JPH04356008A - Infrared-ray wide angle lens - Google Patents

Abstract

PURPOSE:To obtain a sharp image by the infrared-ray wide-angle lens over a 112 deg. visual field and to prevent infrared rays from an aperture stop, a lens barrel, etc., from being made incident on a focal plane type detector except from an image picked-up object. CONSTITUTION:This wide-angle lens consists of an objective OL which consists of four lenses, i.e., a 1st concave meniscus lens L1, a 2nd concave lens L2, a 3rd convex lens L3 and a 4th convex lens L4 in order from an object side and forms an intermediate image and a relay lens RL which consists of four lenses, i.e., a 5th convex meniscus lens L5, a 6th concave lens L6, a 7th convex lens L7, and an 8th convex lens L8 in order from the object side as well and re-images the intermediate image on an infrared detector I; and the eight lenses are made of two kinds of infrared-ray transmitting material, the 2nd lens L2 and 6th lens L6 are made of infrared-ray transmitting materials which are larger in volume than the infrared-ray transmitting materials constituting the 1st, 3rd-5th, and 7th-8th lenses, and the aperture stop 5 which is cooled is arranged between the 8th lens and an image plane.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] This invention relates to an infrared lens used in an infrared imaging device, and particularly relates to an infrared lens with a wavelength of 3 to 5 μm.
This invention relates to an infrared wide-angle lens that is used in the m-band and provides a field of view of 90° or more.

0002

[Prior Art] Infrared imaging devices that obtain video signals by detecting infrared rays emitted by an object according to its temperature use a two-dimensionally spread infrared detector such as a pyroelectric vidicon or a charge-coupled device. Focal plane detectors are in practical use. One of the characteristics that an infrared lens for forming an infrared image on a focal plane detector should have is that aberrations be appropriately removed within a required angle of view and within a required wavelength range.

In recent years, in the infrared wavelength range of 3 to 5 μm,
Multi-element charge-coupled devices comparable to those in the visible and near-infrared regions have been put into practical use, and it has become possible to image a wide field of view with high resolution. Conventionally, infrared wide-angle lenses have been used with a wavelength of 2.2 μm and a field of view of 60 μm, as shown in Japanese Patent Application Laid-Open No. 61-219015, for example.
There is a five-layer structure made of silicon and quartz that has a

0004

[Problems to be Solved by the Invention] In the above conventional example, it is possible to use it in the wavelength band of 3 to 5 μm by changing the quartz to a suitable transparent material and changing the lens shape accordingly, but the field of view is At an angle of view of 60 degrees or more, aberration correction is insufficient and a clear image cannot be obtained. In addition, since the first lens on the object side is an aperture diaphragm, there is a problem that infrared rays emitted by the aperture diaphragm and the lens barrel pass through the lens and enter the focal plane detector, causing noise and charge saturation. there were.

[0005] This invention was made to solve the above-mentioned problems, and in addition to obtaining a clear image over a field of view of 112°, infrared rays from sources other than the object to be imaged, such as the aperture stop and lens barrel, are detected by the focal plane detector. The objective is to obtain a wide-angle lens that prevents infrared light from entering the infrared rays.

[0006]

[Means for Solving the Problems] An infrared wide-angle lens according to the present invention is constructed of two types of infrared transmitting materials with different dispersions, and uses an infrared transmitting material with a large dispersion for the concave second lens and the concave sixth lens. , the intermediate image formed by the objective lens is re-imaged by the relay lens, a cooled aperture stop is disposed outside the lens exit side, and the following conditions are satisfied. (1) 13f〈L〈29f (2) 1.4f〈d19〈2.8f (3) 0.
7〈m〈1.2 However, f: Absolute value of the focal length of the entire system L: Total length of the lens from the vertex of the object side surface of the first lens to the image plane d19: Distance between the cooling aperture stop and the image plane m: Relay lens The imaging magnification is

[0007]

[Operation] Infrared transmitting materials used in infrared lenses usually have a positive dispersion value, that is, the longer the wavelength, the lower the refractive index, and the lens that images the object light on the infrared detector acts as a convex lens as a whole. Therefore, a lens made of a material with a positive dispersion value generally produces axial chromatic aberration in which the longer the wavelength, the weaker the refractive power.

[0008] Therefore, the above-mentioned longitudinal chromatic aberration can be corrected by using two types of infrared transmitting materials with large dispersion and different dispersions for the concave second lens and the concave sixth lens, excluding the first lens.

Furthermore, by providing an aperture stop on the outside of the exit side and cooling it, the infrared radiation from the aperture stop becomes negligibly small, and the infrared radiation from the inside of the lens barrel and the inside of the imaging device is blocked. In addition, by using a relay lens to relay the intermediate image formed by the objective lens to an infrared detector, the lens diameter can be reduced and an aperture stop can be installed outside the lens exit side.

0010

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view of an infrared wide-angle lens according to the present invention. In the figure, L1 to L8 are the first to eighth in order.
L1 to L4 constitute an objective lens OL, and L5 to L8 constitute a relay lens RL. The material of the second concave lens L2 and the sixth concave lens L6 is germanium, and the others are all silicon. S is a cooled aperture stop, and I is an infrared detector placed on the focal plane. W is a Dewar window for installing and cooling the infrared detector I and the aperture stop S in a vacuum.

The reason why the infrared wide-angle lens is configured as described above is as follows, and the following effects can be produced. The reason why the lens is constructed of two types of infrared transmitting materials with different dispersions and the infrared transmitting materials with large dispersions are used for the concave second lens and the concave sixth lens is to correct chromatic aberration. Infrared transparent materials usually have a positive dispersion value, that is, the longer the wavelength, the lower the refractive index.

[0012] Since the lens that forms an image of the object light on the infrared detector acts as a convex lens as a whole, a lens made of a material having a positive dispersion value generally produces axial chromatic aberration in which the longer the wavelength, the weaker the refractive power.

In order to correct this axial chromatic aberration, it is effective to construct a lens using two types of infrared transmitting materials and to use a material with a large dispersion value for the concave lens.

However, if a material with a large dispersion value is used for the concave first lens, lateral chromatic aberration will be overcorrected, and the second concave lens will be overcorrected.
It is no longer easy to correct it after the lens. Therefore, the second lens, which is a concave lens excluding the first lens, and the sixth lens are concave lenses.
Use a material with a large dispersion value for the lens.

By providing an aperture stop on the outside of the ejection side and cooling it, it is possible to make the infrared radiation from the aperture stop negligible and to block the infrared radiation from the inside of the lens barrel and the inside of the imaging device. can. Infrared detectors are usually covered with a so-called cold shield, which is a cooled material that does not reflect or transmit infrared rays, in order to minimize the incidence of unnecessary infrared rays emitted from sources other than the object being photographed, such as the lens barrel.

By providing an aperture stop outside the exit side of the lens, it is also possible to match the aperture stop with the cold shield without placing an excessive load on the cooler. Although such a configuration may include only an objective lens, the height of incidence of the light beam on the front group of the lens becomes high, leading to an increase in the diameter of the lens, especially in the case of a wide angle.

Furthermore, in order to ensure sufficient back focus, it is necessary to place a lens with strong negative refractive power in the front lens group, and it is necessary to sufficiently correct various aberrations that increase in conjunction with high incident light. becomes less easy. However, by using a relay lens to relay the intermediate image formed by the objective lens to the infrared detector, the lens diameter can be reduced and it becomes easy to install an aperture stop outside the lens exit side.

Further, the following setting conditions (1) to (3) for the infrared wide-angle lens are determined in consideration of the following.

(1) 13f〈L〈29f(2)
1.4f〈d19〈2.8f(3) 0.7〈m〈1
．． 2 Where, f: Absolute value of the focal length of the entire system L: Total lens length from the vertex of the first lens object side surface to the image plane d19: Distance between the cooling aperture stop and the image plane m: Imaging magnification of the relay lens .

Condition (1) defines the total length of the lens. If the lower limit is exceeded, the refraction angle at the image-side surface of the fifth lens becomes large, which increases the deterioration of imaging performance due to manufacturing errors and may cause total internal reflection. Also. If the upper limit is exceeded, the overall length becomes longer, and if the distance between the aperture stop and the infrared detector is kept below a certain value in order to reduce the load on the cooler, the diameter of the relay lens increases, making it difficult to correct aberrations.

Condition (2) defines the distance between the aperture stop and the infrared detector. If the lower limit is exceeded, the diameter of the relay lens will increase, making it difficult to correct aberrations. Moreover, when the upper limit is exceeded, the heat load on the cooler increases.

Condition (3) is for obtaining a bright lens with a small aperture ratio (F number). When the imaging magnification of the relay lens exceeds the lower limit, the lens diameter on the incident side of the relay lens becomes small, the numerical aperture on the incident side of the relay lens increases, and the configuration of the objective lens and the relay lens becomes difficult.

[0023] Although specified numerically below, r in FIG.
1 to r16 are the curvature radius of the lens corner surface, d1 to d16
is the wall thickness or air spacing between each lens surface. The exit side surface of the fifth lens L5 is an aspherical surface, and its shape is defined by y being the distance from the optical axis, z being the intersection of the optical axis and the aspherical surface, and the displacement of the aspherical surface from the reference plane perpendicular to the optical axis. When , it is expressed as .

[0024] However, C is the principal curvature 1/r10, k is the conic constant, and when D=E=F=G=0, K<-1: hyperboloid K=-1: paraboloid -1<K <0: Elliptical surface (major axis rotation center) K=0
: Spherical surface 0<K : Ellipsoidal surface (minor axis rotation center) D, E,
F and G are high-order aspherical coefficients added to the conical surface.

Since the incident height of the fifth lens L5 differs depending on the angle of view, by introducing an aspherical surface on its entrance or exit side, it is effective to correct coma and astigmatism around the visual field. It is also effective for setting the aperture stop to a desired location.

The table shows the structural data of the examples.
Absolute value of table focal length f=1, aperture ratio F/
1, 2, Angle of view 2ω = 112°, Wavelength λ = 4μm Surface Radius of curvature Surface spacing Optical material
Refractive index Abbe number
r d
(λ=4μm)
n4-1

n4 ν=──
─

n3-n5
1 1.553 0.234
Si 3.4254
101 2
1.207 0.703
1
3 17
．． 452 1.148 Ge
4.0244
235 4 16.164
0.290
1
5 -3.821
0.563 Si 3.
4254 101
6 -2.227 0.2
36 1

7 2.144 1.148
Si 3.4254
101 8
1.666 2.532
1
9-3.1
34 1.148 Si
3.4254
101 10 -2.800
4.574
1
11 -16.743 0.
230 Ge 4.0244
235 1
2 13.909 0.073
1
13 1
7.031 0.557 Si
3.4254
101 14 -7.080
0.078
1
15 3.575
1.004 Si 3.4
254 101
16 4.171 0.541
1
17
∞ 0.195 S
i 3.4254
101 18 ∞
0.091
1
19 ∞
1.843 1

However, ∞ of the radius of curvature r of surface 9 is (aperture stop)
Parameter of aspheric surface 10 C=-0.3571,
K=-0.1591 D= 8.061× 10-4, E= 3.2
29×10-5F=-3.922×10-5,
G= 1.204×10-5

In this embodiment, the total length L=17.2f, the interval d19 between the aperture stop and the infrared detection period=1.84f, and the relay lens imaging magnification m=0.933. FIG. 2 shows an aberration diagram of the example based on the structural data in the table. In the figure, S represents a sagittal screen, and M represents a meridional screen. As is clear from the figure, it shows good imaging characteristics in the wavelength range λ = 3 to 5 μm, and is of great practical benefit as an infrared wide-angle lens that can be used with a focal plane infrared detector to obtain wide-field images. .

[0028]

As described above, according to the present invention, since the aperture stop is provided on the outside of the lens exit side, it is possible to use the cold shield as the aperture stop, and as a result, the inside of the lens barrel and the inside of the imaging device are It is possible to obtain an infrared wide-angle lens that can shield unnecessary infrared rays emitted by the sensor, reduce noise, prevent saturation of the charge generated by the detector, and can be used in a wide environmental temperature range.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing the configuration of an infrared wide-angle lens according to the present invention.

FIG. 2 is an aberration diagram of the infrared wide-angle lens according to the present invention.

[Explanation of symbols]

L1 to L8 1st to 8th lenses OL Objective lens RL Relay lens S
aperture diaphragm

Claims (1)

[Claims] Claim 1: In order from the object side, a concave meniscus first lens, a concave second lens, a convex third lens, and a convex fourth lens.
an objective lens consisting of a number of lenses and forming an intermediate image;
Similarly, from the object side, the fifth convex meniscus lens and the sixth concave lens.
The infrared lens consists of four lenses: a convex seventh lens, and a convex eighth lens, and a relay lens that re-images an intermediate image on an infrared detector. of the infrared transmitting material, and the second
The lens and the sixth lens are the first, third to fifth, and seventh to seventh lenses.
An infrared transmitting material with larger dispersion than the infrared transmitting material constituting the eighth lens is used, and a cooled aperture stop is arranged between the eighth lens and the image plane, (1) 13f〈L〈29f (2) 1 .4f〈d19〈2.8f(3) 0.
An infrared wide-angle lens characterized by satisfying the following conditions: 7〈m〈1.2〉. Here, f: Absolute value of the focal length of the entire system L: Total lens length from the vertex of the object side surface of the first lens to the image plane d19: Distance between the cooling aperture stop and the image plane m: Imaging magnification of the relay lens.