BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an optical scanning device according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a first example of the optical scanning device;
FIG. 3 is a table of relation between appearance (direction) of scanning-line curve and deviation (color misregistration) in stations M and Y caused by temperature change;
FIGS. 4 and 5 are comparative examples of the optical scanning device shown in FIG. 2;
FIG. 6 is a schematic diagram of a second example of an optical scanning device;
FIGS. 7 and 8 are schematic diagrams of comparative examples of the optical scanning device;
FIG. 9 is a top view of the optical scanning device shown in FIG. 8;
FIG. 10 is a schematic diagram of a third example of an optical scanning device;
FIG. 11 is a schematic diagram of a modified example of the optical scanning device shown in FIG. 10;
FIG. 12 is a schematic diagram of a fourth example of an optical scanning device;
FIG. 13 is a schematic diagram of a comparative example of the optical scanning device shown in FIG. 12;
FIG. 14 is a schematic diagram of an image forming apparatus according to an embodiment of the present invention;
FIGS. 15A to 15C are views of a first scanning lens shown in FIG. 14;
FIG. 16 is a schematic diagram of a first example of an image forming apparatus;
FIG. 17 is a schematic diagram of a second example of an image forming apparatus;
FIG. 18 is a top view of the image forming apparatus shown in FIG. 17;
FIG. 19 is a schematic diagram of relevant parts of a third example of an image forming apparatus;
FIG. 20 is a schematic diagram of a fourth example of an image forming apparatus;
FIG. 21 is a schematic diagram of a modified example of the image forming apparatus shown in FIG. 20;
FIG. 22 is a schematic diagram of relevant parts of a fifth example of an image forming apparatus;
FIG. 23 is a schematic diagram of relevant parts of a sixth example of an image forming apparatus;
FIG. 24 is a schematic diagram of a comparative example of the image forming apparatus;
FIG. 25 is a schematic diagram of relevant parts of an seventh example of an image forming apparatus;
FIG. 26 is an exploded plan view of an optical path from a polygon mirror to a photosensitive drum in an eighth example of an image forming apparatus;
FIGS. 27A to 27C are views of a first scanning lens shown in FIG. 26;
FIG. 28 is exploded plan view of an optical path from a polygon mirror to a photosensitive drum in a comparative example for the image forming apparatus shown in FIG. 26;
FIGS. 29A to 29C are views of a first scanning lens shown in FIG. 28;
FIGS. 30A and 30B are graphs of change in scanning-line shape caused by temperature rise along with the rotation of a polygon scanner shown in FIG. 26;
FIG. 31 is schematic diagram of a ninth example of an image forming apparatus;
FIG. 32 is a schematic diagram of relevant parts of an tenth example of an image forming apparatus;
FIG. 33 is a schematic diagram of an eleventh example of an image forming apparatus; and
FIG. 34 is a schematic diagram of a comparative example for the image forming apparatus shown in FIG. 33.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Exemplary embodiments of the present invention are explained in detail below referring to the accompanying drawings.
In the following, while a scanning lens is explained as a focusing element constituting a scanning optical system, a scanning mirror can also be used as a focusing element. An optical path of a laser beam from an light source to a photosensitive drum (photoconductor) having a surface to be scanned (target surface) via a coupling lens, a cylindrical lens, a deflector (polygon mirror), a scanning lens, a folding mirror and the like, as well as mechanical structures for supporting them is hereinafter collectively referred to as “station”. The following description pertains mainly to an optical scanning device of single beam scanning system in which a single laser beam is scanned on a photoconductor corresponding to each of a plurality of stations. However, the same can be applied to a multi-beam scanning system in which a plurality of laser beams is scanned simultaneously on a single photoconductor.
FIG. 1 is a schematic diagram of an optical scanning device according to an embodiment of the present invention. A laser beam emitted from an light source such as semiconductor laser (not shown) is converted into a generally parallel light flux by action of a coupling lens (not shown), and then focused as a linear image which is longer in the main scanning direction on a deflecting and reflecting surface of a polygon mirror (deflector) 14 by action of a cylindrical lens (not shown). The polygon mirror 14 is mounted on a so-called polygon motor 14a to configure a polygon scanner, and rotates at high speed (several tens thousand rpm), so that it may cause noise or generate heat due to windage loss in an edge part of the polygon mirror 14. Further in the polygon motor 14a, heat generation may be caused by a bearing or a driving circuit substrate. Therefore, for the purpose of preventing noise and diffusion of heat by air flow, the polygon scanner 14b is often surrounded by a wall 19. In such a case, a transparent substrate (soundproof glass) 18 is disposed for allowing passage of beams.
A laser beam deflected and reflected by the polygon mirror 14 scans on a surface of a photosensitive drum (target surface) 16 as light spots via a scanning optical system 15. The scanning optical system 15 includes a first scanning lens 15-1 and a second scanning lens 15-2.
The above described cylindrical lens, polygon scanner 14b, scanning optical system 15 and the like are housed in an optical housing 23. The optical housing 23 is fabricated by aluminum dye casting, resin molding or the like. The one made of resin is often employed in a low-end machine because it can be fabricated with low cost. However, such housing often leads the problems of low heat conductivity, low rigidity and the like. For this reason, in a high-end machine, the one made of aluminum is often employed because it has good heat conductivity and it can realize high rigidity.
Generally, since a high-end machine often outputs a large number of prints (for example, several tens prints to one hundred and several tens of prints per a minute), rotation speed of the polygon scanner 14b is as high as several tens of thousands of rpm or more, and accordingly the amount of heat generation is tremendous. For this reason, it is necessary to surround the polygon scanner 14b with the wall 19 (and soundproof glass 18) as described above.
Although diffusion of heat by air flow can be suppressed by surrounding the polygon scanner 14b with the wall 19, it is impossible to prevent heat from diffusing in the member of optical housing 23 through heat conductivity. This is significant in the case of an aluminum optical housing. Further, when the optical scanning device is used as an exposing device of an image output apparatus using electrophotography, it may possibly be influenced by heat generated at a fixing device or the like that transfers a developed toner image onto a recording medium.
In this way, there is a possibility that heat transmits through the member of the optical housing 23 to reach the first scanning lens 15-1 and the second scanning lens 15-2. When the first/second scanning lens 15-1, 15-2 is made of resin, in particular, temperature distribution (deviation) may arise due to difference in heat conductivity with respect to the optical housing 23. This temperature distribution causes changes in physical property values such as refractive index or shape of the optical surface (or straightness of the entire lens) of the first/second scanning lens 15-1, 15-2, so that change (deterioration) in optical performances such as beam spot diameter or scanning-line shape on the target surface 16 occurs. Change in scanning-line shape causes deviation in superimposing toner images resulting in color misregistration, i.e., in the sub-scanning direction between each stations in a tandem image forming apparatus, and the deterioration appears more significantly in a half-tone image, in particular.
First, the structure in which curving direction of scanning lines due to temperature change are matched between each stations is explained. FIG. 2 is a schematic diagram of a color image (two-color image) forming apparatus with two optical scanning devices as shown in FIG. 1. A developing device, a transferring device, a fixing device and the like that do not enter into the present invention are not shown. A double-station type optical scanning device (two-image forming apparatus) is explained below. However, it may be applied to any double-stations in an optical scanning device (multicolor image forming apparatus) having more stations. As described above, the term “station” refers to an optical path and an optical element located between a light source in an optical scanning device and a target surface (photoconductor), as well as mechanical structures for supporting them. The light source can be a multi-beam light source, and a multi-beam scanning system that simultaneously scan one target surface with a plurality of laser beams can be employed.
Optical scanning devices 20M1 and 20Y1 each forms an independent station, and a laser beam that is modulated according to image data draws a scanning line on the photosensitive drums 16M and 16Y. After a toner image is formed by electrophotography in correspondence with the shape of the scanning line, the toner image is transferred onto an intermediate transfer belt 31. If the shape of the scanning line differs between the optical scanning devices 20M1 and 20Y1, toner images to be superimposed onto the intermediate transfer belt 31 are misregistered.
As is described previously, in an optical scanning device, the heat generated at the polygon scanner rotating at high speed reaches the first scanning lens 15-1 and the second scanning lens 15-2 via the optical housing 23 to change the refractive index and shape of optical surface (straightness of the entire lens). As a result, the scanning-line shape changes as shown in FIG. 3, in the image forming apparatus in FIG. 2.
FIG. 3 is a table of relation between the appearance (direction) of scanning-line curve due to temperature change and its deviation (color misregistration on recording medium) between stations M and Y. Note that, in FIG. 3, the horizontal direction represents the main scanning direction, while the vertical direction represents the sub-scanning direction. The amount of color misregistration (iv)=curve component of the station M (i)-curve component of the station Y (ii). For example, when toner images are superimposed together while curving direction of scanning lines of both the stations are matched, color misregistration is the most severe at the center image height in the cases of (c) and (d) of FIG. 3. In FIG. 3, (a) to (d) depict shapes of scanning lines on the photosensitive drums 16M and 16Y before rotation of the polygon scanner 14b ((a) in FIG. 3) and after rotation of polygon scanner ((b) to (d) in FIG. 3). The shape of the scanning line may be represented by a sum of component of the first scanning lens 15-1 and component of the second scanning lens 15-2.
In the case of (a) in FIG. 3, the scanning-line shape is straight, and therefore, color misregistration does not occur. As shown in FIG. 2, by making the secured face side of the first/second scanning lens 15-1M and 15-2M (contact face with optical housing 23M1), and the secured face side of the first/second scanning lens 15-1Y and 15-2Y (contact face with optical housing 23Y1) coincidence with each other in the optical scanning devices 20M1 and 20Y1, the scanning-line shape on the photoconductor changes into a similar shape as shown in (b) in FIG. 3, so that color misregistration can be minimized. That is, in the case of (b) in FIG. 3, the scanning line is in a shape of upward convex in both the stations M and Y, and there is substantially no deviation between them. Consequently, color misregistration does not occur.
On the other hand, FIG. 4 is a comparative example of the first example, in which the secured face side of the first/second scanning lens 15-1M and 15-2M (contact face with optical housing 23M1), and the secured face side of the first/second scanning lens 15-1Y and 15-2Y (contact face with optical housing 23Y1) are inverted between both optical scanning devices 90M1 and 90Y1. Therefore, color misregistration occurs as shown in (c) in FIG. 3. That is, in the case of (c) in FIG. 3, curving direction of the scanning-line curves are opposite (upward convex and downward convex) between both the stations M and Y. Therefore, when scanning lines of both the stations M and Y are matched, there is no color misregistration at the outermost image height, while color misregistration is severe at the center image height.
FIG. 5 is another comparative example of the first example, in which, since distance along the optical housing 93M2 (distance in which heat transmits) from the polygon scanner 14b to the first/second scanning lens 15-1M and 15-2M is long on the side of station M (optical scanning device 90M2), change in scanning-line shape is relatively small. Therefore, color misregistration occurs as shown in (d) in FIG. 3. That is, in the case of (d) in FIG. 3, although the scanning lines of both the stations M and Y curve in the same direction, they are different in curving angle. Thus, color misregistration occurs.
A bonding technique to secure scanning lenses is explained below. In securing the first scanning lens 15-1 and the second scanning lens 15-2 to the optical housing 23, it is preferred to employ a bonding technique for achieving low costs. When length of scanning lens in the main scanning direction is denoted by L, it is preferred that the region where top face or bottom face of the scanning optical element contacts the housing member is provided in the area within ±L/4 from the center part of the scanning optical element.
As described above, by securing the scanning lens to the optical housing so that curving direction of scanning lines due to temperature change match between target surfaces, it is possible to reduce color misregistration even if characteristics (refractive index, surface shape, etc.) of the scanning lens change. It is, however, more desired that the change in characteristics of scanning lens is small.
When a scanning lens made of resin is secured on an optical housing made of aluminum by bonding, the scanning lens made of resin deforms due to difference in amount of thermal expansion between, as temperature rises due to rotation of polygon scanner, for example, which may lead occurrence of scanning-line curve. By providing the bonding part in a region within ±L/4 from the center part of the scanning lens, it is possible to reduce deformation of the scanning lens and to suppress change in scanning-line shape.
Next, pressure securing using a spring is explained as another technique for securing the scanning lens. In order to prevent deformation of the scanning lens due to difference in amount of heat expansion, the scanning lens can be secured under pressure to the optical housing by a spring so that it can substantially freely expand (scaling up/scaling down). In the condition that temperature distribution occurs inside the member after temperature change, distribution of refractive index of the scanning lens and shape of the scanning lens also change, which may cause the scanning line-curve. However, when the temperature distribution stabilizes to certain distribution over the time, the distribution of refractive index and change in shape are recovered to the original condition, so that it is possible to reduce the change in scanning-line shape.
In the first example, two independent optical scanning devices form the respective stations is described. This structure can reduce occurrence of deviation in optical property (scanning-line shape, in particular) between each stations (optical scanning devices) as a design value, so that assembling/adjustment of the optical scanning devices can be facilitated. Further, since the optical scanning devices are independent from each other, it is possible to facilitate the replacing operation when a part such as polygon scanner fails to operate properly. For example, when repair is conducted at the site of the user, the optical scanning device itself may be replaced by new one rather than replacing the failed part at the site, and the failed optical scanning device may be separately repaired in a repair plant or the like.
In contrast to this, two stations may be housed in an optical housing 23. By housing them in one optical housing, it is possible to reduce the number of assembling steps if assembling and adjustment are required for the optical scanning device (optical housing) in assembling the image forming apparatus in an assembling plant. In such a case, to reduce color misregistration, it is desired to mount the polygon scanner and the scanning lens as shown in FIG. 6. That is, on the optical housing 232, the secured face side of the first/second scanning lens 15-1M and 15-2M (contact face with optical housing 232) and the secured face side of the first/second scanning lens 15-1Y and 15-2Y (contact face with optical housing 232) are made into coincidence with each other in the optical scanning devices 20M2 and 20Y2 so that the relationship of (b) in FIG. 3 is satisfied.
As comparative examples, FIG. 7 depicts optical scanning devices 90M3 and 90Y3 in which two stations M and Y are housed in an optical housing 933. FIGS. 8 and 9 depict optical scanning devices 90M4 and 90Y4 in which two stations M and Y are housed in an optical housing 934. As shown in FIG. 8, for miniaturization of optical scanning device, and limitation of mechanical layout such as photosensitive drum interval in the image forming apparatus, each station is provided with one folding mirror that folds optical path of laser beam. FIG. 9 is an exploded plan view of optical layout without a folding mirror where the stations M and Y are disposed symmetrically on the right and left sides about the polygon mirror 14.
In the structure of FIGS. 8 and 9, first scanning lenses 15-1M and 15-1Y are provided on the side where the polygon scanner 14b is installed in the optical housing 934. Accordingly, heat generation from the polygon scanner 14b transmits to the first scanning lenses 15-1M and 15-1Y which are generally at the same distances, and as a result, the scanning-line shape changes. At this time, difference in the number of folding mirror 24 in the optical path from the first scanning lens 15-1 to the photosensitive drum 16 in both stations is even number (one for each station). Therefore, curving direction of scanning lines are opposite to each other in photosensitive drums 16M and 16Y. That is, color misregistration in toner image on the intermediate transfer belt (not shown) appears as difference in the amount of scanning-line curve between both stations ((c) in FIG. 3).
In contrast to this, when difference in the number of folding mirrors provided in both stations is an even number, as shown in FIG. 10, one first scanning lens 15-M may be disposed on the same face side, while the other first scanning lens 15-1Y may be disposed on the opposite face side, with respect to the face where the polygon scanner 14b is installed in the optical housing 233. With this structure, curving direction of scanning lines on the photosensitive drums 16M and 16Y can be matched, so that color misregistration can be reduced ((b) in FIG. 3).
More preferred structure, a modified example of the third example, is shown in FIG. 11. In FIG. 11, the shape of the optical housing 234 on the side of station M (optical scanning device 20M4) is set so that distance from the polygon scanner 14b to the first scanning lens 15-1 (distance of heat conduction along the optical housing 234) is substantially equivalent.
The comparative example examines the case where difference in the number of folding mirrors provided in both stations is an even number. When the difference in the number of folding mirrors is an odd number for the reason of mechanical layout, the first scanning lenses 15-1M and 15-1Y can be disposed on the face side where the polygon scanner 14b is installed in the optical housing 235 shown as a fourth example in FIG. 12. In FIG. 12, the numbers of folding mirrors are two on the side of station M, and three on the side of station Y, and the difference is 1 (odd number). By setting numbers of folding mirrors to be provided so that curving direction of scanning line coincides on each photosensitive drum, it is possible to reduce color misregistration.
Since scanning-line curve on the photosensitive drum 16 is represented by the sum of component of the first scanning lens 15-1 and component of the second scanning lens 15-2, it is possible to make overall variation in scanning-line curve smaller by making at least either one of the variable component smaller. As the distance from the polygon scanner 14b to the scanning lens (distance of heat conduction along the optical housing 23) increases, influence of heat generation at the polygon scanner 14b decreases, and generation amount of the variable component can also be reduced. Although a folding mirror is never disposed between the first scanning lens 15-1 and the polygon scanner 14b, in general, it is often the case that a folding mirror is disposed between the first scanning lens 15-1 and the second scanning lens 15-2.
When a folding mirror is disposed between the first scanning lens 15-1 and the second scanning lens 15-2, distance from the polygon scanner 14b to the second scanning lens 15-2 (distance of heat conduction along the optical housing 23) should be set to be longer than the distance to the first scanning lens 15-1. With such structure, it is possible to make temperature change near the second scanning lens 15-2 smaller, so that the component of the first scanning lens 15-1 is dominant with respect to the scanning-line curve on the photosensitive drum 16, and the second scanning lens 15-2 may be installed in any manner.
Generally, in the case of a scanning optical system made up of a plurality of scanning optical elements, the scanning optical element located at a position closer to the target surface has larger power in the sub-scanning direction (along optical path of laser beam), and deterioration in straightness more largely influences on the scanning-line shape in the target surface. Therefore, it is desired to reduce heat conduction to the scanning optical element disposed near the target surface.
FIG. 13 depicts a comparative example of the fourth example. In the station Y (optical scanning device 90Y5), a second scanning lens 15-2Y is disposed between the polygon scanner 14b that is heat source, and the first scanning lens 15-1Y from the view point of heat conductivity within the optical housing member. Therefore, in examples of FIGS. 10 and 11, the second scanning lens 15-2Y is little influenced by heat generation at the polygon scanner 14b. However, the structure in FIG. 13 may cause some problem.
The second scanning lens 15-2M of station M (optical scanning device 90M5) is disposed at a position that is generally symmetrical to the second scanning lens 15-2Y with respect to the polygon scanner 14b, and influence of heat generation at the polygon scanner 14b is also comparable. Further, since difference in the number of folding mirror 24 disposed in the optical path after second scanning lenses 15-2M and 15-2Y in both stations M and Y is even (0 for both stations), curving direction of components of scanning lines are opposite to each other. Therefore, this structure is undesirable because of increasing color misregistration.
In the third and fourth examples and comparative examples (FIG. 8 to 12), distance from the polygon scanner 14b to the second scanning lens 15-2 is set longer than the distance to the first scanning lens 15-1, to reduce the influence of component of the second scanning lens 15-2.
When the optical scanning device as described above is employed as an exposing device of an image forming apparatus that outputs a color image by superimposing toner images formed on a plurality of photoconductors by electrophotography, on a recording medium, it is possible to obtain an output image of high quality with less color misregistration.
In a conventional image forming apparatus, by forming a toner image for detecting color misregistration that is irrelevant to an output image on an intermediate transfer belt and sensing it with a predetermined detection sensor, it is possible to detect the degree of overlapping of toner images between different stations (i.e., degree of scanning-line curve) and to adjust the writing start timing in the sub-scanning direction on the basis of the detecting results. Therefore, this can reduce the amount of color misregistration.
By further applying the optical scanning device of the embodiment to such a color image recording apparatus, it is possible to reduce occurrence of deviation of scanning-line shape caused by temperature change. Therefore, it is possible to reduce the amount of toner image for detecting color misregistration that is unnecessary for output image, and frequency of stopping job during continuous outputting is reduced, so that reduction in the number of prints can be prevented (environment-responsive).
FIG. 14 is a schematic diagram of a color image (bicolor image) forming apparatus with two stations inside the optical scanning device as shown in FIG. 1. A developing device, a transferring device, a fixing device and the like that do not enter into the present invention are not shown. As a bicolor image, for example, superimposing two color toner images of yellow (Y) having low visual sensitivity and black (K) having high visual sensitivity is assumed (superimposed on any one of the intermediate transfer belt and the recording medium or both).
The term “station” refers to an optical path and an optical element located between a light source in an optical scanning device and a target surface (photoconductor), as well as mechanical structures that support them. The light source can be a multi-beam light source, and a multi-beam scanning system that simultaneously scans one target surface with a plurality of laser beams can also be used. The station corresponding to the laser beam that is deflected and reflected at the polygon mirror 14-1 is referred to as ST1, and the station corresponding to the laser beam deflected and reflected at the polygon mirror 14-2 is referred to as ST2. The polygon scanner 14b includes the polygon motor 14a, and the two polygon mirrors 14-1 and 14-2 mounted thereon.
In FIG. 14, the laser beams emitted from a light source (not shown) such as two semiconductor lasers each are converted into generally parallel light fluxes by action of two coupling lenses (not shown) and focused as linear images that are longer in the main scanning direction (linear images focused in the sub-scanning direction) on (deflecting and reflecting surfaces of) the polygon mirrors 14-1 and 14-2 by action of two cylindrical lenses (not shown). The two laser beams deflected and reflected at the polygon mirrors 14-1 and 14-2 respectively pass through the first scanning lenses 15-1-1 and 15-1-2, and through the second scanning lenses 15-2-1 and 15-2-2, and reach two photosensitive drums 16-1 and 16-2. In the present example, each one folding mirror 24 is disposed between the first scanning lens and the second scanning lens, and the optical path of laser beam is folded so that the optical element is housed inside the optical housing 23.
The first scanning lenses 15-1-1 and 15-1-2, and the second scanning lenses 15-2-1 and 15-2-2 are preferably made of resin which allows mass production by molding (low cost) and generation of complicated surface shape (high performance).
The first scanning lenses 15-1-1 and 15-1-2 having substantially the same shape are housed in the optical housing 23 while they are aligned in the sub-scanning direction. With such a structure in which individual scanning lenses are aligned, it is possible to keep individual optical performance, especially surface shape of the optical surface accurately, and enables use in a single-station optical scanning device such as monochrome image forming apparatus.
As shown in FIGS. 15A to 15C, the bottom face (back face) of the first scanning lens 15-1-1 (15-1-2) is formed with three bosses, which function as positioning unit (and reference for height) in the height direction (sub-scanning direction). As a result, it is possible to keep the height (of sub-scanning direction) from a mounting reference by forming accurately only near the bosses without molding the entire scanning lens with high accuracy even in the case of piling up in the sub-scanning direction.
Because the polygon scanner 14b rotates at such high speed as several tens of thousands of rpm, heat generated by friction at the polygon motor 14a and by windage loss at the polygon mirrors 14-1 and 14-2 transmit in the member of the optical housing 23 and reach near the first scanning lens 15-1-1 (15-1-2) [arrow in FIG. 14]. Similarly, heat reaches near the first scanning lens also from heat source outside the optical scanning device, for example, from a fixing device.
Due to the influence of heat transmitted through the member of the optical housing 23 from the polygon scanner 14b, temperature deviation arises between near the bosses (three positions) provided in the scanning lens 15-1-1 of the lower layer, and the optical housing 23 (mounting surface 30).
This temperature deviation may cause local change in optical surface shape of the scanning lens 15-1-1 near the bosses, or distribution in physical property values in the scanning lens 15-1-1, for example, refractive index, and for this reason, shape of the scanning line (scanning-line curve) on the photosensitive drum 16-1 in station ST1 changes. When a toner image is formed in such a station (photosensitive drum), the resultant image is of low quality. The influence is significant particularly in the case of color component having high visual sensitivity (black toner), so that it is important for the station containing the first scanning lens 15-1-1 to be compatible to a color component having low visual sensitivity (yellow toner) for obtaining an image of high quality.
In the case of such a bicolor image, change in one shape of the scanning lines results in color misregistration. Therefore, the necessity of selecting a station from the level of visual sensitivity is small. However, even in a bicolor image forming apparatus, image outputs are often conducted in a single color (black) having higher visual sensitivity, for example, for image of characters, so that it is desired to realize high quality of image of the color component having high visual sensitivity.
Since the scanning lens 15-1-2 is overlaid on the scanning lens 15-1-1, the scanning lens 15-1-1 made of resin functions as heat insulating member, temperature deviation between the scanning lenses 15-1-1 and 15-1-2 is so small that it is not be substantially problematic. As described later, although there is influence of natural convection inside the optical housing 23, the effect of heat transmission in the optical housing member is more significant in the case of a metallic optical housing.
In the foregoing, the structure in which two stations are aligned (two stages) in the sub-scanning direction in an optical path at least from the polygon mirror to the first scanning lens is explained. However, structures having three or more stages can be applied. An example of such structure is explained below.
FIG. 16 depicts part of a tricolor image forming apparatus of a first example. The polygon scanner 14b in FIG. 16 includes the polygon motor 14a and three polygon mirrors 14-1, 14-2 and 14-3 mounted thereon. The station corresponding to the laser beam deflected and reflected at the polygon mirror 14-1 is referred to as ST1, the station corresponding to the laser beam deflected and reflected at the polygon mirror 14-2 is referred to as ST2, and the station corresponding to the laser beam deflected and reflected at the polygon mirror 14-3 is referred to as ST3.
Likewise the case of the bicolor image forming apparatus (color image forming apparatus) in the fifth example, heat generated at the polygon scanner 14b (or fixing device, etc.) transmits in the member of the optical housing 23 and reaches near the first scanning lens. The heat transmits to the scanning lenses 15-1-2 and 15-1-3 via the scanning lens 15-1-1 (of lowermost layer) which contacts the optical housing 23 (mounting surface 30), of the first scanning lenses.
As a result, as described previously, temperature deviation arises between the optical housing 23 (mounting surface 30) and the scanning lens 15-1-1, and local temperature deviation arises near boss parts of the scanning lens 15-1-1, so that the scanning-line shape of this station (ST1) largely changes. On the other hand, since the scanning lens 15-1-1 in the lowermost layer functions as a heat insulating member, temperature deviations between 15-1-1 and 15-1-2, and between 15-1-2 and 15-1-3 are so small that they do not cause substantial problem.
Since quality of image that is formed in the photosensitive drum 16-1 corresponding to ST1 is more likely to deteriorate than those formed in stations ST2 and ST3, it is preferred to make toner of a color component having low visual sensitivity into correspondence with ST1. On the other hand, it is preferred to make toner of a color component having high visual sensitivity into correspondence with station ST2 or ST3. For example, when the three colors are formed of toner of black (K), cyan (C) and yellow (Y), yellow toner having lowest visual sensitivity may be made into correspondence with station ST1.
Similarly, a tetra-color image forming apparatus can be constructed by piling up four first scanning lenses and adding toner of magenta (M) to the three colors. Also in this case, yellow toner having lowest visual sensitivity may be made into correspondence with station ST1. Note that an image forming apparatus for images of more colors can be achieved in a similar manner.
As described later, since natural convection occurs inside the optical housing, the scanning lens in the uppermost layer is exposed to air flow of relatively high temperature. As a result, the scanning-line shape corresponding to the scanning lens in the uppermost layer (lesser extent to the lowermost layer) may simultaneously change. Therefore, more preferred structure is to make the color component having the highest visual sensitivity (usually black toner) into coincidence with an intermediate layer other than the uppermost layer and the lowermost layer.
In this manner, since visible deterioration of color image quality due to color misregistration can be effectively suppressed, it is possible to reduce the frequency of detecting color misregistration from the degree of overlapping of toner images of each colors compared to a conventional image forming apparatus (environment-responsive).
FIGS. 17 and 18 depict part of a tetra-image forming apparatus of a second example. It is assumed that toner of four colors, for example, black (K), magenta (M), cyan (C), and yellow (Y) are superimposed.
As shown in FIG. 17, for miniaturization of optical scanning device, and limitation of mechanical layout such as photosensitive drum interval in the image forming apparatus, each station is provided with one folding mirror 24 for folding optical path of laser beam. FIG. 18 is an exploded plan view of optical layout without folding mirror where the stations ST1-L and ST2-L, the stations ST1-R and ST2-R are disposed symmetrically on the right and left sides about the polygon mirror 14.
Also in FIG. 18, likewise the case as previously described, the heat generated at the polygon scanner 14b, a fixing device or the like transmits in the member of the optical housing 23, and reaches near the first scanning lens. The heat transmits to the scanning lenses 15-1-2-R (15-1-2L) via the scanning lens 15-1-1R (15-1-1-L) (of the lower layer) contacting the optical housing 23 (mounting surface 30), of the first scanning lenses 15-1-1R, 15-1-1-L, 15-1-2-R and 15-1-2-L. Although temperature deviation arises between the scanning lens 15-1-1-R (15-1-1-L) in the lower layer and optical housing 23, temperature deviation between the scanning lens 15-1-2-R (15-1-2-L) in the upper layer and the scanning lens 15-1-1-R (15-1-1-L) in the lower layer is too small to substantially cause a problem. Therefore, by disposing a photosensitive drum for yellow toner having the lowest visual sensitivity in a station, on lower layer side of the first scanning lens (ST1-R or ST1-L) and disposing a photosensitive drum for black toner having the highest visual sensitivity in a station, on upper layer side of the first scanning lens (ST2-R or ST2-L), it is possible to visually reduce the influence of color misregistration caused by change in scanning-line shape due to temperature change.
FIG. 19 depicts part of a bicolor image forming apparatus of a third example. A heat insulating member 32 is arranged between the first scanning lens 15-1-1 and the optical housing 23 that satisfies:
H/κ>0.008
where H [m] is thickness of heat-insulating base material, and κ[W/(m·K)] is heat conductivity of insulating base material. Even when the optical housing 23 is made of metal having high heat conductivity, heat of the optical housing 23 cannot be transmitted to the first scanning lens 15-1-1 because of the heat insulating member 32, and temperature distribution is not likely to occur near a boss part of the first scanning lens 15-1-1.
For example, in the heat insulating member 32 made of resin (PET+30% glass fiber), since heat conductivity κ is generally 0.4 [w/(m·K)], thickness of the heat insulating member H may be selected so that H=0.0032 (meter)=3.2 (millimeter) is satisfied.
in the case of an optical housing made of resin
In the foregoing, the case where the optical housing is made of metal having high heat conductivity is explained. In this section, the case where the optical housing is made of resin having low heat conductivity is explained.
An image forming apparatus shown in FIG. 20 as a fourth example are of basically the same as that shown in FIG. 14 except that the optical housing 23 is made of resin in FIG. 20. Accordingly, the soundproof glass 18 and the wall 19 are disposed to surround the polygon scanner 14b, so that heat generated at the polygon scanner 14b does not diffuse together with air flow with rotation of the polygon mirrors 14-1 and 14-2, and since the optical housing 23 is made of a material having low heat conductivity, the distance of diffusion due to heat conductivity in the member of the optical housing is very small compared to the case of metallic optical housing.
Also in FIG. 20, heat generated due to rotation of the polygon scanner 14b transmits inside the optical housing 23 by natural convection from the chamber (region) surrounded by the soundproof glass 18 and the wall 19. Convection represented by the arrows in FIG. 20 generates, and as a result, temperature change starts from the top face side of the scanning lens 15-1-2 disposed on the upper side in FIG. 20 (in the direction opposite to the gravity acceleration vector) of the first scanning lenses 15-1-1 and 15-1-2. Therefore, temperature distribution occurs inside the scanning lens 15-1-2, and shape of the scanning line on the photosensitive drum of station corresponding to this scanning lens changes, leading deterioration in image quality.
FIG. 21 is a modified example of the fourth example. Also in this case, natural convection arises inside the optical housing 23 made of resin, and the scanning lens 15-1-2 disposed on the upper side with respect to the direction of the gravity is exposed to air flow of relatively high temperature, so that shape of the scanning line on the photosensitive drum of station corresponding to the scanning lens 14-2 changes, leading deterioration in image quality.
Therefore, in both structures shown in FIGS. 20 and 21, it is preferred that when the optical housing is made of resin, an image of color component having low visual sensitivity is formed on the photoconductor of the station corresponding to the side of the scanning lens 15-1-2 disposed on the upper side with respect to the direction of the gravity.
As shown in FIG. 22 for explaining a fifth example, by disposing a cover 33 over the first scanning lens 15-1-2 in the structure of FIG. 20, it is possible to prevent the air flow due to natural convection inside the optical housing 23 (air warmed by heat generated at the polygon scanner 14b) from coming into direct contact with the scanning lens 15-1-2. As a result, it is possible to reduce occurrence of temperature distribution inside the scanning lens 15-1-2.
When a scanning lens made of resin having relatively large coefficient of thermal expansion is housed in an optical housing made of material having relatively small coefficient of thermal expansion (such as aluminum), it is preferred to secure the scanning lens under pressure by a spring. By employing such structure, free expansion of scanning lens is not prevented when temperature of the optical scanning device changes, so that it is possible to prevent the scanning-line curve from changing.
FIG. 23 is a schematic diagram of relevant parts of a sixth example of an image forming apparatus viewed from the arrow in FIG. 22, and springs 35 (three positions) are disposed between the cover 33 and the scanning lens 15-1-2 explained in FIG. 22. It is preferred for assemblability and assembling accuracy, to integrally form the spring 35 with the cover 33. It is preferred that the springs 35 are disposed at the positions opposing to the bosses provided in the first scanning lenses 15-1-1, 15-1-2 (FIG. 23). This is because if the points of action of the springs 35 are misaligned from the positions of bosses, deformation of the first scanning lenses 15-1-1, 15-1-2 may occur.
When heat generation amount at the polygon scanner 14b is large and the optical housing is made of metal having high heat conductivity, a large amount of heat transmits through the member and reach the first scanning lens, so that the measures described in the embodiments are sometimes inadequate. When the optical housing is made of resin having low heat conductivity, the influence of natural convection inside the optical housing may affect not only on the scanning lens on the upper layer side but also on the scanning lens of the lower layer side.
In such a case, for example, as described in FIG. 14, by disposing folding mirrors (number) so that curving direction of components of scanning lines generated on the first scanning lenses 15-1-1 (lower layer) and 15-1-2 (upper layer) match on the photosensitive drums 16-1 and 16-2, it is possible to reduce color misregistration. At this time, difference in the number of folding mirrors between upper and lower layers is an even number.
In contrast, in a comparative example of FIG. 24, which is a comparative example, since the curving direction of components of scanning lines generated on the first scanning lenses 15-1-1 (lower layer) and 15-1-2 (upper layer) are opposite to each other on the photosensitive drums 16-1 and 16-2, the amount of color misregistration increases and quality of color image is deteriorated. At this time, difference in the number of folding mirrors between upper layer and lower layer is an odd number.
Part of a bicolor image forming apparatus (color image forming apparatus) of a seventh example is shown in FIG. 25. In the image forming apparatus of the present example, curvature in the sub-scanning direction of the optical surface (any one of incident side and exit side or both) of the first scanning lenses 15-1-1a and 15-1-2a is zero (radius of curvature is 8) in the structure shown in FIG. 14.
In this manner, by employing the structure in which curvature of at least either of the incident side or exist face side of the first scanning lenses 15-1-1a and 15-1-2a is zero, even when the heat generated at the polygon scanner 14b transmits inside the member of optical housing and reaches the lower stage of the first scanning lens 15-1-1a, and leads local change in shape of optical surface near a boss, and distribution in physical property values (for example, refractive index) inside the scanning lens 15-1-1a, it is possible to reduce the influence compared to the structure in which curvature of optical surface is not zero.
Even when incident position of laser beam into the first scanning lenses 15-1-1a and 15-1-2a (sub-scanning direction) deviates due to variations in parts and mounting tolerance such as light source, cylindrical lens, first scanning lenses 15-1-1a and 15-1-2a themselves, it is possible to reduce deterioration of optical performance in the target surface because the curvature of optical surface is zero. It is more effective to make curvatures of the incident side and exit side of the first scanning lenses 15-1-1a and 15-1-2a be zero and to make refractive index of the optical element be zero.
FIG. 26 is an exploded plan view (top view) of an eighth example of an optical path (on the side of station ST1) from the polygon mirror 14-1 to the surface of the photosensitive drum 16-1 in FIG. 14. The positions where bosses are disposed in the first scanning lens 15-1-1 (three positions) are shown in FIGS. 27A to 27C (three-side view).
FIGS. 28 and 29A to 29C depict comparative examples of the eighth example. In the eighth example (FIG. 26, FIGS. 27A to 27C), two bosses provided on both sides in the main scanning direction of the first scanning lens 15-1-1 are disposed outside the main scanning direction of an effective area. On the other hand, in the comparative examples (FIG. 28, FIGS. 29A to 29C), two bosses provided on both sides in the main scanning direction of the first scanning lens 15-1-1 are disposed inside the main scanning direction of the effective area.
Change in scanning-line shape (photosensitive drum 16-1 surface) caused by temperature rise of the apparatus due to heat generation caused by rotation of the polygon scanner 14b in the present example in shown in FIGS. 30A and 30B. FIG. 30A is for the case of structure of the eighth example (FIG. 26, FIGS. 27A to 27C), and FIG. 30B is for the case of structure of the comparative example (FIG. 28, FIGS. 29A to 29C). Assumed size of output image is “A3”, and width of image area is 300 millimeters (Y=±150 millimeters or less).
In the case of the comparative example (FIG. 30B), the laser beam passing near the bosses of the first scanning lens 15-1-1c (two positions on both sides) reaches near the coordinate of Y=±100 millimeters in the photosensitive drum 16-1, so that the scanning-line shape drastically changes near the coordinate Y=±100 millimeters (within image area). In the case of application to two or more color polychromatic image, if the degree of change in scanning-line shape differs between stations, or the curving direction of scanning lines are opposite due to difference in the number of provided folding mirrors, color misregistration occurs in an output image.
On the other hand, in the case of the eighth example (FIG. 30A), since the laser beam passing near bosses of the first scanning lens 15-1-1 (two positions on both sides) corresponds near the coordinate Y=±160 millimeters outside the image area in the photosensitive drum 16-1, it is possible to reduce change in scanning-line shape in the image area. Further, shift in the sub-scanning direction of the scanning line position can be compensated, for example, in the following manners:
- Adjust starting timing of scanning in the sub-scanning direction.
- Adjust rotation phase of photosensitive drum.
- Separately provide an optical path deflecting element, and deflect the light path at a very small angle in the sub-scanning direction, thereby suppressing occurrence of color misregistration in an output image.
A structure of a bicolor image forming apparatus (color image forming apparatus) of a ninth example is shown in FIG. 31. Unlike the case of the fifth example (FIG. 14), a single first scanning lens 15-1-12 (focusing element a) is shared by two stations ST1 and ST2. In this structure, heat generated at the polygon scanner 14b transmits inside the optical housing member as indicated by the arrow, and reaches the first scanning lens 15-1-12 via the mounting surface 30, and forms continuous temperature distribution inside the first scanning lens 15-1-12. On the other hand, in the case of the fifth example, double-layered structure is employed, so that temperature distribution is intermittent (discontinuous) between the lower stage 15-1-1 and the upper stage 15-1-2 of first scanning lens. This difference in phenomenon is significant at the time of transition of temperature change. Therefore, in the present example, it is possible to reduce generation amount of deviation in variation of scanning-line curves between two stations ST1, ST2, and to reduce color misregistration in an output image.
Part of a bicolor image forming apparatus (color image forming apparatus) of a tenth example is shown in FIG. 32. In the structure of the tenth example (FIG. 32), curvatures in the sub-scanning direction of the optical surface (incident side and exit side) of the first scanning lens 15-1-12a are set at zero (radius of curvature is 8). However, curvature of either one side may be set at zero. According to this structure, the effects of the tenth example (FIG. 31) and the seventh example (FIG. 25) can be obtained concurrently.
In the bicolor image forming apparatus (color image forming apparatus) of FIG. 31, as the three disposing positions of bosses provided in the first scanning lens 15-1-12, the two bosses provided on both sides of the main scanning direction may be disposed outside the main scanning direction of the effective area regarding optical scanning. As a result, it is possible to reduce generation amount of deviation in scanning-line shape between the upper stage side and the lower stage side of the first scanning lens 15-1-12, and to reduce occurrence of change in scanning-line shape in the image area as in the case of the eighth example.
Likewise the structures of the seventh example (FIG. 25) and the tenth example (FIG. 32), it is preferred to implement the lens that is optically closest to the polygon scanner 14b which is a heat source, by the scanning lens (15-1-1a, 15-1-2a, 15-1-12a) having curvature in the sub-scanning direction of zero. For example, in FIG. 25 (FIG. 14), in a group of first scanning lenses 15-1-1a and second scanning lens 15-2-1 corresponding to the light beam from the polygon mirror 14-1, the first scanning lens 15-1-1a is the lens that is optically closest to the polygon scanner 14b. The expression “optically closest” means that the distance along the optical path of the laser beam is smallest.
With such structure, color misregistration (in the sub-scanning direction) caused by heat generation of the polygon scanner described in the tenth and thirteenth examples can be reduced. Additionally, since the first scanning lens which is closer to polygon scanner 14b (15-1-1a and 15-1-2a in FIG. 25) does not have power (refracting power) in the sub-scanning direction, the second scanning lens (15-2-1 and 15-2-2) on the side of the photoconductor (target surface) has positive strong power in the sub-scanning direction, so that imaging magnification in the sub-scanning direction of the scanning optical system decreases, leading the advantages that deterioration of performance due to mounting error of parts, shape error and the like can be prevented, and small-diameter beam spot can be realized, and change in magnification according to image height can be easily corrected. As a result, high quality of output image can be realized.
Further, in the seventh example and the tenth example, it is preferred to construct mechanical layout of the optical housing member (mechanical design aspect) so that the physical distance along the optical housing member (distance of heat conduction in the optical housing member) from the polygon scanner to the first scanning lens is shortest, as well as to dispose the first scanning lens having curvature in the sub-scanning direction of zero at the position which is optically closest to the polygon scanner which is a heat source (optical design aspect).
As such a structure of an eleventh example based on the structure of the tenth example is explained. FIG. 33 is a structural view depicting a structure of a bicolor image forming apparatus (color image forming apparatus) of the eleventh example. A comparative example of the eleventh example is shown in FIG. 34. In the case of the comparative example shown in FIG. 34, heat generated at the polygon scanner 14b transmits through the optical housing member, and reaches the second scanning lens 15-2-2c which is mounted on the back side of the polygon scanner 14b mounting surface as indicated by the arrow. Since distance D2 from the polygon scanner 14b to the second scanning lens 15-2-12c is smaller than distance D1 from the polygon scanner 14b to the first scanning lens 15-1-12a, the second scanning lens 15-2-12c is influenced by heat generation at the polygon scanner 14b more strongly. Since the curvature in the sub-scanning direction of the first scanning lens 15-1-12a is set at zero (in terms of optical design), and the power in the sub-scanning direction is concentrated to the side of the second scanning lens 15-2-2-2c, influence of heat generation of the polygon scanner 14b is more likely to be stronger.
On the other hand, in the case of the eleventh example shown in FIG. 33, since the mounting surface of the polygon scanner 14b and mounting surfaces of the second scanning lenses 15-2-1 and 15-2-2 are completely separated or sufficiently distanced from the first scanning lens 15-1-12a (distance D2 from the polygon scanner 14b to the second scanning lens 15-2-2 is longer than distance D1 from the polygon scanner 14b to the first scanning lens 15-1-12a), the amount of heat generated at the polygon scanner 14b and that transmits to the second scanning lenses 15-2-1 and 15-2-2 is very small. Heat generated at the polygon scanner 14b is transmitted to the first scanning lens 15-1-12a. However, as described above, since the power in the sub-scanning direction of the first scanning lens 15-1-12a is set at zero, influence exerted on the scanning-line shape is small, and this is a preferred structure.
As set forth hereinabove, according to an embodiment of the present invention, it is possible to reduce color misregistration as well as the amount of color misregistration. Thus, a high-quality image can be obtained.
Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.
Patent Application
20080069585
