Optical scanner with anamorphic optical system

An optical scanner includes a multi-faceted rotating mirror which reflects light beam from a laser unit, and a condenser lens which focuses the beam as a scanning spot on a scanning surface. An anamorphic optical system comprising a cylindrical lens is disposed between the mirror and the surface, and focuses the beam on the surface in the scan direction, but focuses the beam on a plane short of the scanning surface in a direction perpendicular to the scan direction. An optical element having a magnification of projection which is equal to or less than unity and having a refracting or focussing force only in a direction perpendicular to the scan direction is disposed between the plane and the scanning surface.

BACKGROUND OF THE INVENTION 
The invention relates to an optical scanner, and more particularly, to an 
arrangement for compensating for a displacement of a scanning spot which 
results from an inclination from a reference plane of a multi-faceted 
rotating mirror used in light beam scanning apparatus. 
An optical scanner generally used in facsimile systems, optical printers or 
the like comprises a light beam scanner which employs a multi-faceted 
rotating mirror. Such a scanner directs a collimated light beam from a 
light source to a facet of a polyhedron such as hexahedron or octahedron, 
which reflects and focuses the beam onto a scanning surface. Rotation of 
the mirror in one direction performs a scanning of the focussed light spot 
across the scanning surface. 
FIG. 1 shows an example of the prior art optical scanner. Multi-faceted 
rotating mirror 2 is rotatably mounted on support shaft 1 for rotation in 
one direction. A collimated light beam from a laser (not shown) impinges 
on one facet thereof. After reflection by reflecting surface 2a of the 
mirror, the beam is passed through condenser lens 3, which comprises a 
spherical lens, and is focussed onto a scanning surface 4 disposed in the 
focal plane of the condenser lens 3 as a scanning light spot. As mirror 2 
rotates, the spot travels across and thus scans the surface 4. For the 
convenience of subsequent description, the direction indicated by optical 
axis 5 will be referred to as X-axis, the direction along which the beam 
scan takes place as Y-axis, and the axis of shaft 1 as Z-axis. 
In the optical scanner thus constructed, the rotation of mirror 2 at a high 
speed may cause an oscillation thereof, producing an inclination of the 
reflecting surface to cause the light spot to be offset from the normal 
scan direction. Where the scanning surface represents an original and is 
conveyed in the direction of the Z-axis, this produces an error in the 
line-to-line spacing which is usually referred to as a pitch error. 
Considering this arrangement more closely with reference to FIG. 2, if the 
reflecting surface 2a has an angle of inclination .DELTA..theta. with 
respect to the Z-axis as a result of an error in the manufacturing process 
or an oscillation during the rotation of the mirror, the collimated beam 
from the light source will impinge on the reflecting surface 2a with an 
angle of incidence .DELTA..theta. and be reflected at an angle of 
reflection .DELTA..theta., so that the reflected beam will have an angle 
of 2.DELTA..theta. with respect to the X-axis when passing through 
condenser lens 3. Consequently, the spot will be displaced on the scanning 
surface 4 by a distance .DELTA.Z=2.DELTA..theta..multidot.f.sub.s 
displaced from the optical axis 5 where f.sub.s represents the distance 
between condenser lens 3 and surface 4 or the focal length at the lens. 
For a small angle .DELTA..theta., we have 2.DELTA..theta..apprxeq.tan 
2.DELTA..theta., and hence 
EQU .DELTA.Z=f.sub.s tan 2.DELTA..theta. 
Such disadvantage can be overcome by the use of a cylindrical lens which 
prevents a pitch error in the event the reflecting surface is inclined, as 
disclosed in U.S. Pat. application Ser. No. 190,024. 
Such scanner is illustrated in FIG. 3. Referring to this Figure, a light 
beam from laser 16 is adapted to impinge on reflecting surface 12a of a 
multi-faceted rotating mirror 12. The scanner includes a first convex, 
cylindrical lens 17 disposed in the optical path extending between the 
laser 16 and the mirror 12 for focussing the impinging beam in the axial 
direction of shaft 11 which is associated with the mirror 12, or in the 
direction of Z-axis, and a second convex, cylindrical lens 18 disposed 
between the mirror 12 and a condenser lens 13, which is a spherical lens 
similar to lens 3 of FIGS. 1 and 2, for refracting the reflected beam only 
in the direction of the Z-axis or in a direction perpendicular to the scan 
direction. Mirror 12 is coaxially secured to the free end of shaft 11, 
which is rotatably supported by bearing 19. The opposite end of shaft 11 
fixedly carries drive pulley 20, which is engaged by endless belt 23 which 
also extends around pulley 22 fixedly mounted on the output shaft 21a of 
drive motor 21, thus driving mirror 12 for rotation in one direction. 
In the arrangement described, the beam impinging on reflecting surface 12a 
is focussed in the axial direction of shaft 11 or in the direction of the 
Z-axis by convex lens 17 which is disposed so that the reflecting surface 
12a is located on the focal position thereof, whereby a light image on 
reflecting surface 12a will be a linear image which is perpendicular to 
the Z-axis. The reflected image passes through second lens 18, and is 
focussed by condenser lens 13 onto the surface 14 as a light spot in a 
manner similar to the prior arrangement as far as the scan direction or 
the direction of Y-axis is concerned. Like condenser lens 13, the second 
convex cylindrical lens 18 is disposed so that the reflecting surface 12a 
and the scanning surface 14 are located conjugate to each other as far as 
the Z-axis component of ray is concerned. In this manner, the image which 
is focussed onto the surface 14 represents a spot, the position of which 
in the direction of the Z-axis remains unchanged if surface 12a is 
inclined from Z-axis to result in an incidence of the linear image 
reflected thereby onto the second lens 18 at an angle to change the 
optical path along which it passes from the convex cylindrical lens 18 to 
the condenser lens 13. Thus, a pitch error which usually results from an 
inclination of the rotating mirror is substantially completely avoided. 
However, in the optical scanner arrangement described above, it is 
necessary to focus a light image of a narrow line on the surface 12a. If 
the scanning surface 14 comprises a recording member of a low sensitivity 
which is scanned by a high output beam from argon laser having a power 
output exceeding 10 W, the incidence of the beam onto the reflecting 
surface 12a will cause a temperature rise at the spot where it impinges, 
disadvantageously causing a deformation or damage of the reflecting 
surface 12a or burning of foreign matters deposited thereon which result 
in a degraded reflectivity. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an optical scanner which 
eliminates the described disadvantages of the prior art by the use of an 
anamorphic optical system having different focal positions for the scan 
direction and for another direction which is at right angles thereto. 
In accordance with the invention, the reflection by the rotating mirror 
takes place without forming the reflected beam into a linear form or 
reducing the size thereof, thus avoiding a deformation, damage or a 
degraded reflectivity which may result from a temperature rise of the 
reflecting surface of the rotating mirror while maintaining a sufficiently 
small light spot and reducing a pitch error associated with an inclination 
of the rotating mirror to a degree which is sufficient for practical 
purposes.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 4 is a front view of an optical scanner according to one embodiment of 
the invention. As shown, light from a light source 36 formed by a laser 
unit is passed through an expander lens 39 comprising a convex lens 39a 
and a concave lens 39b and which converts it into a collimated light beam 
having a given diameter. The beam impinges on the reflecting surface 32a 
of a multi-faceted rotating mirror 32, which is mounted on a shaft 31 for 
rotation in a direction indicated by an arrow. As the mirror 32 rotates, 
the collimated beam is reflected toward a scanning surface 34. 
An anamorphic optical system comprising a first convex cylindrical lens 37 
and a condenser lens 33 which is formed by a spherical lens is disposed 
intermediate the multi-faceted rotating mirror 32 and the scanning surface 
34. The system focuses a spot on the scanning surface 34 in the scan 
direction or in the direction of the Y-axis while it focuses a spot on a 
plane 40 or short of the scanning surface 34 in the direction of the 
Z-axis (see FIG. 5). A second convex cylindrical lens 38 is disposed 
between the focal plane 40 and the surface 34, and has a refracting or 
focussing power only in a direction perpendicular to the scan direction or 
in the direction of the Z-axis and a magnification of projection which is 
equal to or less than unity. The purpose of the lens 38 is to project the 
light which is focussed on the plane 40 to and focuses it on the surface 
34 (see FIG. 5). 
Thus, both the first and second cylindrical lenses 37, 38 have a refracting 
power only in the direction of the Z-axis, but have no refractive power in 
the direction of the Y-axis. The condenser lens 33 is spaced from the 
scanning surface 34 by a distance f.sub.s which is equal to its focal 
length. Thus the reflected beam will be focussed on the surface 34 as far 
as the Y-axis is concerned. 
The refracting power of the first cylindrical lens 37 is chosen such that 
the beam is focussed on the plane 40 short of the second cylindrical lens 
38 as far as the Z-axis is concerned since the composite focal length of 
the elements 37, 33 in refracting the beam in the direction of the Z-axis 
is 1/m times the focal length f.sub.s of the condenser lens 33 where m is 
greater than unity. The second cylindrical lens 38 is disposed so that the 
plane 40 and the surface 34 are optically conjugate to each other with 
respect to the lens 38. The lens 38 has a refractive index so that its 
magnification of projection is equal to 1/n where n is greater than unity. 
When thus arranged, the conjugacy of the focal plane 40 and the scanning 
surface 34 with respect to the second cylindrical lens 38 assures that the 
beam which is focussed on the plane 40 in the direction of the Z-axis will 
be again focussed on the surface 34, as shown in FIG. 5, forming a 
scanning spot on the latter surface 34. 
In operation, assuming that the reflecting surface 32a of the multi-faceted 
rotating mirror 32 has an angle of inclination of .DELTA..theta. as viewed 
in the direction of the Z-axis, the beam reflected by the reflecting 
surface 32a will deviate from the Z-axis by angle of 2.DELTA..theta., 
whereby its focussed position on the plane 40 will be displaced by a 
distance of d.sub.1 =(f.sub.s /m)2.DELTA..theta. from the X-Y plane 
including optical axis 35. Since the cylindrical lens 38 has a 
magnification n when it projects an image on the plane 40 onto the 
scanning surface 34, the corresponding spot formed on the surface 34 will 
be displaced by an amount of d.sub.1 /n in the direction of the Z-axis. 
Thus resulting deviation .DELTA.Z.sub.O will be equal to 2f.sub.s 
.multidot..DELTA..theta./mn. 
However, it is to be noted that the magnitude of the deviation 
.DELTA.Z.sub.O can be said to be minimal when it is compared with the 
corresponding magnitude of deviation which occurs in the conventional 
arrangement which does not include the cylindrical lenses 37, 38. 
Specifically, the magnitude of the deviation or pitch error will be 
2f.sub.s .multidot..DELTA..theta. in the conventional arrangement, but can 
be reduced by a factor of mn in the arrangement of the invention. By way 
of example, by choosing m=2 and n=10, the magnitude of the pitch error can 
be reduced by a factor of 20 as compared with the prior art arrangement, 
thus affording practical utility. 
It should be understood that the invention is not limited to the precise 
construction shown and described. By way of example, the first cylindrical 
lens 37 may be transposed to the other side of the condenser lens 33. 
Alternatively, the second cylindrical lens 38 may be replaced by a 
cylindrical, concave mirror 41 to scan a scanning surface 42 which is 
disposed in parallel relationship with the optical axis 35. The 
multi-faceted mirror may comprise a pyramid. Additionally, each of the 
first and second cylindrical lenses may comprise a combination of convex 
and concave cylindrical lenses to provide a color correction, provided the 
total refracting power is positive. Also, the anamorphic optical system 
need not be a combination of a cylindrical lens and a condenser lens.