Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE IN~7ENTION
This invention relates to a flying spot scanning
system for communicating video information to a scanned
medium, and more particularly to a scanning system which
utilizes a multifaceted rotating polygon for controlling
the scanning cycles.
Much attention has been given to a various
optical approaches in flying spot scanning for the
purpose of imparting the information content of a
modulated lig~t beam to a scanned medium. Galvanometer
arrangements have been used to scan the light across
a document for recording its information content thereon.
Such arrangements have included planar reflecting
mirrors which are driven in an oscillatory fashion. Other
approaches have made use of multifaceted mirrors which
are driven continuously. Various efforts have been made
to define the spot size in order to provide for an
optimum utilization of the scanning system.
~ ne such effort is that described in United
States Patent No. 3,675,016. The approach used was to
make the spot size invariant and as small as possible by
defining the dimensions of the focused beam so that only
part, preferably half, of a mirror facet is illuminated
during ~canning. This teaching alludes to generalized
techniques for assuring the constancy of the size of the
aperture of a rotating mirror scanning system. ~y either
illuminating several facets of the mirror or by directing
light in a beam that is sufficiently narrow to assure
that less than a full facet is the most that can ever be
illuminated by the beam and limiting scanning to that
portion of the rotary travel of the facet when such facet
i~ illuminated by all of such light beam. However, such
system apertures are dimensionally invariant because the
dimensions of the rotating facets have no influence on such
apertures.
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While the system as described in U.S. Patent No.
3,675,016 may have advantages over the prior art, nevertheless,
various constraints must be imposed upon the spot size and
other relationships of optical elements within the system which
are not always desirable.
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SUMMARY OF THE INVENTION
In accordance with an aspect of this invention there
is provided apparatus for recording information from an
electrical signal onto a scanned medium comprising: means
for providing a beam of high intensity light, means for
modulating the light beam in accordance with the information
content of an electrical signal, means for focusing said
modulated beam to a spot upon the surface of a light sensitive
medium at a predetermined distance from said focusing means,
scanning means positioned in the optical path of said modulated
beam for scanning the spot across said medium to impart the :
information content of said spot to said medium, a first
cylindrical lens so positioned in the optical path of the
focused beam between said scanning means and said medium that
runout errors are substantially corrected at said medium,
the plane of no power of said cylindrical lens being oriented
in the direction of scan, means for deflecting a portion of
the scanned spot to provide a second beam directed at a
predetermined start of scan position located spatially from
said medium, means for detecting the start of the scan located
at said start of scan position at a distance from said focus-
ing means approximately equal to said predetermined distance,
and a second cylindrical lens so positioned in the path of
said second beam between said detecting means and said
deflecting means that runout errors are substantially corrected
at said detecting means, the plane of no power of said second
cylindrical lens being oriented in the direction of scan.
In accordance with another aspect of this invention
there is provided apparatus for recording information from
an electrical signal onto a scanned medium comprising: means
for providing a beam of high intensity light, means for
modulating the light beam in accordance with the information
content of an electrical signal, first optical means for
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expanding said modulated beam, second optical means in convolu-
tion with said first optical means for imaging said expanded
beam to a spot in a focal plane at a predetermined distance
from said second optical means, scanning means positioned
between said second means and the focal plane for scanning the
spot across a light sensitive medium in said focal plane to
impart the information content of said spot to said medium,
means for detecting the start of the scan located spatially
from said medium at said predetermined distance from said ;~
second optical means, means for deflecting a portion of the
scanned spot at a predetermined start of scan position to form
a second beam directed away from said medium to illuminate
said detecting means, a first cylindrical lens so positioned
in the optical path of the imaged beam between said second
optical means and said medium that runout errors are substan-
tially corrected at said medium, the plane of no power of
said cylindrical lens being oriented in the direction of scan,
and a second cylindrical lens so positioned in the second beam
path between said deflecting means and said detecting means
that runout errors are substantially corrected at said detect-
ing means, the plane of no power of said second cylindrical
lens being oriented in the direction of scan.
A feature of an embodiment of the invention is that
the beam of light incident upon the multifaceted polygon
illuminates at least two contiguous facets of the polygon
during each scanning cycle to provide the desired sequence
of spot scanning. Such feature provides a flying spot scan-
ning system which has an extremely high duty cycle.
Another feature of an embodiment of the invention
is that a very large depth of focus is provided for the spot
at the contact loci at the surface of the scanned medium.
This feature is provided by utilizing a finite conjugate
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imaging system in convolution with the light beam and
the rotating polygon. A doublet lens, in series with a
convex imaging lens between the light source and the
medium provides such an arrangement. The doublet lens
enables the original light beam to be sufficiently expanded
for illuminating multiple contiguous facets of the polygon,
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whereas the imaging lens converges the expanded beam to
focus at the contact loci on the surface of the scanned
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medium. Employing such an optical system assures a
uniform spot size at the scanned medium even though a
substantial scan width is traversed by the spot.
Still another feature of an embodiment of the
invention is the modulation of the original light beam by
means of a video signal. The information content within
the video signal is thereby imparted to the light beam
itself. The medium to be scanned is one which is responsive
to the modulated beam and records its information content as
contained within the scanning spot in a usable form on its
! surface across the scan width.
Also, another feature of an embodiment of the
invention is that a start/stop of scan detection apparatus
is in combination with and substantially matches the con-
volution of imaging elements which focus the flying spot
at the surface of the scanned medium, although such detection
apparatus is spacially distant from the scanned medium.
Yet another feature of an embodiment of the
invention includes an embodiment of the flying spot scanning
system for utilization in high speed xerography. The scanned
medium in such an embodiment would consist of a xerographic
drum which rotates consecutively through a charging station,
an exposure station where the spot traverses che scan width
of the drum, through a developing station, and a transfer
station where a web of copy paper is passed in contact with
the drum and receives an electrostatic discharge to induce
the transfer of the developed image from the drum to the
30 copy paper. A fusing device then fixes the images to the
copy paper as it passes to an output station.
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DESCRIPTION OF THE DRAWINGS
Figure 1 is an isometric illustration of a flying
spot scanning system in accordance with the invention.
Figure 2(a) is a partial perspective view illus-
trating the-use of the scanning beam generated by the system
of Figure 1.
Figure 2(b) is a top perspective view of the
optical system shown in Figure 2(a). ~ -
Figure 3 is an isometric illustration of a first
alternate to the flying spot scanner of Figure 1 and with
the same numbers indicating the same or similarly operating
parts.
Figure 4 is a perspective view of the utiliza-
tion of the scanning beam according to the first alternate
embodiment.
Figure 5 is an isometric illustration of a second
alternative to the flying spot scanning system of Fisure 1
and wherein the same number indicates same or similar parts.
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Figure 6(a) is a side perspective view of the
utilization of the cylindrical correction lens of Figure 6.
Figure 6(b) is a top perspective view of the
utilization of the cylindrical lens.
Figure 7 is an isometric illustration third alternative
to that of Figure 1 of a flying spot scanning system with the
same numerals indicating the same or similar parts.
DESCRIPTION OF THR PREFERRED EMBODIMENT
In Figure 1, an embodiment of a flying spot scanning
system in accordance with the invention is shown. A light
; source 1 provides the original light beam for utilization by
the scanning s~stem. The light source 1 is preferably a laser
which generates a collimated beam of monochromatic light which
may easily be modulated by modulator 4 in conformance with the
information contained in a video signal.
Modulator 4 may be any suitable electro-optical
modulator for recording the video information in the form of
a modulated light beam 6 at the output of the modulator 4.
The modulator 4 may be, for example, a Pockel's cell comprising
a potassium dihydrogen phosphate crystal, whose index of re-
fraction is periodically varied by the application of the vary-
ing voltage which represents the video signal. The video
signal may contain information either by means of binary pulse
code modulation or wide-band frequency code modulation. In
any event, by means of the modulator 4 the information within
the video signal is represented by the modulated light beam 6.
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The light beam 6 is reflected from mirror 8 in
convolution with a doublet lens 10. The lens 10
may be any lens, preferably of two elements, which elements
are in spaced relation to each other such that the external
curved suxfaces are provided in symmetry with the internal
surfaces. Preferably the internal surfaces of lens 10 are
cemented together to form a common contact zone. Of course,
as is often the case in the embodiment of such a lens as
a microscope objective, the elements may be fluid spaced.
The lens 10 is required to image the axial point of beam
6 through a focal point on the opposite side of lens 10.
At the focal point, beam 6 diverges or expands to form
beam 12 which impinges upon at least two contiguous facets
of a scanning polygon 16.
In this embodiment, the rotational axis of polygon
16 is orthogonal to the plane in which light beams 6 travels.
The facets of the polygon 16 are mirrored surfaces for the
reflection of any illuminating light impinging upon them.
With the rotation of the polygon 16, a pair of light beams
are reflected from the respective illuminated facets and
turned through a scan angle for flying spot scanning.
Alternatively, flying spot scanning could be provided by
any other suitable device, such as mirrored piezoelectric
crystals or planar reflecting mirrors which are driven in
an oscillatory fashion.
In all of these arrangements, however, the
reflecting surfaces would be at a distance S from the
originating focal point of light beam 12 and in orthogonal
relation to the plane bounded by the beam 6 such that the
reflected beams would be in substantially the same
plane as beam 6.
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At a distance a from the leading illuminated facet :
of polygon 16 is positioned an imaging lens 20. Lens 20 is -
of a diameter D to cooperate with the respective reflected
light beams throughout an angle of 2 ~ to render convergent beams
22 which define a focal plane 24 at a distance f from the imag- :
ing lens 20. In this preferred embodiment, imaging lens 20 is
; a five element compound lens as disclosed in United States
Patent 3,741,521 issued January 10, 1973 and assigned to the
assignee of the present invention. The focal plane 24 is
proximate a recording medium 25 whose surface 26 is brought in
contact with the respective focal spots of the convergent light
beams throughout a scan width x.
A uniform spot size is assured throughout the scan
width x even though a curved focal plane 24 is defined through-
out the scanning cycle. The lens 10 in convolution with the
imaging lens 20 provides a finite conjugate imaging system
which allows a large depth of focus d which is coextensive with
the contact loci of a spot throughout the scan width x on the :
surface 26 of the medium 25.
The beam of light from the light source 1, through
the optical elements and reflected from the scanning polygon to
the scanning medium 25 defines a light path.
As shown in figure 2(a), medium 25 may be a xero-
graphic drum which rotates consecutively through a charging
station depicted by corona discharge device 27, exposure surface
26 where the beam from the rotating polygon 16 traverses the
scan width x on the drum 25, through developing station 28
depicted by a cascade development enclosure, transfer station
30 where a web of copy paper is passed in contact with the
drum 25 and receives
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an electrostatic discharge to induce a transfer of the
developed image from the drum 25 to the copy paper.
The copy paper is supplied from the supply reel 31, passes
around guide rollers 32 and through drive rollers 33 into
rec~iving bin 35. A fusing device 34 fixes the images to
~e copy paper as it passes to bin 35.
Usable images are provided in that the information
c~ntent of the scanning spot is represented by the modulated
or variant intensity of light respective to its postion
within the scan width x. As the spot traverses the charged
surface 26 th~ough a given scan angle c7~, the spot
dissipates t~e electrostatic charge in accordance with its
light intensity. The electrostatic charge pattern thus
produced i~ developed in the developing station 28 and then
transferred to the final copy paper. The xerographic drum
25 is cleaned by some cleaning device such as a rotating
brush 36 bef~re being recharged by charging device 27. In
this manner, the information content of the scanned spot
i8 recorded on a more permanent and useful medium. Of
course, alternative prior art techniques may be employed
to cooperate with a scanned spot in order to utilize the
information contained therein.
m e polygon 16 is continuously driven by a motor 40
and synchronized in rotation to a synchronization signal
representative of the scan rate used to obtain the
original video signal. The rotation rate of the xerographic
drum 25 determines the spacing of the scan lines. It also
may be peferable to synchronize the drum 25 in some manner
to the signal source to maintain image linearity. The source
image is reproduced in accordance with the signal and is
transferred to prlntout paper for use or storage.
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The number of facets has been found to be optimum
if at least 20 to 30 facets are employed. The scan angle o~
traversed would be equal to the number of facets chosen in
relation to one complete revolution of the polygon 16. An
extremely useful arrangement would have the polygon 16 with
24 facets and a scan anglec~ of 15 degrees. Since the depth
of focus d of the converging beam 22 is related to the scan
angle in that as the scan angle c~ increases the radius of
curvature of the focal plane 24 increases, it is important
to define a scan angle in relation to the desired scan width
x. For a scan width x of approximately ll inches it has
been found that the scan angle o~ of 12 to 18 degrees, with
20 to 30 facets on the polygon 16, is optimum. Figure 2(b)
is a top perspective view of the optical system shown in
Figure 2(a).
The optical system of the present invention provides
a virtually lO0~ duty cycle scan for the entire scan angle
c~ by virtue of the illumination of at least two contiguous
facets. The illumination of two contiguous facets is
preferred. ~With such illumination, another scanning spot
is provided at a distance equal to the scan width x behind
the leading scanning spot with virtually no wait between
successive scans. With the continuous rotation of the polygon
16 additional contiguous facets are subsequently illuminated,
thereby providing successive convergent beams following the
leading convergent beam 22 by no more than the scan angle,
if so desired. Thus, a flying spot scanning system which
has an extremely high duty cycle is provided.
The optical system of the present invention projects
the highest energv portion of the laser beam onto the rotating
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polygon and to the photoreceptor surface. The energy output
of the laser beam has a Gaussian distribution with the highest
intensity distribution being concentrated near the axis of
projection. In the Gaussian distribution the highest inten-
sity energy is located at the centre of the beam. By
spreading the beam out over a plurality of facets, the beam
centre portion having the highest intensity radiation energy
fills all the polygon facets reflecting the beam onto the
photoconductive surface. The fringes of the beam having the
lowest Gaussian energy intensities are spread out beyond
those polygon facets and are excluded from the information
signal scanned across the photoreceptor surface. Shaping the
beam to spread it across a plurality of facets thereby
filters the lower energy radiation within the beam and
excludes it, permitting the utilization of the maximum energy
content of the beam.
In the first alternate embodiment as shown in
Figure 3, the light beam 6 is reflected from mirror 8 in
convolution with a doublet lens 10. The lens 10 may be any
lens, preferably of two elements, which elements are in
spaced relation to each other such that external curved
surfaces are provided in symmetry with the internal surfaces.
Preferably the internal surfaces of lens 10 are cemented
together to form a common contact plane. Of course, as is
often the case in the embodiment of such a lens as a micro-
scope objective, the elements may be fluid spaced. The lens
10 is required to image either a virtual or real axial point
of beam 6 through a focal point, for example, on the opposite
side of lens 10 for a real image. At the focal point, beam 6
diverges or expands to form beam 12 which would be more than
sufficient to impinge upon a given facet of a scanning polygon
16.
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At a distance S2 from the leading illuminated facet
of polygon 16 is positioned an imaging lens 18. Lens 18 is ~
of a diameter D to cooperate with the expanded light beam 12 ;
to render a convergent beam 2 which illuminates the desired
facets to reflect respective light beams 22 to focus to focal
plane 24 at a distance d from the polygon 16. In this first
alternate embodiment, imaging lens 18 is a l-n element lens.
The focal length f of lens 18 is related to Sl, S2 and d by the
following thin lens equation: 1 +
Sl S2 +d f
The rotational axis of polygon 16 is orthogonal to the
plane in which light beam 6 travels. The facets of the polygon
16 are mirrored surfaces for the reflection of any illuminating
light impinging upon them. With the rotation of the polygon
16, assuming two contiguous facets are illuminated at a given
time, a pair of light beams 22 are reflected from the respective
illuminated facets and turned through a scan angle ~ for flying
spot scanning. Alternatively, flying spot scanning could be
provided by any other suitable device, such as mirrored piezo-
electric crystals or planar reflecting mirrors which are driven
( in oscillatory fashion.
In all of these arrangements, however, the reflectingsurfaces would be at a distance Sl from the originating focal
point of light beam 12 and in orthogonal relation to the plane
bounded by the beam 6 such that the reflected beams would be
in substantially the same plane as beam 6.
The focal plane 24 is proximate a recording medium
25 whose surface 26 is brought in contact with the respective
focal spots of the convergent light beams throughout a scan
width x.
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A uniform spot size is assured throughout the scan
width x even though a curved focal plane 24 is defined through-
out the scanning cycle. The lens 10 in convolution with the
imaging lens 18 provides a finite conjugate imaging system which
allows a large depth of focus of which is coextensive with the
contact loci of a spot throughout the scan width x on the
surface 26 of the medium 25.
As shown in Figure 4, medium 25 may be a xerographic
drum and in all respects is similar to the arrangement of
Figure 2 except for the position of the imaging lens as shown
in Figure 3.
The second alternative embodiment is as shown in
Figure 5. At a distance a from the leading illuminated facet of
polygon 16 is positioned an imaging lens 20. As shown, the
lens 20 is located between the polygon 16 and the medium 25.
Alternatively, the lens 20 may be located between the polygon
16 and the lens 10 as shown in Figure 3. In this embodiment,
lens 20 is of a diameter Dl to cooperate with the respective
reflected light beams throughout each scan of 2 d~ to render
convergent beams 22 which define a focal plane 24 at a distance
f, from the imaging lens 20. In the first embodiment, imaging
lens 20 is a five element compound lens as disclosed in United
States Patent 3,741,621 to G.L. McCrobie issued June 26, 1973
and assigned to the assignee of the present invention. The
focal plane 24 is proximate a recording medium 25 whose surface
26 is brought in contact with the respective focal spots of :~
the convergent light beams throughout a scan width x. .
Since runout errors and polygon facet errors may
cause poor results in terms of the quality of image transfer to
the scanned medium, a cylindrical lens 36 is positioned in the
optical path between the polygon and the scanned medium with its
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aperture aligned with the aperture of the polygon 16. The
lens 23 may be either bi-convex, plano-convex, or meniscus.
The light beam 22 impinges on the convex surface of the lens 36
to focus at plane 24 at a predetermined position in an ordinate
perpendicular to the direction of scan on the surface 26 of
the medium 25.
- As shown in Figure 6, the polygon 16 is continuously
driven by a motor 40 and may be synchronized in rotation to a
synchronization signal representative of the scan rate used
to obtain the original video signal. In the case of the
utilization of a xerographic drum, the rotation rate of the
drum determines the spacing of the scan lines. The rotation
of the polygon 16 off-axis from that desired causes runout
errors or, in this case, a deflection of the beam 22 in the
vertical direction away from the desired scan line. Assuming
an angular deviation of ~ from the desired axis of rotation
for the polygon 16, a runout angle of ~ defines the deflection
from the intended direction of scan. Other misalignments of
optical elements within the system, such as facet misalignment,
also may cause the same runout effects.
The disposition of the cylindrical lens 36 in Figures
6(a), 6(b), the optical path, though, compensates for such
effects. The lens 36 is located at a distance b from the origin
of the angular deflection ~ and a distance u from the imaging
lens 20. The compensation is effected in that the off-axis
beam passes through the convex surfaces of lens 36. Then, the
lens 36 focuses the beam 22 to a spot at a predetermined line -~
of scan in the focal plane 24 at a distance b' from the lens 36.
To insure that lens 36 is sufficiently wide a length L is pro-
vided approximately equal to or greater than the scan width
s. The runout dimensions at lenses 20 and 36 are Dl' and D2',
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respectively.
In this embodiment, in order to determine the optimum
relationship between the imaging elements 20 and 36, the
aperture Dl of the imaging lens 20 shall be equal to approximately
2ax/fl, with an (f/number)l equal to approximately s/2a tan2 ~
Furthermore, the focal length of lens 36 is f2, which is defined
as l/b + l/b = l/f2. From this we may define the optimum focal
length f2 for determining the distance of the focused spot from
the lens 36.
Since it is also necessary to define the aperture
D2 of the lens 36 to practice the invention, the following
relationship is preferred:
D2 ~ 2b tan ~
Having defined f2 and D2, it is helpful to determine
the necessary (f/number)2 for the lens 36:
(f/number)2 = f2/D2 = b'/2 tan ~ (b' + b)
An optimum relationship is that the cylindrical
lens 36 be located at a distance from the surface 26 of the
medium 25 approximately equal to the focal length f2 of the
lens 36.
A third alternative embodiment shown in Figure 7,
wherein the same numerals correspond to the same or similar
parts as in Figure 3.
The light beam 6 is reflected from mirror 8 in con-
volution with a doublet lens 10. The lens 10 may be any lens,
preferably of two elements, which elements are in spaced -~
relation to each other such that the external curved surfaces
are provided in symmetry with the planar internal surfaces.
Preferably the internal surfaces of lens 10 are cemented
together to form a common contact zone. Of course, as is often
the case in the embodiment of such a lens as a microscope
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objective, the elements may be fluid spaced. The lens 10 is
required to image either a virtual or real axial point of beam
6 through a focal point, for example, on the opposite side of
lens 10 for a real image. At the focal point, beam 6 diverges
or expands to form beam 12 which would be more than sufficient
to impinge upon a given facet of a scanning polygon 16.
At a distance S2 from the leading illuminated facet
of polygon 16 is positioned an imaging lens 18. Lens 18 is of
a diameter D to cooperate with the expanded light beam 12 to
render a convergent beam 20 which illuminates the desired facets
to reflect respective light beams 22 through a positive
cylindrical lens 23 to focus to the focal plane 24 at a distance
S3 from the polygon 16. In this preferred embodiment, imaging
lens 18 is a five element compound lens as disclosed in afore-
mentioned U.S. Patent 3,741,521.
The rotational axis of polygon 16 is orthogonal to
the plane in which light beam 6 travels. The facets of the
polygon 16 are mirrored surfaces parallel to the axis of
rotation for the reflection of any illuminating light imping-
ing upon them. With the rotation of the polygon 16, assumingtwo contiguous facets are illuminated at a given time, a pair
of light beams 22 are reflected from the respective illuminated
facets and turned through an angle 2 dL for flying spot scanning~
Alternatively, flying spot scanning could be provided by any
other suitable device, such as mirrored piezoelectric crystals
of planar reflecting mirrors which are driven in an oscillatory
fashion.
In all of these arrangements, however, the reflecting
surfaces would be at a distance Sl from the originating focal
point of light beam 12 and in orthogonal relation to the plane
bounded by the beam 6 such that the reflected beams would be in
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substantially the same plane as beam 6.
The cylindrical lens 23 is positioned in the optical
path between the polygon 16 and the desired line of scan in the
focal plane 24 with its aperture aligned with the aperture of
the polygon 16.
- The function of the lens 23 is to compensate for
runout errors in the scanning system. The lens 23 may be
either bi-convex, plano-convex or meniscus and further relates
to the scanning system as described with respect to Figure 6.
The focal plane 24 is proximate a recording medium
25 whose surface 26 is brought in contact with the respective
focal spots of the convergent light beams throughout a scan
width x.
A substantially uniform spot size is assured through-
out the scan width x even though a curved focal plane 24 is
defined throughout the scanning cycle. The lens 10 in con-
volution with the imaging lens 18 provides a finite conjugate
imaging system which allows a large depth of focus d which is
coextensive with the contact loci of a spot throughout the
scan width x on the surface 26 of the medium 25.
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~ As is further shown in figure 7, a mirror 42 is
positioned proximate the start of scan location to deflect
at least a portion of the beam 22 to direct a beam 44
through a positive cylindrical lens 46 to focus at a
detector 48. The detector 48 includes a photodiode (not
shown), or other optically sensitive element, which produces
an eled~rical pulse to indicate the start of scan upon
illumination by the beam 44. The detector 48 further
includes a timing element (not shown) in combination with
the optically sensitive element which is responsive to the
start of scan pulse. The timing element through well known
techniques times out the predetermined duration of a
scanning cycle to produce a stop of scan pulse. An example
of such a technique would be a capacitive element charged
at the start of scan pulse which charge decays in relation
to a predetermined time constant to trigger a one-shot
multivibrator at the stop of scan. The start/stop of scan
signa~s are then used to slave the video signal to the scan
rate of the scanning system.
The detection elements 46 and 48 are substantially
matched with the convolution of imaging elements which focus
( the flying spot at the surface of the scanned medium. The
cylindrical lens 46 i5 distanced along it9 respective optical
path from the lens 18 precisely at the same length as the
cylindrical lens 23 is distanced along its respective optical
path from the lens 18. Furthermore, the aperture, focal
lèngth, and focal number of the lens 46 is substantially
identical to that required for lens 23. Therefore, the
focused spot of the beam 44 is in a focal plane at a distance
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S3 along the optical path from the polygon 16, where the
detector 48 is located. Thereby, an effective detection
system is provided which contributes to a high degree of
synchronization accuracy and constancy, with no interference
with the spot scanning elements within the system.
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