Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to facsimile reproduction
equipment and in particular to a system for shaping the scanning
spot of facsimile transmitting and receiving equipment for optimum
reproduction.
Facsimile equipment is used to transmit visual images such
as photographs and the like from one location to another. Images
reproduced by a facsimile receiver generally exhibit a line or
dot structure due to the scanning nature of the electro-optical
reproduction process employed. Heretofore, attempts have been
made, utilizing recent advances in the fields of electronics,
physics and optics to minimize and ideally remove the lines or
dots from the reproduction. To this end, facsimile equipment
utilizing lasers have been employed with the laser beam passed
through a suitable aperture arrangement to produce a generally
rectangular scanning spot with a dimension transverse to the scan
direction approximately equal to the scan spacing and the dimen-
sion along the scan direction somewhat smaller.
The shortcoming of the above system is that the natural
spot profile for a laser beam is a circularly symmetrical Gaussian
shape. By placing an aperture in the system, part of the Gaussian
beam is cut off so that there is a light loss and hence a reduc-
tion in the efficiency of the system. In addition, diffraction
problems arise from introducing an edge (of the aperture) into the
beam resulting in (a) a reduction in the depth of focusing of the
system and/or (b) ringing at the outer edges of the spot image.
Further, adjustments to such systems require skilled technicians
capable of diagnosing, measuring and varying the optics of the
system.
The present invention provides a method of scanning
for receiving and transmitting in facsimile equipment wherein the
natural spot profile for a laser beam is utilized. The present-
invention further provides such a method wherein the circular
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beam is utilized to provide an elongated spot profile covering
substantially the entire height of a scan line so that the repro-
duced image is virtually free of scan lines. The present invention
also provides a method wherein adjustments may be made electron-
ically and without requiring any optical adjustments.
The above and other beneficial objects and advantages
are attained in accordance with the present invention by pro-
viding a facsimile system comprising a transmitter and receiver.
The transmitter includes means for scanning an image to be repro-
duced and for generating an electrical energy signal the intensityof which varies in correspondence to density variations in the
image scanned. ~he system further includes a receiver which
utilizes a laser focused on a sheet of sensitized paper or other
substrate material which hereinafter are all termed "paper". The
spot profile of the laser is scanned across the sheet along scan
lines greater in width than the diameter of the spot profile. An
acousto-optic modulator cell is interposed between the laser and
paper to modulate the intensity of the laser in response to vari-
ations in an applied electrical signal. The applied signal com-
prises the electrical energy signal transmitted from the opticalcell. The receiver further includes means for cyclically varying
the frequency of the applied electrical energy signal whereby to
deflect the laser substantially between the top and bottom of
the scan line transverse to the direction of scan. The deflection
of the laser beam perpendicular to the direction of scan occurs
at a rate much faster than movement in the scan direction. The
transmitter may also utilize a laser beam scanner and an acousto-
optic modulator interposed between the laser and image.
According to the present invention therefore there is
provided a facsimile system comprising:
a transmitter including: means for scanning an image
to be reproduced; means for generating an electrical energy signal
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the intensity of which varies in correspondence to density varia-
tions in the image scanned; and means for transmitting said elec-
trical signal to a receiver; and,
a receiver including: a laser having a beam focused on
a sheet of sensitized paper; a sheet of light sensitized paper;
means for scanning said laser beam across said sheet to produce a
scan line, the variations in the exposure of the paper corresponding
to the intensity of said laser beam; an acousto-optic modulator
cell means interposed between said laser and paper to modulate
the intensity of said laser beam in response to variations in
an applied electrical signal; means for applying said transmitter
electrical energy signal to said optical cell on an rf carrier; and,
means for cyclically varying the frequency of said carrier
whereby to deflect said laser beam substantially between the top
and bottom of said scan line in an undulating path generally trans-
verse to the direction of scan.
The present invention also provides the method of
facsimile reproduction comprising the steps of:
scanning an image to be reproduced along a scan line and
: 20 generating an electrical signal varying in correspondence with
variations in the color of the image along said scan line; trans-
mitting said varying signal to a receiver; applying said varying
signal to an acousto-optical modulator cell within said receiver,
said cell being adapted to vary the intensity of a laser beam
passing therethrough in accordance with variations in said signal;
scanning a sheet of sensitized paper with a focused laser
: beam through said cell to produce image variations in the tone of
which corresponds to variations in the intensity of said laser
beam; and
modulating the frequency of said applied varying signal
whereby to deflect said laser beam in an undulating path generally
transverse to the direction of scan.
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The present invention also includes a receiver for use
in a facsimile reproduction system, said receiver comprising:
means for supporting and transporting a sheet of sensitized paper;
a laser beam focused on a sheet of sensitized paper supported by
said aforementioned means; means for scanning said laser beam
across said sheet along a scan line; an acousto-optic modulator
cell means interposed between said laser and said sheet supporting
means; means for applying an rf signal to said acousto-optic
modulator cell means; first means for modulating the intensity of
said rf signal in response to variations in a transmitted signal
of an image to be reproduced; and, second means for cyclically
varying the carrier frequency of said rf signal.
The present invention further includes a transmitter for
use in a facsimile reproduction system, said transmitter comprising
an image supporting means; a laser; means for scanning a beam for
. said laser across said image; acousto-optic modulator cell means
interposed between said laser and image supporting means; means
for applying a constant amplitude signal on an rf carrier to said
: cell means; means for cyclically varying the frequency of said
aforementioned rf carrier to deflect said laser beam in an undu-
lating path generally transverse to the direction of scan; photo-
. electric cell means; and means for reflecting said laser beam off
:: said image onto said photoelectric cell means to generate an
image transmission signal.
In one aspect thereof the present invention provides a
: receiver for use in a facsimile reproduction system, said receiver
comprising: a laser having a beam focused on a sheet of sensitized
paper; a sheet of light sensitized paper; means for scanning said
laser beam across said sheet to produce a scan line, the variations
in the exposure of the paper corresponding to the intensity of thelaser beam reaching the paper; an acousto-optical modulator cell
interposed between said laser and said paper; means for applying
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an rf signal to said acousto-optical cell; means for modulating
the intensity of said laser beam in response to variations in a
transmitted electrical energy signal of an image to be reproduced;
and means for cyclically varying the frequency of said rf signal
whereby to deflect said laser beam substantially between the top
and bottom of said scan line in an undulating path generally
transverse to the direction of scan.
The present invention will be further illustrated by
way of the accompanying drawings which similar components bear
the same reference numerals throughout the several views and in
which:
Fig. l is a block diagram representation of the trans-
mitter of a facsimile system in accordance with the present
invention;
Fig. 2 is a block diagram representation of a receiver in
accordance with the present invention; and,
Fig. 3 is a simplified plan representation of a surface -
scanned in accordance with the present invention. ;
The facsimile system of the present invention comprises
a transmitter 10 depicted in Fig. 1 and a receiver 12 depicted inFig. 2.
Referring to Fig. 1, the transmitter portion 10 of the
present system includes a helium-neon laser 14. The coherent
light of the laser is focused as a precisely dimensioned beam on
the sheet 16 containing the image to be reproduced through a
reciprocating mirror 18 and an acousto-optic modulator cell 20.
The intensity of the light reflected off the image is sensed by
photoelectric cells 22. In response to the reflected light, the
photoelectric cells 22 generate an electrical energy signal which
is suitably amplified and transmitted to an appropriate receiver
through telephone lines or the like. The intensity of the light
reflected at any instance to the photoelectric cells is a function
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of the density tone of the portion of the image scanned at that
time. Thus, variations in the intensity of the output signal of
the photoelectric cells are representative of tonal changes in
the image.
As stated, the light beam to the image is reflected off
mirror 18. Mirror 18 is reciprocated by a galvanometer motor 24
which, in turn, is precisely driven by control 26. The deflection
of the light beam off mirror 18 thus serves to enable -the beam
to scan across the width of the image sheet 16. At the same time,
the sheet 16 is supported and transported longitudinally by rollers
28 driven by motor 30. Synchronization between drive motor 30
and the galvanometer drive 26 is maintained by sequence logic 32.
Each time the reflected light scans across paper 16, a
scan line results having a line height in the longitudinal direc-
tion (i.e., the direction of movement of paper 16). In order that
a reproduction of the image not result with visible lines, it is
important that the intensity of the scanning beam be kept constant
across the entire height of the scan line during both transmitting
and receiving. In accordance with the present invention, this
is accomplished by utilizing a scanning spot having a diameter
smaller than the height of the scan line and then rapidly deflect-
ing the spot between the upper and lower limits of the scan line.
This has the effect of an efficient aperture without its dis-
advantages and permits a blending of scan lines at top and bottom
; to reduce or eliminate their visibility in the facsimile product.
To this end, the laser beam passes through an acousto-
optic modulator (AOM) cell 20. The AOM cell is a small section
of glass upon which a piezoelectric transducer has been fused.
The transducer reacts to the application of current sending
acoustic waves into the glass, changing its refractive index in
precise relationship to the presence of the acoustic signal. The
details of the construction and properties of such cells are
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described in the literature, as for example, the article "A Review
of Acoustooptical Deflection and Modulation Devices" by E.I. Gordon,
published in the October, 1966 Applied Optics. It suffices to
say for the present application, that such cells possess the
following two properties:
1. The intensity of a laser beam passing through the
cell is modulated as a function of the strength of
an rf signal applied to the cell; and,
2. The angular deflection of a laser beam passing
through the cell is a function of the frequency of
an rf signal applied to the cell.
The transmitter 10 makes use only of the second of the
above described properties of the acousto-optic modulator. As
will be described forthwith, the receiver 12 makes use of both.
Referring still to Fig. 1, it can be seen that an rf
signal is applied to the acousto-optic modulator 20 through a
drive circuit 34. The drive circuit 34 of the transmitter pro-
duces a constant strength signal on a 10 KHz carrier to enable
narrow band signal detection at the 10 KHz carrier to eliminate
low frequency interference (such as 60 Hz interference) ensuring
constant laser light signal intensity on the photograph being
transmitted. The frequency of the rf signal varies cyclically
between the upper and lower limits chosen to deflect the scanning
spot 36 so that it undulates as it traverses across the scan line
as shown in Fig. 3. Deflecting the spot across the height of a
scan line at a very high rate produces a signal out of the photo-
detectors as if a rectangular shaped spot were used. Since the
intensity of the 10 KHz laser beam directed onto the image is
constant, variations in the output of the photodetector circuit
represent variations in the tonal intensity of the image.
- The receiver of the present system is depicted in Fig. 2.
The construction of the receiver 12 is substantially the same as
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the transmitter 10. The principal difference is that the receiver
AOM is driven by a frequency modulated signal of varying strength.
Thus, the receiver 12 comprises a helium-neon laser identical in
construction with laser 14 of the transmitter. The coherent
light beam of the laser is focused to produce a precisely dimen-
sioned Gaussian spot on a sheet of sensitized paper 40. The
sensitized paper 40 may, for example, comprise a drysilver paper
such as that developed and produced by the 3M Company of Minneapolis,
Minnesota. This paper, upon being exposed to a laser beam can be
heat processed to produce an image the tonal intensity of which is
a function of the intensity of the exposing laser.
The beam of laser 38 is reflected onto paper 40 off a
reciprocating mirror 42 and through an acousto-optic modulator
cell 44. As before, mirror 42 is reciprocated by a galvanometer
motor 46 to scan across the width of the paper as the sensitized
paper is advanced by rollers 48. The galvanometer drive 50 and
roller drive 52 are synchronized by suitable logic 54.
The drive circuit 56 for the acousto-optic modulator
produces an rf signal both amplitude and frequency modulated.
Thus, one input 58 to drive circuit 56 is the suitably amplified
output of the photodetector circuit of the transmitter. The other
input 60 to drive circuit 56 is a frequency modulated carrier.
Thus, the intensity of the laser beam impinging the sensitized
paper varies with tonal variations of the image being reproduced.
Additionally, the scanning spot undulates up and down within
controlled limits as it traverses each scan line.
In a successful practice of the present invention, the
velocity of the up-and-down motion of both the transmitting and
receiving spots (as oriented in Fig. 3) was approximately 100 times
faster than the horizontal scan velocity of the beam. That is, the
dithering motion of the spot was at a rate of 2,000 in./sec. and
the scan velocity was 19 in./sec. The band width of the signal
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was 2 KHz (500 Hz-2,500 Hz) and the dithering frequency was 120
KHz. It has been found that the dithering frequency must be at
least 10 times greater than the signal band width to prevent cross-
talk. The rf signal was frequency modulated by + 10 MHz. The
transmitting and receiving scan line height was approximately .010
inches and the spot diameter .004 inches. The facsimile trans-
mission from transmitter 10 to receiver 12 resulted in a reproduc-
tion virtually free of scan lines.
It should be appreciated that while the transmitter and
receiver of the present invention are disclosed herein in a com-
plete system, the components may be used with compatible conven-
tional equipment. That is, the receiver could receive transmitted
images from a conventional transmitter and the transmitter could
transmit images to a conventional receiver.
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