JPS631561B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a telecentric projection lens that is used at a magnification of around 1/10, has a relatively bright F number, and has various aberrations well corrected. In recent years, a method has been used in which a solid-state image sensor is used as a scanning means of a reading device, and the solid-state image sensor is placed on an image plane as a scanning light-receiving element to scan an original. In this case, a projection lens is used to transmit the original to the solid-state image sensor, and a color separation prism is inserted between the projection lens and the imaging surface, that is, the solid-state image sensor, to further transmit the color signals of the original to the solid-state image sensor. There is a need. Therefore, if a general non-telecentric lens is used as a projection lens, off-axis light rays will enter the color separation prism obliquely, causing shading. In order to avoid this shedding, we make it telecentric, that is, the principal ray of the incident light passes through the focal point on the object world side, so that the principal ray of the emitted light on the image field side exits parallel to the optical axis even off-axis. This prevents shading from occurring due to the color separation prism. Such a projection lens is generally required to have the following points. First, in order to scan documents at high speed using a solid-state image sensor, it is necessary to increase the amount of exposure per unit time to the device, and to use a light source with as low an intensity as possible for the lamp that illuminates the document, the lens A bright F number is required. Secondly, in order to make the apparatus smaller, that is, to reduce the distance between the document surface and the image surface, it is necessary to widen the angle of view of the lens. Thirdly, since each solid-state image sensor has a thickness of approximately 15 μm, high resolution is required. Fourth, since it is necessary that the amount of light be equal throughout the solid-state image sensor, it is necessary to set the off-axis aperture efficiency of the lens to 100%. Fifth, it is necessary that the surface of the original is projected uniformly, that is, that distortion is small. Sixthly, it is necessary to lengthen the back focus of the lens in order to insert the color separation prism between the lens and the solid-state image sensor. The purpose of the present invention is to satisfy the above requirements by providing a bright F-number, high resolution, and high contrast, that is, spherical aberration, coma, field curvature, and distortion are well corrected, and the aperture efficiency is 100% and magnification is This provides a telecentric projection lens that is used at around 1/10x magnification. The projection lens according to the present invention has a pupil at the focal point on the object world side of the entire lens system, and the lens system is composed of three subsystems in order from the object world side: a first group, a second group, and a third group. , the first group is a positive single lens, the second group is a biconcave single lens, the third group is made up of four lenses or three lenses, and any one lens in the first group has a bonding surface. And, it satisfies the following conditions. (1) 1.72≦｜f ₁ /f ₂ |≦2.58 (2) −0.33≦f ₂ /f≦−0.19 (3) 0.41≦f ₃ /f≦0.59 However, f ₁ is the focal length of the first group lens, f ₂ is the focal length of the lens group, f ₃ is the focal length of the lens group, and f is the focal length of the entire system. Furthermore, in the projection lens according to the present invention, the surface of the object-world side of the lens of the first group is a surface facing the object-world side, and the second group has the shape of a biconcave lens. Furthermore, the lens placed closest to the object world in the group is a meniscus lens with a concave surface facing the object world, and the lens placed closest to the image field in the group is a meniscus lens with a concave surface facing the image field. A positive meniscus lens having a positive meniscus is desirable. The present invention will be explained in detail below. First, the above conditions will be explained. Condition (1) is for keeping the refractive power of the first group lens and the second group lens well balanced and correcting spherical aberration favorably. This lens is a telecentric system, and the distance between the principal points of the first group and the second group is wider than the distance between the principal points of the first group and the second group, so the position where the paraxial ray passes through the first group is far apart, resulting in a large amount of spherical aberration. will occur. In other words, when |f ₁ /f ₂ | becomes less than the lower limit of 1.72, the refractive power of the first group increases, and paraxial rays passing through this surface are strongly refracted in the optical axis direction, causing a large amount of negative spherical aberration. do. Moreover, when the upper limit value is 2.58 or more, the refractive power of the first group becomes large, and positive spherical aberration occurs which overcorrects the negative spherical aberration occurring in the first group. Condition (2) is for correcting field curvature,
When f ₂ /f becomes larger than the upper limit value -0.19, the Petzvaar sum becomes over-corrected and the curvature of field becomes over-corrected. In order to compensate for this drawback, it is necessary to increase the absolute value of the refractive power of the first group, but if this is done, a large amount of distortion will occur in the first group, which will be difficult to correct, as described in condition (3) below. becomes. On the other hand, if the lower limit is exceeded, it becomes difficult to correct the Petzval sum of the entire system, resulting in insufficient correction of the curvature of field. Condition (3) is for correcting field curvature and distortion aberration, and because this lens is a telecentric system, the position where the principal ray passes through the first group is far away from the optical axis. In other words, f ₃ /f is the lower limit value
When it is smaller than 0.41, that is, when the refractive power of the first group becomes large, the chief ray passing through the first group is strongly refracted in the optical axis direction, and a large amount of negative distortion occurs. Furthermore, when f ₃ /f is outside the upper limit value of 0.59, the field curvature worsens to the extent that it is difficult to correct. Next, the lens shape of the projection lens of the present invention will be explained. In the projection lens according to the present invention, in order to brighten the F number, it is advantageous to correct spherical aberration in the lens group in which the paraxial rays are farthest from the optical axis of the lens. The field side surface has a shape convex toward the image field side. In addition, in this type of lens, correction of the Petzvaar sum depends mostly on the power of the first group. Therefore, the power of the first group tends to become too strong, and various aberrations tend to occur in the second group. In order to suppress the aberrations generated in this lens group as much as possible, the shape of the lens group is biconcave. Furthermore, in the lens of the present invention, since the lens of the present application is a telecentric lens, the off-axis principal ray must be moved high, that is, far away from the optical axis, by the second group. Therefore, in order to increase the height of the chief ray by the lens disposed closest to the object world side of the lens group, it is preferable that the lens be a positive meniscus lens with a concave surface facing the object world side. Further, the lens disposed closest to the image field side of the group is preferably a meniscus lens with a concave surface facing the image field side. The reason for this is that various aberrations are corrected by the concave surface on the image field side of the lens, but the lens does not have negative power because it is necessary to lengthen the back focus of the entire lens system. Next, examples of the projection lens according to the present invention will be shown. In each example, the focal length f is normalized to 1, the F number is 5.0, the angle of view ω is 25.2°, and the imaging magnification β is −0.12343. In each example, Ri is the radius of curvature of the i-th surface, Di is the axial wall thickness or axial air gap between the i-th surface and the i+l-th surface, and Ni is the axial air gap between the i-th surface and the i+l-th surface. The refractive index of the i-lens for the d-line, νi is the Abbe number of the i-th lens, and fi is the focal length of the i-th group. DO indicates the axial air distance from the pupil plane SL to the Rl plane. The cross section of the lens shown in Example 1 is shown in FIG. 1A, its various aberrations (spherical aberration, field curvature, distortion aberration) are shown in FIG.
Shown in Figure C. Furthermore, in Examples 2 to 9, a cross-sectional view of the lens of Example i is shown in Fig. iA, and its various aberrations are shown in Fig. iB.

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[Table] Next, Table 1 shows the third-order aberration coefficients of each of the above embodiments. In addition, is spherical aberration, is comatic aberration,
is the astigmatism, P is the Petzvaar sum, and is the distortion aberration.

【table】 [Brief explanation of the drawing]

FIG. 1A is a cross-sectional view of the lens of Example 1 according to the present invention, FIG. 1B is a diagram of various aberrations of Example 1, FIG. 1C is a diagram of Gaussian upper pole aberration of Example 1, and FIG. A is for Example 2, FIG. 3 A is for Example 3, FIG. 4 A is for Example 4, FIG. 5 A is for Example 5, FIG. 6 A is for Example 6, and FIG. A is a cross-sectional view of the lens of Example 7, FIG. 8A is Example 8, and FIG. 9A is Example 9. FIG. 2B is Example 2, and FIG. 4B of Example 3, FIG. 5B of Example 5, FIG. 6B of Example 6, FIG. 7B of Example 7, and FIG. 8B of Example 4. FIG. 9B of Example 8 is a diagram showing various aberrations of each lens of Example 9. Ri...Radius of curvature of the i-th lens surface, Di...i-th lens surface
Axial thickness or axial air gap between the lens surface and the i+1th lens surface, SL...pupil surface or aperture,
...Group, ...Group, ...Group, S...
...Sagittal surface, M...Meridional surface.

Claims (1)

[Scope of Claims] 1. A first group consisting of a positive single lens, a third group consisting of a negative single lens, and a third group consisting of three or four lenses are arranged in order from the object world side, and the focal point on the object world side In a telecentric projection lens having a pupil at a position, the first group is a biconcave lens, one of the lenses of the first group includes a bonding surface, the focal length of the first group is f ₁ , and the first group is Let f ₂ be the focal length of _the lens, f ₃ be the focal _length of the first _group , and f be the focal length of the entire lens system. A telecentric projection lens characterized by satisfying the relationship: f ₃ /f≦0.59. 2. The telecentric projection lens according to claim 1, wherein the object-world-side surface of the second group has a convex surface facing the object-world side. 3. A patent claim in which the lens disposed closest to the object world side of the group is a meniscus lens with a concave surface facing the object world side, and the lens disposed closest to the image field side is a meniscus lens with a concave surface facing the image field side. The telecentric projection lens according to item 2.