Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE IN~ENIION
This invention relates to xerographic reproducing apparatus utilizing a
laser recording system responsive to an electronic digital computer and more
particularly to a laser beam monitor therefor.
The interfacing of electronic digital computers in xerographic reprod-
5 ucing apparatus for converting binary coded intelligence to graphic symbols on hardcopy output is highly desirable since by this means it is possible to produce
xerographic reproducing apparatus which can generate copies ~t higher rates thanheretofore possible. TQ accomplish the foregoing it is necessary to synchronize a
scanning array with the changing position of a non-uniformly scanned laser beam.To synchronize a scanning array with the changing position of a non-
uniformly scanned laser beam, a standard practice is to place a binary grating
structure (e.g. a Ronchi ruling~ at or very near the beam focus to intercept a
portion of the beam to thereby produce an optical pulse train. The pulse train
correlates with the beam positions during scanning and therefore serves as a laser
15 beam monitor (LBM). The detected pulses from an LBM can be used to control a
pulse-train driven scanner, for example, a CCD array. The output shift registers of
the CCD array can be clocked by the detected pulses produced by the LBM.
How the optical pulses from the grating are collected greatly affects
the overall size of the LBM. Heretofore, the detector that collects the pulses has
20 been placed at a beam focus which occupies a position different from all other foci
of the optical path traversed by the light beam generated by the laser. ~ one such
devise of the prior art as illustated in U.S. Patent 3,898,627 the beam generated by
a laser is split by a conventional beam splitter into two beams, one of which is used
to discharge a uniformly charged photoreceptor drum in accordance with intel-
Z5 ligence contained in a computer used to modulate said one beam. The other beamwhich is used to monitor the position of said one beam is directed through a Ronchi
ruling or grating having an elliptical reflector dispose there behind such that said
other beam is intercepted by a photodetector pasitioned at one focus of the
elliptical reflector which focus is spatially different from all other foci occupied
30 by other components of the recording apparatus. Thus, as can be seen the position
of the detector is such that it does not lend itself to a compact arrangement ofelements.
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BRIEF SUMMARY OF THE INVENTION
Thus, in accordance with the present teachings, an
improvement is provided in a laser recording apparatus which
includes means ~or scanning a modulated light beam relative
to a uniformly charged photosensitive surface for selectively
discharging the surface in accordace with intelligence to be
recorded and further including a laser beam monitor including
a photodetector. The improvement which is provided comprises
a compact laser monitor which comprises a retro-reflective
grating for generating a plurality of light pulses which are
reflected in the direction of the photodetector for collection
thereby.
Accordingly, the present invention incorporates a retro-
reflective pulse generator ~i.e. a grating or Ronchi ruling withretro-reflective properties~ as part of a laser beam monitor which
allows photodetector to be positioned at or adjacent a common focus
with a scanning reflector such as a polygon scanner or an oscillating
galvo-mirror. The retro-reflective ruling is positioned such that
the light pulses there~y are returned essentially along the instante-
ous scan direction. If all scan eman~te from a fixed aperture
location, then a fixed detector essent:ially at ~his same location,
will generate the desired electrical pulse train. Hence the use
of a retro~reflective ruling permits t:he laser beam monitor to be
arranged compactly.
To make an ordinary transmis~;ion-ty~e Ronchi ruling act in
a retro-reflective mode, a retro-reflective strip (e.g. 3 M's
Scotchlite type) ~7as placed on the back side of the ruling. The
scotchlite comprises a glass beaded reflector construction. Slight
divergence of the reflected light, due to the small size and imper-
fections of the reflectors, facilitates detection. Alternati~el~r~
a gxating with alternate slits composed of small retro-reflective
cells (corner-reflectors~ can be used. Having the cells ~rom a
surface relief structure allo~s inexpensive mass production of the
grating through replication.
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* Trademark
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Another form of relief that is retro reflective in one
plane, .say, the scanning plane, would be reflecting, right-angle
grooves (dispersed in alternate grating slits) parallel to the
slits. The right-angle grooves comprise a pair of mirrors dis-
posed at right-angles to each other. This form has the advantage
that a small tilt of the grating about its longitudinal axis
permits the reflected pulses to be collected conveniently just
above or below the fixed scanning aperture. The "opaque" slits
or lines in the grating may be either clear or absorbing, for
both cases will yield pulsing of the retro-reflected light as the
beam traverses the grating.
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DETAILED DESCRIPTION OF T~IE DRAWINGS
Fig. 1 is a schematic illustration of a recording device utilizing a laser
beam scanner and a compact laser beam monitor therefor;
Fig. 2 is a schematic illustPation of a modified embodiment of the
recording device shown in Fig. l; and
Fig 3 is a schematic illustration of another embodiment of the
recording device shown in Fig.l.
DETAILED DESCRIPTION OF THE PREFERED
EMBODIMENTS OF THE INVENTION
~n accordance with the invention as illustrated in Fig. 1, a controlled
light beam is generated from a source of high energy coherent light such as a
continuous mode laser 12. The beam 10 is projected along an optical path which
includes a beam spliter 14. A beam 16 emerging from the beam spliter 14 travels
through a light modulator 18 of the type well known in the art. The modulator 18responds to the binary states 1 or 0 of electrical information bits transmitted on an
input line Z0. The modulator emits the beam 16 along either of two closely
adjacent but slightly different output paths 22 and 24. When the beam is emittedalong the output path 22 it eventually travels past a knife edge 26 and strikes a
photoconductive surface 28 at a spot 30. The surface 28 is herein disclosed in the
form of a drum. The beam discharges the surface which had previously been
uniformly charged in accordance with well known techniques used in xerography.
When the beam is emitted along the path 24 it strikes the knife edge 26 and
therefore~ does not effect the discharging of the photosensitive surface at thatparticular spot.
As will be appreciated, when the uniformly charged photosensive
surface 28 has been discharged in accordance with information to be printed, toner
material presented to the drum in accordance with well known xerographic
techniques, will adhere to the areas that have not been discharged thereby
3Q rendering the latent image visible.
The beam 16 impinges on a scan mirror 3Z after passing through lens
system 34 and is thereby directed along a line corresponding to the longitudinalaxis of the photosensive drum 28. Drive means 36 serves to rotate the drum Z8 sothat when a subsequent scan takes place another line of latent information is
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formed on the drum. The scan mirror 3Z comprises a galvo-mirror which is
oscillated by a motor 38 which is synchronised with the rotational speed of the
drum 28. The beam splitter 14 produces a second beam 40 which is directed by
- means of a reflecting mirror 42 through a lens system 44 so that it too impinges on
5 the scan mirror 32 but at a difEerent angle than the beam 16. A second reflecting
mirror 46 is positioned to intercept the beam 40 and reflect it in the direction of a
retro-reflective grating 48. The grating 48 comprises retro-reflective cells
forming relief structures disposed at right-angles with respect to each other toform what is commonly referred to as grooved reflectoPs which are indicated by
10 the reference character 50. The grooved reflectors are disposed in alternate slits
in the grating intermediate opaque slits or lines 52.
The beam 40 is reflected by the grating 48 back to the mirror 46 such
that the mirror redirects the beam generally back in the direction of the grating
but at an angle that enables the beam to impinge on a photodetector 54 which is
15 adjacent the scan mirror 32. Suitable tilting of the grating 48 with respect to the
mirror 46 permits positioning of the detector 54 above the scanning mirror 3Z. It
will appreciated that the grading could also be tilted such that the detector 54could be positioned below the mirror 32.
As shown in Fig. 2, a modified form of the invention comprises a
Z number of components which are similar to those found in the embodiment of Fig.
1. Accordingly, like components will be referred to in the drawings with the same
reference characters.
The primary differences of the embodiment of Fig. 2 reside in the
scanner, detector and the pulse generator. The galvo mirror 32 of Fig. 1 is
Z5 replaced by a polygon scanner 60 which is rotated by means of a motor and drive
indicated schematically by reference character 6Z. The pulse generating device or
grating 48 is replaced by a corner reflector type grating 64 and the detector 54 is
replaced by a ring detector 66.
As shown, one facet 70 of the polygon scanner is employed to generate
30 a clock signal from the corner reflector type grating, retro-reflected light being
sampled by the ring-shaped detector 66 located adjacent the facet 70. Another
facet 68 is used to reflect the light beam 16 toward the photorecepter 28 in a
horizontal scanning motion.
Reference is now made to Fig. 3 wherein there is disclosed another
35 modified form of the invention. Components which are common with Fig. 1 and Zhave the same reference characters. In this embodiment, the galvo-mirror 70
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which could also be a polygon scanner uses one surface thereof for the writing
beam (i.e. beam that discharges the photoreceptor~ and the opposite surface for the
clock signal beam. The grating 48 comprises a grooved type grating the same as
the grating of the embodiment of Fig. 1. The grooved type grating allows
5 collection of the light beam above or below the galvo-mirror sirrlply by tilting
thereof at a very small angle.
