Color image forming apparatus

Disclosed is a color image forming apparatus in which it is possible to prevent positional deviation in scanning of each optical scanning device without having to improve the lens accuracy to an extreme degree. In a color image forming apparatus of the type which has a plurality of sets of optical scanning devices and image carriers, each of the plurality of optical scanning devices includes a laser unit having a semiconductor laser and a collimator lens, a cylindrical lens formed of a plastic lens material and adapted to effect image formation on the deflection surface of a light deflector from a beam emitted from the laser unit, and an f.theta. lens system having at least one plastic lens for effecting image formation on the surface of the image carrier from the beam deflected by the light deflector, wherein at least one of the plastic lenses of each of the plurality of optical scanning devices is molded in the same cavity, and wherein at least one of the plastic lenses molded in the same cavity is molded in a multiple cavity mold.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a color image forming apparatus and, in
 particular, to a color image forming apparatus which has a plurality of
 pairs of optical scanning devices and corresponding image carriers,
 optical scanning being performed on each image carrier with a beam emitted
 from each optical scanning device to thereby form an image, or a color
 image forming apparatus which has an optical scanning device including a
 multi-semiconductor laser and a plurality of image carriers, optical
 scanning being performed on the image carriers with a plurality of beams
 emitted from the optical scanning device to thereby form an image. The
 image forming apparatus is suitably applicable, for example, to a high
 speed laser printer apparatus, a plural-drum type color copying machine or
 a digital color copying machine.
 2. Description of the Related Art
 Conventionally, in a color image forming apparatus such as a plural-drum
 type color printer or a plural-drum type color copying machine, a laser
 exposure device, that is, an optical scanning device has been used which
 provides a plurality of image forming sections corresponding to color
 components obtained through color separation with image data corresponding
 to the color components, that is, a plurality of beams.
 Generally speaking, an optical scanning device comprises a semiconductor
 laser device serving as the light source, a first lens unit (collimator
 lens, cylindrical lens, etc.) for reducing the diameter of a beam emitted
 from the semiconductor laser device to a predetermined dimension, a light
 deflector for continuously deflecting and reflecting the beam whose
 diameter has been reduced by the first lens unit in a direction
 perpendicular to the direction in which the recording medium is conveyed,
 a second lens unit (f.theta. lens system) for forming an image at a
 predetermined position of the recording medium from the beam deflected and
 reflected by the light deflector, etc.
 In known examples of such an optical scanning device, a plurality of
 optical scanning devices are arranged in correspondence with the image
 forming sections, or a multi-beam optical scanning device capable of
 providing a plurality of beams is arranged.
 In an example of the multi-beam optical scanning device, lens units such as
 f.theta. lenses are arranged in a number corresponding to the number of
 beams, as disclosed in Japanese Patent Laid-Open No. 3-264970. Further, as
 disclosed in Japanese Patent Publication No. 2-20986, there is arranged in
 the vicinity of the photosensitive member (photosensitive drum) a
 cylindrical lens for causing a plurality of beams to impinge upon a single
 light deflector and to be condensed by a single f.theta. lens (scanning
 lens) to correct any bend of the scanning line.
 However, the above conventional multi-beam optical scanning devices have
 the following problems.
 In the device disclosed in Japanese Patent Publication No. 2-20986, the
 beam impinges upon the f.theta. lens obliquely, so that the scanning line
 is curved, and the curved scanning line is corrected by a cylindrical
 lens. However, the correction cannot be effected completely. When there
 are a plurality of incident beams, there exist a plurality of incident
 angles, and the scanning lines with different curvatures cause positional
 deviation on the surface of the photosensitive member.
 In the device disclosed in Japanese Patent Laid-Open No. 3-264970, the
 number of lens units such as f.theta. lens are arranged to correspond to
 the number of beams. In the case of this arrangement, the plurality of
 beams pass the centers of the respective optical systems, so that the
 scanning line is not easily curved. However, unless the accuracy of the
 optical systems is improved to an extreme degree, positional deviation in
 the scanning direction is generated on the surface of the photosensitive
 member due to errors in production. To avoid this, the lens accuracy is
 improved to an extreme degree, or a lens position adjusting mechanism is
 provided, resulting in the apparatus as a whole being complicated.
 Further, this leads to an increase in production cost.
 SUMMARY OF THE INVENTION
 It is a first object of the present invention to provide a color image
 forming apparatus of the type which has a plurality of pairs of optical
 scanning devices and image carriers, wherein each optical scanning device
 is appropriately constructed, whereby it is possible to prevent scanning
 positional deviation of each optical scanning device without having to
 improve the lens accuracy to an extreme degree.
 It is a second object of the present invention to provide a color image
 forming apparatus of the type which has an optical scanning device
 including a multi-semiconductor laser and a plurality of image carriers,
 wherein the optical scanning device is appropriately constructed, whereby
 it is possible to prevent scanning positional deviation without having to
 improve the lens accuracy to an extreme degree.
 In accordance with the present invention, there is provided a color image
 forming apparatus of the type which has a plurality of sets of optical
 scanning devices and image carriers, in which beams emitted from the
 optical scanning devices are directed to the surfaces of the corresponding
 image carriers, scanning being performed on the surfaces of the image
 carriers with the beams to form images of different colors on the surfaces
 of the image carriers, a color image being formed from the images formed
 on the surfaces of the plurality of image carriers,
 wherein each of the plurality of optical scanning devices comprises
 a laser unit having a semiconductor laser and a collimator lens,
 a cylindrical lens consisting of a plastic lens material and adapted to
 form an image on the deflection surface of a light deflector from a beam
 emitted from the laser unit, and
 an f.theta. lens system having at least one plastic lens for forming an
 image on the surface of the image carrier from the beam deflected by the
 light deflector,
 wherein at least one of the plastic lenses of the plurality of optical
 scanning devices is molded by a multiple cavity mold.
 In particular, in each of the plurality of optical scanning devices, the
 laser unit, the cylindrical lens, and the f.theta. lens system are held by
 a single plastic holding member, and the plastic holding member is molded
 by a multiple cavity mold.
 In each of the plurality of optical scanning devices, the light deflector
 is also held by the plastic holding member.
 The cavity combination of the plastic lens and the plastic holding member
 is specified, and, with another combination, the plastic lens is not
 attached to the plastic holding member.
 The cavity combination of the plastic lens and the plastic holding member
 is constructed such that it can be recognized from the outside of each
 optical scanning device.
 A plurality of optical scanning devices of the same cavity combination of
 the plastic lens and the plastic holding member are mounted in the
 apparatus main body.
 The f.theta. lens system has two plastic lenses, and the cavity combination
 of the two plastic lenses is specified, no attachment being possible with
 any other combination.
 The cavity combination of the two plastic lenses constituting the f.theta.
 lens system is constructed such that it can be recognized from the outside
 of each optical scanning device.
 Optical scanning devices of the same cavity combination of the two plastic
 lenses constituting the f.theta. lens system are mounted in the apparatus
 main body.
 The plurality of optical scanning devices share a light deflector.
 Further, in accordance with the present invention, there is provided a
 color image forming apparatus of the type which has a plurality of sets of
 optical scanning devices and corresponding image carriers, and in which
 beams emitted from the optical scanning devices are directed to the
 surfaces of the corresponding image carriers, scanning being performed on
 the surfaces of the image carriers with the beams to form images of
 different colors on the surfaces of the image carriers, a color image
 being formed from the images formed on the surfaces of the plurality of
 image carriers,
 wherein each of the plurality of optical scanning devices has at least one
 plastic lens and a plastic holding member for holding at least one optical
 element, and at least one of the plastic lens and the plastic holding
 member is molded by the same cavity of the multiple cavity mold, and the
 cavity combination of the plastic lens and the plastic holding member is
 specified, the plastic lens not being attachable to the plastic holding
 member with any other cavity combination.
 In particular, the cavity combination of the plastic lens and the plastic
 holding member can be recognized from the outside of each optical scanning
 device.
 A plurality of optical scanning devices of the same cavity combination of
 the plastic lens and the plastic holding member are mounted in the
 apparatus main body.
 Each of the plurality optical scanning devices has a light deflector, which
 is held by the plastic holding member.
 The plurality of optical scanning devices share a light deflector.
 Further, in accordance with the present invention, there is provided a
 color image forming apparatus of the type which has a plurality of optical
 scanning devices including a multi-semiconductor laser and a plurality of
 image carriers, and in which a plurality of beams emitted from the optical
 scanning devices are directed to the surfaces of the corresponding image
 carriers, scanning being performed on the surfaces of the image carriers
 with the beams to form images of different colors on the surfaces of the
 image carriers, a color image being formed from the images formed on the
 surfaces of the plurality of image carriers,
 wherein the plurality of optical scanning devices comprise
 a laser unit having the multi-semiconductor laser and a collimator lens,
 a cylindrical lens consisting of a plastic lens material and adapted to
 form images on the deflecting surface of the light deflector from a
 plurality of beams emitted from the laser unit, and
 an f.theta. lens system having at least one plastic lens for forming images
 on the surfaces of the image carriers from the plurality of beams
 deflected by the light deflector, and
 wherein at least one of the plastic lenses of the optical scanning device
 is molded in the same cavity of a multiple cavity mold.
 In particular, in the plurality of optical scanning devices, the laser
 unit, the cylindrical lens, and the f.theta. lens system are held by a
 single plastic holding member, and the plastic holding member is molded in
 the same cavity of a multiple cavity mold.
 In the plurality of optical scanning devices, the light deflector is also
 held by the plastic holding member.
 The cavity combination of the plastic lens and the plastic holding member
 is specified, and the plastic lens does not attach to the plastic holding
 member with any other combination.
 The cavity combination of the plastic lens and the plastic holding member
 can be recognized from the outside of the optical scanning device.
 In the plurality of optical scanning devices, the cavity combination of the
 plastic lens and the plastic holding member is the same.
 The f.theta. lens system has two plastic lenses, and the cavity combination
 of the two plastic lenses is specified, the attachment being impossible
 with any other combination.
 The cavity combination of the two plastic lenses constituting the f.theta.
 lens system can be recognized from the outside of the optical scanning
 device.
 In the plurality of optical scanning devices, the cavity combination of the
 two plastic lenses constituting the f.theta. lens system is the same.
 Further, in accordance with the present invention, there is provided a
 color image forming apparatus of the type which has a plurality of optical
 scanning devices including a multi-semiconductor laser and a plurality of
 image carriers, and in which a plurality of beams emitted from the optical
 scanning devices are directed to the surfaces of the corresponding image
 carriers, scanning being performed on the surfaces of the image carriers
 with the beams to form images of different colors on the surfaces of the
 image carriers, a color image being formed from the images formed on the
 surfaces of the plurality of image carriers,
 wherein the plurality of optical scanning devices have at least one plastic
 lens and a plastic holding member holding at least one optical element,
 wherein at least one of the plastic lens and the plastic holding member is
 molded in the same cavity of a multiple cavity mold, and wherein the
 cavity combination of the plastic lens and the plastic holding member is
 specified, and the plastic lens cannot be attached to the plastic holding
 member with any other combination.
 In particular, the cavity combination of the plastic lens and the plastic
 holding member can be recognized from the outside of the optical scanning
 device.
 In the plurality of optical scanning devices, the cavity combination of the
 plastic lens and the plastic holding member is the same.
 The plurality of optical scanning devices have a light deflector, which is
 held by the plastic holding member.
 Further, in accordance with the present invention, there is provided a
 color image forming apparatus of the type which has a plurality of sets of
 optical scanning devices and image carriers, and in which beams emitted
 from the optical scanning devices are directed to the surfaces of the
 corresponding image carriers, scanning being performed on the surfaces of
 the image carriers to form images of different colors on the surfaces of
 the image carriers, a color image being formed from the images formed on
 the plurality of image carriers,
 wherein each of the plurality of optical scanning devices comprises
 a laser unit,
 a light deflector for deflecting the beam emitted from the laser unit, and
 a lens system having at least one plastic lens for forming an image on a
 predetermined surface from the beam emitted from the laser unit, and
 wherein at least one of the plastic lenses of the plurality of optical
 scanning devices is molded in the same cavity of a multiple cavity mold.
 In particular, in each of the plurality of optical scanning devices, the
 laser unit and the lens system are held by a single plastic holding
 member, and the plastic holding member is molded in the same cavity of a
 multiple cavity mold.
 In each of the plurality of optical scanning devices, the light deflector
 is also held by the plastic holding member.
 The laser unit has a multi-semiconductor laser emitting a plurality of
 beams.
 Further, in accordance with the present invention, there is provided a
 color image forming apparatus of the type which has an optical scanning
 device including a light source portion emitting a plurality of beams and
 a plurality of image carriers, and in which a plurality of beams emitted
 from the optical scanning device are directed to the surfaces of the
 corresponding image carriers, scanning being performed on the surfaces of
 the image carriers with the beams to form images of different colors on
 the surfaces of the image carriers, a color image being formed from the
 images formed on the surfaces of the plurality of image carriers,
 wherein the optical scanning device comprises
 a light source portion,
 a light deflector for deflecting a plurality of beams emitted from the
 light source portion, and
 a lens system having at least one plastic lens for effecting image
 formation on a predetermined surface with the plurality of beams emitted
 from the light source portion,
 wherein there are provided a plurality of said lens systems in
 correspondence with the plurality of beams, and wherein at least one of
 the plastic lenses of the lens systems is molded in the same cavity of a
 multiple cavity mold.
 Further, in accordance with the present invention, there is provided a
 color image forming apparatus of the type which has an optical scanning
 device including a multi-semiconductor laser and a plurality of image
 carriers, and in which a plurality of beams emitted from the optical
 scanning device are led to the surfaces of corresponding image carriers,
 scanning being performed on the surfaces of the image carriers with the
 beams to form images of different colors on the surfaces of the image
 carriers, a color image being formed from the images formed on the
 surfaces of the plurality of image carriers,
 wherein the optical scanning device comprises
 a laser unit having a multi-semiconductor laser and a collimator lens,
 a light deflector for deflecting a plurality of beams emitted from the
 laser unit,
 a cylindrical lens consisting of a plastic lens material and adapted to
 effect image formation on the deflection surface of the light deflector
 from the plurality of beams emitted from the laser unit, and
 and an f.theta. lens system having at least one plastic lens for effecting
 image formation on a predetermined surface from the plurality of beams
 deflected by the light deflector,
 wherein there are provided a plurality of said f.theta. lens systems in
 correspondence with the plurality of beams, and wherein at least one of
 the plastic lenses of the f.theta. lens systems is formed in the same
 cavity of a multiple cavity mold.
 Further objects, features and advantages of the present invention will
 become apparent from the following description of the preferred
 embodiments with reference to the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 FIG. 1 is a schematic diagram showing a main part of the color image
 forming apparatus of the first embodiment of the present invention.
 In the drawing, numeral 1 indicates the color image forming apparatus
 (apparatus main body), numerals 2a, 2b, 2c and 2d indicate optical
 scanning devices (hereinafter also referred to as "scanner units")
 constructed as described below, and numerals 4a, 4b, 4c and 4d indicate
 photosensitive drums (photosensitive members) serving as image carriers.
 In this embodiment, beams (laser beams) optically modulated based on image
 information are emitted from the optical scanning devices 2a, 2b, 2c and
 2d, and directed by way of mirrors 3a, 3b, 3c and 3d to the surfaces of
 the photosensitive drums 4a, 4b, 4c and 4d serving as recording media to
 form latent images. These latent images are formed on the photosensitive
 drums 4a, 4b, 4c and 4d uniformly charged by primary chargers (not shown),
 visualized as cyan, magenta, yellow and black images by developing devices
 5a, 5b, 5c and 5d and sequentially transferred electrostatically by
 transfer rollers to a transfer material S conveyed by a transfer belt P to
 thereby form a color image. The mirrors 3a, 3b, 3c and 3d are constructed
 so as to adjust the inclination of scanning lines by adjusting mechanisms
 (not shown).
 FIG. 2 is a main part schematic diagram showing one of the optical scanning
 devices 2a and the image carrier 4a corresponding thereto. In the drawing,
 the components which are the same as those of FIG. 1 are indicated by the
 same reference numerals.
 In the drawing, numeral 6a indicates a semiconductor laser serving as the
 light source means. Numeral 7a indicates a collimator lens which
 transforms the beam emitted from the light source means 6a into a
 substantially parallel beam. Numeral 24a indicates an opening diaphragm,
 which restricts the passing beam (quantity of light). The semiconductor
 laser 6a, the collimator lens 7a and the opening diaphragm 24a are
 components of the laser unit. Numeral 8a indicates a cylindrical lens
 formed of a plastic material (plastic cylindrical lens), which has a
 predetermined refractive power in a sub-scanning direction perpendicular
 to the plane of the drawing, and forms in the sub-scanning plane a
 substantially latent image on a deflection surface 9a1 of a light
 deflector described below from the beam passed through the opening
 diaphragm 24a. Numeral 21a indicates an f.theta. lens system (scanning
 lens system) having an f.theta. characteristic, which is composed of first
 and second plastic lenses (plastic f.theta. lenses) 10a and 11a.
 In the optical scanning device of this embodiment, the beam emitted from
 the semiconductor laser 6a is transformed into a substantially parallel
 beam by the collimator lens 7a, and the beam (quantity of light) is
 restricted by the opening diaphragm 24a before the beam impinges upon the
 plastic cylindrical lens 8a. In the main scanning section, the
 substantially parallel beam impinging upon the plastic cylindrical lens 8a
 is output as it is. In the sub-scanning section, it is converged and forms
 a substantially latent image (which is longitudinal with respect to the
 main scanning direction) on the deflection surface 9a1 of the light
 deflector 9a. The beam deflected by the deflection surface 9a1 of the
 light deflector 9a is passed through the f.theta. lens system 21a, whereby
 its scanning linearity is corrected, and is directed to the surface of the
 photosensitive drum 4a through the mirror 3a. By rotating the light
 deflector 9a in the direction of the arrow A, optical scanning is
 performed in the direction of the arrow B on the surface of the
 photosensitive drum 4a. Then, as described above, latent images of C
 (cyan), M (magenta), Y (yellow) and B (black) are formed on the surfaces
 of the corresponding photosensitive drums. After this, they are
 multi-transferred to the transfer material to form a single color image.
 As shown in FIG. 3, in this embodiment, at least one of the plastic lenses
 forming the optical scanning devices is molded in the same cavity, and at
 least one of the plastic lenses molded in the same cavity is molded in a
 multiple cavity mold (22K, 8K, 10K, 11K).
 Further, in this embodiment, in each optical scanning device, the laser
 unit, the cylindrical lens, and the f.theta. lens system are held by a
 single plastic holding member, and the plastic holding member is molded in
 the same cavity, and the plastic holding member molded in the same cavity
 is molded in a multiple cavity mold. Further, the light deflector of each
 optical scanning device is held by the plastic holding member.
 In this embodiment, the plastic holding member 22a of the scanner unit 2a,
 the plastic cylindrical lens 8a, and the first and second plastic f.theta.
 lenses 10a and 11a are molded in a plurality of cavities, and the cavity
 combination is specified, the attachment of the plastic lens to the
 plastic holding member 22a being impossible with any other combination.
 For example, as shown in FIG. 3, the plastic holding members 22 are molded
 in two cavities, the cylinders 8 are molded in six cavities, the plastic
 lenses 10 are molded in four cavities, and the plastic lenses 11 are
 molded in three cavities.
 In this embodiment, the molded products used are molded in two cavities A1
 and A2. The number of cavities, which is determined depending upon the
 molding tact, is not particularly restricted. Naturally, in some cases,
 only one cavity is used. In FIG. 3, numerals 22K, 8K, 10K, and 11K
 indicate multiple cavity molds.
 As shown, for example, in Table 1, the cavity combinations are specified,
 no attachment being possible with any other combination.
 TABLE 1
 Holding member Plastic Plastic
 Scanner box 22 Cylinder 8 lens 10 lens 11
 Combination 1 A1 A1 A2 A1
 Combination 2 A2 A1 A1 A1
 The above combinations 1 and 2 are determined such that an aberration such
 as scanning line curvature and field curvature is improved in terms of
 tolerance. A plurality of scanner units of the same combination are
 mounted on a single apparatus.
 FIGS. 4, 5 and 6 are diagrams illustrating the plastic cylindrical lens 8a
 shown in FIG. 2. In FIG. 4, numeral 12 indicates a positioning member for
 the positioning on a plastic scanner box 2a', and numeral 13 indicates a
 cavity determining dowel. The cavity determining dowel 13 differs
 depending on the cavity of the plastic scanner box 2a' (plastic holding
 member 22a), and only a plastic cylindrical lens 8a of a specified cavity
 can be attached. FIG. 5 is a diagram illustrating the plastic cylindrical
 lens of another cavity; it differs in the configuration of the portion in
 the vicinity of the cavity determining dowel 13. FIG. 6 is a perspective
 view of the plastic cylindrical lens 8a.
 FIG. 7 is a diagram illustrating the first and second plastic f.theta.
 lenses 10a and 11a constituting the f.theta. lens system 21a. In the
 drawing, numeral 14 indicates pins for positioning the first and second
 plastic f.theta. lenses 10a and 11a, which are above the plastic scanner
 box 2a', in the Y-direction, and numeral 15 indicates pins for positioning
 them in the X-direction. Numeral 16 indicates cavity determining dowels,
 and numeral 17 indicates pins indicating the cavity of the first and
 second plastic f.theta. lenses 10a and 11a. Since the pins 17 indicating
 the cavities differ in position depending upon the lens cavity, the cavity
 combination of the plastic scanner box 2a' (plastic holding member 22a)
 and the first and second plastic f.theta. lenses 10a and 11a is determined
 according to the cavity determining dowel 16.
 In this embodiment, the color image forming apparatus is constructed as
 described above, whereby it is possible to prevent positional deviation in
 scanning of each optical scanning device without having to improve the
 lens accuracy to an extreme degree.
 FIG. 8 is a schematic diagram showing a main part of an optical scanning
 device according a second embodiment of the present invention. In the
 drawing, the components which are the same as those shown in FIG. 2 are
 indicated by the same reference numerals.
 This embodiment differs from the above-described first embodiment in that
 only the cavity combination of the first and second plastic f.theta.
 lenses is determined, no attachment being possible with any other
 combination, and that the cavity combination can be recognized from the
 outward appearance of the plastic scanner box 2a' (optical scanning device
 2a). Apart from the above, the construction and the optical effect of this
 embodiment are substantially the same as those of the first embodiment
 described above, and the same effect can be thereby achieved.
 That is, in the drawing, numeral 17 indicates pins determining the cavity
 combination of the first and second plastic f.theta. lenses 10a and 11a,
 any other cavity combination being impossible. In this embodiment, there
 are two cavities A1 and A2. Numeral 18 indicates a pin indicating the A1
 cavity of the second plastic f.theta. lens 11a, which passes through the
 plastic scanner box 2a' in the Z-direction. The A2 cavity is equipped with
 no pin 17. Therefore, the cavity combination of the first and second
 plastic f.theta. lenses 10a and 11a can be recognized according to whether
 the pin 17 passes through the plastic scanner box 2a' or not.
 In this way, in this embodiment, only the cavity combination of the first
 and second f.theta. lenses is determined, and the cavity combination can
 be recognized from the outward appearance of the plastic scanner box 2a',
 whereby combination error in assembly and replacement is prevented.
 Further, scanner units of the same combination are mounted in the
 apparatus main body.
 While in this embodiment four lenses of the same combination are
 incorporated, it is possible to incorporate lenses of any combination as
 long as the combination of the first and second plastic f.theta. lenses is
 determined without involving any problem in performance.
 Further, while in this embodiment the cavity combination is recognized
 according to whether there is a pin or not, this should not construed
 restrictively. It is also possible, for example, to identify the cavity
 combination by attaching a label or the like to the plastic scanner box at
 the time of assembly. It is possible to adopt a construction in which the
 cavity combination of the plastic lens and the plastic holding member can
 be recognized from the outside of each scanner unit.
 FIG. 9 is a schematic diagram showing a main part of the third embodiment
 of the present invention. In the drawing, the components which are the
 same as those shown in FIGS. 1 and 2 are indicated by the same reference
 numerals.
 This embodiment differs from the first embodiment described above in that
 in each optical scanning device, the laser unit, the plastic cylindrical
 lens, and the f.theta. lens system are held by a single plastic holding
 member and that the light deflector is shared. As in the first embodiment
 described above, this plastic holding member is molded in the same cavity
 and in a multiple cavity mold.
 That is, in the drawing, numeral 23a indicates a plastic holding member,
 which holds a semiconductor laser 6a, a collimator lens 7a, a plastic
 cylindrical lens 8a, and first and second plastic f.theta. lenses 10a and
 11a. As in the first embodiment, the cavity combination of the plastic
 holding member 23a, the plastic cylindrical lens 8a, and the first and
 second plastic f.theta. lenses 10a and 11a is determined.
 In this embodiment, a plastic holding member 23a' which is the same as the
 plastic holding member 23a is arranged symmetrically on the plastic
 scanner box 2a' with the polygon mirror 9a' therebetween, whereby the
 polygon mirror 9a' is shared. That is, one plastic holding member 23a' is
 reversed with respect to the plane of the drawing.
 FIG. 10 is a schematic diagram showing the construction of the entire
 apparatus of this embodiment. As shown in the drawing, in this embodiment,
 two scanner units 2a" and 2b" in which two sets of optical scanning
 devices are provided are mounted.
 By thus constructing the color image forming apparatus, this embodiment
 provides the same effect as the above-described embodiments.
 While in each embodiment a semiconductor laser emitting a single beams is
 used as the light source means, this should not be construed
 restrictively. For example, it is also possible, as shown in FIGS. 11 and
 12, to form the light source means forming the optical scanning devices 32
 and 32' by multi-semiconductor lasers. The present invention is also
 applicable as in the above embodiments when a plurality of beams are
 emitted from the optical scanning device 32 (while two beams are emitted
 from each optical scanning device in FIGS. 11 and 12, it is also possible
 to emit two or more beams) and directed to the surface of the
 photosensitive drum 33a (33b) to form a color image from the images formed
 on the plurality of photosensitive drums 33a and 33b.
 FIG. 13 is a schematic diagram showing a main part of a color image forming
 apparatus of the fifth embodiment of the present invention using a
 multi-semiconductor laser.
 A rotary polygon mirror 41 is formed by two polygon mirror portions 41a and
 41b and an intermediate section connecting these polygon mirror portions
 41a and 41b. These portions are formed by machining a single metal block.
 A laser light source 40 is provided which emits two beams so that they
 impinge upon the polygon mirror portions 41a and 41b. Cylindrical lenses
 45a and 45b are provided between the laser light source 40 and the polygon
 mirror portions 41a and 41b. In the emitting direction of the polygon
 mirror portions 41a and 41b, there are provided upper and lower scanning
 lenses 46a, 47a, 46b and 47b. Further, in the emitting direction of the
 upper scanning lenses 46a and 47b, there is provide an image carrier 48a,
 and, in the emitting direction of the lower scanning lenses 46b and 47b,
 there is provided a reflection mirror 49, and, in the reflecting direction
 of the reflection mirror 19, there is provided an image carrier 48b.
 Due to this construction, the two laser beams which are emitted from the
 laser light source 40 and which are parallel beams are caused to focus on
 the upper polygon mirror portion 41a and the lower polygon mirror portion
 41b of the rotary polygon mirror 41 by the cylindrical lenses 43a and 43b
 having refractive power only in the sub-scanning direction. Further, the
 beams deflected by the rotary polygon mirror 41 are transmitted through
 the scanning lenses 46a, 47a, 46b and 47b having an f.theta.
 characteristic, and the upper laser beam directed to the image carrier
 48a, and the lower laser beam is reflected by the reflection mirror 49 and
 directed to the image carrier 48b to perform optical scanning.
 Here, the laser light source 40 is a laser unit having a
 multi-semiconductor laser and a collimator lens, and parallel beams L1 and
 L2 are emitted from the laser light source 40.
 Further, the laser light source 40, the rotary polygon mirror 41, the
 cylindrical lens 45a, the scanning lenses 46a and 47a, the cylindrical
 lens 45b, and the scanning lenses 46b and 47b are accommodated in a
 plastic holding member 50.
 Here, the first lens system which effects image formation on the image
 carrier 48a from the first beam L1 emitted from the laser light source 40
 is formed by the cylindrical lens 45a and the scanning lenses 46a and 47a,
 at least one of which is a plastic lens.
 Further, the second lens system which effects image formation on the image
 carrier 48b from the second beam L2 emitted from the laser light source 40
 is formed by the cylindrical lens 45b, and the scanning lenses 46b and
 47b, at least one of which is a plastic lens.
 At least one of the plastic lenses of each lens system is a lens molded in
 the same cavity of a multiple cavity mold.
 As described above, in accordance with the present invention, it is
 possible to realize a color image forming apparatus of the type which has
 a plurality of sets of optical scanning devices and image carriers,
 wherein, by appropriately constructing each optical scanning device, it is
 possible to prevent positional deviation in scanning of each optical
 scanning device without having to improve the lens accuracy to an extreme
 degree.
 Further, as described above, in accordance with the present invention, it
 is possible to realize a color image forming apparatus of the type which
 has an optical scanning device including a multi-semiconductor laser and a
 plurality of image carriers, wherein, by appropriately constructing the
 optical scanning device, it is possible to prevent positional deviation in
 scanning of the optical scanning device without having to improve the lens
 accuracy to an extreme degree.
 While the present invention has been described with reference to what are
 presently considered to be the preferred embodiments, it is to be
 understood that the invention is not limited to the disclosed embodiments.
 On the contrary, the invention is intended to cover various modifications
 and equivalent arrangements included within the spirit and scope of the
 appended claims. The scope of the following claims is to be accorded the
 broadest interpretation so as to encompass all such modifications and
 equivalent structures and functions.