JP2021004998A - Imaging lens - Google Patents

Abstract

To provide a Gaussian type lens having good imaging performance for light between a visible range and a short wavelength infrared range (about 400-1700 nm wavelength).SOLUTION: An imaging lens comprises, in order from an object side, a first lens group, an aperture diaphragm and a second lens group. A configuration at front and rear sides of the diaphragm includes, in order from the object side, a positive lens LS1 having a convex surface on the object side, a negative lens LS2 having a concave surface on an image side, an aperture diaphragm, a negative lens LS3 having a concave surface on the object side and a positive lens LS4 having a convex surface on the image side. The two positive lenses and the two negative lenses at front and rear sides of the diaphragm satisfy the following conditional expressions: (θIR_LS2+θIR_LS3)/2+0.0124×(νIR_LS2+νIR_LS3)/2-0.967<-0.01...(1), (νd_LS2+νd_LS3)/2<50...(2), |(θIR_LS1-θIR_LS2)/(νIR_LS1-νIR_LS2)|<0.003...(3) and |(θIR_LS3-θIR_LS4)/(νIR_LS3-νIR_LS4)|<0.003...(4).SELECTED DRAWING: Figure 1

Description

The present invention relates to a Gaussian type single focus lens that can be used in a wide wavelength range (wavelength of about 400 to 1700 nm) from the visible range to the short wavelength infrared range (SWIR: Short-wavelength infrared).

In recent years, surveillance cameras that can be used both day and night have been developed mainly for the purpose of crime prevention. In general, a surveillance camera uses visible light for daytime photography, while it uses near-infrared light (NIR) for nighttime photography. In addition, near-infrared light is less susceptible to scattering than visible light even in dense fog with poor visibility. Therefore, as the lens used for the surveillance camera, a lens whose aberration is corrected in the wavelength range from the visible region to the near infrared region (wavelength of about 400 to 1000 nm) is used. Further, in recent years, high-pixel cameras have appeared even in surveillance applications, and an optical system having better optical performance than before is required.

Patent Documents 1 to 3 are known as conventional single focus lenses in the visible wavelength range to the near infrared wavelength range. Patent Document 1 and Patent Document 2 disclose an optical system for reading an image, which is a Gaussian type in which a positive lens and a negative lens are symmetrically arranged before and after the aperture, and a single focus lens corrected from the visible to the near infrared region. .. Further, Patent Document 3 discloses a Gauss-type imaging lens corrected from the visible to the near-infrared region.

Japanese Patent No. 4278756 Japanese Patent No. 4869813 Japanese Patent No. 5450028

Near-infrared light is used for nighttime photography without lighting, but it may not be possible to obtain a sufficient amount of light, such as when there is very little moonlight near the new moon or when the moon is hidden by clouds. is there. On the other hand, after the hydroxide ions in the atmosphere are excited by sunlight, light called night glow (peak wavelength 1.6 μm) is emitted. By using this light as illumination light, it is possible to take pictures even when there is no moonlight.

However, in the Gaussian type single focus lens described in Patent Documents 1, 2 and 3, although the chromatic aberration from the visible region to the near infrared region with a wavelength of about 1 μm is corrected, the wavelength is as short as 1.6 μm. Aberration correction is not sufficient up to the wavelength infrared region.

The present invention has been made in view of the above, and is a Gaussian type single lens having good imaging performance for light in a wide wavelength range (wavelength 400 to 1700 nm) from the visible region to the short wavelength infrared region. It is an object of the present invention to provide a focal lens.

In order to achieve the above object, the imaging lens according to the present invention is
It consists of a first lens group, an aperture aperture, and a second lens group arranged in order from the object side. The configuration before and after the aperture is a positive lens LS1 with a convex surface on the object side and a negative lens with a concave surface on the image side. It is composed of an LS2, an aperture diaphragm, a negative lens LS3 having a concave surface on the object side, and a positive lens LS4 having a convex surface on the image side. ..
(ΘIR_LS2 + θIR_LS3) / 2 + 0.0124 × (νIR_LS2 + νIR_LS3) /2-0.967 <−0.01 ・ ・ ・ Equation 1
(Νd_LS2 + νd_LS3) / 2 <50 ... Equation 2
| (ΘIR_LS1-θIR_LS2) / (νIR_LS1-νIR_LS2) | <0.003 ... Equation 3
| (ΘIR_LS3-θIR_LS4) / (νIR_LS3-νIR_LS4) | <0.003 ... Equation 4
However,
ν is the Abbe number for a wavelength of 587.6 nm νIR = (N1050-1) / (N400-N1700)
θIR = (N400-N1050) / (N400-N1700)
ν_LS2-3 is ν of lenses LS2-3
νIR_LS1-4 is νIR of lenses LS1-4
θIR_LS1-4 is θIR of lenses LS1-4
N400 is the refractive index at a wavelength of 400 nm N1050 is the refractive index at a wavelength of 1050 nm N1700 is the refractive index at a wavelength of 1700 nm

According to the present invention, it is possible to provide a Gauss type single focus lens in which aberrations are reduced over a wide wavelength range from the visible region to the short wavelength infrared region.

Cross-sectional view of the lens in the single focus lens of Example 1. Aberration diagram in the single focus lens of Example 1 Cross-sectional view of the lens in the single focus lens of Example 2 Aberration diagram in the single focus lens of Example 2 Cross-sectional view of the lens in the single focus lens of Example 3 Aberration diagram in the single focus lens of Example 3 Cross-sectional view of the lens in the single focus lens of Example 4 Aberration diagram in the single focus lens of Example 4

FIG. 1 is a cross-sectional view of a lens in the single focus lens of the first embodiment according to the embodiment of the present invention.

2 (A) and 2 (B) are aberration diagrams of the single focus lens of Example 1 at an object distance of infinity and a close distance of 1000 mm, respectively. The aberration-corrected wavelength range is 400 to 1700 nm.

FIG. 3 is a cross-sectional view of a lens in the single focus lens of the second embodiment according to the embodiment of the present invention.

4 (A) and 4 (B) are aberration diagrams of the single focus lens of the second embodiment at an object distance of infinity and a close proximity of 750 mm, respectively. The aberration-corrected wavelength range is 400 to 1700 nm.

FIG. 5 is a cross-sectional view of a lens in the single focus lens of the third embodiment according to the embodiment of the present invention.

6 (A) and 6 (B) are aberration diagrams in which the object distances of the single focus lens of Example 3 are infinity and close to 2000 mm, respectively. The aberration-corrected wavelength range is 400 to 1700 nm.

FIG. 7 is a cross-sectional view of a lens in the single focus lens of the fourth embodiment according to the embodiment of the present invention.

8 (A) and 8 (B) are aberration diagrams of the single focus lens of Example 4 at an object distance of infinity and a close distance of 1000 mm, respectively. The aberration-corrected wavelength range is 400 to 1700 nm.

In each lens cross-sectional view, the left side is the subject side (object side) and the right side is the image side. DP is an optical block corresponding to a dichroic prism and CG is an optical filter or the like. The IMG is an image plane, and a solid-state image sensor (photoelectric conversion element) such as a CCD sensor or a CMOS sensor is arranged. The aberration diagram is shown in millimeters. In the spherical aberration diagram, aberrations related to 1700 nm, 1050 nm, 587 nm, and 435 nm are shown, and in the astigmatism diagram, m and s represent the meridional image plane and the sagittal image plane. .. Hereinafter, unless otherwise specified, the lens configurations will be described in the order in which they are arranged in order from the object side to the image side.

The imaging lens of the present invention is composed of a first lens group, an aperture diaphragm, and a second lens group arranged in order from the object side, and the configuration before and after the diaphragm is a positive lens LS1 having a convex surface on the object side in order from the object side. The image side is composed of a concave negative lens LS2, an aperture aperture, an object side is a concave negative lens LS3, and an image side is a convex positive lens LS4. The two positive lenses before and after the aperture and the two negative lenses have the following conditional expression. It is characterized by satisfying.
(ΘIR_LS2 + θIR_LS3) / 2 + 0.0124 × (νIR_LS2 + νIR_LS3) /2-0.967 <−0.01 ・ ・ ・ Equation 1
(Νd_LS2 + νd_LS3) / 2 <50 ... Equation 2
| (ΘIR_LS1-θIR_LS2) / (νIR_LS1-νIR_LS2) | <0.003 ... Equation 3
| (ΘIR_LS3-θIR_LS4) / (νIR_LS3-νIR_LS4) | <0.003 ... Equation 4
However,
νd is the Abbe number for a wavelength of 587.6 nm νIR = (N1050-1) / (N400-N1700)
θIR = (N400-N1050) / (N400-N1700)
νd_LS2-3 is ν of lenses LS2-3
νIR_LS1-4 is νIR of lenses LS1-4
θIR_LS1-4 is θIR of lenses LS1-4
N400 is the refractive index at a wavelength of 400 nm N1050 is the refractive index at a wavelength of 1050 nm N1700 is a refractive index at a wavelength of 1700 nm This represents the abnormal dispersibility of the glasses of the negative lenses LS2 and LS3 arranged before and after the aperture. By satisfying Equation 1, chromatic aberration can be corrected in a wide wavelength region from a wavelength of 400 nm to a wavelength of 1700 nm.

In addition, Equation 2 is the average Abbe number in the visible range with respect to the d-line of the negative lenses LS2 and LS3 arranged before and after the aperture, and if it exceeds the upper limit, it is necessary to increase the power of the negative lens when achromatic. , It is not preferable because a lot of spherical aberration and coma are generated.

Further, in Equation 3, the positive lens LS1 and the negative lens LS2 arranged immediately before the aperture correct the axial chromatic aberration (secondary spectrum) having a wavelength of 400 nm and 1700 nm, and the amount of axial chromatic aberration (secondary spectrum) having a wavelength of 1050 nm generated at that time is calculated. It is an index for estimation.

Further, in Equation 4, the negative lens LS3 and the positive lens LS4 arranged immediately after the aperture correct the axial chromatic aberration (secondary spectrum) having a wavelength of 400 nm and 1700 nm, and the amount of axial chromatic aberration (secondary spectrum) having a wavelength of 1050 nm generated at that time is corrected. It is an index for estimation.

By satisfying Equations 3 and 4 at the same time, axial chromatic aberration can be reduced, and chromatic coma and chromatic aberration of magnification can be corrected by the symmetry before and after the aperture by the Gauss type. Further, when the equation 3 is satisfied, the occurrence of color astigmatism can be reduced when an air gap is provided between the positive lens LS1 and the negative lens LS2 to correct the astigmatism.

Further, in the imaging lens of the present invention, it is preferable that the negative lenses LS2 and LS3 before and after the aperture satisfy the following equation 5 (Nd_LS2 + Nd_LS3) / 2 <1.7 ... Equation 5
However,
Nd is the refractive index for a wavelength of 587.6 nm Nd_LS2-3 is the Nd of lenses LS2-3.
If the upper limit is exceeded in Equation 5, the Petzval sum becomes large and it becomes difficult to correct curvature of field, which is not preferable.

Further, in the imaging lens of the present invention, it is preferable that the negative lenses LS2 and LS3 before and after the aperture satisfy the following equation 6 | νd_LS2-νd_LS3 | <15 ... Equation 6
If the upper limit is exceeded in Equation 6, the symmetry before and after the aperture is broken, and it becomes difficult to correct the chromatic aberration, which is not preferable.

Further, in the image pickup lens of the present invention, it is preferable that at least one of the negative lenses LS2 and LS3 before and after the aperture satisfies the following formulas 7, 8 and 9.
θIR + 0.0124 × νIR −0.967 <−0.015 ・ ・ ・ Equation 7
νd <47.5 ... Equation 8
Nd <1.65 ... Equation 9
By applying a glass material satisfying Equations 7, 8 and 9 to at least one of the negative lenses LS2 and LS3 before and after the aperture, it becomes easy to correct chromatic aberration and curvature of field.

Further, in the imaging lens of the present invention, it is preferable that the second lens group is moved to the object side in focusing at a short distance, and the second lens group satisfies the following equation 7.
0.5 <fG2 / f <1.5 ... Equation 10
However,
fG2 is the focal length of the second lens group. If f exceeds the upper limit in the focal length equation 7 of the entire system, the power of the second lens group is small and the amount of movement is large, which is not preferable. Further, if it is below the lower limit, the power of the second lens group is large and the aberration fluctuation due to the shooting distance is large, which is not preferable.

More preferably, the numerical values of Equations 3, 4 and 6 are set as follows.
| (ΘIR_LS1-θIR_LS2) / (νIR_LS1-νIR_LS2) | <0.002 ... Equation 3a
| (ΘIR_LS3-θIR_LS4) / (νIR_LS3-νIR_LS4) | <0.002 ... Equation 4a
| Νd_LS2-νd_LS3 | <10 ... Equation 6a
Further, when an image pickup is performed using a lens corrected from a wavelength of 400 nm to a wavelength of 1700 nm, an image sensor having sensitivity over the entire wavelength range is very expensive, so that the wavelength is branched between the second lens group and the image plane. It is desirable to arrange a dichroic prism for use and divide it into a wavelength region including a visible region and a wavelength region including a short wavelength infrared region for imaging using two image sensors.

As described above, according to the present invention, it is possible to provide a single focus imaging lens in which aberrations are reduced over a wide wavelength range from the visible region to the short wavelength infrared region.

The single focus lens of the first embodiment will be described in detail with reference to FIGS. 1 and 2.

As shown in FIG. 1, the single focus lens according to the first embodiment is composed of a first lens group G1 from the object side, an aperture diaphragm STO that determines a predetermined aperture, and a second lens group G2, and is a close-up lens. The second lens group G2 is moved to the object side for focusing. A dichroic prism DP for wavelength branching is arranged between the second lens group G2 and the image plane IMG. One of the wavelength-branched lights forms an image on the image plane IMG1, and the other forms an image on the image plane IMG2.

The first lens group G1 includes a lens L1 having a negative refractive power, a lens L2 having a positive refractive power, a lens L3 (LS1) having a convex surface on the object side and a positive refractive power, and a concave surface on the image side. It is composed of L4 (LS2) having an optical power, and the negative lens L1 and the positive lens L2 are joined.

The second lens group G2 is composed of a lens L5 (LS3) having a concave surface on the object side and having a negative refractive power, a lens L6 (LS4) having a convex surface on the image side and having a positive refractive power, and a lens L7 having a positive refractive power. The negative lens L5 and the positive lens L6 are joined to each other.

As shown in the aberration diagram of FIG. 2, the single focus lens of Example 1 reduces the aberration over a wide wavelength range from the visible region to the short wavelength infrared region, and the aberration does not fluctuate significantly depending on the shooting distance. I understand.

The single focus lens of the second embodiment will be described in detail with reference to FIGS. 3 and 4.

As shown in FIG. 3, the single focus lens according to the second embodiment is composed of a first lens group G1 from the object side, an aperture diaphragm STO that determines a predetermined aperture, and a second lens group G2, and is a close-up lens. The second lens group G2 is moved to the object side for focusing. An optical block CG is arranged between the second lens group G2 and the image plane IMG. The optical block CG can be omitted if it is not necessary.

The first lens group G1 is composed of a lens L1 having a positive refractive power, a lens L2 (LS1) having a convex surface on the object side and having a positive refractive power, and an L3 (LS2) having a negative refractive power on the image side. The negative lens L2 and the positive lens L3 are joined.

The second lens group G2 includes a lens L4 (LS3) having a concave surface on the object side and having a negative refractive power, a lens L5 (LS4) having a convex surface on the image side and having a positive refractive power, and a lens L6 having a positive refractive power. It is composed of L7 having a negative refractive power and L8 having a positive refractive power, and the negative lens L4 and the positive lens L5 and the positive lens L6 and the negative lens L7 are joined.

As shown in the aberration diagram of FIG. 4, the single focus lens of Example 2 reduces the aberration over a wide wavelength range from the visible region to the short wavelength infrared region, and the aberration does not fluctuate significantly depending on the shooting distance. I understand.

The single focus lens of the third embodiment will be described in detail with reference to FIGS. 5 and 6.

As shown in FIG. 5, the single focus lens according to the third embodiment is composed of a first lens group G1 from the object side, an aperture diaphragm STO that determines a predetermined aperture, and a second lens group G2, and is a close-up lens. The second lens group G2 is moved to the object side for focusing. A dichroic prism DP for wavelength branching is arranged between the second lens group G2 and the image plane IMG. One of the wavelength-branched lights forms an image on the image plane IMG1, and the other forms an image on the image plane IMG2.

The first lens group G1 includes a lens L1 having a negative refractive power, a lens L2 having a positive refractive power, a lens L3 (LS1) having a convex surface on the object side and a positive refractive power, and a concave surface on the image side. It is composed of L4 (LS2) having an optical power, and the negative lens L1 and the positive lens L2 are joined.

The second lens group G2 includes a lens L5 (LS3) having a concave surface on the object side and having a negative power, a lens L6 (LS4) having a convex power on the image side and having a positive power, and a lens L7 having a positive power. , L8 having a negative refractive power, and the negative lens L5 and the positive lens L6 are joined.

As shown in the aberration diagram of FIG. 6, the single focus lens of Example 3 reduces the aberration over a wide wavelength range from the visible region to the short wavelength infrared region, and the aberration does not fluctuate significantly depending on the shooting distance. I understand.

The single focus lens of the fourth embodiment will be described in detail with reference to FIGS. 7 and 8.

As shown in FIG. 7, the single focus lens according to the fourth embodiment is composed of a first lens group G1 from the object side, an aperture diaphragm STO that determines a predetermined aperture, and a second lens group G2, and is a close-up lens. The second lens group G2 is moved to the object side for focusing. A dichroic prism DP for wavelength branching is arranged between the second lens group G2 and the image plane IMG. One of the wavelength-branched lights forms an image on the image plane IMG1, and the other forms an image on the image plane IMG2.

The first lens group G1 includes a lens L1 having a positive refractive power, a lens L2 having a negative refractive power, a lens L3 (LS1) having a convex surface on the object side and a positive refractive power, and a concave surface on the image side. It is composed of L4 (LS2) having an optical power, and the negative lens L2 and the positive lens L3 are joined.

The second lens group G2 is composed of a lens L5 (LS3) having a concave surface on the object side and having a negative refractive power, a lens L6 (LS4) having a convex surface on the image side and having a positive refractive power, and a lens L7 having a positive refractive power. The negative lens L5 and the positive lens L6 are joined to each other.

As shown in the aberration diagram of FIG. 8, the single focus lens of Example 4 reduces the aberration over a wide wavelength range from the visible region to the short wavelength infrared region, and the aberration does not fluctuate significantly depending on the shooting distance. I understand.

Although the preferred examples of the present invention have been described above, it goes without saying that the present invention is not limited to these examples, and various modifications and changes can be made within the scope of the gist thereof. For example, in this embodiment, a single focus lens having a corrected wavelength range of 400 to 1700 nm is shown, but the correction wavelength range is not limited, and the present invention can be applied to a single focus lens having a narrowed or widened corrected wavelength range. ..

Hereinafter, numerical examples of each example will be shown. The plane numbers are in the order of the optical planes counted from the object plane to the image plane. r is the radius of curvature of the i-th optical surface. d is the interval between the i-th and the i + 1-th (the sign is positive when measured from the object side to the image plane side (when the light approaches), and negative in the reverse direction).

Nd and νd indicate the refractive index and Abbe number of the material with respect to the wavelength of 587.6 nm, respectively. Table 1 shows the relationship between the above-mentioned conditional expressions and the numerical examples.

(Numerical Example 1)
(Numerical Example 2)
(Numerical Example 3)
(Numerical Example 4)
Table-1

G1 1st lens group, G2 2nd lens group, STO aperture,
IMG image plane, DP dichroic prism, L1 to L7 lenses

Claims (7)

It consists of a first lens group, an aperture aperture, and a second lens group arranged in order from the object side. The configuration before and after the aperture is a positive lens LS1 with a convex surface on the object side and a negative lens with a concave surface on the image side in order from the object side. It is composed of an LS2, an aperture diaphragm, a negative lens LS3 having a concave surface on the object side, and a positive lens LS4 having a convex surface on the image side. Imaging lens.
(ΘIR_LS2 + θIR_LS3) / 2 + 0.0124 × (νIR_LS2 + νIR_LS3) /2-0.967 <−0.01 ・ ・ ・ Equation 1
(Νd_LS2 + νd_LS3) / 2 <50 ... Equation 2
| (ΘIR_LS1-θIR_LS2) / (νIR_LS1-νIR_LS2) | <0.003 ... Equation 3
| (ΘIR_LS3-θIR_LS4) / (νIR_LS3-νIR_LS4) | <0.003 ... Equation 4
However,
ν is the Abbe number for a wavelength of 587.6 nm νIR = (N1050-1) / (N400-N1700)
θIR = (N400-N1050) / (N400-N1700)
ν_LS2-3 is ν of lenses LS2-3
νIR_LS1-4 is νIR of lenses LS1-4
θIR_LS1-4 is θIR of lenses LS1-4
N400 is the refractive index at a wavelength of 400 nm N1050 is the refractive index at a wavelength of 1050 nm N1700 is the refractive index at a wavelength of 1700 nm The imaging lens according to claim 1, wherein the negative lenses LS2 and LS3 before and after the aperture satisfy the following equation 5.
(Nd_LS2 + Nd_LS3) / 2 <1.7 ... Equation 5
However,
Nd is the refractive index for a wavelength of 587.6 nm Nd_LS2-3 is the Nd of lenses LS2-3. The imaging lens according to claim 1, wherein the negative lenses LS2 and LS3 before and after the aperture satisfy the following equation 6.
| Νd_LS2-νd_LS3 | <15 ... Equation 6 The imaging lens according to claim 1, wherein at least one of the negative lenses LS2 and LS3 before and after the aperture satisfies the following equations 7, 8 and 9.
θIR + 0.0124 × νIR −0.967 <−0.015 ・ ・ ・ Equation 7
νd <47.5 ・ ・ ・ Equation 8
Nd <1.65 ・ ・ ・ Equation 9 The imaging lens according to claim 1, wherein the second lens group is moved to the object side for focusing on a short distance, and the second lens group satisfies the following equation 7.
0.5 <fG2 / f <1.5 ... Equation 10
However,
fG2 is the focal length of the second lens group f is the focal length of the entire system A dichroic prism for wavelength branching is arranged between the second lens group and the image plane, divided into a wavelength region including a visible region and a wavelength region including a short wavelength infrared region, and imaging is performed using two image sensors. The image pickup lens according to claim 1. A surveillance imaging device equipped with the imaging lens according to any one of claims 1 to 6.