Title: Lens unit for multibeam scanning device
Abstract: A lens unit for a scanning device includes a frame having a hollow cylindrical shape, the frame being defined with a lens contact portion therein, a lens accommodated in the frame contacting the lens contact portion, and a retainer accommodated in the frame to retain the lens in position, the retainer having a hollow cylindrical shape, one end side face of the retainer contacting a peripheral portion of the lens received by the frame, an other end portion of the retainer being secured to the frame so that the retainer presses the lens toward the lens contact portion of the frame to fix the lens to the frame. Deformation of the frame, lens and retainer due to the load generated as the retainer presses the lens absorbs deformation of the frame, lens and retainer due to temperature change at least within a predetermined temperature range.
Patent Number: 7,002,759 Issued on 02/21/2006 to Hama, et al.
| Inventors:
|
Hama; Yoshihiro (Saitama-ken, JP);
Hirano; Masakazu (Tokyo, JP)
|
| Assignee:
|
PENTAX Corporation (Tokyo, JP)
|
| Appl. No.:
|
804130 |
| Filed:
|
March 19, 2004 |
Foreign Application Priority Data
| Mar 20, 2003[JP] | 2003-078157 |
| Current U.S. Class: |
359/820; 359/819; 359/811 |
| Current Intern'l Class: |
G02B 7/02 (20060101) |
| Field of Search: |
359/820,819,811
348/202,195
|
References Cited [Referenced By]
U.S. Patent Documents
| 4723833 | Feb., 1988 | Yamada.
| |
| 6198562 | Mar., 2001 | Hayashi et al.
| |
| 6867848 | Mar., 2005 | Ebinuma et al.
| |
| 2002/0163741 | Nov., 2002 | Shibazaki.
| |
| 2003/0179470 | Sep., 2003 | Sudoh.
| |
| Foreign Patent Documents |
| 6-94957 | Apr., 1994 | JP.
| |
| 6-130267 | May., 1994 | JP.
| |
| 7-191247 | Jul., 1995 | JP.
| |
| 7-225953 | Aug., 1995 | JP.
| |
| 8-300725 | Nov., 1996 | JP.
| |
| 11-38344 | Feb., 1999 | JP.
| |
| 2000-75226 | Mar., 2000 | JP.
| |
| 2000/-249948 | Sep., 2000 | JP.
| |
| 2001-4941 | Jan., 2001 | JP.
| |
| 2001/-194605 | Jul., 2001 | JP.
| |
Other References
English Language Abstract of JP 6-130267.
English Language Abstract of JP 6-94957.
English Language Abstract of JP 7-225953.
English Language Abstract of JP 11-38344.
English Language Abstract of JP 2000-75226.
|
Primary Examiner: Schwartz; Jordan M.
Assistant Examiner: Stultz; Jessica
Attorney, Agent or Firm: Greenblum & Bernstein, P.L.C.
Claims
What is claimed is:
1. A lens unit for a scanning device, comprising:
a frame having a hollow cylindrical shape, the frame being defined with a lens
contact portion therein;
a lens accommodated in the frame contacting the lens contact portion defined
in the frame; and
a retainer accommodated in the frame to retain the lens in position, the retainer
having a hollow cylindrical shape, one end side face of the retainer contacting
a peripheral portion of the lens received by the frame, another end portion of
the retainer being secured to the frame so that the retainer presses the lens toward
the lens contact portion of the frame to fix the lens to the frame,
wherein deformation of the frame, lens and retainer due to the load generated
as the retainer presses the lens absorbs deformation of the frame, lens and retainer
due to temperature change at least within a predetermined temperature range so
that a fixed status of the lens with respect to the frame is not released due to
the temperature change within the predetermined temperature range,
wherein the lens has a linear expansion coefficient ρ
1, a longitudinal
elastic modulus E
1, and a cross-sectional area S
1 orthogonal
to an optical axis direction,
wherein the frame has a linear expansion coefficient ρ
2, a longitudinal
elastic modulus E
2, and a cross-sectional area S
2 orthogonal
to the optical axis direction, in which the optical system is installed,
wherein the retainer has a linear expansion coefficient ρ
3,
a longitudinal elastic modulus E
3, and a cross-sectional area S
3
orthogonal to the optical axis direction, the retainer applying the lens with a
load P,
wherein the lens unit being configured to satisfy following condition:
##EQU3##
wherein,
L
1 represents a length of the lens from a contact point of the lens
and the lens contact portion of the frame to a contact point of the lens and the
retainer in the optical axis direction at a predetermined temperature t
0,
L
2 represents a length of the frame from the contact point of the
lens and the lens contact portion of the frame to a lens side end of the other
end portion of the retainer at a predetermined temperature t
0,
L
3 represents a length of the retainer from the contact point of the
lens and the retainer to the lens side end of the other end portion of the retainer
at a predetermined temperature t
0,
wherein L
2=L
1+L
3, and
wherein Δt represents a change of temperature with respect to the predetermined temperature.
2. The lens unit according to claim 1, wherein the scanning device is a multibeam
scanning device which simultaneously scans a plurality of light beams emitted by
multiple light emitting elements on a scan target surface by dynamically deflecting
the light beams by use of a deflecting system, the lens unit being used for each
of the multiple light beams.
3. The lens unit according to claim 1, wherein the other end portion of the retainer
is formed of a screw thread portion, and where an inner surface of the frame at
a portion facing the other end portion of the retainer is formed of a screw thread
portion to engage with the screw thread portion of the retainer.
4. The lens unit according to claim 1, wherein materials and lengths of the frame,
lens and retainer are determined to satisfy a following condition:
ρ
2L2=ρ
1L1+ρ
3L3.
5. The lens unit according to claim 1, wherein the predetermined temperature
range is a range from -20° C. to +70° C.
6. The lens unit according to claim 1, wherein the predetermined temperature
t
0 is closer to the upper end of the predetermined temperature range
than the lower end thereof, and wherein ρ
2L
2<ρ
1L
1+ρ
3L
3.
7. The lens unit according to claim 1, wherein the predetermined temperature
t
0 is closer to the lower end of the predetermined temperature range
than the upper end thereof, and wherein ρ
2L
2>ρ
1L
1+ρ
3L
3.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a lens unit for a multibeam scanning device,
which simultaneously scans a plurality of light beams emitted by multiple light
emitting elements on a scan target surface (e.g. the surface of a photoconductive
drum) by dynamically deflecting the light beams by use of a deflecting system.
Scanning devices for scanning a light beam emitted by a light emitting element
on a scan target surface by dynamically deflecting the light beam by a deflecting
system have been widely known. However, image formation speed of such single-beam
scanning devices (forming images by scanning only one light beam on the scan target
surface) is generally low. For increasing the image formation speed, multibeam
scanning devices, which simultaneously scan a plurality of light beams emitted
by multiple light emitting elements on the scan target surface by dynamically deflecting
the light beams by use of a deflecting system, have recently been proposed (e.g.,
Japanese Patent Provisional Publication P2001-194605A) and have been in practical
use widely.
A lens holding mechanism capable of preventing distortion of a lens by absorbing
deformation of the lens caused by temperature variation has been proposed in Japanese
Patent Provisional Publication No. HEI 07-191247 (pages 2-5, FIGS. 1, 6 and 9).
The lens holding mechanism places elastic material between the lens and a holding
frame which holds the lens so that temperature-dependent variations (especially,
lens deformation caused by thermal expansion at high temperatures) will be absorbed
by the elastic material. The elastic material prevents the lens distortion by absorbing
the lens deformation mainly in the radial direction.
The patent document Japanese Patent Provisional Publication No. HEI 07-191247
also proposes a countermeasure against lens deformation in the thrust direction,
in which lens distortion caused by the lens deformation in the thrust direction
is avoided by fixing the lens by vertically sandwiching the lens between the holding
frame and a lens retainer, by bonding the elastic material to the lens and vertically
sandwiching only the elastic material between the holding frame and the lens retainer, etc.
In these multibeam scanning devices, it is essential to maintain each distance
between beam spots (formed on the scan target surface by the light beams) with
high precision corresponding to a prescribed desired resolution throughout the
scanning period. In other words, relative positions among optical paths of the
light beams have to be maintained substantially constant. However, when an expensive
one-chip (integrated) multibeam laser diode unit or a multi-chip laser diode unit
is employed for the multibeam scanning device, it is impossible in many cases to
maintain the distances among the beam spots formed on the scan target surface (measured
in the scanning direction) since each component of the chip moves slightly due
to temperature variation.
Further, the position of the collimating lens employed in the multibeam
scanning device, as the outlet of the laser diode unit for the light beam, has
an important effect on the position of the beam spot on the scan target surface.
For example, if the collimating lens slightly moves relative to the light beam
emitted from the laser diode, the beam spot on the scan target surface moves three
to ten times as long as the displacement of the collimating lens. Therefore, in
conventional multibeam scanning devices, a sensor is generally placed at a position
equivalent to the scan target surface and each distance between the beam spots
is monitored and fed back, that is, each distance between the beam spots is maintained
to be constant by means of a closed-loop system.
However, providing the multibeam scanning device with such a feedback control
mechanism (closed-loop control mechanism) leads to upsizing, complication and high
cost of the device.
In the method proposed in Japanese Patent Provisional Publication No. HEI 07-191247,
the lens are vertically sandwiched between the holding frame and the lens retainer
in order to resolve the lens deformation problem in the thrust direction, each
component of the lens holding mechanism contracts in low ambient temperatures,
by which clearance occurs among the components and the lens moves relative to the
holding frame. Therefore, in the highly sensitive multibeam scanning devices, the
lens holding mechanism having such composition can not successfully maintain the
beam spot intervals on the scan target surface (measured in the scan direction)
to be constant.
Similarly, in the method of Japanese Patent Provisional Publication No.
HEI 07-191247 bonding the elastic material to the lens and vertically sandwiching
only the elastic material between the holding frame and the lens retainer as the
countermeasure against lens deformation in the thrust direction, injecting adhesives
between the small-sized lens installed in the multibeam scanning device and the
elastic material is difficult, and the increase of steps in the manufacturing process
leads to high cost. Further, once the elastic material is bonded to the lens, the
lens cannot be separated from the elastic material and thus both have to be discarded
when quality of one of them deteriorates. Further, in cases where such adhesives
are used, thermal expansion of the adhesives accompanying temperature variation
might have ill effects on the lens.
SUMMARY OF THE INVENTION
The present invention is advantageous in that an improved lens unit for a multibeam
scanning device is provided. Employing the lens unit, the multibeam scanning device
can maintain the beam spot intervals on the scan target surface with high accuracy
even if the ambient temperature changes, without the need of the mechanism for
monitoring the beam spot intervals and executing the feedback control and without
the use of adhesives for fixing the optical system to the frame or lens holding mechanism.
According to the invention, there is provided a lens unit for a scanning
device, which includes a frame having a hollow cylindrical shape, the frame being
defined with a lens contact portion therein, a lens accommodated in the frame with
contacting the lens contact portion defined in the frame, and a retainer accommodated
in the frame to retain the lens in position, the retainer having a hollow cylindrical
shape, one end side face of the retainer contacting a peripheral portion of the
lens received by the frame, an other end portion of the retainer being secured
to the frame so that the retainer presses the lens toward the lens contact portion
of the frame to fix the lens to the frame. In the lens unit constructed as above,
deformation of the frame, lens and retainer due to the load generated as the retainer
presses the lens absorbs deformation of the frame, lens and retainer due to temperature
change at least within a predetermined temperature range so that a fixed status
of the lens with respect to the frame is not released regardless of the temperature
change within the predetermined temperature range.
Optionally, the scanning device is a multibeam scanning device which
simultaneously scans a plurality of light beams emitted by multiple light emitting
elements on a scan target surface by dynamically deflecting the light beams by
use of a deflecting system, the lens unit being used for each of the multiple light beams.
Further optionally, the other end portion of the retainer is formed of a
screw thread portion, and where an inner surface of the frame at a portion facing
the other end portion of the retainer is formed of a screw thread portion to engage
with the screw thread portion of the retainer.
Still optionally, the lens has a linear expansion coefficient ρ
1,
a longitudinal elastic modulus E
1, and a cross-sectional area S
1
orthogonal to an optical axis direction, the frame has a linear expansion coefficient
ρ
2, a longitudinal elastic modulus E
2, and a cross-sectional
area S
2 orthogonal to the optical axis direction, in which the optical
system is installed, and the retainer has a linear expansion coefficient ρ
3,
a longitudinal elastic modulus E
3, and a cross-sectional area S
3
orthogonal to the optical axis direction, the retainer applying the lens
with a load P. The lens unit may be configured to satisfy the following condition:
##EQU1##
wherein,
L
1 represents a length of the lens from a contact point
of the lens and the lens contact portion of the frame to a contact point of the
lens and the retainer in the optical axis direction at a predetermined temperature t
0,
L
2 represents a length of the frame from the contact point
of the lens and the lens contact portion of the frame to a lens side end of the
other end portion of the retainer at a predetermined temperature t
0,
L
3 represents a length of the retainer from the contact
point of the lens and the retainer to the lens side end of the other end portion
of the retainer at a predetermined temperature t
0,
L2=L1+L3, and
Δt represents a change of temperature with respect to the predetermined temperature.
In a particular case, the materials and lengths of the frame, lens and retainer
are determined to satisfy a condition: ρ
2L
2=ρ
1L
1+ρ
3L
3.
Optionally, the predetermined temperature range is a range from -20°
C. to +70° C.
Still optionally, when the predetermined temperature to is closer to the upper
end of the predetermined temperature range than the lower end thereof, and ρ
2L
2<ρ
1L
1+ρ
3L
3.
On the contrary, when the predetermined temperature to is closer to the lower end
of the predetermined temperature range than the upper end thereof, and ρ
2L
2>ρL
1+ρ
3L
3.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1 is a schematic diagram showing the optical composition of a multibeam
scanning device in accordance with an embodiment of the present invention;
FIG. 2 is an enlarged view showing part of the multibeam scanning device around
a prism;
FIG. 3A is a top view showing an example of a light source unit that is employed
for the multibeam scanning device:
FIG. 3B is a side view of the light source unit of FIG. 3A;
FIG. 3C is a front view of the light source unit of FIG. 3A;
FIG. 4 is a cross-sectional view showing a cross section around the optical
axis of a lens unit which is mounted on a support of the light source unit; and
FIG. 5 shows a cross-sectional side view of a modification of the lens unit
shown in FIG. 4.
DESCRIPTION OF THE EMBODIMENTS
Referring now to the drawings, a description will be given in detail of
preferred embodiments in accordance with the present invention.
FIG. 1 is a schematic diagram showing the optical composition of a multibeam
scanning device
100 in accordance with an embodiment of the present invention.
As shown in FIG. 1, the multibeam scanning device
100 includes first and
second light emitting elements
102 and
104. The first and second
light emitting elements
102 and
104 (e.g. laser diodes) emit first
and second light beams
106 and
108, respectively. In this embodiment,
the first and second light beams
106 and
108 are emitted from the
first and second light emitting elements
102 and
104 to be on a plane
orthogonal to the rotation axis of a polygon mirror which will be described below,
and preferably, to be parallel with each other.
The first light beam
106 emitted by the first light emitting element
102
is collimated by a collimating lens
122 into a parallel light beam, and
is incident on a prism
124. The prism
124 shifts the optical paths
of the first light beam
106 toward the second light beam
108, which
is emitted by the second light emitting element
104. The first light beam
106 passes through a cylindrical lens
112 and a slit
128,
and is incident upon a reflecting surface
114a of the polygon mirror
114. The cylindrical lens
112 has refractive power for converging
the light beam only in the direction parallel to the rotation axis
114b
of the polygon mirror
114 (auxiliary scanning direction) so that the
light beam is converged in the auxiliary scanning direction in the proximity of
the reflecting surface
114a.
The first light beam
106 is then reflected by the reflecting surface
114a,
passes through an fθ lens
118, and is focused on the scan target surface
120. According to the rotation of the polygon mirror
114 at a constant
revolving speed, a beam spot of the first light beam
106 focused on the
scan target surface
120 moves on the scan target surface
120 at a
substantially constant speed. The direction of the movement of the first beam spot
106 on the scan target surface will be referred to as a "main scanning direction".
A direction perpendicular to the main scanning direction and parallel to the scan
target surface
120 will be referred to as an "auxiliary scanning direction."
Incidentally, the "main scanning direction" can be defined not only
on the scan target surface
120 but also at any point on the optical path
of the light beam, as a direction regarding the main scan of the light beam, that
is, the direction in which the light beam is dynamically deflected by the polygon
mirror
114 or the direction in which the light beam moves according to the
revolution of the polygon mirror
114. The "auxiliary scanning direction"
can also be defined at any point on the optical path of the light beam as a direction
orthogonal to the main scanning direction.
Meanwhile, the second light beam
108 emitted from the second light
emitting element
104 is collimated by a collimating lens
110 into
a parallel light beam, passes through a position adjustment element
126,
the cylindrical lens
112, the slit
128, and is incident upon the
polygon mirror
114. It should be noted that the second light beam
108
incident on substantially the same incident position on the reflecting surface
114a as the first light beam
106. Therefore, the first and
second light beams
106 and
108 are not exactly parallel with each
other on the polygon-mirror side of the prism
124, rather a tilt angle θ
is formed therebetween in the revolving direction of the polygon mirror
114
as shown in FIG. 1.
The second light beam
108 reflected by the reflecting surface
114a
of the polygon mirror
114 further proceeds to pass through the fθ
lens
118 and is then incident upon the scan target surface
120 to
form a beam spot which moves in the main scanning direction.
On the optical path of the second light beam
108 between the collimating
lens
110 and the cylindrical lens
112, a position adjustment element
126 for adjusting the position of the second light beam
108 is placed.
The position adjustment element
126 is a wedge prism which has a wedge-shaped
sectional form on a plane parallel to its optical axis, for example. In this embodiment,
the height of incident position of the second light beam
108 on the cylindrical
lens
112 is adjusted by adjusting the placement of the position adjustment
element
126. Here, the "height" means a position in the auxiliary scanning
direction. The incident height of the second light beam
108 is adjusted
to be a preset distance (height) different from that of the first light beam
106,
by which the second light beam
108 incident on and reflected by the reflecting
surface
114a of the polygon mirror
114 has a slight tilt angle
with the first light beam
106. In other words, the position adjustment element
126 adjusts the angle (in the auxiliary scanning direction) of the second
light beam
108 incident on the polygon mirror
114. As a result of
the angle adjustment, the second light beam
108 scans on a scan line on
the scan target surface that is a preset interval apart in the auxiliary scanning
direction from a scan line formed by the first light beam
106. Incidentally,
the angle adjustment (adjustment of the incident angle of the second light beam
108 on the polygon mirror
114 in the auxiliary scanning direction
by use of the position adjustment element
126) is made as a step in the
manufacturing process of the multibeam scanning device
100.
As described above, the multibeam scanning device
100 includes the slit
128 which is placed between the cylindrical lens
112 and the polygon
mirror
114. The slit
128 has a narrow opening extending in a direction
parallel to a plane orthogonal to the rotation axis of the polygon mirror
114.
The shapes (beam widths, etc.) of the first and second light beams
106 and
108 are regulated by the narrow opening of the slit
128 so as to
have substantially identical sectional forms, by which effective beams of the first
and second light beams
106 and
108 are formed.
FIG. 2 is an enlarged view showing part of the multibeam scanning device
100
around the prism
124. As shown in FIG. 2, the prism
124 has an entrance
surface
124a through which the first light beam
106 enters
the prism
124, first and second reflecting surfaces
124b and
124c coated with reflective layers for reflecting the first light
beam
106, and an emerging surface
124d from which the first
light beam
106 is emeerged.
The first light beam
106 enters the prism
124 through part of the
entrance surface
124a including the angular part formed by the entrance
surface
124a and the first reflecting surface
124b.
The entrance surface
124a may be coated with an antireflective layer
in order to promote the transmission of the first light beam
106.
After entering the prism
124, the first light beam
106 is reflected
by the first reflecting surface
124b (having a total reflection coating
or reflective coating thereon) toward the second reflecting surface
124c.
The first light beam
106 is reflected again by the second reflecting surface
124c and then emerges from the emerging surface
124d toward
the polygon mirror
114.
In another angular part formed by the second reflecting surface
124c
and the emitting surface
124d, a chamfered part
124e
is formed. The first light beam
106 reflected by the first reflecting
surface
124b is incident not only on the second reflecting surface
124c but also on the chamfered part
124e. In this case,
if the edge part of the second reflecting surface
124c nearby the
chamfered part
124e is a mirror surface, there is a possibility that
the first light beam
106 passes through or is reflected by the edge part
toward a particular direction and affects the image formation; however, the surface
of the chamfered part
124e is processed so as to defuse light incident
thereon. For example, the chamfered part
124e of this embodiment
is formed to have a frosted or ground surface having surface roughness of approximately
#400-#800. Therefore, the first light beam
106 incident on the chamfered
part
124e is scattered around, without such transmission or reflection
causing high light intensity in a particular direction.
An edge part of the second reflecting surface
124c adjoining the
emitting surface
124d is placed in the optical path of the second
light beam
108. Therefore, part of the second light beam
108 is incident
on the edge part of the second reflecting surface
124c. Since the
second reflecting surface
124c is provided with a total reflection
coating or reflective coating as mentioned above, the second light beam
108
incident on the edge part is reflected away and does not travel toward the polygon
mirror
114. In other words, the edge part of the second reflecting surface
124c blocks a small part of the second light beam
108.
As above, the edge part of the second reflecting surface
124c reflects
the first light beam
106 toward the polygon mirror
114 while blocking
a small part of the second light beam
108. Thus, on the light emerging side
of the prism
124, the first light beam
106 emerges from the area
blocking the second light beam
108 and consequently, the first and second
light beams
106 and
108 adjoin each other with no gap at the emerging
surface
124d. As mentioned before, the first and second light beams
106 and
108 incident upon the polygon mirror
114 have a slight
tilt angle θ (in the revolving direction of the polygon mirror
114)
therebetween. Since the first and second light beams
106 and
108
adjoin each other with no gap at the emitting surface
124d of the
prism
124 (i.e., the interval between the two light beams is extremely small),
the tilt angle θ between the two light beams also becomes extremely small.
The multibeam scanning device
100 shown in FIG. 1 can be manufactured
by, for example, preparing a light source unit having the first and second light
emitting elements
102 and
104, etc. mounted on a support or frame,
and thereafter installing the light source unit in the cabinet of the multibeam
scanning device
100. FIGS. 3A,
3B and
3C are a top view, a
side view and a front view showing an example of such a light source unit
150,
respectively. The light source unit
150 includes a support (substrate)
152
on which the first and second light emitting elements
102 and
104,
lens units
130 and
140, the position adjustment element
126,
the prism
124, the cylindrical lens
112 and the slit
128 are mounted.
The first and second light emitting elements
102 and
104 are mounted
on the support
152 so that they can emit light beams substantially on the
same plane and almost in the same direction, that is, light beams almost parallel
with each other. Such a structure of mounting the light emitting elements
102
and
104 is convenient in that electric circuits (unshown) for driving the
light emitting elements can be placed on the back of the light emitting elements
(opposite to the emitting side of the light emitting elements) as a single unit.
FIG. 4 is a cross-sectional view showing a cross section around the optical
axis of the lens unit
130 which is mounted on the support
152 of
the light source unit
150. The lens unit
130, placed between the
second light emitting element
104 and the position adjustment element
126,
includes the collimating lens
110, a lens support frame
132 having
a cylindrical shape for supporting the collimating lens
110, and a lens
retainer ring
134 having a cylindrical shape for pressing and retaining
the collimating lens
110 inside the lens support frame
132. In the
embodiment, the lens unit
140 mounted on the support
152 of the light
source unit
150 has the similar structure, and thus detailed description
thereof is omitted here.
In the following, a process for assembling the lens unit
130 will be explained
in detail.
First, the lens support frame
132 is held so that its positioning protrusion
132a (formed on its interior surface) faces downward and its optical
axis is oriented in the vertical direction. Subsequently, the collimating lens
110 is dropped and set in the lens support frame
132 letting its
surface
110a contact the positioning protrusion
132a of
the lens support frame
132.
The interior surface of the lens support frame
132 is provided with a
screw thread part
132b. Meanwhile, the exterior surface of the lens
retainer ring
134 is provided with another screw thread part
134b
to engage with the screw thread part
132b. As the lens retainer
ring
134 is screwed into the lens support frame
132, the front face
of the lens retainer ring
134 approaches the collimating lens
110,
and eventually the collimating lens
110 is sandwiched between the positioning
protrusion
132a and the front face of the lens retainer ring
134.
By further screwing the lens retainer ring
134, the collimating lens
110
pressed by the front face of the lens retainer ring
134 is fixed in the
lens support frame
132 and thereby the assembly of the lens unit
130
is completed.
The collimating lens
110 employed for this embodiment is implemented by,
for example, a glass lens formed of BK7 or quartz (silica) having a linear expansion
coefficient ρ
1, a longitudinal elastic modulus E
1,
and a cross-sectional area (taken along a plane perpendicular to the optical axis
thereof, and including a peripheral portion thereof) of approximately S
1.
The lens support frame
132 is implemented with, for example, a brass frame
having a linear expansion coefficient ρ
2, a longitudinal elastic
modulus E
2, and a cross-sectional area (which is an annular area) of
approximately S
2 orthogonal to the optical axis. It should be noted
that the lens support frame
132 has different cross-sectional shapes depending
on a position in the longitudinal direction, and the area S
2 represents
a mean value of the cross-sectional areas within an L
2 part of the lens
support frame
132. The lens retainer ring
134 is implemented by,
for example, an aluminium ring having a linear expansion coefficient ρ
3,
a longitudinal elastic modulus E
3, and a cross-sectional area of approximately
S
3 orthogonal to the optical axis. It should be noted that the lens
retainer ring
134 also has different cross-sectional shapes depending on
a position in the longitudinal direction, and the area S
3 represents
a mean value of the cross-sectional areas within an L
3 part of the lens
retainer ring
134.
When the assembly of the lens unit
130 is completed, the length from
the contacting surface of the positioning protrusion
132a (contacting
the surface
110a of the collimating lens
110) to a contact
point P
1 of the collimating lens
110 (contacting the lens retainer
ring
134) becomes L
1, and the length from the contact point P
1
to an end (i.e., the left-hand side end in FIG. 4) of the engaging part where
the screw thread part
132b engages with the screw thread part
134b
on the collimator-lens side becomes L
3. Therefore, the distance
from the contacting surface of the positioning protrusion
132a to
the end of the engaging part becomes L
2=L
1 +L
3.
Hereinafter, an expression "L
2 part of the lens support frame
132"
means a part of the lens support frame
132 between the contacting surface
of the positioning protrusion
132a and the end of the engaging part.
"L
1 part of the collimating lens
110" and "L
3 part
of the lens retainer ring
134" are also defined similarly. That is, L
1
part, L
2 part and L
3 part refer to portions of respective
elements, while L
1, L
2 and L
3 refer to lengths
thereof. It should be noted that the lengths L
1, L
2 and L
3
are those when no pressure is applied and the circumferential temperature
is a reference temperature t
0, which is 20° C. in the embodiment.
The lens unit
130 is designed (i.e., material and size of each member
is determined) to satisfy the following condition (1) in its completed state when
the ambient temperature t is within a range between -20° C. and 70° C.
##EQU2##
where, L
2=L
1+L
3, and Δt=t-t
0.
The following TABLE 1 shows a numerical configuration of each component of the
lens unit
130. In TABLE 1, lengths and areas are those at the ambient temperature
is 20° C., and no pressure is applied.
| TABLE 1 |
|
| |
|
|
longitudinal |
linear |
| |
|
|
elastic |
expansion |
| |
length L |
cross-section |
modulus |
coefficient |
| |
(mm2) |
area S (mm2) |
(Kgf/mm2) |
(×10-5/° C.) |
|
| |
| L1 part of |
3 |
25 |
8 × 104 |
0.1 |
| Collimating |
| Lens |
| L2 part of |
10 |
25 |
10 × 104 |
1.62 |
| Lens Support |
| Frame |
| L3 part of |
7 |
18 |
7 × 104 |
2.3 |
| Lens Retainer |
| Ring |
|
The left side of the inequality (1) indicates the difference of the lengths between
the length L
2 and the sum of the lengths L
1 and L
3 in
the optical axis direction due to the temperature variation.
For example, when the ambient temperature increases by 20° C. in the case
of Table 1, the change ΔL of the length L of each part (L
1 part
of the collimating lens
110, L
2 part of the lens support frame
132, L
3 part of the lens retainer ring
134) caused by
the temperature increase of 20° C. becomes 0.06×10
-3 [mm],
3.24×10
-3 [mm], and 3.22×10
-3 [mm], respectively.
In this example, the increase of the lengths of the parts (L
1 part and
L
3 part) of the collimating lens
110 and the lens retainer ring
134 added together in the optical axis direction due to thermal expansion
(Δt(ρ
1L
1+ρ
3L
3)) is
slightly greater than the increase of the length of the L
2 part of the
lens support frame
132 in the optical axis direction due to thermal expansion
(Δt×ρ
2L
2). In this case, the L
1 part
of the collimating lens
110 and the L
3 part of the lens retainer
ring
134 both expanding are locked up in the L
2 part of the lens
retainer ring
132 (expanding by almost the same length) in a way canceling
out their movement, by which the displacement of the collimating lens
110
in the lens support frame
132 is prevented.
When the ambient temperature decreases, the decrease of the lengths of the parts
(L
1 part and L
3 part) of the collimating lens
110
and the lens retainer ring
134 added together in the optical axis direction
due to thermal contraction (Δt(ρ
1L
1+ρ
3L
3))
becomes slightly greater than the change of the length of the L
2 part
of the lens support frame
132 in the optical axis direction due to thermal
contraction (Δt×ρ
2L
2). In this case, the
L
1 part of the collimating lens
110 and the L
3 part
of the lens retainer ring
134 both contracting are locked up in the L
2
part of the lens retainer ring
132 (contracting by almost the same
length) by almost constant fastening force, by which the occurrence of clearance
between components can be avoided and the displacement of the collimating lens
110 in the lens support frame
132 is prevented. Incidentally, the
length L of each component shown in the Table 1 is a value when the ambient temperature
is 20° C.
For example, when the ambient temperature decreases by 20° C. in the case
of TABLE 1, the change ΔL of the length L of each part (L
1 part
of the collimating lens
110, L
2 part of the lens support frame
132, L
3 part of the lens retainer ring
134) caused by
the temperature decrease of 20° C. is similar to that in the case where the
temperature increase, except that each part contracts by the amount. In this example,
a clearance may be formed between the right-hand side of the L
1 part
and the left-hand side of the L
3 part.
According to the embodiment, however, since the sum of the elastic deformation
of the parts (L
1, L
2 and L
3 parts) is greater
than the clearance which is formed due to the temperature decrease, the fastening
load is retained, and the displacement of the collimating lens
110 in the
lens support frame
132 is prevented.
The right side of the inequality (1) indicates the sum of elastic deformations
[mm] of the L
1 part of the collimating lens
110, L
2 part
of the lens support frame
132 and L
3 part of the lens retainer
ring
134 in the optical axis direction caused by a fastening load P of the
lens retainer ring
134 on the collimating lens
110.
Specifically, when the fastening load P (P≧0) is generated by
fastening the lens retainer ring
134, the L
1 part and L
3
part elastically contract, while the L
2 part elastically expands.
For example, the elastic deformation ΔL′ of each part (L
1
part of the collimating lens
110, L
2 part of the lens support
frame
132, L
3 part of the lens retainer ring
134) caused
by a load (fastening load P: 100 [N]) in the case of TABLE 1 becomes 1.5×10
-4
[mm], 4.2×10
-4 [mm] and 5.5×10
-4 [mm], respectively.
Accordingly, the right side of the inequality (1) becomes 1.12×10
-3 [mm],
which is greater than the left side of the inequality (1) in the above example
(i.e., Δt=20°). Therefore, in this case, the deformation of the parts
(L
1, L
2 and L
3) can be absorbed by the elastic deformation
due to the fastening load P, and the displacement of the collimating lens
110
in the lens support frame
132 is prevented.
The elastic deformation of each component gets larger as the load gets heavier.
Therefore, as the fastening load P gets larger, fastening force of the elastically
deformed components becomes larger, by which relative position of each component
becomes more fixed and stabilized. Consequently, the displacement of the collimating
lens
110 in the lens support frame
132 is prevented. Incidentally,
the fastening load P is maintained below a load level that can excessively deform
the collimating lens
110 and deteriorate optical performance of the collimating
lens
110.
When the temperature drops, the L
1 part of the collimating lens
110
and the L
3 part of the lens retainer ring
134 added together
contract slightly more than the L
2 part of the lens support frame
132,
by which clearance tends to occur among the components and the displacement of
the collimating lens
110 in the lens support frame
132 becomes a
possibility. However, in the case where the lens unit
130 satisfies the
condition (1), the change of the length of the L
2 part of the lens support
frame
132 in the optical axis direction caused by the temperature drop minus
the sum of the change of the lengths of the L
1 part of the collimating
lens
110 and the change of the L
3 part of the lens retainer ring
134 in the optical axis direction [mm] caused by the same temperature drop
is smaller than the sum of elastic deformations of the collimating lens
110,
the lens support frame
132 and the lens retainer ring
134 caused
by the fastening load P. Therefore, even when the clearance tends to be generated
between the collimating lens
110 and the lens retainer ring
134 due
to the temperature change, if the amount of deformation due to the temperature
drop is less than the amount of deformation due to the fastening load P, each component
tends to restore its original shape as the clearance increases, and as a result,
the lens retainer ring
134 keeps applying the fastening load P to the collimating
lens. Therefore, the displacement of the collimating lens can be prevented.
In the above description, the lengths and coefficients are determined such that
the clearance between the collimating lens
110 and the lens retainer ring
134 tends to be formed when the temperature drops (i.e., Δt<0).
It can be understood easily that, when the relationship of the lengths and coefficients
is opposite (i.e., when the clearance between the collimating lens
110 and
the lens retainer ring
134 tends to be formed when the temperature increases
(i.e., Δt>0)), the displacement of the collimating lens
110 can
be prevented.
Therefore, the effect of deformations of the components due to deformation
(caused by temperature change) is absorbed by the elastic deformations of the components.
As above, the collimating lens
110, the lens support frame
132
and
the lens retainer ring
134 are not practically affected by the change of
temperature and no displacement of the collimating lens
110 occurs in the
lens support frame
132. The same applies to the collimating lens
122
of the lens unit
140 for the first light beam
106. By the prevention
of the displacement of the collimating lenses
110 and
122 in the
lens units
130 and
140, the relative positions of the optical paths
of the first and second light beams
106 and
108 are maintained substantially
fixed and thereby the interval of the beam spots formed on the scan target surface
120 (beam spot interval in the auxiliary scanning direction, etc.) can be
maintained constant.
It should be noted that, if the material and length of each of the parts L
1,
L
2 and L
3 are appropriately determined so that the difference
between the expansion/contraction amount due to the temperature variation of the
L
2 part and the sum of the expansion/contraction amounts due to the
temperature variation of the L
1 part and L
3 part is smaller,
the deformation amount due to the fastening load P can be decreased. That is, the
fastening load P can be set smaller. As the fastening load P is set smaller, possible
deformation of the collimating lens
110 due to the fastening force of the
lens retainer ring
134 can be made smaller.
In particular, when the condition (2):
ρ
2L2=ρ
1L1+ρ
3L3 (2)
is satisfied, the clearance between the collimating lens will not be formed, and
thus, the fastening load P can be minimized.
As described above, in the lens unit in accordance with the embodiment of the
present invention, the effect of deformations of the components due to expansion/contraction
caused by temperature variation can be absorbed by the elastic deformations of
the components, by which the displacement of the collimating lenses
110,
122 due to temperature variation can be prevented. Therefore, the intervals
of the beam spots formed on the scan target surface measured in each scanning direction
can be maintained with high precision needing only the initial positioning in the
assembly process. Consequently, the need of installing the feedback control mechanism
in the multibeam scanning device for maintaining the beam spot intervals is eliminated,
and thereby miniaturization, simplification and cost reduction of the multibeam
scanning device are realized. Further, the manufacturing process of the lens unit
can be simplified since the manufacturing steps using adhesives become unnecessary
When the reference temperature t
0 is relatively high, that is, when
Δt tends to be negative, it is preferable that the following condition is satisfied:
ρ
2L2<ρ
1L1+ρ
3L3.
If the opposite condition is satisfied, as the temperature changes (decreases),
the fastening load P applied to the collimating lens
110 increases, which
may deteriorate the optical characteristics of the collimating lens
110.
On the contrary, when the reference temperature t
0 is relatively low,
Δt tends to be positive. In such a case, it is preferable that the following
condition is satisfied:
ρ
2L2>ρ
1L1+ρ
3L3.
If the opposite condition is satisfied, as the temperature changes (increases),
the fastening load P applied to the collimating lens
110 increases, which
may deteriorate the optical characteristics of the collimating lens
110.
While the present invention has been described with reference to the particular
illustrative embodiments, it is not to be restricted by those embodiments but only
by the appended claims. It is to be appreciated that those skilled in the art can
change or modify the embodiments without departing from the scope and spirit of
the present invention.
FIG. 5 shows a modification of the above-described embodiment. In this modification,
a part of the lens retainer ring
134 within the L
3 part is replaced
with elastic ring member
160. In such a structure, since the elasticity
of the L
3 part increase considerably, a difference between the deformation
amount of the L
2 part due to the temperature variation in the optical axis
direction and the deformation amount of the sum of the changes of the L
1
part and L
3 part due to the same temperature variation in the optical
axis direction can be made relatively large, which provides more selections of
the materials and sizes of respective components.
The present disclosure relates to the subject matter contained in Japanese Patent
Application No. 2003-078157, filed on Mar. 20, 2003, which is expressly incorporated
herein by reference in its entirety.
*
