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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a scanning optical apparatus for scanning a light
beam from a light source on a scan surface through a deflector and a lens
system, and in particular to a scanning optical apparatus provided with a
mechanism for detecting and correcting the focus position deviation of the
imaged spot of a light beam on the scan surface which is attributable to
the fluctuation of the environmental conditions such as temperature, etc.
2. Related Background Art
In recent years, as scanning optical apparatuses, use has widely been made
of laser beam printers in which a laser source is modulated in conformity
with an image signal and a laser light from said modulated laser source is
periodically deflected by a deflector and is converged into a spot on a
scanning surface, e.g. a photosensitive recording medium, by a lens
system, and is exposure-scanned to thereby accomplish image recording.
Now, the conventional laser beam printers have suffered from the
disadvantage that each member constituting the lens system is thermally
deformed by a change in the environmental temperature and the converged
position of the laser beam on the photosensitive member (the scan surface)
deviates to deteriorate the quality of image.
As a means for overcoming such disadvantage, there is disclosed in Japanese
Laid-Open Pat. application No. 60-100113 a laser beam printer provided
with focus deviation detecting means for receiving a part of a beam
scanned on a photosensitive member and detecting focus deviation, and a
lens moving device having a correction lens for correcting said focus
deviation.
However, in the aforementioned Japanese Laid-Open Patent application No.
60-100113, an astigmatism system is employed as the focus deviation
detecting system. This system is effective for a stationary light beam,
but is unsuitable for the detection of the focus deviation of a light beam
scanned at a high speed.
Also, in Japanese Laid-Open Patent application No. 62-81873, there is shown
a system in which a plurality of light receiving elements arranged in a
direction perpendicular to the direction of scanning of a light beam are
used as focus deviation detecting means and the number of the light
receiving elements which have received the light beam is used as
information for focus adjustment.
However, in the system shown in this Japanese Laid-Open patent application,
the size of the diameter of the imaged spot in the sub-scanning direction,
which is a direction, perpendicular to the scanning direction, can be
detected, but the size of the diameter of the imaged spot in the main
scanning direction, which is, in the scanning direction cannot be
detected. In, a laser beam printer, the size of the diameter of the imaged
spot in the main scanning direction is usually smaller than the size of
the diameter of the imaged spot in the sub-scanning direction and
therefore, the imaged spot in the main scanning direction is more liable
to be affected by the fluctuation of the environmental conditions and
greatly changed than the imaged spot in the sub-scanning direction.
Consequently, the above-described system cannot provide a scanning optical
apparatus which prevents the deviation of the imaged spot attributable to
the fluctuation of the environmental temperature and is capable of
accurately scanning a laser beam. Further, in the system shown in the
above-mentioned publication, it is necessary that a predetermined number
of bits of information be memorized in advance in the apparatus, and if
the memory of the predetermined number of bits of information is lost due
to some trouble or other, detection of the focus position deviation
becomes impossible.
SUMMARY OF THE INVENTION
It is the object of the present invention to eliminate the above-noted
disadvantages peculiar to the prior-art apparatuses and to provide a
method of readily detecting and correcting any focus position deviation by
a simple construction.
The above object of the present invention is achieved by a scanning optical
apparatus provided with a detecting mechanism for scanning a light beam
from a light source on a scan surface through a deflector and a lens
system and detecting the converged state of the light beam on said scan
surface and a focus position moving mechanism for moving the converged
position of the light beam on said scan surface and wherein a detector for
receiving the light beam deflected by the deflector is used as the
detecting mechanism for detecting the converged state of the light beam on
said scan surface, and the light beam from said light source is scanned on
said detector a plurality of times through the deflector and the lens
system while the converged position of the light beam is moved by the
focus position moving mechanism for moving the converged position of the
light beam on said scan surface, and a signal from said detector during
each of said plurality of times of scanning is compared or
calculation-processed to thereby adjust and determine the converged
position of the light beam on said scan surface.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of the scanning optical apparatus of the
present invention.
FIG. 2 schematically shows the structure of a photodetector.
FIG. 3 shows signals corresponding to spots in various states which are
output from the photodetector.
FIGS. 4A and 4B are a concept diagram of a focus position detecting method
and a block diagram of the control thereof, respectively.
FIG. 5 is a flow chart of the method of FIG. 4.
FIGS. 6A and 6B are a concept diagram of another focus position detecting
method and a block diagram of the control thereof, respectively.
FIG. 7 is a flow chart of the method of FIG. 6.
FIGS. 8A and 8B are a concept diagram of still another focus position
detecting method and a block diagram of the control thereof, respectively.
FIG. 9 is a flow chart of the method of FIG. 8.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1 which illustrates a first embodiment of the scanning
optical apparatus of the present invention, the reference numeral 1
designates a light source device comprising a semiconductor laser and a
collimator lens, the reference numeral 2 denotes a lens system for
movement of the focus position, the reference numeral 3 designates a
rotational polygonal mirror which is a deflector, the reference numeral 4
denotes a drive motor for the rotational polygonal mirror, the reference
numeral 5 designates a scanning lens system, the reference numeral 6
denotes a photosensitive recording drum, the reference numeral 7
designates a spot detector, the reference numeral 8 denotes a detection
signal processing part for processing the electrical signal from the spot
detector 7, and the reference numeral 9 designates a controlling part for
generating a driving signal to the lens system 2 for movement of the focus
position in conformity with a signal output from the detection signal
processing part 8.
FIG. 2 shows a cross-sectional view in a horizontal plane of the spot
detector 7 having an opening limited in the scanning direction and the
detection signal processing part 8 in FIG. 1. The reference numeral 10
designates an opening plate having a predetermined opening 12, the
reference numeral 11 denotes a photoelectric converting surface, and the
reference numeral 13 designates a support member.
When a laser spot is scanned in the direction from A to B in FIG. 2 without
the laser source being modulated, that is, with the laser source emitting
light, the electrical signal 14 output from the spot detector 7 is
generally represented as shown in FIG. 3. If the opening plate 10 is
disposed on the extension of a laser beam scanning line (broken line) on
the photosensitive member surface 6 of FIG. 6, i.e., at a position
optically equivalent to the photosensitive member surface with respect to
the light source, and scanning is effected with the position of the laser
beam waist changed by moving the lens system 2 for movement of the focus
position, the output signal I has its output pattern changed as indicated
by S.sub.1, S.sub.2 and S.sub.3 relative to time t. That is, as a result
of the spot detector having been scanned with the position of the beam
waist changed during each scan, there appears an output pattern in which
the central intensity becomes greatest or an output pattern in which the
half value width becomes smallest. The lens system 2 for movement of the
focus position is operated by the controlling part 9 so as to assume the
position of the beam waist (the converged position of the light beam)
corresponding to that pattern. The control of detecting and correcting
such focus position deviation may be effected before recording of
information of a certain definite unit is effected, or may be executed
with the laser oscillated near only the spot detector during the blanking
period (the period during which formation of an image is not effected for
a long time) of the time during which recording and scanning is effected
(the time during which the light source is modulated and formation of an
image is effected on the photosensitive member surface). Of course, said
controlling operation is stopped when recording an scanning is being
effected. Alternatively, said controlling operation may be executed during
the blanking period (the period from after image recording on a certain
recording sheet is completed until image recording on the next recording
sheet is started) of the time during which image recording is effected on
a predetermined plurality of recording sheets, for example, of size A4, A3
or the like. Of course, said controlling operation is stopped when image
recording is being effected on the recording sheets. Also, a timer may be
connected to the controlling part 9 of the apparatus, the controlling part
may be operated for each predetermined time, the laser beam may be scanned
a plurality of times and adjustment of the focus position may be effected.
Also, a sensor for detecting any change in the environmental conditions
such as temperature and humidity may be provided in the apparatus, and
when a great change in the environmental conditions is detected by this
sensor, said focus position adjustment operation may be performed.
The signal processing system of the first embodiment will now be described.
In the present invention, it is to be understood that curvature of image
field is sufficiently corrected by the scanning lens system 5. Therefore,
adjustment of the focus position ca be effected only at a point on the
scanning line.
Peak Value Detection Focus Adjustment
A system for effecting focus position adjustment on the basis of the peak
value of the quantity of light passed through an opening (slit) will
hereinafter be described with reference to FIG. 4.
The quantity of light passed through the spot detector 7 in FIG. 1 has its
peak value varied by the position in the direction of the optical axis.
The peak value becomes maximum at the focus position and therefore, it can
be found out. The spot detector 7 is comprised of photoelectric conversion
101 and a predetermined opening.
FIG. 4A is a concept diagram of it, and FIG. 4B is a block diagram.
The signal processing system for focus position adjustment operates with
the detection signal processing part 8 as the center. FIG. 5 is a flow
chart.
Prior to recording an information signal, adjustment of the focus position
is effected. The signal of the controlling part 9 is sent to a scan
position counter 107, the scan position counter is set to a position 1,
and by the position command of the scan position counter 107, a driving
device 110 for focusing moves the lens system 2 to the initial position 1
in the direction of the optical axis. As shown in FIG. 1, scanning of the
scan beam is effected by scan means in a direction orthogonal to the
optical axis. The position information of the position in the direction of
the optical axis which is always smaller by 1 is input from the scan
position counter 107 to a scan position counter-1 108. Thereafter, at each
scan, the scan position counter 107 is stepwisely advanced by the output
of the controlling part 9.
The output of the scan position counter 107 acts as a position command on
the driving device 110 for focusing. B this command, the driving device
110 for focusing moves the position of the scan beam in the direction of
the optical axis to a position 2, . . . , a position k, . . . , a position
n, and effects scanning. Here, it is to be understood that the focus
position is between these moved positions. If the focus position is not
between these moved positions, the initial position can be moved.
The output of the photoelectric conversion 101 is shown in FIG. 4A. As
shown, the output of the photoelectric conversion 101 is an electrical
output proportional to the quantity of light passed through the silt, and
the peak value thereof varies correspondingly to a change in the position
in the direction of the optical axis.
The output of the photoelectric conversion 101 has its peak value held by a
peak value hold 102. This state is shown by the peak value hold circuit
output of FIG. 4A. At each scan (i.e., each position in the direction of
the optical axis), the peak value obtained by the last scan is reset by
the output of the controlling part.
The held peak value is converted from an analog voltage into a digital
amount by an A/D converter 103. This digital amount is held by a register
104 for the latest peak value. By the starting of the next scan, the value
held by the register 104 for the latest peak value is transferred to a
register 105 for the peak value one before. The register 105 for the peak
value one before is cleared prior to measurement.
The output of the register 104 for the latest peak value and the output of
the register 105 for the peak value one before are discriminated by a
magnitude discrimination circuit 106 as to whether the magnitudes thereof,
i.e., the peak values thereof, are increased or decreased. If the polarity
of the output of the magnitude discrimination circuit 106 has changed, it
is seen that it has passed the position in the direction of the optical
axis at which the peak value becomes maximum. The position information one
before the current position is input to the scan position counter-1 108
and therefore, if the polarity of the output of the magnitude
discrimination circuit 106 has changed, that position in the direction of
the optical axis is memorized. This position is the focus position. The
position information of the scan position counter-1 108 is transferred to
the scan position counter-1 108. Lens system 2 is returned to the position
one before by the driving device 110 for focusing, and a stop command is
put out from the scan position counter 107 to the controlling part 9,
whereby adjustment of the focus position is completed. For that purpose,
the position of the scan beam in the direction of the optical axis is
fixed at the focus position by the driving device 110 for focusing. The
focus position of the scan beam is always fixed on the photosensitive
recording drum 6 which is at the same position in the direction of the
optical axis as the spot detector 7.
Variance .sigma. Detection Focus Adjustment
A system for effecting focus position adjustment by the variance .sigma.
value of the distribution of the quantity of light passed through the
opening (slit) will hereinafter be described with reference to FIG. 6.
The distribution of the quantity of light passed through the spot detector
7 in FIG. 1 has its variance .sigma. value varied by the position in the
direction of the optical axis. The variance .sigma. value becomes minimum
at the focus position and therefore, it can be found out. FIG. 6A is a
concept diagram of it, and FIG. 6B is a block diagram. FIG. 7 is a flow
chart.
Prior to recording an information signal, adjustment of the focus position
is effected. The signal of the controlling part 9 is sent to the scan
position counter 107, and the scan position counter is set to a position
1. The operations of the scan position counter 107, the driving device 110
for focusing, the scan position counter-1 108 and the photoelectric
conversion 101 are similar to those previously described and therefore
need not be described.
The output of the photoelectric conversion 101 is an electrical output
proportional to the quantity of light passed through the slit, and the
variance .sigma. value thereof varies correspondingly to a change in the
position in the direction of the optical axis.
The output of photoelectric conversion 101 is sampled in a sampling circuit
111 by the sampling clock of a sampling clock generation circuit 115 and
the sample value thereof is held. The sampling clock may desirably be
thirty or more times the scan repetition frequency. The held sample value
is converted from an analog voltage into a digital amount by an A/D
converter 103.
This digital amount has its variance .sigma. value calculated by .sigma.
calculation means 112 during each scan. The .sigma. calculation means 112
may preferably be a combination of a microprocessor, ROM and RAM. The
output of the .sigma. calculation means 112 is memorized by a register 113
for the latest .sigma..
By the starting of the next scan, the value held by the register 113 for
the latest .sigma. is transferred to a register 114 for .sigma. one
before. A value much greater than the variance value .sigma. coming out
during the measuring operation is input to the register 114 for .sigma.
one before, prior to measurement.
The output of the register 113 for the latest .sigma. and the output of the
register 114 for .sigma. one before are discriminated by the magnitude
discrimination circuit 106 as to whether their magnitudes, i.e., the
variance .sigma. values, are increased or decreased. If the polarity of
the output of the magnitude discrimination circuit 106 has changed, it is
seen that it has passed the position in the direction of the optical axis
at which the variance .sigma. value becomes minimum. The position
information one before the current position is input to the scan position
counter-1 108 and therefore, if the polarity of the output of the
magnitude discrimination circuit 106 has changed, that position in the
direction of the optical axis is memorized. This position is the focus
position. The operation thereafter is similar to that of the previously
described example.
Time Measuring Focus Adjustment
A system for effecting focus position adjustment by measuring the time for
which the quantity of light passed through the slit is at a predetermined
level or higher will hereinafter be described with reference to FIG. 8.
The quantity of light passed through the spot detector 7 in FIG. 1 differs
in the distribution in the scan direction depending on the position in the
direction of the optical axis and therefore, the time for which said
quantity of light is at a predetermined level or higher. This time becomes
minimum at the focus position and therefore, it can be found out. FIG. 8A
is a concept diagram of it, and FIG. 8B is a block diagram. FIG. 9 is a
flow chart.
Prior to recording an information signal, adjustment of the focus position
is effected. The signal of the controlling part 9 is sent to the scan
position counter 107. The operations of the scan position counter 107, the
driving device 110 for focusing, the scan position counter-1 108 and the
photoelectric conversion 101 are similar to those previously described and
therefore need not be described.
The output of the photoelectric conversion 101 is an electrical output
proportional to the quantity of light passed through the slit, and the
time vs. electrical output thereof varies correspondingly to a change in
the position in the direction of the optical axis.
The output of the photoelectric conversion 101 is binarized by a binarizing
circuit 116. The output wave form thereof is shown in the middle stage of
FIG. 8A. A time measuring circuit 117 measures time with a clock 118 for
time measuring as the measurement unit. The clock for time measuring may
be arbitrary, but may desirably be thirty or more times the scan
repetition frequency.
The output of the time measuring circuit 117 is memorized by a register 119
for the latest time. By the starting of the next scan, the value held by
the register 119 for the latest time is transferred to a register 120 for
the time one before. Prior to measurement, a value much greater than the
value coming out during measurement is input to the register 120 for the
time one before.
The output of the register 119 for the latest time and the output of the
register 120 for the time one before are discriminated by the magnitude
discrimination circuit 106 as to whether their magnitudes, i.e., the time
of a predetermined level or higher, is increased or decreased. If the
polarity of the output of the magnitude discrimination circuit 106 has
changed, it is seen that it has passed the position in the direction of
the optical axis at which the time of a predetermined level or higher
becomes minimum.
The position information one before the current position is input to the
scan position counter-1 108 and therefore, if the polarity of the output
of the magnitude discrimination circuit 106 has changed, that position in
the direction of the optical axis is memorized. This position is the focus
position. The operation thereafter is similar to that of the previously
described example.
In the examples shown above, the collimator lens system i | | |
