JPH0416766B2 - - Google Patents

Description

[Detailed description of the invention]

(1) Technical Field of the Invention The present invention relates to an optical system of a printing device configured using the principles of electrophotography in a laser printer or the like, and particularly to an optical scanning device thereof. (2) Background of the technology Printing devices such as electrophotographic printers use light as the print head. For example, laser printers use collimated laser beams as print heads. This type of printer is provided with an optical scanning device for scanning this laser beam onto an image plane (photoreceptor). This type of optical scanning device includes a rotating polygon mirror and a scanning lens. The rotating polygon mirror and the scanning lens cooperate to scan the laser beam incident on the rotating polygon mirror onto the photoreceptor. In this type of optical scanning device, the focal length of the scanning lens and the size of the rotating polygon mirror are required to correspond to a predetermined scanning width. Therefore, when there is only one scanning lens, the optical scanning device tends to be large and often occupies a relatively large space. Therefore, there is a need for an optical scanning device that is small, has a wide scanning width, and has good resolution. (3) Prior Art and Problems FIG. 1 is a schematic diagram of a laser printer equipped with a conventional optical scanning device, and FIG. 2 is a plan view of the optical system of the optical scanning device shown in FIG. 1. In FIGS. 1 and 2, the outer periphery of a drum 1 is coated with a photoconductive material, and the outer periphery is formed as a photosensitive plate, that is, an image surface 1a. The charging mechanism 2 serves to charge the photosensitive plate and impart photosensitivity to it. The exposure mechanism 3 has an optical scanning device including a rotating polygon mirror 4, a scanning lens 5, and a light source 6. A laser beam 7 from the light source 6 is transmitted to a photosensitive plate (drum) by the rotating polygon mirror 4 and the scanning lens 5. 1
The surface of the photosensitive plate is scanned and the photosensitive plate is exposed to print (image formation). Reference numeral 8 indicates a developing mechanism, 8a indicates toner for development, and numerals 9 and 9a indicate a transfer mechanism and paper for transfer, respectively. Reference numeral 10 indicates a fixing mechanism, 11 indicates a transferred paper, and 12 indicates a cleaning mechanism for cleaning the photosensitive plate (drum 1). 13 indicates a form overlay exposure device. As the scanning lens 5 used in combination with the rotating polygon mirror 4, a lens system called an F/θ lens is often used. Next, the characteristics of this F/θ lens will be explained. Figure 3 shows this F・θ
FIG. 3 is a diagram for explaining the principle of a lens. In other words, as shown in FIG. 3, when θ is the scanning angle and the focal length of the F/θ lens 5 is ₀ , the incident beam 7 is reflected by the rotating mirror surface 4a, passes through the lens 5, and appears on the image plane 1a. Form an image. When the image height from the optical axis 5a at this time is y, a lens that satisfies the relationship of the following equation (1) is called an F·θ lens. y= ₀・θ (1) As shown in equation (1), the F・θ lens 5 has the role of converting the uniform angular velocity scanning of the laser beam 7 by the rotating polygon mirror 4 into uniform velocity scanning on the image plane 1a. do. Now, as shown in FIGS. 1 and 2, the conventional optical scanning device has one light source 6 and one scanning lens (F/θ lens) 5 for one rotating polygon mirror. It is composed of: Due to this configuration, this conventional device has the following drawbacks. That is, in order to print by scanning a predetermined scan width at a predetermined scan angle, an F/θ lens with a long focal length is required, and the lens 5 becomes large. Therefore, the distance from the rotating polygon mirror 4 to the image plane 1a becomes large. Also,
In order to obtain a predetermined beam spot, it is necessary to increase the thickness (diameter) of the incident laser beam 7.
In order to efficiently reflect this incident laser beam 7, the rotating polygon mirror 4 must be made large. Therefore, since each element is large in size, there is a drawback that the entire device becomes large in size. Furthermore, a lens with a long focal length is disadvantageous in terms of resolution than a lens with a short focal length. Next, these drawbacks will be explained. The size of the optical scanning device is mainly determined by the size of the F/θ lens 5, the size of the rotating mirror 4, and the distance from the F/θ lens 5 to the image plane 1a. In particular, the size of the focal length ₀ of the F/theta lens 5 has the greatest influence on the size of the optical scanning device. In other words, the smaller the focal length of the lens 5, the more compact the device can be. In FIG. 2, the focal length of the F/θ lens 5 is set to ₀ , the total scanning width on the image plane 1a is set to When the practical maximum scanning angle (angle of view) is θmax, the relationship between and θmax is expressed by the following equation (2) (see equation (1) above). /2= ₀・θmax (2) Considering this equation (2), if is predetermined, θmax should be increased in order to reduce ₀ . However, in practice, the magnitude of θmax is limited by other components. That is, assuming that the number of rotating mirror surfaces 4a of the rotating polygon mirror 4 is m, the theoretical maximum scanning angle that can be scanned by the mirror surface 4a in this case (the angle that the light beam incident on the F/θ lens 5 makes with the optical axis 5a) ψmax is expressed by the following equation (3). ψmax=360°/m (3) However, the laser beam 7 incident on the rotating mirror 4 has a predetermined diameter. Therefore, when the scanning angle is ψmax, a portion of the incident beam 7 is eclipsed at the boundary edge of the rotating mirror surface 4a. Therefore, the practical maximum scanning angle θmax is smaller than the theoretical maximum scanning angle ψmax. Here, when the scanning efficiency η is expressed as the following equation (4), it is said that η of around 70% is practically good. η=θmax/ψmax×100 [%] (4) For example, if η=70 (%) and the number of rotating mirror surfaces 4a is m
When the value of θmax is calculated from equations (3) and (4) above, the values shown in the table below are obtained.

[Table] However, the smaller the scanning angle (angle of view) θmax, the easier it is to design the F/θ lens 5, and the larger the scanning angle (angle of view) θmax, the more difficult it is. Therefore, θmax is usually 20°~
An F/θ lens 5 of about 30° is used, and θmax is
F/θ lenses exceeding 40° are not used.
Since the size of θmax is limited in this way, (2)
As is clear from the formula, there is a limit to how small the value of ₀ can be. Therefore, it is necessary to take a large focal length of ₀ . In addition to the above, some optical scanning devices that scan on the same straight line using two optical polarizers (rotating polygon mirrors) have also been proposed (for example, see Japanese Utility Model Application No. 56-30313). However, using two rotating polygon mirrors in this way clearly goes against miniaturization of the device and is not preferable. Apart from this, an optical system that simultaneously scans two beams using only one rotating mirror is also known (for example, see Japanese Patent Laid-Open No. 161566/1983). However, such a conventional optical system uses the same surface of a rotating polygon mirror and uses the same lens system, so there is a problem that it is not possible to scan a wide range on the same straight line. (4) Purpose of the Invention In view of the above-mentioned conventional drawbacks, an object of the present invention is to provide an optical scanning device that is compact, has a wide scanning width, and has high resolution by improving the configuration of the optical system of the optical scanning device. It is something. (5) Structure of the Invention In order to achieve this object, the present invention provides a single rotating polygon mirror and two sets of scanning optics each including a laser light source and a scanning lens system. These two sets of scanning optical systems are arranged around the rotating polygon mirror so that different surfaces of the rotating polygon mirror can be utilized, and the scanning light from the different surfaces of the rotating polygon mirror is used to generate a scanned image. The scanning optical systems are arranged so as to be in the same straight line on the surface, and a reflecting mirror is attached behind each of the scanning lens systems to prevent the scanning lights from the two sets of scanning optical systems from overlapping with each other. Therefore, an optical scanning device is provided in which the scanning width of the entire scanning optical system is approximately ₁ + ₂ , when the scanning width of each of the scanning lens systems is ₁ and ₂ , respectively. (6) Embodiments of the invention Hereinafter, embodiments of the invention will be described in detail based on the drawings. 4 and 5 are diagrams for explaining the principle of the present invention, FIG. 6 is a first embodiment of the present invention, FIG. 7 is a second embodiment of the present invention, and FIG. 8 is a diagram showing a second embodiment of the present invention. 7
FIG. In FIGS. 4 to 8, the same parts as those in FIGS. 1 to 4 are given the same reference numerals. Figures 6 and 7
As shown in the figure, the optical scanning device of the present invention includes one rotating polygon mirror 21 and a mirror surface 21 of the rotating polygon mirror 21.
Two semiconductor laser light sources 1 arranged toward a
5, 15', and a light source section consisting of collimating lenses 20, 20' that convert the output light of the semiconductor laser into parallel light, and two F/θ lenses (scanning lenses) forming the light sources 15, 15', respectively. and the lens 1 corresponding to each of these F/θ lenses 16, 16'.
6, 16' and plane mirrors (17, 17', 18, 18' in FIG. 6, 19, 19' in FIG. 7) placed behind each mirror. The plane mirrors are arranged so that the scanning directions on the image plane 1a of the irradiated light projected onto the image plane 1a from these plane mirrors are substantially parallel to each other.
In this device, the total scanning width is divided into two parts ₁ ,
_The optical system is configured such that the scanning of each portion ₁ and ₂ is carried out by two F/θ lenses 16 and 16', respectively. For this reason,
Since the focal length of the F/theta lens is extremely shortened, this device can be made very compact. Next, the advantages of an optical system using two F/θ lenses will be explained. The focal length of the F/θ lens 5 of the conventional device using one F/θ lens (see Figure 2) is ₀ , and the F/θ lenses 16, 16' of this device using two F/θ lenses. The focal lengths of (Figs. 6 and 7) are ₁ and ₂ , respectively, the total scanning width is 1, and the scanning width of the two F/θ lenses 16 and 16' is ₁ and ₂ , respectively, and the maximum practical scanning angle is (angle of view) is θmax, the relationship between these elements is shown by the above equation (2) and the following equation (5). /2= ₀・θmax (4) Here, if ₁ = ₂ , two F/θ lenses 1
6 and 16' have the same scanning speed on the image plane 1a. This facilitates printing control. Therefore, in the above equation (5), if ₁ = ₂ , then the following equation (6)
(7) is obtained. ₁ = ₂ = /2 = ₀・θmax (6) ∴ ₁ = ₂ = ₀ /2 (7) Therefore, as shown in equation (7), the focal length of the F・θ lenses 16 and 16' of this device is It can be seen that ₁ and ₂ can be half the focal length ₀ of the F/θ lens 5 of the conventional device (FIG. 2). Next, the beam diameter d of the collimated light incident on the F/θ lens and the beam spot diameter dsp on the image plane
Explain the relationship between The resolution of the scanning optical system is determined by the spot diameter dsp on the image plane. The beam diameter d is calculated by the following formula assuming a Gaussian distribution of the beam:
It is shown in (8). _d = 4/π When the diameter of the incident collimated beam in the apparatus of the present invention is d ₁ , the following equation (9) is obtained from equation (8). d ₀ = 4/π×λ ₀ /dsp d ₁ =4/π×λ ₁ /dsp ∴d ₀ /d ₁ = _0/1 (9) Therefore, from equations (7 ₎ and (9), the following equation is obtained. (10) is obtained. d ₁ =d ₀ /2 (10) As shown by the equation (10), it can be seen that the incident collimated beam diameter d ₁ in the device of the present invention may be half the incident collimated beam diameter d ₀ in the conventional device. Next, by transforming equation (8), the following equation (11) is obtained. dsp=4/π×λ/d (11) /d in equation (11) is the value of the F number of a photographic lens, etc., and is expressed as F=/d. Therefore, equation (12) can be transformed into the following equation (12). dsp=4/πλF (12) As mentioned above, the size of this dsp determines the quality of resolution. Therefore, equation (12) shows that the resolution is determined by the size of F. Furthermore, equation (12) is a theoretical equation when there is no lens aberration. Therefore, when designing an actual lens, it is necessary to correct the spherical aberration in order to bring the resolution close to the theoretical value of equation (12). Generally, when F is constant, it is easier to correct spherical aberration when the focal length is short than when it is long, and the same holds true for correction of other aberrations. Next, the size of the rotating polygon mirror 4 will be explained with reference to FIG. In the figure, when the diameter of the incident collimated beam 7 is di, the minimum required radius γm of the rotating polygon mirror 4 is expressed by the following equation (13). λ _n =di/2cosΦ・sin(φmax−θmax/2) (13) However, Φ: Angle between the incident light 7 and the optical axis 5a ψmax: 2π/mm m: Number of rotating mirror surfaces 4a θmax: Practical maximum scanning angle Equation (13) above shows that γm is proportional to di. That is, the size of the incident collimated beam diameter di determines the size of the outer diameter (2γm) of the rotating polygon mirror 4. Therefore, as shown in (10) above,
Since the incident collimated beam diameter d ₁ in this device is half the beam diameter d ₀ in the conventional device, the rotating polygon mirror 21 (FIGS. 6 and 7) of this device can be significantly miniaturized. Further, in the rotating polygon mirror 21 (FIGS. 6 and 7), the tilting error of each mirror surface 21a poses a manufacturing problem. If the inclination of the mirror surface 21a is ψt, the positional deviation error ε of the imaging point on the image plane 1a is ε= ₁ tanψt≒ ₁ ψt∴ψt≒ε/ ₁ . ₁ is the focal length of the F/θ lens. Therefore, the smaller the focal length, the larger the allowable range of mirror tilt error, which is advantageous in manufacturing a rotating mirror. As described above in detail, there are many advantages when using the two F/θ lenses 16 and 16'.
However, in practice, as shown in FIG.
a and 16a' must be parallel and incident on the image plane 1a. In this case, as shown in the figure, the F/θ lens 1
Since the respective scanning sections ₁ and ₂ by 6 and 16' overlap, in this state, two F・θ
The advantage of using lenses 16, 16' is lost. In order to eliminate this overlap, it is conceivable to make the rotating mirror 21 very large and to increase the distance between the optical axes 16a and 16a' to about half the pre-scanning width. However, it is practically impossible to increase the size of the rotating mirror 21 in this way. Therefore, in the apparatus of the present invention, opposing plane mirrors are disposed behind each of the F/.theta. lenses 16, 16' to solve the above-mentioned overlapping problem. In other words, 1st in Figure 6
In this embodiment, in addition to the optical system shown in FIG.
Reflection plane mirrors 17, 18 and 17', 18' are respectively arranged behind the lenses 16, 16'. These plane mirrors 17, 18 and 17',
18' is a set of two lenses each with F/θ lens 16,
16', respectively, and serve to solve the above-mentioned overlap problem. Also,
The second embodiment shown in FIG. 7 is applied to a laser printer. In this embodiment, one plane mirror 19, 19' is arranged behind each F/theta lens 16, 16'. In this case, the plane mirrors 19, 19' are F/θ lenses 16, 1
One mirror for each of the mirrors 6' serves the same role as the plane mirror of the first embodiment. (7) Effects of the Invention As explained in detail above, the optical scanning device of the present invention has the advantage that it can be extremely miniaturized by including two scanning lenses that share the entire scanning width. There is. In other words, although it is small, it has the great effect of being able to scan a wide scanning width.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram of a laser printer equipped with a conventional optical scanning device as an exposure device, Fig. 2 is a plan view of the optical system of the optical scanning device shown in Fig. 1, and Fig. 3 shows the principle of an F/θ lens. Figures 4 and 5 are diagrams for explaining the principle of the present invention. Figure 6 is a perspective view of the first embodiment of the invention, and Figure 7 is a second embodiment of the invention. An example plan view, FIG. 8, is a perspective view of the apparatus shown in FIG. 1... Photocon drum, 1a... Image surface, 1
5, 15'... Semiconductor laser, 16, 16'...
F/θ lens, 16a, 16'a...optical axis, 17,
17', 18, 18', 19, 19'... Reflecting plane mirror, 20, 20'... Collimating lens, 21
... Rotating polygon mirror, 21a ... Rotating mirror surface, ... Total scanning width, ₁ , ₂ ... Divided scanning width.

Claims (1)

[Claims] 1. A single rotating polygon mirror, two sets of scanning optical systems each having a laser light source section and a scanning lens system, and these two sets of scanning optical systems are connected to a rotating polygon mirror. The scanning optical system is arranged around a rotating polygon mirror so that different surfaces of the rotating polygon mirror can be used, and the scanning optical system is arranged so that scanning lights from different surfaces of the rotating polygon mirror are aligned in the same straight line on the scanning image plane. , and a reflecting mirror for preventing the scanning lights from the two sets of scanning optical systems from overlapping with each other is attached to the rear of each of the scanning lens systems, so that the scanning width of each of the scanning lens systems is reduced to ₁ . , ₂ , the scanning width of the entire scanning optical system is approximately = ₁ + ₂ . 2. The optical scanning device according to claim 1, wherein the two scanning lenses have the same focal length. 3. The optical scanning device according to claim 1, wherein the laser light source section includes a semiconductor laser and a collimating lens that converts the output light from the semiconductor laser into parallel light.