Optical scanner and color image forming apparatus using the same

The present invention has an object to provide a highly accurate optical scanner of a simple construction which permits reduction of jitter caused by a division error of the deflection surface of a polygonal deflector, and to provide a color image forming apparatus using the same. The optical scanner of the invention comprises a plurality of scanning units each having a polygonal deflector onto which a flux emitted from a light source is directed, and a scanning optical system which forms the flux reflection-deflected by the polygonal deflector into spots on a surface to be scanned. In the optical scanner of the invention, two or more scanning units commonly use a single polygonal deflector. The two or more scanning units use fluxes reflection-deflected on different deflection surfaces of the polygonal deflector, and the write timing of the two or more scanning units onto the surface to be scanned is determined by detecting fluxes from different deflection surfaces of the polygonal deflector by a single write position detector, and by using signals from that detector.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a color image forming apparatus. More particularly, the invention relates to a color image forming apparatus suitable for use in a laser printer, a digital copying machine, or a multi-function printer, in which a plurality of fluxes emitted from a plurality of light source means scan the corresponding surfaces to be scanned via a single polygonal deflector (polygonal mirror).

2. Description of the Related Art

It is the conventional practice to scan the surfaces of a plurality of photosensitive drums via a single polygonal mirror by means of fluxes emitted from a plurality of light source means, and deflection-scan on two or more different deflection surfaces. The write positions of the surfaces of the photosensitive drums are detected by any of the following two methods.(1) Japanese Patent Laid-Open No. 5-19586 discloses a method of determining a write position on a photosensitive drum on the basis of retrieval information obtained from write position detecting means; and(2) Japanese Patent Laid-Open No. 4-313776 shown inFIG. 11discloses a method of detecting a write position by a single write position detecting means, and determining other write positions on the basis of write position information of a deflection surface from among a plurality of deflection surfaces.

More specifically, all four different stations S1to S4begin writing simultaneously on the basis of respective image signals with reference to a write position detection signal (BD signal)801for the station S1(right end station in FIG.11).

FIG. 1is a timing chart illustrating the timing of writing an image after detection of a write position detection signal by a write position detecting means in the color image forming apparatus shown in FIG.11. As shown inFIG. 12, all the stations S1to S4begin writing simultaneously on the basis of respective image signals with reference to a BD signal for the station S1.

The above-mentioned conventional write position detecting methods have the following problems.

In the method (1) above, it is necessary to provide an optical element, a light receiving element (a light receiving sensor), an electric base and the like for each of the plurality of write position detecting means. This results in a complicated apparatus as a whole, causing a higher cost. A unit for controlling rotation of the polygonal deflector is required. This makes it difficult to downsize the apparatus as a whole.

In the method (2) above, a write positional shift (jitter) is produced by a manufacturing error such as a division error of the polygonal mirror deflection surfaces. This causes deterioration of the image quality, and makes it difficult to obtain a highly accurate image. When employing this apparatus as a color image forming apparatus, a difference in write positions in the main scanning direction between colors causes problems such as the occurrence of color shift.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical scanner of a highly accurate and simple configuration which permits reduction of jitter caused by division errors of the deflection surfaces of the polygonal deflector.

Another object of the invention is to provide a color image forming apparatus which yields a highly accurate color image free from jitter when applying the optical scanner in a color image forming apparatus.

A first aspect of the present invention provides an optical scanner comprising

a plurality of scanning units each having a polygonal deflector serving as deflecting means onto which a flux emitted from light source means is directed, and a scanning optical system which forms the flux reflection-deflected by the polygonal deflector into spots on a surface to be scanned, wherein:

two or more scanning units from among the plurality of scanning units commonly use the polygonal deflector, and the two or more scanning units use fluxes reflection-deflected on different deflection surfaces of the polygonal deflector; and

a write timing of the two or more scanning units onto the surface to be scanned is determined by applying fluxes from different deflection surfaces of the polygonal deflector to a single write position detecting means, and by using a signal from the write position detecting means.

A second aspect of the invention provides an optical scanner comprising

a plurality of scanning units each having a polygonal deflector serving as deflecting means onto which a flux emitted from light source means is directed, and a scanning optical system which forms the flux reflection-deflected by the polygonal deflector into spots on a surface to be scanned, wherein:

two or more scanning units from among the plurality of scanning units commonly use the polygonal deflector, and the two or more scanning units use fluxes reflection-deflected on different deflection surfaces of the polygonal deflector; and

the write timing of each of the two or more scanning units, when the number of the deflection surfaces is three, detects a flux coming via a deflection surface preceding by two surfaces in the rotating direction, thereby obtaining a beam write position detecting signal.

A third aspect of the invention provides an optical scanner comprising

a plurality of scanning units each having a polygonal deflector serving as deflecting means onto which a flux emitted from light source means is directed, and a scanning optical system which forms the flux reflection-deflected by the polygonal deflector into spots on a surface to be scanned, wherein:

two or more scanning units from among the plurality of scanning units commonly use the polygonal deflector, and the two or more scanning units use fluxes reflection-deflected on deflection surfaces not opposite to each other on the polygonal deflector; and

the write timing of each of the two or more scanning units, when the number of the deflection surfaces is four, detects a flux coming via a deflection surface preceding by three surfaces in the rotating direction, thereby obtaining a beam write position detecting signal.

A fourth aspect of the invention provides an optical scanner comprising

a plurality of scanning units each having a polygonal deflector serving as deflecting means onto which a flux emitted from light source means is directed, and a scanning optical system which forms the flux reflection-deflected by the polygonal deflector into spots on a surface to be scanned, wherein:

two or more scanning units from among the plurality of scanning units commonly use the polygonal deflector on deflection surfaces of the polygonal deflector; and

the write timing of each of the two or more scanning units, when the number of the deflection surfaces is five, detects a flux coming via a deflection surfaces preceding by three or four surfaces in the rotating direction, thereby obtaining a beam write position detecting signal.

A fifth aspect of the invention provides an optical scanner comprising

a plurality of scanning units each having a polygonal deflector serving as deflecting means onto which a flux emitted from light source means is directed, and a scanning optical system which forms the flux reflection-deflected by the polygonal deflector into spots on a surface to be scanned, wherein:

two or more scanning units from among the plurality of scanning units commonly use the polygonal deflector, and the two or more scanning units use fluxes reflection-deflected on deflection surfaces not opposite to each other on the polygonal deflector; and

the write timing of each of the two or more scanning units, when the number of the deflection surfaces is six, detects a flux coming via a deflection surface preceding by four surfaces in the rotating direction, thereby obtaining a beam write position detecting signal.

A sixth aspect of the invention provides an optical scanner comprising

a plurality of scanning units each having a polygonal deflector serving as deflecting means onto which a flux emitted from light source means is directed, and a scanning optical system which forms the flux reflection-deflected by the polygonal deflector into spots on a surface to be scanned; wherein:

two or more scanning units from among the plurality of scanning units commonly use the polygonal deflector, and the two or more scanning units use fluxes reflection-deflected on deflection surfaces not opposite to each other on the polygonal deflector; and

the write timing of each of the two or more scanning units, when the number of the deflection surfaces is eight, detects a flux coming via a deflection surface preceding by six surfaces in the rotating direction, thereby obtaining a beam write position detecting signal.

A seventh aspect of the invention provides an optical scanner comprising

a plurality of scanning units each having a polygonal deflector serving as deflecting means onto which a flux emitted from light source means is directed, and a scanning optical system which forms the flux reflection-deflected by the polygonal deflector into spots on a surface to be scanned; wherein:

two or more scanning units from among the plurality of scanning units commonly use the polygonal deflector, and the two or more scanning units use fluxes reflection-deflected on deflection surfaces opposite to a rotation axis of the polygonal deflector; and

the write timing of each of the two or more scanning units, when the number of the deflection surfaces is an even number N (N is 2 or a larger even number), detects a flux coming via a deflection surface preceding by (N−N/2) surfaces in the rotating direction, thereby obtaining a beam write position detecting signal.

An eighth aspect of the invention provides an optical scanner according to the first aspect of the invention, wherein the plurality of scanning units comprise two or more scanning units in which fluxes enter into surfaces not opposite to each other relative to the rotation axis of polygonal deflector.

A ninth aspect of the invention provides an optical scanner according to the first aspect of the invention, wherein the plurality of scanning units comprise two or more scanning units in which fluxes enter into surfaces opposite to each other relative to the rotation axis of the polygonal deflector.

A tenth aspect of the invention provides an optical scanner according to the first aspect of the invention, wherein at least two of the light source means of the plurality of scanning units are arranged on a substrate.

An eleventh aspect of the invention provides an optical scanner according the first aspect of the invention, wherein the flux in the main scanning direction incident on the deflection surface of the polygonal deflector has a flux width wider than the width of the deflection surface in the main scanning direction.

A twelfth aspect of the invention provides an optical scanner according to the first aspect of the invention, wherein the optical scanner has write position detecting means for detecting the flux for write timing of the surface to be scanned; the write position detecting means has a synchronization detecting element; and the synchronization detecting element is arranged on the same scanning units.

A thirteenth aspect of the invention provides an optical scanner according to the first aspect of the invention, wherein the optical scanner has write position detecting means for detecting the flux for write timing of the surface to be scanned; and the flux running toward the write position detecting means via the polygonal deflector does not pass through the scanning lens system.

A fourteenth aspect of the invention provides an optical scanner according to the first aspect of the invention, wherein the optical system forming the flux emitted from the light source means into a line image in the main scanning direction is formed in a plastic mold.

A fifteenth aspect of the invention provides a color image forming apparatus having a plurality of scanning units according to any one of the first to seventh aspects of the invention; and a plurality of image carriers arranged on the surface to be scanned of each of the scanning units and forming images of colors different from each other.

A sixteenth aspect of the invention provides a color image forming apparatus according to the fifteenth aspect of the invention, having a printer controller which converts color signals entered from an external device into image data of different colors, and enters the image data into the respective scanning unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

FIG. 1is a partial sectional view in the main scanning direction (main scanning sectional view) of a first embodiment of the present invention;FIG. 2is a partial sectional view in the sub-scanning direction (sub-scanning sectional view) of the first embodiment of the invention; andFIGS. 3A and 3Bare partial sectional views of write position detecting means (BD optical systems), illustrating only an optical path running toward a synchronous detecting element of a part of fluxes (BD fluxes) reflection-deflected on the deflection surface of the polygonal detector, in an exploded form:FIG. 3Ais a main scanning sectional view, andFIG. 3Bis a sub-scanning sectional view.

In this specification, a plane formed by the optical axis of the scanning lens system and the flux deflected by the optical deflector is defined as the main scanning cross-section, and a plane perpendicular to the main scanning cross-section including the optical axis of the scanning lens system is defined as the sub-scanning cross-section.

In the drawings, S1and S2represent first and second scanning units, respectively (hereinafter referred to as the “stations”). Each of the first and second scanning units comprises first optical elements (2aand2b) which convert fluxes from light source means (1aand1b) into other fluxes; second optical elements (3aand3b) which forms a line image long in the main scanning direction; opening stops (4aand4b) which limit the flux; a polygonal deflector5serving as deflecting means; and a scanning lens systems (6aand6b) which form the fluxes from the polygonal deflector into spots on the surface to be scanned.

In this embodiment, the first and second scanning units S1and S2commonly use a polygonal deflector5, and the first and second scanning units S1and S2use fluxes reflection-deflected on different deflection surfaces of the polygonal deflector5.

In this embodiment, the write timing of the first and second scanning units S1and S2onto the photosensitive drum surfaces (7aand7b) serving as surfaces to be scanned is determined by detecting fluxes from different deflection surfaces of the polygonal deflector5by a single write position detecting means64(BD optical system), and using signals from the single write position detecting means.

The first and second scanning units S1and S2are configured so that the fluxes are incident on the polygonal deflector5from the same direction.

The manner of use of the deflection surfaces of the polygonal deflector5covers the case where the fluxes do not use two opposite surfaces.

In the first and second scanning units S1and S2, the light source means1aand1beach comprises a semiconductor laser (beam source), and the light source means1aand the light source means1bare arranged on a flat substrate. The light source means1aand the light source means1bmay be arranged independently of each other.

The first optical elements2aand2beach comprises a collimator lens, and convert fluxes emitted from the light source means1aand1binto substantially parallel fluxes, divergent fluxes or convergent fluxes.

Each of the second optical elements3aand3bcomprises a cylindrical lens, formed in a plastic mold, and having a prescribed refractive force only in the sub-scanning direction.

The opening stops4aand4bform the fluxes emitted from the cylindrical lenses3aand3binto a desired beam shape.

The elements such as the light source means (1aand1b), the collimator lenses (2aand2b), the cylindrical lenses (3aand3b), and opening stops (4aand4b) form component elements of incident optical means (8aand8b).

The polygonal deflector5comprises, for example, a polygonal mirror of which the number of deflection surfaces N is 4. The mirror is rotated at a certain speed in the arrow A direction in the drawing by a driving means (not shown) such as a motor. In this embodiment, as described above, the first and second scanning units S1and S2commonly use this polygonal deflector5, and the first and second scanning units S1and S2use fluxes reflection-deflected on different deflection surface of the polygonal deflector5.

A scanning lens system comprises independent scanning lenses6aand6b,and forms the fluxes reflection-deflected by the polygonal deflector5into spots on the surfaces to be scanned7aand7b. The scanning lenses6aand6bhave a falldown correcting function by keeping a conjugated relationship between the proximity of the deflection surface of the optical deflector5and the proximity of the surfaces to be scanned7aand7bin the sub-scanning cross-section.

Reference numeral64represents write position detecting means(BD optical system), which has a synchronization detecting lens61(hereinafter denoted as the “BD lens”) for synchronization detecting; a slit62(hereinafter denoted as the “BD slit”); and a synchronization detecting element63(hereinafter denoted as the “BD sensor”), and determines a write timing for the surfaces to be scanned7aand7bof the scanning units S1and S2.

The BD lens61comprises an anamorphic lens in which the curvature in the main scanning direction differs from the curvature in the sub-scanning direction; forms the BD fluxes into an image on the BD slit62surface both in the main scanning direction and the sub-scanning direction; scans the BD slit62within the main scanning cross-section; and, in the sub-scanning cross section, serves as a falldown correcting system of the deflection surface, because the deflection surface and the BD slit62is substantially in a conjugated relationship.

The BD slit62determines a write position of the image. The BD slit62has a knife edge shape and determines an incident position for the substantially formed spots by the BD lens61scanned in the main scanning direction to be incident on the BD sensor63surface. The BD detecting accuracy is therefore set higher for the case of the knife edge of the BD slit62than for the case of the light receiving and of the BD sensor63.

The BD sensor63, being arranged independently of the light source means1aand1bin this embodiment, may be arranged on the same substrate as the light source means1aand1b.

In the optical path of the BD optical system64, a member for turning back the flux such as a turn-back mirror may be arranged. This provides an advantage of downsizing the optical scanner. While the BD lenses61may be individually provided as in the embodiment, a part of the scanning lens6aof the first scanning unit S1may also be used. When using a part of the scanning lens6a,the BD slit62is usually arranged at an optically equivalent position to that of the photosensitive drum surface7a.

Since the BD flux is formed into an image on the BD slit62surface in both the main scanning direction and in the sub-scanning direction, the spot diameter on the BD sensor surface is set so as to be larger than the diameter on the BD slit surface. This configuration thus makes it difficult to be sensitive to a sensitivity irregularity caused by a manufacturing error of the BD sensor or a sensitivity irregularity caused by dust or other adhering matters.

In this embodiment, in the first station S1, the fluxes optically modulated and emitted from the light source means1ain response to image information are converted by the collimator lens2ainto substantially parallel fluxes or converging fluxes, and enter into the cylindrical lens3a.The fluxes incident on the cylindrical lens3ain the main scanning cross-section are emitted as they are and pass through the opening stop4a(fluxes are partially shielded). In the sub-scanning cross-section, the fluxes converge, pass through the opening stop4a(partially shielded), and formed into substantially a line image (a line image long in the main scanning direction) on the deflection surface5aof the polygonal deflector5. The fluxes reflection-deflected on the deflection surface5aof the polygonal deflector5are formed into a spot image by the scanning lens6aonto the photosensitive drum surface7a. The photosensitive drum7ais thus optically scanned in the arrow B direction (main scanning direction) by rotating the polygonal deflector5in the arrow A direction. The image is thus recorded on the photosensitive drum7aserving as a recording medium.

At this point in time, the timing of the scanning starting position on the photosensitive drum surface7a,prior to optically scanning the photosensitive drum surface7ais adjusted. For this purpose, after gathering a part of the fluxes reflection-deflected on the deflection surface5aof the polygonal deflector5(BD fluxes) on the BD slit62surface by means of the BD lens61, the fluxes are introduced into the BD sensor63. The timing of the scanning start position for recording the image onto the photosensitive drum surface7ais adjusted by use of the write position detecting signal (BD signal) resulting from detection of an output signal from the BD sensor63.

In the second station S2, the flux emitted from the light source means1benters the deflection surface5aof the polygonal deflector5from the same direction as that of the first scanning unit S1, and the flux reflection-deflected on the deflection surface5bis formed into a spot image onto the photosensitive drum surface7by means of the scanning lens6b,and is subjected to optical scanning.

Prior to optically scanning the photosensitive drum surface7b,the timing of the scanning start position on the photosensitive drum surface7bis adjusted. After collecting a part of the fluxes reflection-deflected on the deflection surface5bpreceding by three surfaces of the deflection surface5aused for scanning of the first station S1(BD fluxes) onto the BD slit62surface through the BD lens61, the fluxes are introduced into the BD sensor63. The timing of scanning start position for image recording to the photosensitive drum surface7bby use of a write position detection signal (BD signal) is obtained through detection of an output signal from the BD sensor63.

In general, in an optical arrangement in which, with a number of deflection surfaces of four of the polygonal deflector, fluxes are incident on the polygonal deflector from the same direction as in this embodiment, the write timing of the second station S2is determined on the basis of the beam write position detection signal preceding by three surfaces of the deflection surface used for scanning of the first station S1.

The number of deflection surfaces is determined by counting the surfaces in the rotating direction as shown by an arrow and displayed as “preceding by N surfaces”.

That is, when the number of deflection surfaces is four, the write timing of the second station S2is determined by detecting BD fluxes deflected on the deflection surface preceding by three surfaces in the rotating direction by the BD optical system64, thereby obtaining a beam write position detecting signal.

FIG. 4is a timing chart illustrating a write timing of an image from detection of a BD signal by the BD optical system in this embodiment.

In this embodiment, a flux emitted from the light source means1aof the first station S1passes through the BD optical path and is received by the BD sensor63. As shown inFIG. 4, after the lapse of a certain period of time (certain clock) from startup of a BD signal, an image is formed on the photosensitive drum surface7aon the basis of image signals from the first station S1.

After the lapse of a certain period of time (certain clock), the second station S2causes the light source means1bto emit light on the basis of the BD signal detected three surfaces preceding the current one in the BD optical system64in the first station S1, and forms an image on the photosensitive drum surface7bon the basis of the image signals of the second station S2.

In this embodiment, as described above, jitter produced from a division error of the deflection surface of the polygonal deflector5is overcome by detecting BD signals (write position detection signals) to the two photosensitive drum surfaces7aand7bby means of a single BD optical system (write position detecting means)64, thereby achieving an optical scanner, free from a color shift, having a high accuracy and a simple construction.

When applying the present apparatus to a two-color image forming apparatus in this embodiment, it suffices to use only one such apparatus. When using the same for a four-color image forming apparatus as described later, it suffices to form the image by use of two such apparatuses.

In this embodiment, the cylindrical lenses3aand3bof the first and second scanning units S1and S2are independently provided. The present invention is not however limited to this configuration, but for example, the cylindrical lenses may be integrally formed by a plastic mold or the like. In the first and second scanning units S1and S2, the fluxes from the light source means (1aand1b) may be introduced into the polygonal deflector5directly via the opening stops (4aand4b), without using collimator lenses (2aand2b) or cylindrical lenses (3aand3b). In the first and second scanning units S1and S2, each of the scanning lens systems (6aand6b) comprises a single lens. This is not however limitative, but for example, two or more lenses may be employed.

In this embodiment, when the number of deflection surfaces of the polygonal mirror is six or eight (even number), a beam write position detection signal is obtained by detecting two fluxes, when using surfaces not opposite to each other from among the deflection surfaces of the polygonal mirror, via the deflection surface preceding by four or six surfaces in the rotating direction as shown inFIGS. 13 and 14.

Also in this embodiment, when the number of deflection surfaces of the polygonal mirror is four, six or eight (even number), a beam write position detection signal is obtained by detecting two fluxes, when using surfaces opposite to each other from among the deflection surfaces of the polygonal mirror, via the deflection surface preceding by two, three or four surfaces in the rotating direction as shown inFIGS. 15,16and17.

More specifically, for one of the write timings of two or more scanning units, a beam write position detection signal is obtained, when the number of the deflection surfaces is N (N is two or a larger even number), by detecting a flux deflected on the deflection surface preceding by (N−N/2) surfaces in the rotating direction.

When the number of deflection surfaces of the polygonal mirror is three, and when using deflection surfaces different from each other in this embodiment, a beam write position detection signal is obtained by detecting a flux deflected on the deflection surface preceding by two surfaces as shown in FIG.18.

When the number of deflection surfaces of a polygonal mirror is five, and when using deflection surfaces different from each other from among the deflection surfaces of the polygonal mirror in this embodiment, a beam write position detection signal is obtained by detecting a flux deflected on the deflection surface preceding by three or four surfaces in the rotating direction as shown inFIGS. 19 and 20.

As a result, this embodiment can provide the same advantages as in the above-mentioned first embodiment.

FIG. 5is a partial schematic view of the color image forming apparatus of an embodiment of the present invention. This embodiment covers a tandem-type color image forming apparatus in which two optical scanners are arranged, and image information is recorded on a photosensitive drum surface serving as an image carrier in parallel.

InFIG. 5, reference numeral60represent a color image forming apparatus;11and12, optical scanners having the same configuration as in the first embodiment;21,22,23and24, photosensitive drums serving as image carriers;31,32,33and34, developers; and51, a conveyor belt.

Color signals R (red), G (green) and B (blue) are entered from an external device52such as a personal computer into the color image forming apparatus60. These color signals are converted by a printer controller53provided in the apparatus into image data (dot data) of C (cyan), M (magenta), Y (yellow) and B (black). These image data are entered into optical scanners11and12, respectively. Light beams41,42,43and44modulated in response to the image data are emitted from these optical scanners, and the photosensitive surfaces of the photosensitive drums21,22,23and24are scanned by these light beams in the main scanning direction.

The color image forming apparatus of this embodiment has two optical scanners (11and12) which record image signals (image information) in parallel onto the surfaces of the photosensitive drums21,22,23and24in response to the individual colors C (cyan), M (magenta), Y (yellow) and B (black), and prints a color image at a high speed.

The color image forming apparatus of this embodiment forms, as described above, latent images of the individual colors on the corresponding surfaces of the photosensitive drums21,22,23and24by using light beams based on the individual image data by means of the two optical scanners11and12. Then, a full-color image is formed by multi-transferring the image on a recording medium.

As the external device52, for example, a color image reader having a CCD sensor may be employed. In this case, this color image reader and the color image forming apparatus60form a color digital copying machine.

Second Embodiment

FIG. 6is a partial sectional view in the main scanning direction of a second embodiment of the present invention (main scanning sectional view). InFIG. 6, the same component elements as inFIG. 1are represented by the same reference numerals.

The second embodiment differs from the first embodiment in that a polygonal deflector having a number of deflection surfaces N of six is used; two incident optical means78aand78bare arranged at positions opposite to each other relative to the polygonal deflector75in the main scanning cross-section; and two light source means71aand71bare built by a monolithic multi-laser having two light emitting points capable of being independently modulated. The other construction and optical functions are substantially the same as in the first embodiment, thus providing similar advantages.

More specifically, inFIG. 6,78aand78brepresent incident optical means arranged at positions opposite to each other relative to the polygonal deflector75in the main scanning cross-section. The incident optical means78aand78bhave light source means71aand71bcomprising monolithic multi-lasers, collimator lenses72aand72b,cylindrical lenses73aand73b,and opening stops74aand74b,respectively.

Reference numeral75represents the polygonal deflector comprising a polygonal mirror having a number of deflection surfaces N of six, and is rotated at a certain speed in the arrow A direction by driving means (not shown) such as a motor. In this embodiment, as in the above-mentioned first embodiment, the first and second scanning units (stations) S1and S2commonly use the polygonal deflector75, and the first and second scanning units S1and S2use fluxes reflection-deflected on different deflection surfaces of the polygonal deflector75.

In this embodiment, incident optical means78aand78bare arranged at positions opposite to each other relative to the polygonal deflector75. The configuration comprises a polygonal mirror having an even number of deflection surfaces N (N is two or larger) and uses an optical arrangement using deflection surfaces opposite to each other. The write timing of the second station is determined on the basis of the write position detecting signal of the deflection surface preceding by (N−N/2) surfaces of the deflection surface used for scanning of the first station S1.

More specifically, in the second station S2, the timing of the scanning start position on deflection surface75bis adjusted by use of a part of fluxes (BD fluxes) reflection-deflected on the deflection surface75a(preceding by three surfaces of the deflection surface75b) used for scanning the first station S1.

In this embodiment, as described above, the polygonal deflector75comprises a polygonal mirror having six surfaces, and the two light source means71aand71bcomprise monolithic multi-lasers, thereby achieving an optical scanner having a higher speed than in the above-mentioned first embodiment.

Third Embodiment

FIG. 7is a partial sectional view in the main scanning direction (main scanning sectional view) of a third embodiment of the present invention; andFIG. 8is a partial sectional view in the sub-scanning direction (sub-scanning sectional view). InFIGS. 7 and 8, the same component elements as those inFIG. 1are represented by similar reference numerals.

The third embodiment differs from the above-mentioned first embodiment in that a polygonal deflector85has deflection surfaces in two stages within the sub-scanning cross-section and the apparatus comprises first, second, third and fourth scanning units (stations) S1to S4; and four fluxes emitted from the light source means81aand81bfor the scanning units S1to S4enter into the polygonal deflector85at an incident angle of 70°. Other configurations and optical configurations are substantially the same as in the first embodiment, providing similar advantages.

More particularly, inFIGS. 7 and 8, S1and S3represent the first and third scanning units (also referred to as “stations”). Each of the first and third scanning units has a collimator lens82awhich converts two fluxes emitted from the light source means81acomposed of two light sources arranged in the sub-scanning direction; a cylindrical lens83aforming a line image long in the main scanning direction; an opening stop84awhich regulates fluxes; a polygonal deflector85; and a scanning lens86which forms the two fluxes reflection-deflected by the polygonal deflector by the polygonal deflector85into spots on the surface87of the photosensitive drum87a.

S2and S4represent second and fourth scanning units (hereinafter referred to also as “stations”). Each of the second and fourth scanning units S2and S4has a collimator82bwhich converts two fluxes emitted from light source means81bcomposed of two light sources arranged in the sub-scanning direction; a cylindrical lens83bwhich forms a line image in the main scanning direction; an opening stop84bwhich regulates fluxes; a polygonal deflector85; and a scanning lens86bwhich forms the two fluxes reflection-deflected by the polygonal deflector85into spots on the photosensitive drum87surface serving as a surface to be scanned.

In this embodiment, the scanning units S1to S4commonly use a single polygonal deflector85, and the individual scanning units S1to S4use fluxes reflection-deflected on different deflection surfaces of the polygonal deflector85.

In this embodiment, the write timing of the scanning units S1to S4onto photosensitive drum surfaces87ato87dis determined by detecting fluxes from different deflection surfaces of the polygonal deflector85by a write position detecting means (BD optical system), and using signals from this single write position detecting means.

The scanning lenses86aand86bare made by integrally forming two lenses arranged in parallel in the sub-scanning direction, and forms four fluxes reflection-deflected by the polygonal deflector85into spots on the photosensitive drum surfaces87ato87dof the corresponding first to fourth scanning units S1to S4. The scanning lenses86aand86bhave a falldown correcting function by keeping a conjugating relationship between the proximity of the deflection surfaces of the optical deflector85and the proximity of the photosensitive drum surfaces.

In this embodiment, the four fluxes emitted from the light source means81aand81bin the first to fourth scanning units S1to S4are all incident on the different deflection surfaces at an incident angle of 70°. The term the incident angle means an angle between the main beam of the fluxes emitted from the light source means81aand81band the optical axis of the scanning lenses86aand86b. As a result, it is possible to reduce the pitch unevenness more remarkably than in the first embodiment, thus permitting achievement of a more accurate optical scanner.

In an optical arrangement in which four fluxes emitted from the light source means81aand81bare incident on the polygonal deflector in a single direction as in this embodiment, the write timing of the second and fourth stations S2and S4onto the photosensitive drums is determined on the basis of a beam write position detection signal for a deflection surface preceding by (N−1) surfaces from the deflection surface used for scanning of the first and third stations S1and S3.

More specifically, in the second and fourth stations S2and S4, the timing of the scanning start position on the photosensitive drum surfaces87band87dby use of a part (BD flux) of the fluxes reflection-deflected on the deflection surface preceding by three surfaces from the deflection surface used in scanning of the first and third stations S1and S3.

In the polygonal deflector85, the deflection surfaces are arranged in two stages in the sub-scanning direction, and flow fluxes emitted from the light source means81aand81benter the corresponding deflection surfaces of the two-stage polygonal mirror. Since the two-stage polygonal mirror85in this embodiment has deflection surfaces which are simultaneously fabricated into equal flat surfaces, the BD optical system64suffices to detect only the fluxes deflection-reflected on the deflection surfaces85aand85bshown in the upper part of the drawing. That is, for the third and fourth stations S3and S4, it suffices to adjust the scanning start position timing by use of BD signals available for the first and second stations S1and S2.

For the scanning lenses86aand86b,the upper and lower (in the drawing) lenses (in the sub-scanning direction) (86a1,86a2,86b1and86b2) are formed integrally and have the same optical properties. A plurality of turn-back mirrors are therefore arranged so as to adjust the optical path lengths to the photosensitive drum surfaces87ato87d.

The turn-back mirrors suffice to be arranged so that the optical path lengths to the photosensitive mirrors are uniform, and quite the same advantages are available even with other arrangements.

FIG. 9is a timing chart illustrating the timing of the BD optical system from detection of a write position detection signal up to the writing of an image in this embodiment.

In this embodiment, the flux emitted from the light source of the first station S1passes through the BD optical path and is received by the BD sensor63. After the lapse of a certain period of time (certain clock) from startup of a BD signal, an image is formed on the photosensitive drum surface87ain response to the image signal from the first station S1. For the third station S3, as in the first station S1, after the lapse of a certain period of time (certain clock) from the startup of a BD signal, an image is formed on the photosensitive drum surface87con the basis of the image signal from the third station S3.

The second station S2causes the light source to emit light after the lapse of a certain period of time (certain clock) on the basis of a BD signal detected for a deflection surface preceding by three surfaces in the BD optical system64in the first station S1, and forms an image on the photosensitive drum surface87bon the basis of the image signal from the second station S2. The fourth station S4operates in the same manner as the second station S2, and an image is formed on the photosensitive drum surface87don the basis of the image signal of the fourth station S4.

In this embodiment, as described above, the configuration is such that the fluxes emitted from the light sources of the scanning units S1to S4enter the polygonal deflector85at an incident angle of 70°, thus permitting further reduction of pitch unevenness than in the first embodiment and achievement of a higher-accuracy optical scanner. By building the polygonal deflector85with a polygonal mirror having two stages of deflection surfaces in the sub-scanning direction, a four-color image forming apparatus can be achieved with a single optical scanner.

Fourth Embodiment

FIG. 10is a partial sectional view in the main scanning direction of a fourth embodiment of the present invention (main scanning sectional view). InFIG. 10, the same component elements as those shown inFIG. 6are represented by the similar reference numerals.

This embodiment differs from the above-mentioned second embodiment in that first and second scanning units (stations) S1and S2comprise over-field optical systems (OFS optical systems). The other configuration and optical properties are substantially the same as in the first embodiment, giving similar advantages.

More specifically, in this embodiment, fluxes emitted from light source means91ain the first station S1are converted by a collimator lens92into substantially parallel fluxes or diverging fluxes, and enter a cylindrical lens93a. From among the fluxes having entered the cylindrical lens93a,the fluxes in the sub-scanning cross-section converge and are limited through the opening stop94a,enter a deflection surface95aof the polygonal deflector95via a turn-back mirror99aand are formed substantially into a line image (line image long in the main scanning direction) in the proximity of the deflection surface. The fluxes incident on the deflection surface95aenter diagonally relative to a plane perpendicular to the rotation axis of the polygonal deflector, from the sub-scanning cross-section containing the rotation axis of the polygonal deflector95and the optical axis of the scanning lens96a. The fluxes in the main scanning cross-section enter the deflection surface95afrom substantially the center of the deflection angle of the polygonal deflector95. The flux width of the fluxes in this case is set so that, in the main scanning direction, the flux width is sufficiently larger than the facet width of the deflection surface. The fluxes deflection-reflected on the polygonal deflector95are introduced onto the photosensitive drum surface97avia the scanning lens96a. The fluxes perform optical scanning in the arrow B direction (main scanning direction) on the photosensitive drum surface96aby rotating the polygonal deflector95in the arrow A direction in the drawing. An image is thus recorded on the photosensitive drum surface97aserving as a recording medium.

At this point in time, the scanning start position timing on the photosensitive drum surface97ais adjusted, prior to optically scanning the photosensitive drum surface97a,by collecting a part of the fluxes reflection-deflected on the deflection surface95aof the polygonal deflector95(BD fluxes) onto a BD slit62surface by means of a BD lens61, and then introducing the same into a BD sensor63. Using a write position detection signal (BD signal) resulting from detection of an output signal from the BD sensor63, the timing of the scanning start position of image recording on the photosensitive drum surface97ais adjusted.

In the second station S2, as in the first station S1, fluxes emitted from the light source means1benter the deflection surface95bof the polygonal deflector95from the front in a direction opposite to the incident direction of the first scanning unit S1, and the fluxes reflection-deflected on the deflection surface95bare formed into spots through a scanning lens96bonto the corresponding photosensitive drum surface97b,and subjected to optical scanning.

At this moment, the scanning start position timing on the photosensitive surface97bis adjusted, prior to optically scanning the photosensitive drum surface97b,by collecting a part of the fluxes reflection-deflected on the deflection surface95bpreceding by three surfaces to the deflection surface95aused for scanning of the first station S1(BD fluxes) onto the BD slit62surface through the BD lens61, and then introducing the same into a BD sensor62. By using a write position detection signal (BD signal) resulting from detection of an output signal from the BD sensor63, the timing of the scanning start position of image recording on the photosensitive drum surface97bis adjusted.

Also in this embodiment, as in the above-mentioned second embodiment, the write timing of the second station S2is determined on the basis of the write position detection signal for the deflection surface preceding by (N−N/2) surfaces of the deflection surface used for scanning the first station S1.

That is, in the second station S2, the scanning start position timing is adjusted by use of a part of the fluxes reflection-deflected on the deflection surface95bpreceding by three surfaces of the deflection surface95aused for scanning the first station S1.

In this embodiment, as described above, the first and second scanning units S1and S2are built with OFS optical systems, and by detecting the write position detection signal to the two photosensitive drum surfaces97aand97bby use of single write position detecting means64, jitters caused by a division error of the deflection surface of the polygonal deflector95are reduced, and a high-accuracy optical scanner having a simple construction is achieved.

According to the present invention, as described above, the write timing onto the surface to be scanned of two or more the scanning units is determined by detecting fluxes from different deflection surfaces of the polygonal deflector by single write position detecting means, and by using signals from the single write position detecting means. It is thus possible to reduce jitter caused by a division error of the deflection surface of the polygonal deflector, and achieve a high-accuracy optical scanner having a simple construction.

According to the present invention, furthermore, when using the optical scanner of the invention for a color image forming apparatus, it is possible to achieve a color image forming apparatus giving a high-accuracy color image free from a color shift.