JPS61190312A - scanning optical system - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a scanning optical system for scanning and imaging a light beam in a laser printer or the like.

The scanning lenses used in conventional scanning optical systems are made of glass, but the processing process for glass lenses is complicated, and most of them are made of multiple lenses, which require a large number of parts and assembly man-hours, making it difficult to lower prices. It was hindering me. Although there are examples of single lenses, they are large in size and require a glass material with a high refractive index, which limits how low they can be made.

[Conventional technology]

In the conventional scanning single lens (Japanese Patent Laid-Open No. 55-7727), the refractive index is determined by the maximum scanning angle as shown in Table 1.

Table 1 (margin below) From Table 1, the refractive index is 1.62 at the maximum scanning angle of 45'' or less.
5 or more is required. Furthermore, since this single lens is a lens that mainly corrects distortion aberration, it has a large curvature of field and is also large in size.

Figure 12 shows an example of a conventional single lens.The lens data is focal length f = 181 Tsuru, maximum scanning angle ω = 31.5 'r
1 =oo d6-58.2mrz =122
．． 91111 dl -14,1wm n=1.6
7956'it = 173.4 tm (However, the inscribed circle diameter of the rotating polygon mirror is 25 m) 1st
In Figure 2, 1 is a rotating polygon mirror, 2 is a scanning lens, and 3 is a rotating polygon mirror.
is the incident beam, and 4 is the image plane. The constants of the lens are: n is the refractive index, rl+r2 is the radius of curvature of the left surface, radius of curvature of the right surface, do is the distance between the reflection point and the vertex of the left surface of the lens when the scanning angle of the rotating polygon mirror is O'', and dl is the center of the lens. Thickness, d
) is the distance from the apex of the right surface of the lens to the image plane. 1st
Figure 3 shows the field curvature of this lens. In the same figure,
The curve ΔM indicates the calculated value of the meridional curvature of field, and the curve ΔS indicates the calculated value of the sagittal curvature of field, and the position of the image plane 4 in FIG. 12 corresponds to 0 on the horizontal axis of FIG. 13. The image plane 4 is placed closer to the lens than the Gaussian image plane to reduce the amount of defocus.

[Problem that the invention seeks to solve]

Although the above structure prevents the condensed spot diameter of the light beam from expanding due to defocusing due to field curvature, there is a problem in that there is a limit to its effectiveness.

[Means for solving problems]

The present invention provides a scanning optical system that solves the above problems.The present invention provides a scanning optical system that scans and forms an image of a light beam such as a laser, in which a scanning lens that condenses light is made of a single plastic sheet. The curvature of the meridional image surface and the curvature of the sagittal image surface within the scanning angle used by the lens are on the lens side from the Gaussian image surface except on the optical axis, and the curvature of the sagittal image surface is This is done by a scanning optical system that is located closer to the lens than the curvature of the meridional image plane except on the optical axis.

[Function] By using a single plastic lens as the scanning lens, the above scanning optical system can be mass-produced using injection molding technology, and the scanning optical system is less expensive than glass lenses, which require complicated processes such as polishing. The price can be. In addition, the meridional field curvature and sagittal field curvature within the scanning angle used by the lens are on the lens side from the Gaussian image plane except on the optical axis, and the sagittal field curvature is on the lens side except on the optical axis. The amount of field curvature is reduced by arranging the lens to be closer to the lens than the curvature of the meridional image surface.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows a configuration diagram of an embodiment of the present invention.

In this embodiment, as shown in FIG. 1, the light from a laser diode 10 is collimated by a collimating lens 11,
The plastic lens 13 is configured to scan the photosensitive drum 14 and expose the photosensitive drum 14 to light using the plastic lens 13.

Next, the shape of the lens will be explained using FIG. 2. The curvature of field changes when the shape of the single lens changes, and there are roughly three types:

(B) As shown in Figure 2a (ΔM, ΔS are the entire scanning angle range ω~-ω
It is located closer to the lens than the Gaussian image plane, and ΔM is closer to the lens than ΔS.

(b) As shown in Figure 2, ΔM and ΔS are located on the lens side from the Gaussian image plane over the scanning range ω to −ω, and ΔS is Δ
It is located closer to the lens than M.

(C) As shown in FIG. 2C, .DELTA.S is located on the lens side from the Gaussian image plane over the entire scanning angle range .omega. to -.omega., and .DELTA.M is on the right side of the Gaussian image surface in a certain scanning angle range.

From type (a), if the lens data is changed so that ΔS approaches the Gaussian image plane, ΔM approaches the Gaussian image plane more than ΔS. This is type (b). Furthermore, by changing the lens data to bring ΔS closer to the Gaussian image plane, Δ
M approaches the Gaussian image plane and passes through it, and ΔM rapidly increases on the positive side.

This is type (c). The total amount of curvature is (ΔS,
(b) and (c) have the smallest ΔM (significantly increased content).
It is near the type boundary. The easiest type to use as a scanning lens is type (B), which is close to (C).

Examples of plastic lenses designed based on this theory are shown below.

<Example 1>f""181m,ω=31.5" do =28.7 crane r I= -140,56m d, =10.Om
n −1,486rz = −55,36w dt
=178.2 wrThe ray tracing diagram of this lens is shown in Fig. 3, and the field curvature and positional distortion are shown in Fig. 4 a and b. It is shown moved by about half of the total curvature. The positional strain Pe indicates the value of . The sign is positive when moving away from the scanning center (when θ-〇), and negative when approaching. Note that k was set to 184.6.

<Example 2> f=181m, ω=31.5°do ”19.3m r+ = 202.18m dt =30.0++
n n=1.486r t -64,27ts d
t = 182.81111 The ray tracing diagram of this lens is shown in Fig. 5, and the field curvature and positional distortion are shown in Figs. 6a and b. However, k was set to k" 1 B 2. l vm.

<Example 3> f = 181 m, ω = 31.5° tlo = 5.
08m r+ = 267.1(hn dt =60.0+
n n-1,486rz = 71.03 tm
dz = 188.30 m The ray tracing diagram of this lens is shown in Fig. 7, and the field curvature and positional distortion are shown in Fig. 8 a and b.

However, k was -180.5 times l.

<Example 4> f = 181m, (L) = 31.5" do = 23.7mm r l = -220,02m d 1 = 30.0mm
n=1.57rt=73.71
mm dz =183.46 irises A ray tracing diagram of this lens is shown in Fig. 9, and the field curvature and positional distortion are shown in Figs. 10a and b. However, k was set to 181.8 cranes.

Examples 1 to 3 above are lenses having different lens center thicknesses. In Examples 1 and 2.3, the center thickness d1 is 10.3.
It is 0.60 cranes. It can be seen from FIGS. 4, 6, and 8 that when the field curvature is the same, the positional distortion can be reduced as the center thickness increases. When the center thickness is large, there is a method of using a scanning optical system in which only the vicinity of the surface where the beam is scanned is cut out (see FIG. 11). The larger the center thickness of this type of lens, the more difficult and expensive it is to process with glass, but it is easy to process with plastic.

Also, for plastics, the thickness is greater than the center thickness.

The lens height h (Fig. 11) is small, making it lightweight.
Lens settings can be made with high precision.

〔Effect of the invention〕

As explained above, according to the present invention, the scanning lens is
By using a single plastic lens, mass production is possible using injection molding technology, and the scanning optical system can be made at a lower cost than a glass lens that requires complicated processes such as polishing. In addition, the meridional field curvature and sagittal field curvature within the scanning angle used by the lens are on the lens side from the Gaussian image plane except on the optical axis, and the sagittal field curvature is on the lens side except on the optical axis. By arranging it closer to the lens than the curvature of the meridional image plane, there is an effect that the amount of curvature of the field can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an embodiment of the scanning optical system of the present invention;
Fig. 2 is a diagram showing the types of field curvature, Fig. 3 is a ray tracing diagram of the lens according to the first embodiment of the present invention, and Fig. 4 is a diagram showing the field curvature and positional distortion of the lens. , FIG. 5 is a ray tracing diagram of a lens according to a second embodiment of the present invention, FIG. 6 is a diagram showing field curvature and positional distortion of the lens, and FIG. 7 is a diagram showing a third embodiment of the present invention. 8 is a diagram showing the field curvature and positional distortion of the lens, and FIG. 9 is a ray tracing diagram of the lens according to the present invention.
10 is a diagram showing the field curvature and positional distortion of the lens, FIG. 11 is a diagram showing the shape of the lens according to the embodiment of the present invention, and FIG. 12 is a diagram showing the conventional lens. FIG. 13 is a ray tracing diagram of the lens of the scanning optical system, and FIG. 13 is a diagram showing the field curvature of the lens. In the figure, 10 is a laser diode, 11 is a collimating lens, 12 is a rotating polygon mirror, 13 is a plastic lens, 1
4 indicates a photosensitive drum, respectively.

Claims (1)

[Claims] 1. In an optical system that scans and images a light beam such as a laser, the scanning lens that condenses the light is a single plastic lens, and the meridional image plane within the scanning angle used by the lens The curvature of the sagittal image plane is closer to the lens than the Gaussian image plane except on the optical axis, and the curvature of the sagittal image plane is closer to the lens than the curvature of the meridional image plane except on the optical axis. A scanning optical system characterized by being located on the side. 2. The scanning optical system according to claim 1, wherein the lens has a thickness smaller in the direction perpendicular to the scanning direction than its center thickness.