3,946,150 employs a cylindrical lens
near the photoreceptor. This proximity requires the use
of a long cylindrical lens. Also, since this lens is
located close to the developer unit it tends to become
coated with toner which degrades the light and resulting
photocopy.
Rabedeau, 4,123,135 is stated to be an improve-
ment over the apparatus described in Fleischer,
3,750,189. Rabedeau notes that the beam entering the
spherical lens need not be collimated. Rabedeau makes
use of this by employing a negative cylindrical lens
with power in the scan direction following the polygon
to produce the same beam divergence in both azimuths for
the beam that enters the spherical focusing lenses.
The method and structure permits the use of less
expensive cylindrical lenses but also tends to ~latten
the field. However, it remains a very dif ficult problem
to flatten the fie]d over wide angles when relatively
high resolution is required.
SUMMARY OF THE INVENTION
The present invention overcomes the above
difficulties first, by providing a cylindrical meniscus
lens with power in the horizontal scan direction to
focus the beam to a small spot in the scan direction.
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Second, by providing a pair of cylindrical-spherical
lenses to provide cross-scan or vertical focus. The
cylindrical-spherical lenses provide a function similar
to a pair o~ toric lenses but at a more reasonable cost.
For the laser diode scanning system of the present
invention it is also an advantage to have the horizontal
- and vertical focus independent.
The optical system of the present invention is
structured and fabricated as demountable, unitary,
modular assembly independent of the remainder of the
apparatus with which it is or may be operably associ-
ated. The lens assemblies of the optical system, once
adjusted for focus, are fixed in position with.in the
module although each lens is in fact demountable for
replacement without the need for realign~ent upon re-
insertion in the assembly. The module is sealed against
du t and dirt contaminatior. and is provided with its own
source of pressurize~ air to prevent dust and dirt from
accumulating within the assembly from one source or
another. Additionally the module is angularly adjust-
ably positionable relative to the photoreceptor drum and
includes means for preventing accidental ingress of
toner into the optical module. Precisely located pivot
pins enable accurate adjustment of the optics relative
to the photoreceptor drum which, once fixed in position,
need no further adjustment or alteration.
It is, ~herefore, an object for the present
invention to overcome ~ach of these problems in a new,
novel and heretofore unobvious manner and to provide a
photo-optical solid state laser diode scanning system
wherein a solid state laser is caused to produce a
divergent beam of visible electromagnetic radiation
which is collected and collimated and thereafter
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optically shaped to reduce the vertical dimension while
expanding the horizontal dimension for subsequent
impingement upon a rotatable polygonal mirror assembly.
The collimated laser beam is then focused onto a photo-
conductor, for example, a rotatable drum, through acylindrical lens and a light folding mirror, passing
through a spherical lens to the photoconductor itself.
A novel aspect of the invention is the pro-
vision for the apparatus to be modularly related and to
be mounted to a rigid, fixed base mel~ber. Each element
of the novel combination is adjustably, positionable
relative to the base as well as to the axis of the laser
beam, the lenses, mirrors and polygonal mirror facets
thereby insuring an ~ccurate, clear and highly defined,
latent image on the photoconductor,i.e. drum.
Another novel aspect of the present invention
is the yrovision ~f a novel photoconductor drum
charging, exposing, toning and cleaning apparatus for a
laser diode and printing and/or copying apparatus
utilizing a novel folded laser scanning light path in
combination with a corna charging, discharging apparatus
not heretofore available in electrophotographic
processing apparatus.
Other objects, features and advantages of the
invention will be readily apparent from the following
description of two different embodiments thereof, taken
in conjunction with the ~ccompanying drawings, although
variations and modifications may be efected without
departing from the spirit and scope of the novel
concepts of the disclosuxe.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic side elevational view,
not to scale, of apparatus embodying the present
invention;
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Figure 2 is a detailed side elevational view o
the apparatus of Figure l;
Figure 3 is a schematic top end view not to
scale, of the apparatus of Figures 1 and 2;
` Figure 4 is a detailed top plan view of the
apparatus of Figure 3;
Figure 5 is a schematic illustration, not to
scale, of the electrophotographic process sta~ion of the
present invention;
10Figure 6 is a detailed view of the apparatus
illustrated in Fisure 5;
Figure 7 is a schematic representation, not to
scale, o the light beam path for the apparatus
incorporating the dified invention;
1~Figure 8 is an unfolded schematic representa-
tion of the light beam formation as it passes through
diferent lens assemblies in the apparatus of the
invention;
Figure 9 is a perspective view, not to scale,
of the apparatus o the invention;
Figure 10 is a partial top plan view of
apparatus incorporating the invention;
Figure 11 i~ a plan of the organization of the
views of Figure llA and llBi and
25Figures llA and llB are side elevational views
of the modular structure of the invention.
~ESCRIPTIO~ OF ONE EMBODIMENT OF THE INVENTIO~
In one of its broadest aspects the apparatus of
the present invention is typified by the arrangements
set forth schematically in Figure 1. The laser diode
printer apparatus 10 is seen to comprise a laser diode
12, energized in a known manner to produce a beam of
electromagnetic radiation, arrow 14, which is adapted to
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be passed through a lens system 16 which acts to
collimate the light beam 14 and direct the collimated
liyh-t beam 18 into and through a pair of optical prisms
20 and 22, respectively, ~hich act to change the beam
from an elliptical cross section to a circular cross
section.
The now collimated, altered, beam 24 of light
is next directed through a focusing lens 26 to a
rotatable polygonal mirror assembly 28. The focused
light beam 30 is reflected off the faces or facets of
the polyyon 28 as the latter is rotated by drive motor
32 in the direction of arrow 3~. The rays 36 of the
reflected beam are focused onto the photoconductive drum
surface 38 of rotatable drum 40 via a tiltable mirror
assembly 42 and a second focusing lens system 44,
Figure 1.
The focusad laser beam 46 is adapted to scan
the cylindrical photGconductor surface 38 from edge to
edye or side to side to means of the rotating polygon
mirror 28 and drive motor 28. Modulation (by means not
shown) of the laser diode 12 produces a latent electro-
static image upon the surface 38 of photoconductor 40.
Copying and/or printing media 48, Figures 5 and 6, ls
adapted to receive the image o~ the intelligence carried
by the latent electrostatic ima~e by means o~ and ln a
manner to be described 1.ater on herein.
A physical embodiment of the apparatus 1~
schematically` illustrated in Figure 1 is seen most
clearly in the side elevational view of Figure 2, to
comprise a rigid base member 50 on which the entire
assembly is mounted and adapted to be slideably moveable
back and forth or right to left as the case may be. A~
earlier mentioned, mirror 42 is adapted to be tllted
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about its horizontal axis mounting pivot 52 so as to
fold the laser beam 36 upwardly toward the cylindxical
lens 44, Figure l. Tilting adjustment of the mirror 42
is provided by means of the threaded horizontal cross
shaft 54 adjustably moveable by means of thumb wheel 56
against the vertical mirror support column 58 disposed
- in vertical mounting pillar 60 secured to base 50.
Vertical, erectable movement of mirror 42 is provided by
means of slo~ 62 and pin 64, as seen most clearly in
Figure 2.
The laser diode 12 (light generatiny element)
is surrounded by a thermo-electric cooling member 66 and
is gimbally mounted, as at 68 to support 70. A heat
sink 72 of copper or similar material capable of rapidly
and efficiently dissipating large quantities of heat
abuts the laser diode assembly 12. The gimble pivoting
arrangement 68 supports the heat sink 72 and cooling
member 66 enabling the laser diode 12 to be pivoted
about two orthogonal axes that pass directly through the
diode chip. The laser diode temperature is regulated so
as to be constant at approximately 20-21 degreees C by a
feedback controller with a thermistor sensor (not
shown). It is necessary that the heat sink temperature,
which is close to ambient, be somewhat above the c~ntrol
temperature since the thermoelectric device can only
cool and cannot heat.
The laser light 14 emitted by the diode 12 is
; collected by the objective lens 16 which in one
embodiment comprises a microscope objective having a
maynification of 20 times. This is necessary since the
output area ~of the diode is about two tenths micron by
about five micronsj from which light is being emitted
and is extremely small. The objective lens 16 has the
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laser light at its focu~. The light enters the lens as
a diverging set of light rays 14 from the laser diode
12. The objective lens 16 ~ollimates the light, as seen
most clearly in ~igure 2. Since the light from the
diode 12 is diverging on the entering side of lens 16
and is collimated on the exiting side thereof, the bea~
is not generally circular but rather oblong or ellipti-
cal in cross section.
In order to correct ~his9 two separate but
optically complimentary photo-optical elements are
employed. The collimated light is first passed through
prism 20 which is configured so as to compress the beam
in the vertical plane or direction down to approximately
one tenth inch. Therea~ter, the beam is redirected into
and through prism 22 which is constructed such that the
li~ht bealn is expanded slightly in the horizontal
direction. Exiting from the second ~rism 22 the light
beam now has a ciroul2r cross section and is collimated
before entering focusing lens 26.
Except for the tiltable mirror 42, which is
provided with its own separate, individual adju table
mounting ~eans previously described, each of the lenses
and prisms heretofore mentioned are provided with
separate means ~or orthogonally positioning these
eLements relative to each other as well as with respect
to the axis of the laser liyht beam.
A rigid elevating platform 74 is secured to the
base 50 to which is mounted, as by bolts (not shown) a
second rigid mount 76. ~ember 76 provides oppositely
disposed parallel guidin~ tracks (not shown) for slide-
ably moviny support member 78. To one end of the me~ber
78 is secured a rockably, pivoted support member 80
accurately moveable about pivot 82 on the leftward
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projecting end of the member 78. Member 80 provides a
tiltable support for the laser diode 12 including its
heat sink 72 as well as for the objective lens assembly
16 and its adjus~ing means. Vertical, slideable, adjust-
ment for diode 12 is controlled and actuated by means ofmicrometer slides and the knurled thumb wheel 84. The
horizontal adjustment is by means of thumb wheel 8~,
Figure 4. Rockins movement for platforln 8~ to axially
align the laser beam 14 is provided by means of the
threaded adjusting wheel 88 which is adapted to rockably
pivot the member 80 about pivot 82 by means of threaded
shaft 90 against the right end of member 80. Sliding,
focus.i}lg adjustment for objective lens assembly 16 is
provided by thumb wheel and shaft assembly 92 at the
rightward end of member 80.
Laser beam light rays 18 pass, as before
mentioned, through a compression prism 20 which is
angularly, adjusta~]y mounted on a horizontal pivot 94
and tiltable about this pivot by means of thumb wheel
96, cam 9~ and L-shaped follower link 100. Adjustrnent
movement of prism 72 is accomplished by means o~ thumb
wheel 102, Figures 2 and 4, cylindrical cam 104 and L-
shaped cam follower 10~ secured to prism 22. Focusing
lens 26 is threadedly, adjustable backwards and forward
2~ for accurate focus ing by rotation within the lens
support 108.
The focused laser light beam 30 after passing
throuyh the focusing lens 26 is reflectively scanned
across the surface 38 of photoconducting drum member 40
3 in a manner such that the data or intelligence contained
in the modulated beam is placed upon the drum for
copying/printing purposes, as will be hereinaftar
described.
S~.5
Polygon mirror 28, rotating in the direction of
arrow 34 by drive motor 32 carries 20 mirror facets and
rotates at the rate of 12,558 RPM. The laser light is
passed from the ocusing lens 26 to the face of each
mirror facet so as to scan through an angle of 3~
degrees (as the polygon rotates) which is precisely
twice the angle that the facet moves through during the
period of time for one scan line. With the focusi~g
lens 26 positioned in front of or before the polygon 28,
the focal point tends to be on the arc of a sphere.
Rather than in a plane this marks for correction
problems since the beam 24 passes throuyh the same point
of this lens all the time. Additionally, as the polygon
scanner 28 rotates, the deflected beam is rotatiny at a
constant rotational rate so the speed will be constant
on an arc but will not be constant on the straight scan
line. The beam or spot speeds up at the periphery,
which produces a small effect in changing the dimension
of the characters.
In the preferred embodiment as seen in Figure 3
a focusing lens 110 is positioned after the polygon 28.
This lens characterized as a F-theta lens (fe) and
avoids the variation of lineal scan velocity with the
scan angle. Normally, the spot displacement d of a
simple lens varies as the focal length times the tangent
of theta. (d = f tan- e). It is possible to make an
F-theta (fe) lens so that the lineal displacement varies
as the focal length times theta (fe) itself. This
allows a linear relationship between polygon rotation
and spot position. Plus, it produces a flatter field so
that th~ focus is in a plane (including the scan line).
The (fe) lens is triplet lens.
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As the polyyon 28 turns, the focused light is
reflected off tilt mirror 42 and angularly, upwardly
into and through the elongated, focusing cylindrical
lens 44 to be raster scanned across the photoconductor
40. Only about 25 degrees of the total scan is used for
the printed scan line. Located on the right hand side
of the scan line facing the drum 40 is a start of scan
detector (not shown) which is used to time the initia-
tion-of printing.
Lens 44 has a 1at surface on one side and
convex surface Oll the other side and is utilized to
reduce the vertical or facet apex angle error, an error
in the position of the beam due to wobbling of the
facets from one facet to the other, i.e., the so-called
change in the apex angle. (The angle between the axis
of rotation of the pvlygon and the facet, varies from
one facet to the other and this variation causes the
beam to deflect sll~htly in the vertical direction).
Utilization of cylindrical lens 44 reduces the effect of
the wobble. The rotating polygon causes the light to
scan a full raster scan length i.e. the width of the
dru,n or the width of the lins that is to be printed on
the page one scan length for each facet on the polygon.
Obviously, the more facets there are on the polygon the
easier it is to reduce the RPM's required of this
rotating device. For example, with only eight facets
the device would have to be rotated at a high RPM to get
the same number of scans per seconds. Scans per second
is determined by the speed at whlch it is desired to
3 print. At ninety pages per minute this is approxi~
mately 6,000 lines per minute. This is a lineal surface
velocity on the drum of 17.42 inches per second. The
raster line spacing is determined by the resolution
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desired for 240 dots per inch. Each raster line is
spaced by 1 over 240. Thus, the raster scan lines ars
spaced l/240th of an inch or .00417 inches apart.
Dividing the raster line spacing by the velocity of the
drum gives the time permitted for each scan line. The
reciprocal of the scan line time gives the scan rate.
In other words, the scan rate would be just equal to
17.44 inches per second by .00417 inches. However,
since .00417 is equivalent to 1 over 24~, the result can
be expressed as 17.44 times 240 dots per inch. This
gives a repetition rate for the scan in scans per
second. Each scan occurs in approximately 239 micro-
seconds. Obviously, the more facets on the polygon the
more the total RPM can be reduced. The present polygon
has 20 facets. The number of facets is tied in with the
resolution that is desired to be achieved.
To derive the desired resolution, the beam must
be expanded to a pre~etermined size as it is passed into
the final focusi~g lens. The larger the beam goiny
into the focusing lens~ the smaller the spot size. A
reciprocal relationshlp exists between the spot size and
beam size entering the focusing lens. The larger the
beam going into the focus lens, the smaller the spot
size. In other words, when the aperture is smal]. at the
focusing lens, the di*fraction is greater so the
d~E~action limited spot is larger. Thus, the collimated
beam size (aperture) should be larger to obtain a small
spot.
PRINTING/COPYING STATION
_
Referring now to Figures 5 and 6, with emphasis
first to Figure 5, there is shown a highly schematic or
diagrammatic ~ide elevational view o~ the electrophoto-
graphic process station of the present invention. The
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bloc'~ identified in Figure 5 as "optical scanning
assembly" is ~eant, for purposes of illustration, to
include the complete optical structural arrangement
shown in Figure 2 including the cylindrical ~ocusing
lens 44 through which the modulated laser beam 46 passes
to impinge upon the rotatable drum 40. The cylindrical
lens corrects for any beam motion introduced by the
rotating polygon and its associated vertically disposed
mirrors.
Drum 40 is provided with a relatively hard,
long wearing, photoconductive coatiny 38 having a high
infrared response, Figure 1 and is adapted to be rotated
in the direction of arrow 112. The size of the drum is
calculated to accept 11 inch or 14 inch length sheets o
plain paper for copying/printing in serial fashion, one
after the other, so as to increase the general "through
put" of the apparatus.
Initially the drum 40 has no surface charge on
it and no toner. The charge coratron 114, which
consists of six wires stretched across, parallel to but
out of contact with the drwn surface, is electrically
energized p~acing a uniform electrical char~e across the
photoconductive surface 38. The drum 40 rotates clock-
wise, so that the light from the laser diode 12 strikes
the areas on the drurn surface where no printing is
desired which discharges the background. The laser
diode beam is swept across the length of the drum and
selectively modulated with the intelligence necessary to
produce the printed matter desired. Each scan line at a
resolution of 240 dots per inch will have 240 scan lines
per inch of printing. The dots will be generated by
turning the laser diode 12 on and of to get the
intelligence information on the dru.n. The drum now has
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~elective regions of electrical charge and regions that
are discharged or have no electrical char~e thus forming
a latent electrostatic image thereon.
The drun next passes to the toner station 11~.
; Toner station 116 has an electrical charge bias supply
to the toner 118 with a polarity and magnitude such that
the toner is attracted to the drum surfaces 38 in the
regions corresponding to where the print will be. At
this point in the process, the apparatus has produced a
developed image. As the drum 40 continues to rotate
fur~her, it comes into the transfer area 120 where the
image is to be transferred from the photoconductive drum
to the copy material, e.g. paper 48. Paper 48 is moved
from left to r ght axrow 122. Two implementations are
employed for toner transfer. Both of them use electro-
static means. Nonconductive toner 118 is used. The
paper 48 is charged by means of a transfer coratron 124.
The coratron wires develop an electrostatic charge field
which essentially causes the toner to have a greater
attraction towards the paper 48 and the downstream
(rightward) detac coratron 126 than it does towards the
photoconductive drum 40. The toner effectively lifts
off th~e drum and is applied to the paper. The detac
coratron 126 separates the paper 48 from the drum to
which is electrostatically attracted. Detac coratron
126 applies a DC pulse at the front or leadin~ edge of
the paper to li~t the leading edge up. As the paper
continues to move under member 126 and as soon as the
leading edge is picked up off the drum, an AC electrical
potential is applied to member 126. This di~char~es the
paper, the paper 48 thus is lifted off the drum with the
toner intact.
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The paper carrying the toner image next passes
into the fuser 128 which is a combination of pressure
and heat produced by means of two opposing roller
mernbers 130 and 132, respectively. Thereafter, the
papar is passed into the next station of the machine at
- which time the paper bears an image o~ the intelligence
copied or printed thereon. Although greater than 98 per
cent of the toner is transferred to the paper 48, in
order to offset the problems with residual toner on the
photoconductive druln, if any, a preclean coratron 134
and preclean lamp member 136 are used. An AC coratron
wire is used at this point with the AC switchiny
polarity between positive and negative, discharges the
surface of the drum 40 and also discharges the toner
118. Since light also discharges the surface of the
druln a low wattage (8 watt)fluorescent bar li~ht is used
to make sure tha~ all of the charge is removed in
addition to the toner. A vacuum cleanin~ station 138
provided with a rotating bristle brush 140 of soft
bristles, with a vacuum applied from a source (not
shown~ sucks off residual toner which may be on the
drum. ~t this point the drum is considered to be clean
as far as toner is concerned. However, since toner was
covering some suraces of the photoconductor that the
light from the drum is rotated past a inal discharge
lamp 142. Light from lamp 142 shines onto a completely
cleared drurn removes all residual charge very
effectively. The apparatus is now ready to start the
copy process again at the charge coratron 114.
If however, the paper for some reason, does not
detach or lift up o~f the drum 40, oppositely disposed
stripper finger members 144, which protrude slightly
into the drw~ and into recessed areas at the edyes i.e.
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opposite sides of the paper, catch the paper and tend to
lift the paper away from the drum.
DESCRIPTION OF ANOTHER EMBODIMEL~T OF THE PRESE~T I~EL~TION
_
Briefly, as seen in the highly schematic, views
g of Figures 7 and 8 a laser diode lOA mounted behind a
sapphire window 12A is pulsed by a driver circuit (not
shown) which is fed from the imaging electronics
operably coupled ~o the present apparatus and also not
shown herein. The logic i5 set up such that ~he laser
diode is "off" when intelligible characters or other
information is to be prin~ed. Solid state laser diodes
of the double heterostructure variety produce a
diverging ~eam of light when pulsed by the driver
circuit and therefore, requires collimated optics.
15Light rays 14A emitted from the laser diode lOA
are first collected and collimated by a four element
objective lens assembly the elements of which are
designated 16A, 18A, ~OA and 22A respectively, provided
with spherical elements. The first two lenses act as
converging or positive elements and collect the light
with minimum aberration while the third objective
element 20A is a diverging lens that compensates the
residual aberration from the other three elements.
Collimated light 24A erneryes from fourth element 22A
when the facet of the laser diode chip lOA is located at
the focal point of the objective lens assernbly. The
collimated beam 24A now has an elliptical shape
corresponding to the differing divergence angles of the
beam from the laser diode lOA in planes parallel and
3o perpendicular to the junction of the diode. It is noted
that a three element objec~ive lens could be employed in
place of the present four element lens assembly with
suitable choice of glasses.
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The collimated laser beam 24A is then scanned
and focused onto a rotatable photoreceptor drum 26A,
Figs. 8 and 9 by means of a polygon scanner assembly 28A
and a set of four lens elements designated hereinafter.
An additional function of the four lens set i5 to
correct for the pyramidal angle variation or the varia-
- tion in the angle be~een the facets of the polygon and
the axis of rotation. Without this correction the laser
scan lines on the drum 26A would not overlap uniformly
from one scan line to the next. Since the light from
the la'ser diode lOA discharges the surface potential or
white space between the latent images of characters and
or intelliyible information, the variable overlap of
scan lines would cause a variation in the discharga of
the potential therefore and cause lines to appear in the
"white" space of the toned image.
The above variation in scan line overlap is
reduced in the following manner. First, the collimated
beam 24A is focused in the cross scan direction by a
first cylindrical lens 30A to form a line image on the
facet of the polygon 28A. Lenses that follow tha poly-
gon 28A focus the beam to a small spot on the photo-
receptor 26A.
I the line of focus is strictly in the plane
of the facet, then rotation of the facet about that
focus line would produce no movement of the subsequently
focused spot on the photoreceptor 26A. In practice,
however, the correction for pyramidal angle error is not
absolute and the focus~d spot will move slightly in the
cross scan direction as a line cannot be perfectly
focused on the facet for all positions. Following the
polygon 28A a cylindrical meniscus lens 32A serves to
focus the beam to a small dimension in the scanning or
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horizontal direction. By appropriately curving or
bendiny ~his lens the field of the horizontal ocus can
be ~lattened and the aberration can be minimized. This
lens interacts very weakly with the ver~ical or cross
scan focus since the power is negligible in that
direc~ion. The cross scan or vertical focus~ng is
achieved by a pair of cylindrical-spherical lenses 3~A
and 36A with the corresponding centers of the
cylindrical and spherical surfaces located near the
facet and with power mainly in the vertical or cross
scan direction. Thus, aberrations in the vertical
direction are reduced and the field is flattened in the
scan direction first by using a pair of lenses and
second by bending or curving the lenses slightly in the
horizontal direction. It can be shown from thin lens
theory that the index of refraction of the glass in the
pair should be greater than 1.62 to have a finite separa-
tion between the pair. Plint glasses such as SF6 have a
high index of refraction and allow a comfortable space
between the pair. One novel aspect of thls optical
combination is that the pair approximates a pair of
toroidal lenses which have been used in the prior art
but which are expensive to manufacture. Another novel
aspect is that the horizontal and vertical spot size can
be focused independently. Finally, a mirror 38A,
Figure 9 bends the light at a suitable angle to cause
the light to impinge upon the surface of drum 26A.
Referring to the detailed drawings Fiyure lO
and llA and llB the laser diode lOA is moun~ed on a
copper block 42A, as seen from above. The copper block
in turn is mounted on a thermo-electric cooler 44A that
cools the laser diode lOA to approximately l9 degrees
centigrade. The diode lOA is positioned accurately by
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means of a setup fixture (not shown) rel~tive to a
microscope objective 46A 50 that it is on the center
line of the microscope objective optics. The position
of the laser diode lOA is thus preset relative to the
optical axis before installing it in the laser diode
assembly. The diode ase~bly lOA including cooler 44~ is
- positioned on locating pins (not shown) that locate it
so as to avoid any adjustments while in place on the
copy machine. From the laser diode l~A the light
diverges, more in the hori7ontal direction than in the
vertical direction. The horizontal direction is
characterized herein as "the scan direction" and the
vertical direction is characterized herein as the "cross
scan direction". The light in the scan direction
diverges by 30 degrees to the half intensity points on
the beam and by 10 degrees to the half intensity points
on the beam i~ the cross scan direction. After the
light passes through the microscope ohjective 46A, the
beam is collimated assumin~ that the microscope
objective is positioned properly. This divergence of
the beam produces a different beam si7e in the scan
direction as opposed to the cross scan direction. The
beam produces a diferent beam size in the scan
direction as opposed to the cross scan dir~ction. The
beam will have a larger dimension in the cross scan
direction than in the scan direction with about a three
to one ratio.
~ he collimated beam is next passad through a
cylindrical lens 48A. The ~irst cylindrical lens
(similar to lens 30A) has power in the cross scan
direction and will cause the beam to focus down to a
line image on the rotatable polygon 50A. It does not
have any power in the scan direction. The baam is still
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collimated in the scan or horizontal direc-tion. The
light is then focused by a horizontal meniscus lens 52A
(similar to lens 32A~ which is the first lens following
the polygon 50A. Lens 52A has power in the scan
5 direction only so it focuses the wide horizontal
dimension of the beam down to a narrow spot on the photo-
receptor drum 54A after passing throuyh the third and
fourth lenses 58A and 60A, respectively, ~identical to
lenses 34A and 36A) which individually have no power in
10 the horizontal direction and, after reflecting off of
folding mirror 56A (identical to mirror 38A) is in focus
both horizontally and vertically on the drum 54A. For
the cross scan direction, the bealn is focused by the
third ana fourth lenses 58A and 60A which have a
15 cylindrical surface on the first surface 62A and 64A,
respectively, and a spherical surface 6~A and 68A on the
second su~face, respectively. T~is pair of lenses
(following each other in succession) have power in the
20 cross scan direction and essentially focus the line
which is imaged on the polygon 50A to a point in the
cross scan dirçction. The focusing in the other
direction is performed by the horizontal meniscus lens
52A which is, as before mentioned, the first lens
25 following the polygon.
The lenses are curved 50 as to flatten the
field. The horiæonta} meniscus lens 52A is generally
curved concave toward the facet of the polygon 50A,
almost producing a spherical surface with its center at
30 the facet. The curving or bending of t~e lenses permits
the field of focus at the drum 54A to be flat and also
improves the scan linearity at the drum.
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~ ach one of the four lenses 43A, 52A, 62A and
64A is demountably removable relative to each respective
frame of ref rence. This permits cleaning, polishiny,
replacement, e;c. Fixed locating pins 60A are
arranged to provide ixed reference for each lens and
5 enables these individual elements to be removed at wlll
and replaced without fear of mislocation.
As can be seen in Figures 10, llA and llB the
complete optical assembly is mounted within a rectangu-
- lar casting 70~ which includes a cover 72A that encloses
the top of the casting. The top 72A itself is provided
with an air inlet 74A and a flange 76A that comes down
close over the polygon 50A. Rotation of the polygon 50
by motor 78A creates a vacuum which causes air to be
pulled in through a filter 80A that is centered over the
l; top of the polyson 50A. The filtered air is forced out
inside of the optical castiny 70A to create a positive
air pressure therein. The air flow is through the
casting 70A, through an elongated opening 82A (the beam
scanning aperture) up through a wedge shaped shroud 84A
20 that is mounted to the top cover 72A. The air pressure
is maintained such that the positive air flow prevents
toner from getting .into casting 70A. The shroud 84A is
angled in such a way as to overhang the as~embly, thus
the optics is protected ~rom dust, dirt or falliny
25 objects from falling directly into the optics.
As can be seen in Figures 9 and 10 the beam is
folded off of folding mirror 56A Figure 10 and directed
up through the shroud 84~ to the photoreceptor drum 54A.
Beam scanning begins with each facet rotation starting
3 from the left side of thne drum 26A Figure 9. The beam
crosses a start of scan detector 88A that is mounted up
near the photoreceptor 54A. This member provides the
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timiny for the printing that is to be performed. The
start of scan pulse provided by detector 88A is used to
initiate the print cycle on a scan line and so many
counts after that pulse, the electronics (not shown)
initiates the unloading of the dot line buffers (not
shown) that provides the information for each character
- in bit form.
The polyyon 50A which is six inches in diameter
is provided with 1-3 facets 90A. Obviously the smaller
the number of facets the higher the speed must be. The
size of the present polygon was chosen for a thirty page
per minute copier so as to keep the RPM of the motor
down to a speed around 4900 RPM. At this speed the
polygon is expected to last for the life o the machine.
The size of the facets 90A were selected to provide
uniform illumination across the full scan width. The
beam as it falls on facet 90A is about two tenths of an
inch in diameter bet~een half intensity points. This is
the so-called underilled case,i.e. a facet 90A is under-
filled. The intensity of the scanning beam or scanningspot must remain constant across the full scan width.
If part of the beam is cut of toward the edge of the
facets, this results in a decrease of intensity. The
width of the facets is designed so as to keep the
intensity of the beam constant across a nine and a half
inch scan within five percent variation. The nine and a
half inch dimension is slightly wider than the width of
the drum. The start of the scan detector 88A is
included within this dimension.
3 The complete optical assembly of the present
invention is set into the printer/copier machine on a
pair of pivots 92A. One on each opposite side of
castin-~ 70A to the left in Figure llA. The center of
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these pivot points passes through the center 94A of
folding mirror 56A. The whole assembly is horizontally
angularly adjusted by means of screw 96A and slot 98A on
the underslde of casting 70A located at the opposite end
5 of the casting removed fro~ the pivot points. After the
box like assembly 7~A is placed into the machine, the
adjusting screw 96A tilts the whole assembly until the
beam falls onto the start to scan detector 88A. By
having the pivot points pass through the center of
folding mirror 56A the plane of focus remains sub-
stantially constant or unchanged on the photoreceptor as
the assembly is tilted to make the beam fall on the
start of scan detector.
There has thus been described a new, novel and
heretofore unobvious photo-optical laser diode printing/
copying apparatus which provides a very high speed, very
efficient and very cost effective combination of oper-
ational apparatus tG provide clean, clear, crisp copies
without the attendant problems associated with much of
the prior art devices.
