JPH0534591A - Image forming lens for compensating temperature variation - Google Patents

Abstract

PURPOSE:To reduce the cost and also to decrease an image point movement caused by a temperature variation and moreover to improve the image forming performance by using plastic for a material of a specific lens and allowing it to satisfy a specific condition. CONSTITUTION:In this triplet system lens, a first lens consisting of a positive lens, a second lens consisting of a biconcave negative lens, and a third lens consisting of a biconvex positive lens are placed with a cavity in order from an object side. Materials of a second lens and a third lens are both plastic. Also, this image forming lens satisfies expressions I-IV. In the expression I-IV, n1 and n3, nu3, f2 and f3, d2, and d4 denote refractive indexes to a (d) line of the material of a first and a third lenses, respectively, the Abbe number of the material of a third lens, focal distances of a second and a third lenses, respectively, an interval on an optical axis of a first lens and a second lens, and an interval on an optical axis of a second lens and a third lens. Also, at least the concave surface of the object side of a second lens and the convex surface of the object side of a third lens are aspherical surfaces.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image-forming lens which compensates for a temperature change with a small image point movement against a refractive index change and a linear expansion due to a temperature change.

[0002]

2. Description of the Related Art Recently, a plastic lens has been introduced into a lens system in order to reduce weight and cost.
However, the temperature coefficient of the refractive index of plastic is 30 to 40 times larger than that of optical glass, and the sign is opposite, and the coefficient of linear expansion is about 10 times larger than that of optical glass. Therefore, it is difficult to introduce a plastic lens because the environmental performance is greatly changed, such as a facsimile or an image scanner, and the image forming performance is greatly deteriorated when used in a device having no image point position adjusting function. Was there.

[0003]

DISCLOSURE OF THE INVENTION The present invention provides an imaging lens which can reduce the cost by introducing a plastic lens, has less image point movement due to temperature change, and has good imaging performance. is there.

[0004]

An image forming lens for temperature change compensation according to the present invention is a triplet type lens having a three-lens structure as shown in the configuration diagram of FIG. A lens, a second lens composed of a biconcave negative lens, and a third lens composed of a biconvex positive lens are arranged with an air gap. The material of the positive first lens is optical glass, the material of the negative second lens and the positive lens. The material of the third lens is plastic. As mentioned above, plastic is
Since the temperature coefficient and linear expansion coefficient of the refractive index are remarkably larger than those of optical glass, the amount of image point movement due to temperature change is also large. When the positive first lens is made of plastic, the image point movement due to the temperature change occurs first, and even if the negative second lens and the positive third lens are used to cancel this, the image point movement will occur. It is difficult to reduce the amount. This is as if fire fighting activities took place after the fire became big.
It is also a bad idea because it is similar to leaving the seed of the problem unattended and performing problem analysis and countermeasures later. The feature of the arrangement of the lens material of the lens of the present invention is that the first first lens which is far from the image point does not cause the movement of the image point due to temperature change.
The material of the lens is optical glass, and the material of the negative second lens and the positive third lens is plastic. The above is contrary to the rule that it is advantageous to use an optical glass having a high refractive index as the material of the positive third lens in order to improve the image forming performance of the triplet type lens, so that it is difficult to design. However, there is an advantage that the movement of the image point due to the temperature change can be effectively reduced. The imaging lens for temperature change compensation of the present invention has excellent imaging performance and can compensate for temperature change by satisfying the following conditions. That is, n ₁ −n ₃ > 0.1 (1) 35 <ν ₃ <50 (2) 1.15 <f ₃ / | F ₂ | <1.5 ··· (3) 0.8 <d ₄ / d ₂ <1.2 ··· (4) where n _{1 is} the material of the first lens Refractive index for d-line n ₃ : Refractive index of third lens material for d-line ν ₃ : Abbe number of third lens material f ₂ : Focal length of second lens f ₃ : Focal length of third lens d ₂ : The distance d ₄ between the first lens and the second lens on the optical axis: The distance d ₄ between the second lens and the third lens on the optical axis. The condition (1) relates to the refractive indices of the materials of the positive first lens and the positive third lens. When the material of the third lens is made of plastic, a high refractive index cannot be expected so much, and it remains at 1.49 <n ₃ <1.7, which is disadvantageous in increasing the aperture ratio of the synthetic lens and obtaining the flatness of the image plane. Is. The condition (1) is a positive refractive index n of the material of the first lens.
_{1 is made} to be 0.1 or more larger than the refractive index n ₃ of the material of the positive third lens to compensate for the lack of the refractive index of the material of the third lens. If this condition is not satisfied, a large aperture ratio and an image surface Cannot be flattened. The condition (2) relates to the Abbe number of the material of the positive third lens, and is for keeping both the axial chromatic aberration and the lateral chromatic aberration favorable. If the lower limit of the condition (2) is exceeded, axial chromatic aberration will be undercorrected, and lateral chromatic aberration will also be undercorrected. In this case, when the axial chromatic aberration is corrected by using a glass material having a large Abbe number for the positive first lens, the undercorrection of the chromatic aberration of magnification is further increased. If the upper limit of the condition (2) is exceeded, axial chromatic aberration will be overcorrected, and lateral chromatic aberration will also be overcorrected. In this case, when the axial chromatic aberration is corrected by using a glass material having a small Abbe number for the positive first lens, the overcorrection of the chromatic aberration of magnification is further increased.

The condition (3) is for reducing the amount of movement of the image point due to temperature change. The focal length f ₂ of the negative second lens is equal to the focal length f ₃ of the positive third lens,
And if the temperature coefficient and the linear expansion coefficient of the refractive index are equal,
Image point movements due to temperature changes also cancel each other out and do not increase, but in the case of a triplet system lens, there is one negative lens for two positive lenses, and in order to flatten the image plane, the negative second lens refraction It is necessary to increase the force (1 / f ₂ ). On the other hand, when plastic with a low refractive index is used for the positive third lens, the refractive power (1 / f ₃ ) of the third lens is weakened, and the positive first lens made of optical glass has a shortage of refractive power. If supplemented, the imaging performance can be improved. When the value goes below the lower limit of the condition (3), the amount of movement of the image point due to temperature change is small and good, but the flatness of the image surface deteriorates. When the value exceeds the upper limit of the condition (3), the flatness of the image surface is improved, but the amount of movement of the image point due to temperature change increases.

The condition (4) has a close relationship with the condition (3) and is for reducing the amount of movement of the image point due to temperature change. When the axial distance d ₄ between the negative second lens and the positive third lens is made smaller than the axial distance d ₂ between the positive first lens and the negative second lens, the positive third lens An equivalent synthetic focal length cannot be obtained unless the refractive power of is increased. In this case, the amount of movement of the image point due to the temperature change decreases in the direction of decreasing the value of f ₃ / | f ₂ | of the condition (3). When the lower limit of the condition (4) is exceeded, the lower limit of the condition (3) is also exceeded, and the flatness of the image surface deteriorates. When d ₄ is made larger than d ₂ , an equivalent combined focal length can be obtained even if the refractive power of the positive third lens is weakened. In this case, the flatness of the image surface becomes good in the direction in which the value of f ₃ / | f ₂ | of the condition (3) increases. When the upper limit of condition (4) is exceeded, the result exceeds the upper limit of condition (3),
The amount of image point movement increases due to temperature changes. By satisfying the above four conditions, the present invention provides a good imaging lens with a small amount of image point movement due to temperature change.

By further adding the condition (5), it is possible to obtain an image forming lens having an excellent aperture ratio and a small aberration. The condition (5) is such that at least the object-side concave surface of the negative second lens and the object-side convex surface of the positive third lens are aspherical surfaces, and an aspherical surface is formed on the object-side concave surface of the negative second lens. When introduced, the flatness of the image plane and the correction of the coma aberration of the light flux passing through the peripheral portion of the front lens can be made satisfactory. When an aspherical surface is introduced to the object-side convex surface of the positive third lens, it is possible to favorably correct the spherical aberration and the coma aberration of the light flux passing through the peripheral portion of the rear lens. In addition to the above two aspherical surfaces, by introducing aspherical surfaces into the image-side concave surface of the negative second lens and the image-side convex surface of the positive third lens, further aberration is introduced. This can of course be considered good, and this does not depart from the gist of the present invention.

[0008]

Next, Tables 1 to 7 show Examples 1 to 7 of the imaging lens for temperature change compensation according to the present invention. Since the plate glass for pressing the document on the object side has almost no effect on the optical performance, it is omitted. The symbols used in this explanation are as follows. f: focal length of the whole system m: imaging magnification f ₂ : negative focal length of the second lens f ₃ : positive focal length of the third lens r _i : sequential radius of curvature of spherical surface or apex curvature radius of aspherical surface d _i: sequentially lens thickness or air distance n along the optical axis _i: refractive index sequentially for the material of the d line of the lens [nu _i: sequentially lens material Abbe number d _c: the optical axis of the image-side cover glass Upper thickness n _c : Refractive index of the material of the image side cover glass with respect to the d-line Next, the expression of the shape of the aspherical surface is X: distance from the tangent plane at the vertex of the lens surface on the aspherical surface h: from the optical axis Height C: Curvature of apex of aspherical surface (C = 1 / r) K: Conical constant A _2i : Aspherical coefficient

[Equation 1] It is represented by. Δn / ΔT: temperature coefficient of refractive index of material α: coefficient of linear expansion of material Δf ₂₀ : amount of change in focal length of the entire system when the temperature rises by 20 ° C. from the reference temperature Δs ′ ₂₀ : reference temperature, imaging magnification The distance between the object and the lens at m is fixed, and the amount of change in the image point distance when the temperature rises by 20 ° C. is set.

[Table 1] Example 1 F8 f = 24.77 m = -0.112 Object height 108 Angle of view 47.5 ° (Δn / ΔT) × 10 ⁻⁶ α × 10 ⁻⁶ r ₁ = 6.150 d ₁ = 1.90 n ₁ = 1.80420 ν ₁ = 46.5 4 6.3 r ₂ = 9.116 d ₂ = 0.50 r ₃ = -21.072 d ₃ = 0.70 n ₂ = 1.58983 ν ₂ = 31.0 -108 60 r ₄ = 5.858 d ₄ = 0.50 r ₅ = 9.058 d ₅ = 1.50 n ₃ = 1.53285 ν ₃ = 41.6 -109 66.9 r ₆ = -10.784 dc = 0.7 nc = 1.523 f ₃ / | f ₂ | = 1.233 d ₄ / d ₂ = 1.0 Δf ₂₀ = 0 .014 Δs' ₂₀ = 0.001

Table 2 Example 2 F5.6 f = 24.76 m = -0.112 Object height 108 Angle of view 47.5 ° (Δn / ΔT) × 10 ⁻⁶ α × 10 ⁻⁶ r ₁ = 6.854 d ₁ = 2.19 n ₁ = 1.80420 ν ₁ = 46.5 4 6.3 r ₂ = 12.252 d ₂ = 0.61 r ₃ = -13.944 d ₃ = 0.80 n ₂ = 1.58983 ν ₂ = 31.0 -108 60 r ₄ = 6.485 d ₄ = 0.60 r ₅ = 8.753 d ₅ = 1.55 n ₃ = 1.53285 ν ₃ = 41.6 -109 66.9 r ₆ = -12.103 dc = 0.7 nc = 1.523

[Table 3] Example 3 F5.6 f = 24.76 m = −0.112 Object height 108 Field angle 47.5 ° (Δn / ΔT) × 10 ⁻⁶ α × 10 ⁻⁶ r ₁ = 7.036 d ₁ = 2.19 n ₁ = 1.80420 ν ₁ = 46.5 4 6.3 r ₂ = 14.131 d ₂ = 0.614 r ₃ = -14.334 d ₃ = 0.91 n ₂ = 1.58983 ν ₂ = 31.0 -108 60 r ₄ = 6.457 d ₄ = 0.686 r ₅ = 9.107 d ₅ = 1.55 n ₃ = 1.53285 ν ₃ = 41.6 -109 66.9 r ₆ = -13.085 dc = 0.7 nc = 1.523

[Table 4] Example 4 F5.6 f = 20.35 m = −0.088 Object height 108 Angle of view 46.6 ° (Δn / ΔT) × 10 ⁻⁶ α × 10 ⁻⁶ r ₁ = 5.715 d ₁ = 1.80 n ₁ = 1.80420 ν ₁ = 46.5 4 6.3 r ₂ = 10.540 d ₂ = 0.52 r ₃ = -10.937 d ₃ = 0.75 n ₂ = 1.58983 ν ₂ = 31.0 -108 60 r ₄ = 5.298 d ₄ = 0.48 r ₅ = 6.859 d ₅ = 1.26 n ₃ = 1.53285 ν ₃ = 41.6 -109 66.9 r ₆ = -10.234 dc = 0.7 nc = 1.523

[Table 5] Example 5 F5.6 f = 20.35 m = -0.088 Object height 108 Angle of view 46.6 ° (Δn / ΔT) × 10 ⁻⁶ α × 10 ⁻⁶ r ₁ = 5.81 3 d ₁ = 1.81 n ₁ = 1.80420 ν ₁ = 46.5 4 6.3 r ₂ = 12.036 d ₂ = 0.52 r ₃ = -12.101 d ₃ = 0.75 n ₂ = 1.58983 ν ₂ = 31.0 -108 60 r ₄ = 5.200 d ₄ = 0.586 r ₅ = 7.550 d ₅ = 1.26 n ₃ = 1.53285 ν ₃ = 41.6 -109 66.9 r ₆ = -10.820 dc = 0.7 nc = 1.523

[Table 6] Example 6 F6.3 f = 30.63 m = -0.112 Object height 128 Angle of view 45.7 ° (Δn / ΔT) × 10 ^-6 α × 10 ^-6 r ₁ = 8.407 d ₁ = 2.70 n ₁ = 1.80420 ν ₁ = 46.5 4 6.3 r ₂ = 14.934 d ₂ = 0.79 r ₃ = -14.615 d ₃ = 1.00 n ₂ = 1.58983 ν ₂ = 31.0 -108 60 r ₄ = 8.000 d ₄ = 0.68 r ₅ = 9.730 d ₅ = 1.90 n ₃ = 1.53285 ν ₃ = 41.6 -109 66.9 r ₆ = -14.876 dc = 0.7 nc = 1.523

[Table 7] Example 7 F6.3 f = 29.11 m = -0.112 Object height 128 Angle of view 47.8 ° (Δn / ΔT) × 10 ^-6 α × 10 ^-6 r ₁ = 8.171 d ₁ = 2.57 n ₁ = 1.80420 ν ₁ = 46.5 4 6.3 r ₂ = 15.231 d ₂ = 0.75 r ₃ = -14.263 d ₃ = 1.00 n ₂ = 1.58983 ν ₂ = 31.0 -108 60 r ₄ = 7.809 d ₄ = 0.68 r ₅ = 9.731 d ₅ = 1.82 n ₃ = 1.53285 ν ₃ = 41.6 -109 66.9 r ₆ = -14.549 dc = 0.7 nc = 1.523 As described in each of the embodiments of the present invention, the change amounts of the focal length and the image point distance due to the temperature change are not particularly taken into consideration in the temperature change in the triplet type lens including two plastic lenses. Compared to 1/3 to 1/5, it is small. Further, the aberration when the temperature changes is close to that at the reference temperature. The aberration curves of Example 1 of the present invention are shown in FIG. 2, and the aberration curves of Examples 2 to 7 are sequentially shown in FIGS.
It shows in FIG. In any of the examples, among the triplet lenses having the three-lens structure, although two lenses are plastic lenses, coma, axial chromatic aberration, and lateral chromatic aberration are all favorable, and an image with good contrast is obtained. To be Further, as can be seen from the aberration curve, it is also an advantage of the present invention that the sagittal image plane is flat without returning to the (+) side even if the angle of view becomes high.

[0009]

As described above, the temperature change compensating imaging lens according to the present invention uses the plastic lens for two of the triplet lenses of the three-lens structure, but the temperature change does not occur. It is possible to reduce the amount of movement of the image point due to the change in the refractive index and the linear expansion. Therefore, the image forming performance can be maintained and can be sufficiently applied to a device such as a facsimile or an image scanner, which has a large change in environmental temperature and does not have a function of adjusting the image point position.
That is, according to the present invention, the cost of the lens system can be reduced and the cost of the device can be reduced.

[Brief description of drawings]

FIG. 1 is a second embodiment of an imaging lens for temperature change compensation according to the present invention.
FIG.

FIG. 2 is an aberration curve diagram for Example 1.

FIG. 3 is an aberration curve diagram for Example 2.

FIG. 4 is an aberration curve diagram for Example 3.

5 is an aberration curve diagram for Example 4. FIG.

FIG. 6 is an aberration curve diagram for Example 5.

FIG. 7 is an aberration curve diagram for Example 6.

8 is an aberration curve diagram for Example 7. FIG.

Claims (2)

[Claims] 1. A first lens comprising a positive lens in order from the object side.
In a triplet type lens in which a lens, a second lens composed of a biconcave negative lens, and a third lens composed of a biconvex positive lens are arranged with an air gap, both materials of the negative second lens and the positive third lens are plastic. And a temperature change compensation imaging lens characterized by satisfying the following conditions. _{n 1 -n 3> 0.1 ········· (} 1) 35 <ν 3 <50 ············ (2) 1.15 <f 3 / | f ₂ | <1.5 ... (3) 0.8 <d ₄ / d ₂ <1.2 ... (4) where n _{1 is} for the d-line of the material of the first lens Refractive index n ₃ : Refractive index of third lens material with respect to d-line ν ₃ : Abbe number of third lens material f ₂ : Focal length of second lens f ₃ : Focal length of third lens d ₂ : First Distance between optical axis of lens and second lens d ₄ : Distance between optical axis of second lens and third lens 2. The temperature-compensation imaging lens according to claim 1, wherein at least the object-side concave surface of the negative second lens and the object-side convex surface of the positive third lens are aspherical surfaces. Imaging lens for temperature change compensation. ...
(5)