JPH06230278A - Lens system of symmetric constitution with two groups and two lenses - Google Patents

Abstract

PURPOSE:To obtain a reading optical system for an image scanner as bright as about F4 by means of a lens system of symmetric constitution with two groups and two lenses. CONSTITUTION:This system is a lens system of symmetric constitution with two groups and two lenses having a first lens L1 composed of a positive meniscus lens whose convex surface confronts the object side, next a diaphragm and a second lens L2 composed of a meniscus lens whose effective surface has at least the same shape as that of the first lens L1 and the convex surface confronts the image side in the order from the object side, both surfaces of the first lens L1 and the second lens L2 are formed as aspherical surfaces, respectively, and the lens system satisfies the following conditions: (1)... 0.50<R1/R2<0.75 (2)... 1.0<F1/F<1.9 where, R1 resents the radius of curvature of the vertex of the first surface, R2 the radius of curvature of the vertex of the second surface, F the focal length of a whole system and F1 the focal length of the first lens or the second lens.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-group two-lens symmetrical lens system suitable for use in, for example, a reading optical system for a scanner.

[0002]

2. Description of the Related Art Conventionally, as a desired optical performance of a reading lens for a scanner, a high MTF from the center to the periphery of the screen, an aperture efficiency of nearly 100% in order to minimize a shortage of light amount in the periphery, and reading of the periphery. There has been a demand for a lens having a small distortion so that an error does not occur. Further, in order to increase the reading speed, the reading lens is required to be as bright as possible, and it has been desired to simplify the lens structure in order to further reduce the cost and the weight.

Japanese Patent Laid-Open No. 3-116109 discloses a lens of this type whose lens structure is simplified. This uses an aspherical surface in order to satisfactorily correct various aberrations by using a lens having a symmetrical structure and using an aspherical surface, and particularly to ensure a good tangential direction resolution necessary for a barcode reading lens.

[0004]

However, since the present invention requires a lens for a scanner that can read not only a bar code but also the type on a document, it cannot be said that the tangential is sufficient. , Resolution in the radial direction is also required. Furthermore, as far as the examples are concerned, the brightness is at most insufficient with respect to F6.5 and the brightness required by the present invention.

The present invention has been made in view of the above-mentioned circumstances, and an object thereof is to obtain a wide-angle, bright and high-performance two-group two-lens symmetrical lens. About F4, angle of view about 40 degrees, distortion
It is to obtain a lens that is kept within 0.2%.

[0006]

In order to achieve this object, the present invention relates to a first lens composed of a positive meniscus lens having a convex surface directed toward the object side, and an aperture stop in order from the object side.
And a second lens comprising a meniscus lens whose convex surface faces the image side and which has the same effective surface as that of the first lens.
It is a lens system with a two-group two-element symmetric structure with lenses,
Both surfaces of the first lens and the second lens are formed as aspherical surfaces, ... 0.50 <R ₁ / R ₂ <0.75 ... 1.0 <F ₁ / F <1.9, where R ₁ ; The radius of curvature of the apex of the surface R ₂ ; the radius of curvature of the apex of the second surface F; the focal length of the entire system F ₁ ; the focal length of the first lens or the second lens .

[0007]

In the lens system having the two-group two-element symmetric structure constructed as described above, at least the effective surface portion of the lens has the same shape.
That is, it has a symmetrical configuration with two lenses sandwiching the diaphragm. With this configuration, distortion, lateral chromatic aberration, coma, etc. are sufficiently corrected, and further, spherical aberration must be sufficiently corrected to satisfy the bright specification of F4. In the present invention, as a countermeasure, an aspherical surface is used to maintain optical performance, and the aspherical surface works well for correction of astigmatism,
The balance with spherical aberration is properly maintained.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the optical system configuration diagram of a lens system having a two-group two-element symmetric configuration shown in FIGS.

A lens system L having a two-group two-element symmetric structure according to the present invention is a two-group two-lens system consisting of _one symmetrical lens L ₁ and L ₂ arranged with an aperture A _P interposed therebetween. The two symmetrical lenses L ₁ and L ₂ are configured to have the same shape.

In this case, a lens system L having a two-group two-element symmetric structure
The first lens L ₁ forming the front group of is formed as a positive meniscus lens with the convex side facing the object side, and the first surface 1 formed with the convex surface facing the object side, and the first surface 1 On the other hand, the second surface 2 which is formed as a concave surface toward the image surface side, which is located apart from the predetermined axial lens thickness D ₁ , is formed as a symmetric aspherical surface that satisfies the respective formulas described below. ing.

The first lens L ₁ is arranged in front of a diaphragm A _P which is arranged with a predetermined axial air space D ₂ therebetween.

On the other hand, the second lens L ₂ constituting the rear part of the lens system L having a two-group two-element symmetric structure is formed as a positive meniscus lens having the same shape as the first lens L ₁ and has a predetermined axial air gap. at a D _3, they are arranged symmetrically in a state opposite to the first lens L ₁ to the rear position of the a _P diaphragm.

That is, the second lens L ₂ has a third surface 3 formed as an aspherical surface which is concave toward the object side, and a predetermined axial lens thickness D ₄ with respect to the third surface 3. It is formed as a positive meniscus lens having a fourth surface 4 which is a convex aspherical surface facing the image side.

The first lens L ₁ and the second lens L
_2, since at least the effective portion as described above has the same shape, Explaining its shape on behalf of the first lens L _1, the first surface 1 and the second surface of the first lens L ₁ 2 Is as in Equation 1.

[0015]

[Equation 1]

Where φ is the aspherical surface measured in the optical axis direction from a point on the tangent plane of the aspherical surface vertex of the first surface 1 to a point on the first surface 1 at a height H from the optical axis. Deformation amount (including displacement amount due to spherical surface) The same applies to the second surface C; paraxial curvature H of the aspherical surface of the first surface or the second surface; height from the optical axis K; first surface or the first surface It is formed as an aspherical surface represented by a conical constant A _{n of} two surfaces; an aspherical coefficient of the nth order of the first surface or the second surface.

.. 0.50 <R ₁ / R ₂ <0.75 ..1.0 <F ₁ / F <1.9 where R _{1 is} the radius of curvature of the apex of the first surface R _{2 is} the second surface The radius of curvature of the apex F; the focal length of the entire system F ₁ ; the focal length of the first lens or the second lens are satisfied.

G is designed as a cover glass provided on an image pickup device (not shown) arranged on the image side of the lens system L having a two-group two-lens symmetrical structure. This cover glass is made of a highly precise parallel flat glass plate.

As described above, according to the present invention, in the lens system L having a two-group two-element symmetric structure, two lenses L ₁ having the same shape,
By using L ₂ , the cost is reduced, and by disposing the aperture A _P to sandwich it, distortion, lateral chromatic aberration, coma, etc. are sufficiently corrected, and the aspherical surface is for astigmatism correction. It also worked well, and it was possible to obtain a reading optical system for a scanner that can properly maintain the balance with spherical aberration.

In this case, in the present invention, the condition indicates the ratio of the radii of curvature of R ₁ and R ₂ , and particularly relates to correction of spherical aberration. If the condition is exceeded, spherical aberration will be overcorrected and the balance with the image plane will not be achieved. On the other hand, if the value goes below the lower limit, spherical aberration will be undercorrected and field curvature will increase.
Even if both are attempted to be corrected with an aspherical surface shape, the aspherical surface amount becomes too large and the processing accuracy becomes strict, which is not a preferable configuration.

The condition is the focal length of F _1. If the focal length exceeds this upper limit and the focal length becomes longer, the lens is enlarged, which is not preferable. On the other hand, when the length becomes shorter, the coma aberration becomes worse due to the wider angle of view, and it becomes difficult to secure high optical performance up to the most peripheral portion of the image plane.

Further, although the lens shape of the present invention is a meniscus lens, it lowers the height of the light rays going out to the concave surface on the image side with respect to the height of the light rays entering from the convex surface on the object side. This is to reduce the Petzval sum.
Therefore, in the present invention, while satisfying the condition,
It is more preferable to satisfy the following conditions.

.. 0.17 <R ₂ /F<0.59 This condition regulates the power of the air lens between the symmetrically arranged lenses with respect to the radius of curvature on the concave side of the meniscus lens.

If R ₂ becomes larger than the upper limit, the Petzval sum becomes large and the image surface becomes too under, and the spherical aberration cannot be balanced.
On the other hand, if the size becomes smaller, there is no space for arranging the fixed diaphragm, and the lens becomes thicker to secure the edge thickness of the lens, which is not preferable.

2 set under the conditions described above
Specific examples of the types will be given. Examples 1 and 2
Table 1 shows the specifications.

[0026]

[Table 1]

In the symbols other than the above-mentioned symbols used in the examples, R is the radius of curvature of each lens surface, D is the center thickness of the lens or the air gap, and N _d is the refractive index of the glass material with respect to the d-line. , V _d is the Abbe number for the d-line, F is the focal length of the entire lens system, 2ω is the angle of view, Fno is the F number, and Y is the maximum image height. In the rear of this embodiment, there is a glass block corresponding to BK7, which is designed as a cover glass provided on the image sensor.

Example 1 F = 14.80 Fno = 3.4 Y = 6.3 2ω = 40 ° RD N _d ν _d 1 1st lens 4.185 1.50 1.49200 57.0 2 1st lens 5.622 2.24 3 2nd lens −5.622 1.50 1.49200 57.0 4 Second lens -4.185 0.00 5 Cover lens ∞ 0.80 1.51633 64.1 6 Cover lens ∞ First surface aspherical coefficient K = -6.73 450 x 10 ^-1 A ₄ = 2.30662 x 10 ^-3 A ₆ = 4.69944 x 10 ^-5 A ₈ = 1.01577 × 10 ⁻⁴ A ₁₀ = −2.24930 × 10 ⁻⁵ A ₁₂ = 1.88896 × 10 ⁻⁶ Second surface aspherical coefficient K = −1.95691 × 10 ⁻² A ₄ = 2.82992 × 10 ⁻³ A ₆ = 3.45518 × 10 ⁻⁴ A ₈ = −4.46965 × 10 ⁻⁶ A ₁₀ = 1.58451 × 10 ⁻⁵ A ₁₂ = −1.59545 × 10 ⁻⁶ Third surface aspherical coefficient K = −1.95691 × 10 ⁻² A ₄ = −2.82992 × 10 ^{_{-3 A 6 = -3.45518 × 10 -4}} A 8 = 4.46965 × 10 -6 A 10 = -1.58451 × 10 -5 A 12 = 1.59545 × 10 -6 fourth surface aspheric coefficients K = -6.73450 × 10 ^-1 A ₄ = −2.30662 × 10 ⁻³ A ₆ = −4.69944 × 10 ⁻⁵ A ₈ = −1.01577 × 10 ⁻⁴ A ₁₀ = 2.24930 × 10 ⁻⁵ A ₁₂ = −1.88896 × 10 ⁻⁶ [Example 2] F = 14.82 Fno = 4.0 Y = 6.3 2ω = 40 ° R D N _d ν _d 1 1st lens 4.454 1.50 1.49200
57.0 2 1st lens 6.205 2.24 3 2nd lens −6.205 1.50 1.49200 57.0 4 2nd lens −4.454 0.00 5 Cover lens ∞ 0.80 1.51633 64.1 6 Cover lens ∞ First surface aspherical coefficient K = −6.86086 × 10 ⁻¹ A ₄ = 1.83440 × 10 ^-3 A ₆ = 8.90528 × 10 ^-5 A ₈ = 5.39226 × 10 ^-5 A ₁₀ = -1.44373 × 10 ^-5 A ₁₂ = 1.33504 × 10 ^-6 2nd surface aspherical coefficient K = -2.28024 × 10 ^-2 A ₄ = 2.27412 × 10 ^-3 A ₆ = 2.44938 × 10 ^-4 A ₈ = -1.39594 × 10 ^-5 A ₁₀ = 7.36081 × 10 ^-6 A ₁₂ = 2.64477 × 10 ^-7 Third surface aspherical coefficient K = −2.28024 × 10 ⁻² A ₄ = −2.27412 × 10 ⁻³ A ₆ = −2.44938 × 10 ⁻⁴ A ₈ = 1.39594 × 10 ⁻⁵ A ₁₀ = −7.36081 × 10 ⁻⁶ A ₁₂ = −2.64477 × 10 ^-7 fourth surface aspheric coefficients _{^{K = -6.86086 × 10 -1 A 4}} = -1.83440 × 10 -3 A 6 = -8.90528 × 10 -5 A 8 = -5.39226 × 10 -5 A 10 = 1.44373 × 10 - ⁵ a ₁₂ = -1.33504 × 10 ^-6 these examples 1 and 2, as shown in the aberration diagram of each of the following examples, Very good correction is also made for the aberrations of the displacement, it has a lens system excellent 2 group two symmetrical configuration.

That is, in the first embodiment, as shown in FIG.
As shown in the spherical aberration diagram, the astigmatism diagram, and the distortion diagram of (B) and (C), where one scale is set to 1, good correction results are obtained.

As for the second embodiment, as shown in FIG.
As shown in the spherical aberration diagram, the astigmatism diagram, and the distortion diagram of (B) and (C), where one scale is set to 1, good correction results are obtained.

The present invention is not limited to the above-described embodiments, but can be variously modified and implemented without departing from the scope of the invention.

Further, the first lens L _1, need not be arranged symmetrically with respect to the second lens L ₂ is squeezed A _P, short stop A _P
It suffices to dispose them across.

[0033]

As described above, according to the present invention, it is possible to provide a cheap and high-performance new reading lens.

This reading lens has the above-mentioned construction and is as bright as Fno 4,3.4, has a wide angle of view of about 40 degrees, and has a distortion aberration of 0.2% or less, and high optical performance from the center of the screen to the periphery of the screen. Have Further, by using a symmetrical lens, the cost of the mold can be greatly reduced and an inexpensive lens can be realized.

[Brief description of drawings]

FIG. 1 is an optical system configuration diagram of a lens system having a two-group two-element symmetric configuration according to Example 1 of the present invention.

FIG. 2 is an optical system configuration diagram of a lens system having a two-group two-element symmetric configuration according to Example 2 of the present invention.

FIG. 3 is a characteristic diagram of Example 1 of the present invention. In this case, (A) is a spherical aberration diagram, (B) is an astigmatism diagram, and (C).
Shows distortion diagrams, respectively.

FIG. 4 is a characteristic diagram of Example 2 of the present invention. In this case, (A) is a spherical aberration diagram, (B) is an astigmatism diagram, and (C).
Shows distortion diagrams, respectively.

[Explanation of symbols]

L 2 group 2 lens symmetric lens system L ₁ 1st lens L ₂ 2nd lens 1 1st surface 2 2nd surface A _P diaphragm 3 3rd surface 4 4th surface G Cover glass S Sagittal M Meridional

Claims (1)

[Claims] 1. A first lens comprising, in order from the object side, a positive meniscus lens having a convex surface directed toward the object side, and then a diaphragm, and at least the effective surface of the lens and the first lens have the same shape, and the convex surface is directed toward the image side. A lens system having a two-lens two-element symmetric structure having a second lens composed of a meniscus lens, wherein the first lens and the second lens are formed as aspherical surfaces on both sides, ... 0.50 <R ₁ / R ₂ <0.75 ... 1.0 <F ₁ / F <1.9 where R ₁ ; radius of curvature of vertex of first surface R ₂ ; radius of curvature of vertex of second surface F; focal length of entire system F ₁ A lens system having a two-group two-element symmetric structure, characterized in that it is configured to satisfy various conditions such as the focal length of the first lens or the second lens.