JPH08160296A - Image pickup system - Google Patents

Abstract

PURPOSE: To exactly and surely read out an image by making an image circle larger, shifting an effective image pickup surface, inclining it according to the curvature of field of an optical system and satisfying a specified condition of the ratio of maximum to minimum of the distance from the optical axis. CONSTITUTION: The effective image pickup surface of an image pickup device is arranged by inclining it to the direction of the curvature of field included in an image forming optical system against a surface perpendicular to the optical axis. Among the perpendicular lines from points on the effective image pickup surface to the optical axis, the ratio of the maximum point yu of the distances from the optical axis to the minimum point yt;yu:yt satisfies the following condition i.e., 0.85YUO/YLO<yu/yt<1.15YUO/YLO, where, YUO, YLO are ideal image heights corresponding to yu, yt on the Gauss image plane, respectively, the object position is -256.55mm from a first surface of the image forming optical system (Y value at the maximum point UO of distance on a glass surface; YUO=7.54mm, Y value at the minimum point LO: YLO=5.13mm).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image pickup system for optically reading information recorded by being encoded by, for example, a bar code or a dot code.

[0002]

2. Description of the Related Art Conventionally, there has been known an apparatus for optically reading a code coded in an image pattern such as a bar code or a dot code. When an image of a code pattern printed, engraved or added to an article is read by such a reading device to decode the content, the code pattern should intersect perpendicularly to the optical axis of the imaging optical system of the imaging system. Move items or move the imaging system. However, when moving goods, it is difficult to move when the goods are piled up high in a warehouse, for example, so it is not always possible to bring the code pattern so that it intersects perpendicularly to the optical axis of the optical system. Often not. In this case, a code pattern is imaged not perpendicular to the optical axis or outside the optical axis. Therefore, it is impossible to focus on the entire code pattern having a finite size, and the image is formed in a distorted shape on the image pickup surface, or is formed outside the image pickup surface.
The image circle may deviate from the image circle determined by the effective diameter of the imaging optical system, and reading may not be successful.

[0003]

SUMMARY OF THE INVENTION The present invention provides an imaging optical system capable of accurate and reliable reading even when the code pattern is tilted with respect to the optical axis of the imaging optical system or is not located on the optical axis. It is provided.

[0004]

In the image pickup system of the present invention, the effective image pickup surface of the image pickup element is arranged outside the optical axis of the image forming optical system, and the effective image pickup surface of this image pickup element is perpendicular to the optical axis. It is placed at an angle in accordance with the direction of the field curvature of the imaging optical system with respect to the surface, and is the distance from the optical axis on the effective image pickup surface that is perpendicular to the optical axis from the effective image pickup surface. The ratio y _U / y _L of the point y _U where y is the maximum and the point y _{L where the} minimum is y satisfies the following condition (1).

(1) 0.85y _U0 / y _L0 <y _U / y
_L <1.15 y _U0 / y _L0 Here, y _U0 and y _L0 are ideal image heights corresponding to y _U and y _L on the Gaussian image plane, respectively.

The present invention is a so-called shift lens in which the image circle of the image forming optical system is enlarged to some extent, and an effective image pickup surface is arranged in a part of this image circle substantially perpendicularly to the optical axis of the image forming optical system. The imaging system is adopted. This makes it possible to read images accurately and reliably with less image distortion and out-of-focus. Here, the optical performance of the imaging optical system needs to be ensured only in a certain range of the effective image pickup surface, so that even if there is field curvature or tilt, the effective image pickup surface is fitted so as to fit the curve. By tilting from the state perpendicular to the optical axis, out-of-focus can be reduced over the entire effective image pickup surface, and it becomes possible to accurately and surely read even a code pattern having a finer pitch. As a result, even in the design of the imaging optical system, as long as astigmatism is corrected, some field curvature is allowed, and the number of optical system components is reduced, which is advantageous for downsizing.
However, when the effective image pickup surface is tilted from a state perpendicular to the optical axis, a rectangular subject is subject to trapezoidal distortion, which causes distortion and reading errors, and the tilt is preferably as small as possible. For this reason, in the present invention, the effective image pickup surface of the image pickup device is arranged outside the optical axis of the image forming optical system and tilted according to the direction of the field curvature of the image forming optical system.

In FIG. 7, A is an imaging optical system, O is an optical axis, L _e is a chief ray, B is an image sensor, and G is a Gaussian image plane. And y _U is a point having a large distance from the optical axis out of the points perpendicular to the optical axis 0 from the point on the image sensor B, that is, the distance from the point U in the figure to the optical axis 0, and similarly y _L is the point L. To the optical axis L. Further, y _U0 and y _L0 are points U on the Gaussian image plane, respectively.
0, the distance from the point L0 to the optical axis. And the ratio of y distance y from the optical axis is y _U and the minimum is the maximum _L y _U /
The y _L was made to satisfy the above condition (1).

For example, the distortion of a rectangular object image appears in a form in which the effect of tilting the effective image pickup surface or the object surface with respect to the optical axis and the effect of distortion aberration are combined. In order to remove this distortion, it is ideal that y _U / y _L be equal to the ideal image height ratio y _U0 / y _L0 on the Gaussian image plane, and the larger the deviation from that value, the greater the distortion tends to become. . However, in practice, there is no problem as long as the amount of deviation is less than half the code pattern pitch. Further, the effective image pickup surface of the image pickup device is arranged outside the optical axis of the image formation optical system, and the effective image pickup surface or the object surface is arranged in the direction of the field curvature of the image formation optical system with respect to the surface perpendicular to the optical axis. In the tilted imaging system, if the distance a of the exit pupil position from the final lens surface of the imaging optical system is 5 times or more the focal length of the entire imaging optical system, the effective imaging surface or the object surface is tilted. The change in magnification with time is small.

The above is good when a code pattern having a coarse pitch is read, or when a signal processing system after the image pickup system is provided with a distortion processing function 8 and an allowable distortion amount has a margin, but it is optically to some extent. If the reading error is likely to occur if the distortion correction of No. is not performed, the graphic distortion due to the distortion aberration and the graphic distortion when the effective image pickup surface is tilted from the state perpendicular to the optical axis are output in the opposite directions. Is good. For example,
When the distortion due to the imaging optical system is barrel-shaped, the rectangular image has a trapezoidal shape that becomes shorter on the side far from the optical axis, so it is better to tilt the effective image plane away from the optical axis toward the side away from the optical system. As for the field curvature, in the range of the effective image pickup surface, it is preferable to make the direction away from the optical system as the distance from the optical axis increases. In that case, of the points on the normal line to the optical axis of the imaging optical system that passes through the point C that is on the effective image pickup surface and serves as a reference on the effective image pickup surface,
The distortion aberration and field curvature of the optical system are set so that the ratio y _u / y _L of the point y _U where the distance y from the optical axis is the maximum and the point y _L where the distance y is the minimum is within the range satisfying the following condition (2). It is desirable to correct the above and set the tilt amount of the effective image pickup surface.

(2) 0.90y _U0 / y _L0 <y _U / y
_L <1.10y _U0 / y _L0 However, it is limited to the case where the distortion aberration of the imaging optical system changes monotonically as it moves away from the optical axis. Further, as the information amount of the code pattern is increased, the pitch of the code pattern becomes finer, and if it goes out of the range of the condition (2), a read error is likely to occur.

Next, the image forming optical system used in the image pickup system of the present invention comprises one or two convex surfaces facing the object side in order from the object side.
First group including one negative meniscus lens and one or two
An optical system having a wide comprehensive angle of view, which includes a second group including a positive lens, a third group of negative lenses having a concave surface on the image side, and a fourth group of positive lenses. Since the comprehensive angle of view is wide as described above, the code pattern can be accurately and reliably read even if it is largely deviated from the optical axis of the imaging optical system. Further, if the above-mentioned imaging optical system satisfies the following conditions (3), (4) and (5), a code pattern having an extremely fine pitch can be read accurately and surely.

(3) 1.5 <(R ₁₁ + R ₁₂ ) / (R
_{11 -R 12) <5.0 (4} ) 0.6 <(R 31 + R 32) / (R 31 -R 32) <
2.5 (5) 0.1 <D ₁₂ /f<0.8 where R ₁₁ and R ₁₂ are the radii of curvature of the object-side surface and the image-side surface of the negative meniscus lens of the first group, respectively. ₃₁ , R ₃₂
Is the radius of curvature of the object-side surface and the image-side surface of the negative lens of the third group, respectively, and D ₁₂ is the distance between the first group and the second group, that is, the most image-side lens of the first group and the most of the second group. The distance between the lenses on the object side is f, and f is the focal length of the imaging optical system.

The condition (3) defines the shape factor of the most object-side lens of the first group, that is, the negative meniscus lens, and the condition (4) is the most object-side lens of the third group, which is negative. It defines the shape factor of the lens. Unlike a general imaging optical system for imaging, this type of imaging optical system is required to have good imaging performance at some off-axis angle of view. The image pickup system of the present invention has a small astigmatic difference between the point y _U having the largest distance from the optical axis and the point y _L having the smallest distance from the optical axis among the points on the effective image pickup surface, and the lateral aberration with respect to the ideal image height. Since it suffices that the value obtained by removing the field curvature and the distortion aberration from is small, when performing aberration correction with the point P at this point, the shape factor of the negative lens closest to the object side in the second group is
It is advantageous to set it on the positive side, as compared with the case where correction is performed by balancing the entire axis. Further, in the image forming optical system, it is not preferable that the curvature of field, in particular, when the angle of view is large, the abruptness of the curvature changes abruptly. Therefore, the shape factor of the negative meniscus lens closest to the object side in the first lens group is the axial factor. Therefore, it is advantageous to set the value on the positive side as compared with the case where the correction is performed while keeping the overall balance.

The condition (3) is provided for the above reason.
Therefore, if the lower limits of the conditions (3) and (4) are exceeded, the value obtained by removing the field curvature and the distortion aberration from the field curvature and the lateral aberration in a certain part having a large angle of view. It becomes smaller and harder. On the other hand, when the value exceeds the upper limit of the condition (4), the lateral aberration is apt to increase due to the spherical aberration.

The condition (5) defines the apex distance between the image-side lens of the first group and the object-side lens of the second group.
When the value goes below the lower limit, it becomes difficult to correct the astigmatic difference at a large angle of view, and when the value goes above the upper limit, the diameter of the negative meniscus lens on the object side of the first lens unit becomes large and the lens system cannot be made compact.

It is more preferable to set the lower limit of condition (4) to 0.7. Further, if the lower limit value is set to 0.8, the above effect can be more favorably obtained. Further, it is desirable to set the upper limit of condition (4) to 2.0. Further, if it is 1.5, it is more effective.

[0017]

EXAMPLES Examples of the present invention will be described below. Example 1 f = 10.512, F number = 4.2, image height = 7.54 r ₁ = 6.5399 d ₁ = 0.7831 n ₁ = 1.51633 ν ₁ = 64.15 r ₂ = 3.4849 d ₂ = 1.5932 r ₃ = 6.5249 d ₃ = 2.5116 n ₂ ＝ 1.72000 ν ₂ ＝ 50.25 r ₄ ＝ 344.2085 d ₄ ＝ 0.7967 r ₅ ＝ ∞ (aperture) d ₅ ＝ 0.0000 r ₆ ＝ -68.7776 d ₆ ＝ 0.7964 n ₃ ＝ 1.80518 ν ₃ ＝ 25.43 r ₇ ＝ 7.4690 d ₇ ＝ 0.2093 _{r 8 = 12.1562 d 8 = 1.9911} n 4 = 1.60311 ν 4 = 60.70 r 9 = -5.3304 (R 11 + R 12) / (R 11 -R 12) = 3.281 (R 31 + R 32) / (R 31 - R ₃₂ ) = 0.804 D ₁₂ /f=0.152

Example 2 f = 9.944, F number = 4.0, image height = 8.62 r ₁ = 6.6579 d ₁ = 0.8102 n ₁ = 1.56873 ν ₁ = 63.16 r ₂ = 3.9587 d ₂ = 3.8430 r ₃ = 8.2198 d ₃ = 1.8428 n ₂ = 1.83400 ν ₂ ＝ 37.17 r ₄ ＝ -32.0169 d ₄ ＝ 0.7023 r ₅ ＝ ∞ (aperture) d ₅ ＝ 0.0000 r ₆ ＝ 79.5807 d ₆ ＝ 0.8147 n ₃ ＝ 1.84666 ν ₃ ＝ 23.78 r ₇ ＝ 7.6046 d _{7 = 0.4500 r 8 = -39.2075 d} 8 = 2.6000 n 4 = 1.69680 ν 4 = 55.53 r 9 = -5.7740 (R 11 + R 12) / (R 11 -R 12) = 3.933 (R 31 + R 32) / _{(R 31 -R 32) = 1.211} D 12 /f=0.386

Example 3 f = 8.584, F number = 4.0, image height = 1.56 r ₁ = 13.6901 d ₁ = 1.0251 n ₁ = 1.48749 ν ₁ = 70.21 r ₂ 4.9946 d ₂ = 2.3300 r ₃ = 13.7560 d ₃ = 0.8000 n ₂ = 1.51633 ν ₂ = 64.15 r ₄ = 5.3060 d ₄ = 1.7048 r ₅ = 38.2063 d ₅ = 1.6000 n ₃ = 1.67270 ν ₃ = 32.10 r ₆ = -55.7132 d ₆ = 1.1624 r ₇ = 9.4189 d ₇ = 1.6102 _{n 4 = 1.70154 ν 4 = 41.24} r 8 = -17.2827 d 8 = 0.7193 r 9 = ∞ ( _{stop) d 9 = 0.3002 r 10 =} 31.2101 d 10 = 1.0426 n 5 = 1.80518 ν 5 = 25.43 r 11 = 7.2806 d 11 _{= 0.3000 r 12 = -20.5388 d 12} = 2.3124 n 6 = 1.60311 ν 6 = 60.70 r 13 = -4.8734 (R 11 + R 12) / (R 11 -R 12) = 2.149 (R 31 + R 32) / ( R ₃₁ −R ₃₂ ) = 1.609 D ₁₂ /f=0.199 The data of the above-mentioned examples are those of the imaging optical system, r ₁ ,
r ₂ , ... Is the radius of curvature of each surface of the lens, d ₁ , d ₂ ,.
Is the wall thickness and spacing of each lens, n ₁ , n ₂ , ... Is the refractive index of each lens, and ν ₁ , ν ₂ , ... Is the Abbe number of each lens.

In these examples, the example 1 has the structure shown in FIG. 1, and the object point position is − from the first surface of the image forming optical system.
256.55 mm, y value of point U0 with the maximum distance from the optical axis on the Gaussian plane y _U0 = 7.54 mm, y of minimum point L0
The value of y _L0 = 5.13 mm. The field curvature of the image forming optical system of Example 1 is as shown in the aberration diagram of FIG. 4, and therefore the distance S _KV = 1 from the final surface of the image forming optical system to the Gaussian surface.
Distance to point U for 0.422 mm S _U = -0.95
By tilting the effective image pickup surface so as to move by 2 mm or the distance S _L = −0.652 mm to the point L, it is possible to focus on the entire surface within the range of the effective image pickup surface.

With the above-mentioned structure, graphic distortion occurs, which is as follows.

The values of y at the points U and L after tilting are as follows.

Y _U = (β _U / β ₀ ) · y _U0 · (1 + DT
_U ) = 0.908 y _U0 y _L = (β _L / β ₀ ) · y _L0 · (1 + DT _L ) = 0.9
40y _L0 where (β _U / β ₀ ) and (β _L / β ₀ ) are as follows.

Β _U / β ₀ = {(S _KV + S _U ) -P _ET } /
(S _KV −P _ET ) = 0.926 β _L / β ₀ = {(S _KV + S _L ) −P _ET } / (S _KV −
P _ET ) = 0.949 where P _ET is the exit pupil position from the final surface of the imaging optical system, and P _ET = −2.409.

From the aberration diagram shown in FIG. 4, D _TU = −0.0
195, D _TL = -0.0095. Here, DT can be expressed as follows.

DT = β ₁₀ y ₀ + β ₂₀ y ₀ ² + β ₃₀ y ₀ ³ + β
₄₀ y ₀ ⁴ + ... Therefore, y _U / y _L = 0.966 · y _U0 / y _L0 Example 2 has the configuration shown in FIG.
The value of y at the point U0, which is −240.90 mm from the plane and has the maximum distance from the optical axis on the Gaussian image plane, is y _U0 = 8.62.
The value of y at the minimum point L0 in mm is y _L0 = 6.03 mm.

The field curvature of the optical system of the second embodiment is shown in FIG.
The distance from the final surface of the optical system to the Gaussian surface is S _KV = 10.486 mm, and the distance S _U on the optical axis from this Gaussian surface to the point U on the imaging surface is S _U
= -0.826 mm, and the distance S _L to the point _L is S _L =-
If the effective image pickup surface of the image pickup device is inclined so as to move by 0.826 mm, the focus can be adjusted within the range of the effective image pickup surface.

The figure distortion caused by this is as follows.

Y _U = (β _U / β ₀ ) · y _U0 · (1 + DT
_U ) = 0.958y _U0 y _L = (β _L / β ₀ ) · y _L0 · (1 + DT _L ) = 0.9
57y _U0 The values of (β _U / β ₀ ) and (β _L / β ₀ ) in the above formula are as follows.

Β _U / β _o = {(S _KV -S _U ) -P _ET } /
(S _KV -P _ET ) = 0.940 β _L / β _o = {(S _KV -S _L ) -P _ET } / (S _KV −
P _ET ) = 0.940 However, P _ET = −3.358 Further, from the aberration diagram, DT _U = −0.0190, DT _L = −
It is 0.0185. Therefore y _U / y _L = 1.00
1y _U0 / y _L0 .

The third embodiment has a configuration as shown in FIG. 3, the object point position is -229.37 mm from the first surface of the image forming optical system, and the point U0 having the maximum distance from the optical axis on the Gaussian surface. The y value of y is y _U0 = 10.00 mm, and the y value of the minimum point UL is y _L0 =
It is 7.00 mm.

The field curvature of the optical system of the third embodiment is shown in FIG.
Since the distance from the final surface of the optical system to the Gaussian surface is S _KV = 12.717 mm, the point U is S _U = −
By tilting the effective image pickup surface so that the point L moves by 0.200 mm and S _L = −0.800 mm, it becomes possible to focus on the entire surface within the range of the effective image pickup surface.

The figure distortion caused by this is as follows.

Y _U = (β _U / β ₀ ) · y _U0 · (1 + DT
_U ) = 0.886 y _U0 y _L = (β _L / β ₀ ) ・ y _L0・ (1 + DT _L ) = 0.8
84y _L0 Here, (β _U / β ₀ ) and (β _L / β ₀ ) are as follows.

Β _U / β _o = {(S _KV -S _U ) -P _ET } /
(S _KV -P _ET ) = 0.988 β _L / β _o = {(S _KV -S _L ) -P _ET } / (S _KV −
P _ET ) = 0.951, but P _ET = −3.641 or from the aberration diagram, DT _U = −
0.103 and DT _L = -0.070.

Therefore, y _U / y _L = 1.002 y _U0 /
y _L0 .

[0037]

According to the image pickup system of the present invention, the image circle is enlarged, the effective image pickup plane is shifted, and the image pickup system is tilted in accordance with the curvature of field of the optical system. Good imaging was possible, which enabled accurate and reliable reading.

[Brief description of drawings]

FIG. 1 is a sectional view of a first embodiment of the present invention.

FIG. 2 is a sectional view of a second embodiment of the present invention.

FIG. 3 is a sectional view of a third embodiment of the present invention.

FIG. 4 is an aberration curve diagram of Example 1 of the present invention.

FIG. 5 is an aberration curve diagram of Example 2 of the present invention.

FIG. 6 is an aberration curve diagram of Example 3 of the present invention.

FIG. 7 is a diagram showing a relationship between an imaging lens and an effective image pickup surface of an image pickup element.

Claims (3)

[Claims] 1. An effective image pickup surface of an image pickup device is arranged outside the optical axis of an image forming optical system, and the effective image pickup surface is formed with respect to a plane perpendicular to the optical axis of the image forming optical system. The ratio y _u / y of the point y _U where the distance y from the optical axis is the maximum and the point y _L where the distance y from the optical axis is tilted in accordance with the direction and where an arbitrary straight line perpendicular to the optical axis intersects with the effective imaging surface. An imaging system in which _L satisfies the following condition (1). (1) 0.85y _U0 / y _L0 <y _U / y _L <1.15
y _U0 / y _L0 where y _U0 and y _L0 are y _U and y _L on the Gaussian image plane, respectively.
Is the ideal image height corresponding to. 2. An effective image pickup surface of an image pickup device is arranged outside the optical axis of the image forming optical system, and a differential value dz / dy and distortion aberration with respect to a distance y from the optical axis of the field curvature of the image forming optical system. The sign of the differential value dDT / dy with respect to the distance y from the optical axis of DT is made to have a different sign in the range of the effective image pickup surface, and the effective image pickup surface is tilted according to the direction of the field curvature of the image forming optical system, Of the points where an arbitrary straight line perpendicular to the optical axis intersects with the effective imaging surface, the ratio y _U / y _L of the point y _U where the distance y from the optical axis is the maximum and the point y _{L where} it is the minimum is the following condition (2). An imaging system characterized in that (2) 0.90 y _U0 / y _L0 <y _U / y _L <1.10
y _U0 / y _L0 where y _U0 and y _L0 are y _U and y _L on the Gaussian image plane, respectively.
Is the ideal image height corresponding to. 3. The image forming optical system comprises a negative meniscus lens having a convex surface directed from the object side to the object side, a biconvex lens, a negative lens having a concave surface on the image side, and a positive lens. An imaging system characterized by satisfying (3), (4), and (5). (3) 1.5 <(R ₁₁ + R ₁₂ ) / (R ₁₁ −R ₁₂ ) <
5.0 (4) 0.6 <(R ₃₁ + R ₃₂ ) / (R ₃₁ −R ₃₂ ) <
2.5 (5) 0.1 <D ₁₂ /f<0.8 where R _ij is the radius of curvature of the j-th surface of the ith lens from the object side, and D ₁₂ is the first lens and the second lens. Is the air distance between the lenses, and f is the focal length of the entire system.