JPS63124018A - Scanner - Google Patents

Abstract

PURPOSE:To satisfactorily correct a focal shift in a scanning position in the sub-scanning direction, and to execute a scan of a large screen with high resolution, by placing an optical element whose refractive power is varied continuously in only the sub-scanning direction, between a rotary polygon mirror and the scanning surface. CONSTITUTION:Between a rotary polygon mirror 1 and the surface 3 to be scanned, an optical element 5 which does not have a refractive power in the main scanning direction but has a refractive power in only the sub-scanning direction is placed. As a result, a luminous flux in the main scanning direction, among luminous fluxes from a light source 10 is reflected by the polygon mirror 1 through a cylindrical lens 4 having no refractive power in the main scanning direction, passes through the optical element 5 as it is by a condensing lens (f-theta lens system) and reaches on the surface 3 to be scanned. On the other hand, the luminous flux in the sub-scanning direction is condensed to one point PF of one reflecting surface of the polygon mirror in order to correct a surface inclination of the polygon mirror 1 by the lens 4, and focused again onto the surface to be scanned 3 by a lens 2 and the optical element 5. In this case, since the optical element 5 is constituted so as to have a refractive power varied continuously in accordance with an image curvature quantity in each image height of the lens 2, the image curvature in the sub-scanning direction can be corrected satisfactorily.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a scanning device, and particularly to a scanning device that reflects a light beam from a laser light source on a rotating polygon mirror and then scans the surface to be scanned via a condensing lens. 1 for displaying and recording information, such as laser printers, laser 77xmillimeter, and laser CO
M (Computer Output Microf
The present invention relates to a scanning device suitable for the following applications.

(Prior art) Conventionally, the light beam from a laser light source is guided to a rotating polygon mirror, and the reflected light beam from the one-rotation polygon mirror is collected by a condensing lens f-θ.
Various scanning devices have been proposed in which the surface to be scanned is scanned by focusing the light onto the surface to be scanned through a lens and rotating a single-rotation polygon mirror.

For example, Figures 4 and 5 are schematic diagrams of the optical systems of conventional scanning devices, respectively. Figure 4 is a cross-sectional view in the main scanning direction in which scanning is performed by rotating a rotating polygon mirror, and Figure 5 is a sectional view in the main scanning direction. FIG.

In FIG. 4, the light beam from the light [10] in the main scanning direction passes through the cylindrical lens 4 which has no refractive power in the main scanning direction, enters the one-rotation polygon mirror, and is reflected by the two-rotation polygon mirror 1. The light is focused onto the surface to be scanned 3 by a condenser lens 2 which is an f-theta lens system.

On the other hand, the light beam in the sub-scanning direction shown in FIG. 5 is focused by the cylindrical lens 4, formed into an image at a predetermined position PF on the reflecting surface of the two-rotation polygon mirror 1, and then focused onto the scanned surface 3 by the focusing lens 2.

Here, the focusing lens 2 has a toric surface and corrects errors due to surface tilt of the rotating polygon mirror by differentiating the conjugate relationships of imaging in the main scanning direction and the sub-scanning direction.

In general, it is desirable that the focusing lens be constructed so that the focal position coincides with the scanning plane in the scanning direction and the sub-scanning direction over the entire scanning width.

However, in reality, since various aberrations remain in the focusing lens, the position where the cylindrical lens 4 forms an image on the rotating polygon mirror 1 in the sub-scanning direction changes depending on the rotation angle of the rotating polygon mirror, so the focal position is scanned. It was very difficult to match the surface perfectly.

FIG. 6 shows the deviation of the focal position on the scanning plane in FIGS. 4 and 5, with the horizontal axis representing the field curvature and the vertical axis representing the scanning angle. In the figure, the solid line indicates the focal position in the sub-scanning direction, and the broken line indicates the focal position in the main scanning direction. As shown in the figure, the spot shape varies due to focal errors depending on the scanning position on the scanning plane, and as a result, there is a limit to the ability to reduce the beam diameter and scan with high resolution.

On the other hand, US Pat. No. 4,496,209 proposes a method of correcting the focal position of the image plane using a bent cylindrical lens.

However, since this method has a refractive power in the scanning direction, it is very difficult to correct the complex field curvature in both the scanning direction and the sub-scanning direction independently.

(Problems to be Solved by the Invention) The present invention focuses mainly on the focal point at the scanning position in the sub-scanning direction when the light beam from the light source is reflected by a rotating polygon mirror and focused on the surface to be scanned by a condensing lens. It is an object of the present invention to provide a scanning device capable of satisfactorily correcting misalignment, so-called field curvature, and capable of scanning a large screen with high resolution.

(Means for solving the problem of unlucky points) A light beam from a light source is guided to a rotating polygon mirror, and a periodically deflected reflected light beam from the rotating polygon mirror is focused on a scanning surface via a condenser lens. In a scanning device that performs scanning, an optical element having no refractive power in the main scanning direction but whose refractive power changes continuously only in the sub-scanning direction is disposed between the rotating polygon mirror and the scanning surface.

(Example) FIGS. 1 and 2 are schematic diagrams of the optical system of the scanning device of the present invention, respectively. The figure is a cross-sectional view in the sub-scanning direction, which is perpendicular to the main-scanning direction.

In Figures 1 and 2, 1 is a rotating polygon mirror, and the rotation axis 1
It rotates around 1. 2 is a condensing lens and f-θ
It consists of a lens system. 3 is a surface to be scanned, 4 is a cylindrical lens, 10 is a light source, and 5 is an optical element that has no refractive power in the main scanning direction but only in the sub-scanning direction. 3(B) shows cross-sectional views at positions A, B, C, D, and E perpendicular to each position in FIG. 3(A).

The difference between the scanning device of this embodiment shown in FIGS. 1 and 2 and the conventional scanning device shown in FIGS. 4 and 5 is that in this embodiment, there is a Optical element 5 having the above-mentioned shape
is arranged to correct field curvature in the sub-scanning direction due to residual aberrations of the condenser lens 2.

That is, in this embodiment, as shown in FIG. 1, among the light beams from the light source 10, the light beam in the main scanning direction is reflected by one reflecting surface of the rotating polygon mirror 1 through the cylindrical lens 4 which has no refractive power in the main scanning direction. After that, the light is guided onto the scanned surface 3 by the condenser lens 2 via the optical element 5.

As shown in FIG. 3(A), the optical element 5 does not have refractive power in the main scanning direction, so the light beam passes through the optical element 5 as it is and reaches the surface 3 to be scanned.

On the other hand, as shown in FIG. 2, the light beam in the sub-scanning direction is focused by a cylindrical lens 4 onto a point PF on one reflective surface of the rotating polygon mirror 1 in order to correct the surface tilt of the rotating polygon mirror 1. Then, the light is focused again onto the surface to be scanned 3 by the condenser lens 2 and the optical element 5.

Then, by rotating the rotating polygon mirror 1, the surface to be scanned 3 is scanned.
scanning above.

The imaging relationship in this embodiment is that in the main scanning direction, the cylindrical lens 4. The surface to be scanned 3 is in a conjugate relationship with an object at infinity via the focusing lens 2 and the optical element 5. Further, in the sub-scanning direction, the surface to be scanned 3 is configured to be in a conjugate relationship with the reflection point PF of the rotating polygon mirror 1 via the condenser lens 2 and the optical element 5.

In general, among the aberrations in a condenser lens, meridional field curvature in the main scanning direction is relatively easier to correct than sagittal field curvature in the sub-scanning direction, as is clear from the aberration diagram in Figure 6. I can do it.

For this reason, it is important how well the sagittal field curvature in the sub-scanning direction can be corrected in the condenser lenses of many scanning devices. However, it is generally very difficult to satisfactorily correct the sagittal field curvature in the sub-scanning direction within the condenser lens.

Therefore, in this embodiment, the sagittal field curvature in the sub-scanning direction is calculated by taking into account the curvature (2) for each image height, as shown in FIG. 3(A).

Good correction is achieved by arranging an optical element having the shape shown in (B) between the rotating polygon mirror 1 and the surface to be scanned 3.

That is, the optical element 5 is configured to have no refractive power in the main scanning direction but has refractive power only in the sub-scanning direction. The lens is configured to have a refractive power that continuously changes according to the

This makes it possible to satisfactorily correct the curvature of field in the sub-scanning direction, and to scan over the entire scanning range with the light beam on the surface to be scanned being focused to a small extent.

In this example, a case is shown in which the surface inclination of the rotating polygon mirror and the field curvature in the sub-scanning direction are corrected by a combination of a condensing lens with a toric surface and an optical element. Alternatively, the object of the present invention can be similarly achieved by having a condensing lens having a cylindrical lens take on the function of an optical element.

Furthermore, the optical element shown in Fig. 3 has a positive and negative refractive power in the sub-scanning direction, and is constructed in a shape in which the refractive power is continuously changed. It is also possible to have a configuration in which only the first part is continuously changed.

(Effects of the Invention) According to the present invention, when a light beam from a light source is reflected by a rotating polygon mirror and focused on a surface to be scanned by a condenser lens for scanning, an optical element having the above-mentioned shape is placed at a predetermined position. By doing this, the imaging position of the light beam can be aligned with the scanning plane and scanning can be performed with high resolution. Furthermore, the field curvature of the condenser lens can be well corrected, so it is possible to scan with high resolution over a wide scanning width. A scanning device capable of doing this can be achieved.

[Brief explanation of the drawing]

1 and 2 are schematic diagrams of optical systems in the main scanning direction and sub-scanning direction of an embodiment of the present invention, respectively, and FIGS. 3(A) and 3(B)
) is an explanatory diagram of the optical element according to the present invention, FIGS. 4 and 5 are schematic diagrams of conventional optical systems in the main scanning direction and the sub-scanning direction, respectively.
FIG. 6 is an explanatory diagram showing the shift of the focal point position on the scanning plane in the scanning device shown in FIG. 4. In the figure, 1 is a rotating polygon mirror, 2 is a condenser lens, 3 is a scanned surface,
4 is a cylindrical lens, 5 is an optical element, and 10 is a light source. Patent applicant Canon Co., Ltd. 1st issue Figure 2 Figure 30 (A) Vines Figure 3 (B) AB CO Mi

Claims (1)

[Claims] In a scanning device that conducts scanning by guiding a light beam from a light source to a rotating polygon mirror and converging periodically deflected reflected light beams from the rotating polygon mirror onto a scanning surface via a condenser lens, the rotating polygon mirror and the scanning surface, an optical element having no refractive power in the main scanning direction and whose refractive power changes continuously only in the sub-scanning direction is disposed between the scanning surface and the scanning surface.