JPH07318797A - Lens for infrared ray - Google Patents

Abstract

PURPOSE:To obtain a high-performance and low-cost lens having a sufficient back focus even at a wide angle of view of >=40 deg. by forming the lens of retrofocus constitution consisting of a negative first component and a positive second component and forming so the lens as to satisfy specific conditions. CONSTITUTION:This lens for IR rays is composed of an optical material to allow transmission of IR rays and is arranged, successively from the object side, the first component having a negative refracting power and the second component having a positive refracting power to the lens constitution arranged with a cooled full-aperture diaphragm behind the second component. Further, relation between the back focus and the focal length of the entire system and the shape of mainly the object side face on the extremely object side are so set as to satisfy the following two conditions: 4.5<d/(f.tanomega)<7.5, 0.021 <f/(nl.FNO.rl)<0.060. Here, (d) denotes the back focus (air equiv. length); (f) denotes the focal length of the entire system; omega denotes a half angle of view; nl denotes the refractive index of the lens on the extreme object side; rl denotes the radius of curvature of the object side face of the lens on the extreme object side; FNO denotes an F number.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an infrared lens used in an infrared image pickup device or the like, and particularly has an angle of view of 4
The present invention relates to a wide-angle infrared lens that exceeds 0 °.

[0002]

2. Description of the Related Art Recent infrared image pickup devices are becoming smaller, higher in performance and lower in price due to the practical use of two-dimensional image pickup devices and the high integration of the devices, and their application fields are industrial and medical. It has been widely used for business purposes such as night surveillance. In particular, when used for night-time monitoring, it is desired to provide a wide-angle lens having a wide photographing field angle so that a wider photographing range can be obtained.

For example, JP-A-52-37444 and JP-A-61-219015 are known to disclose infrared lenses having an angle of view exceeding 40 °.

The infrared imaging device is a device for detecting infrared rays radiated by an object according to its temperature and outputting it as a thermal image. However, the lens barrel and the aperture stop, which form the imaging optical system, also emit infrared rays. Therefore, the device itself becomes noise, which may cause a decrease in S / N ratio. These noises are caused by Japanese Patent Laid-Open No. 64-88.
This can be prevented by disposing a cooled aperture stop behind the lens final surface, as described in Japanese Patent No. 414. Here, the lens final surface is the final surface of the lens system excluding a flat plate such as a filter placed immediately before the image pickup element.

By the way, since the wide-angle infrared lens disclosed in the above two publications has an insufficient back focus, a cooled aperture stop is arranged behind the final lens surface, and the It is impossible to secure the light amount to the peripheral part.

Further, as an infrared lens having a wide angle of view and a back focus sufficient to dispose a cooled aperture stop behind the last surface of the lens, there is disclosed in Japanese Patent Laid-Open No. 4-356.
The lens described in Japanese Patent Publication No. 008 is known. But,
This lens adopts a relay lens system in order to secure the amount of light in the peripheral portion, and the number of lenses is as large as eight. Since the optical material used in the infrared lens is very expensive, a large number of lenses leads to an increase in cost. Furthermore, it also leads to a decrease in transmittance.

[0007]

In consideration of the above circumstances, the present invention provides a high-performance and low-cost infrared lens having a sufficient back focus even in a wide angle of view of 40 ° or more. The purpose is to do.

[0008]

In order to achieve the above object, the present invention comprises a first component having a negative refracting power and a positive refracting power which are composed of an optical material which transmits infrared rays and which are, in order from the object side. And a cooled aperture stop at the rear of the second component, and further, the relationship between the back focus and the focal length of the entire system, and the lens closest to the object side. It is characterized in that the shape of the object side surface of is properly specified.

[0009]

With the retrofocus configuration of the negative first component and the positive second component, a sufficient back focus can be obtained even in a wide angle of view, and an appropriate definition of the shape of the object side surface of the lens closest to the object side is mainly provided. This reduces the amount of ghost light that is likely to occur in the retrofocus configuration.

[0010]

EXAMPLE An infrared lens of the present invention is made of an optical material that transmits infrared rays, and a first component having a negative refracting power and a second component having a positive refracting power are arranged in order from the object side. The cooled aperture stop is arranged behind the second component, and further, the following condition is satisfied.

(1) 4.5 <d / (f · tanω) <7.5 (2) 0.021 <f / (n1 · FNO · r1) <0.0
60 where d is the back focus (air conversion length), f is the focal length of the entire system, ω is the half angle of view, n1 is the refractive index of the lens closest to the object side, and r1 is the curvature of the object side surface of the lens closest to the object side. The radius is FNO: F number.

In the infrared lens used in the infrared image pickup device and the like, as described in the section "Prior Art", a cooled aperture stop is arranged behind the final lens surface (the image side surface of the lens closest to the image side). By doing so, the influence of infrared rays emitted from the lens barrel and the aperture stop can be suppressed to a negligible extent.

The above conditional expression (1) secures a space for arranging the cooled aperture stop behind the final lens surface, and limits the back focus necessary to secure a sufficient amount of light even in the peripheral portion of the screen. To do. The back focus here is the distance between the final lens surface and the image surface, and is obtained by adding the air gap to the air thickness of the plate thickness of the filter or the like inserted between the final lens surface and the image surface. is there.

If the lower limit of conditional expression (1) is exceeded, the distance between the aperture stop and the image plane will become short, and the angle difference between the light rays incident on the image pickup device will be large in the central portion and the peripheral portion of the screen. This is shown in FIG. When the distance between the image plane and the aperture stop becomes short, the angle α of the light beam at the peripheral portion becomes large, and the light amount difference between the central portion and the peripheral portion of the screen becomes too large based on the cosine fourth law. That is, the difference in sensitivity between the central portion and the peripheral portion of the screen becomes large. On the other hand, if the upper limit of conditional expression (1) is exceeded, the back focus becomes longer than necessary, and compactification of the lens cannot be achieved sufficiently.

Further, the wider the angle of view, the larger the ratio of the back focus to the focal length. Therefore, by adopting a so-called retrofocus type lens configuration in which the first component having the negative refractive power is arranged on the object side and the second component having the positive refractive power is arranged on the image side, the angle of view is 40
Even if the angle exceeds °, it becomes possible to secure the back focus limited by the conditional expression (1).

On the other hand, the two-dimensional image pickup device used in the infrared image pickup device has a reflectance of infrared rays of several tens on its surface.
It is very high at about%. For this reason, the light once imaged on the image sensor by the infrared lens travels backward through the optical system due to the reflection on the surface of the image sensor, is reflected by each lens surface inside the optical system, and is again formed on the image sensor. Ghosting occurs due to the image.

Generally, when a retrofocus type lens system is adopted, a negative distortion aberration generated by the first component having a negative refractive power becomes a problem. When it is attempted to correct this distortion, a ghost caused by reflection on the first surface of the first lens (the object side surface of the lens closest to the object side) becomes a serious problem. FIG. 2 shows how the axial light flux emerging from the image plane is reflected by the first surface of the first lens. Here, β is the reflection angle of the marginal ray on the axis, and as β increases, the image forming position of the ghost moves away from the image plane, and the light amount of the ghost decreases. This β is expressed by the following equation.

Β = arcsin (f / (2n1 · FNO · r1))

Conditional expression (2) defines a range in which the amount of ghost light generated by the reflection on the first surface of the first lens is not a problem. When the lower limit of conditional expression (2) is exceeded, the ghost generated by reflection on the first surface of the first lens increases, whereas when the upper limit of conditional expression (2) is exceeded, the first
It becomes difficult to properly correct the negative distortion aberration generated in the component.

Furthermore, it is desirable that the refractive power of the first component and the refractive power of the second component satisfy the following conditions.

(3) −1.3 <(φ1 / φ2) tan ω <−0.25 where φ1 is the refractive power of the first component, and φ2 is the refractive power of the second component.

Infrared lenses are generally required to have a large aperture ratio. This is because the infrared image sensor is different from the one for visible light, and when the aperture ratio is made smaller, the S / N ratio at the time of conversion into an electric signal in the image sensor is lowered and the image quality is significantly deteriorated. And the reason is that the critical resolution, which is determined by the aperture ratio, decreases.

Conditional expression (3) defines the relationship between the refractive powers of the first component and the second component, and the aberration is satisfactorily corrected while keeping the aperture efficiency high at a large aperture ratio with an F number of 2 or more. It shows the range for obtaining a high-performance infrared lens. Making the aperture efficiency extremely high is effective in reducing the difference in the amount of light between the central part and the peripheral part of the screen, and at the same time,
It also reduces the noise caused by infrared rays emitted from the lens frame that regulates the luminous flux. If the upper limit of conditional expression (3) is exceeded, it is difficult to secure a sufficient back focus because the refractive power of the first component with respect to the second component is too weak. If the lower limit of conditional expression (3) is exceeded, the negative Petzval sum will increase, making it difficult to satisfactorily correct the image surface.

Needless to say, good chromatic aberration can be corrected by using a high dispersion material for the negative lens in the second component.

Further, the infrared lens of the present invention is advantageous in terms of cost since the number of constituent lenses is smaller than that of the conventional one.

Specific numerical examples of the infrared lens according to the present invention will be shown below. However, in each example,
ri (i = 1,2,3 ...) is the radius of curvature of the i-th surface counted from the object side, and di (i = 1,2,3 ...) is the i-th axis counted from the object side. Shows the upper surface spacing, Ni (i = 1,2,3 ...)
3) indicates the refractive index and Abbe number of the i-th lens with respect to infrared rays having a wavelength of 4 μm counted from the object side.
Further, f is the focal length of the entire system, and FNO is the open F number.

<Example 1> ------------------------------------------- ----------------- f = 6.3 FNO. = 2.0 Curvature radius Axial upper surface spacing Refractive index Abbe number r 1 29.006 d 1 4.000 N 1 3.42290 ν 1 226.4 r 2 14.241 d 2 67.989 r 3 1029.718 d 3 3.500 N 2 3.42290 ν 2 226.4 r 4 -51.430 d 4 3.330 r 5 -25.020 d 5 3.000 N 3 4.02420 ν 3 103.2 r 6 -35.864 d 6 8.000 r 7 -30.777 d 7 3.500 N 4 3.42290 ν 4 226.4 r 8 -28.053 d 8 1.500 r 9 53.954 d 9 3.000 N 5 3.42290 ν 5 226.4 r10 60.995 d10 14.442 r11 ∞ d11 2.000 N 6 3.42290 ν 6 226.4 r12 ∞ d12 14.600 r13 ∞ d13 0.400 ν 76.42 90 r14 ∞ <Example 2> ------------------------------------------- ---------------- f = 6.3 FNO. = 2.0 Curvature radius Axial upper surface spacing Refractive index Abbe number r 1 38.358 d 1 4.000 N 1 3.42290 ν 1 226.4 r 2 21.752 d 2 74.989 r 3 -3273.108 d 3 3.500 N 2 3.42290 ν 2 226.4 r 4 -60.483 d 4 4.783 r 5 -22.970 d 5 3 .300 N 3 4.02420 ν 3 103.2 r 6 -36.965 d 6 10.000 r 7 -30.457 d 7 3.000 N 4 3.42290 ν 4 226.4 r 8 -27.764 d 8 1.500 r 9 56.533 d 9 3.000 N 5 3.42290 ν 5 226.4 r10 196.653 d10 10.462 r11 ∞ d11 2.000 N 6 3.42290 ν 6 226.4 r12 ∞ d12 14.600 r13 ∞ d13 0.400 N 7 3.42290 ν 7 226.4 r14 ∞ <Example 3> ------------------ ----------------------------------------- f = 4.5 FNO. = 2.0 Radius of curvature Axis upper surface distance Refractive index Abbe number r 1 25.500 d 1 4.000 N 1 3.42290 ν 1 226.4 r 2 18.218 d 2 4.000 r 3 36.353 d 3 3.000 N 2 3.42290 ν 2 226.4 r 4 20.475 d 4 65.000 r 5 811.912 d 5 3.500 N 3 3.42290 ν 3 226.4 r 6 -60.617 d 6 4.480 r 7 -22.782 d 7 3.300 N 4 4.02420 ν 4 103.2 r 8 -38.133 d 8 10.000 r 9 -32.007 d 9 3.500 N 5 3.42290 ν 5 226.4 r10 -27.721 d10 1.500 r11 46.715 d11 3.000 N 6 3.42290 ν 6 226.4 r12 93.861 d12 10.895 r13 ∞ d13 2.000 N 7 3.42290 ν 7 226.4 r14 ∞ 14 14.600 r15 ∞ d15 0.400 N 8 3.42290 ν 8 226.4 r16 ∞ <Example 4> ------------------------------- ---------------------------- f = 11.5 FNO. = 2.0 Curvature radius Axial surface spacing Refractive index Abbe number r 1 29.411 d 1 2.500 N 1 3.42290 ν 1 226.4 r 2 19.399 d 2 27.000 r 3 78.812 d 3 3.000 N 2 3.42290 ν 2 226.4 r 4 -1230.845 d 4 5.000 r 5 -20.278 d 5 3.500 N 3 4.02420 ν 3 103.2 r 6 -36.300 d 6 6.500 r 7 -21.683 d 7 3.000 N 4 3.42290 ν 4 226.4 r 8 -20.641 d 8 1.000 r 9 4443.453 d 9 3.000 N 5 3.42290 ν 5 226.4 r10 -112.738 d10 1.000 r11 40.089 d11 3.000 N 6 3.42290 r 53.226.4 d12 13.624 r13 ∞ d13 2.000 N 7 3.42290 ν 7 226.4 r14 ∞ d14 14.600 r15 ∞ d15 0.400 N 8 3.42290 ν 8 226.4 r16 ∞ <Example 5> ------------------ ------------------------------------------ f = 6.3 FNO. = 2.0 Curvature Radius Axis Upper surface spacing Refractive index Abbe number r 1 38.358 d 1 4.000 N 1 3.42290 ν 1 22 6.4 r 2 20.188 d 2 50.000 r 3 219.729 d 3 3.500 N 2 3.42290 ν 2 226.4 r 4 -65.254 d 4 4.783 r 5 -20.619 d 5 3.300 N 3 4.02420 ν 3 103.2 r 6 -33.998 d 6 10.000 r 7 -32.865 d 7 3.000 N 4 3.42290 ν 4 226.4 r 8 -27.064 d 8 1.500 r 9 40.452 d 9 3.000 N 5 3.42290 ν 5 226.4 r10 81.928 d10 11.836 r11 ∞ d11 2.000 N 6 3.42290 ν 6 226.4 r12 ∞ d12 8.000 r13 00 N 7 3.42290 ν 7 226.4 r14 ∞ ---------------------------------------------------------- ----------------------------- Abbe number (ν (4)) for infrared rays with a wavelength of 4 μm
Indicates the variance defined by the following equation.

Ν (4) = (n (4) -1) / (n (3) -n (5)) where n (4) is the refractive index for infrared rays having a wavelength of 4 μm, and n (3) is a wavelength of 3 μm. N (5): Refractive index for infrared rays having a wavelength of 5 μm.

The refractive index (n (4), n (3), n (5)) and Abbe of each wavelength of silicon (Si) and germanium (Ge) used as lens materials in the embodiments of the present invention.

The number (ν (4)) is

It shows in [Table 1].

[0031]

[Table 1]

FIGS. 3 to 7 are lens configuration diagrams corresponding to the first to fifth embodiments. The two flat plates provided behind the second component correspond to the window of the cold shield and the face plate of the image pickup device.

In Examples 1, 2, and 5, in order from the object side, the first component consisting of the first lens which is a negative meniscus lens convex to the object side, the positive second lens and the negative meniscus concave to the object side. It is composed of a third lens which is a lens, a fourth lens which is a positive meniscus lens concave to the object side, and a second component which is a fifth lens which is a positive meniscus lens convex to the object side.

In the third embodiment, in order from the object side, the first component is a negative meniscus lens that is convex toward the object side, the second component is positive, and the negative meniscus lens is concave toward the object side. A third lens, a fourth lens which is a positive meniscus lens concave to the object side and a positive fifth lens, and a second lens which is a sixth lens which is a positive meniscus lens convex to the object side
It is composed of ingredients.

In the fourth embodiment, in order from the object side, the first component which is a negative meniscus lens having a convex surface toward the object side and the negative second lens, the positive third lens, and the concave surface toward the object side are concave. It is composed of a fourth lens which is a negative meniscus lens, a fifth lens which is a positive meniscus lens concave to the object side, and a second component which is a sixth lens which is a positive meniscus lens convex to the object side.

8 to 12 are aberration diagrams corresponding to Examples 1 to 5. Further, the solid line represents the spherical aberration for infrared rays having a wavelength of 4 μm, and the broken line (SC) represents the sine condition. Furthermore, the broken line (DM) and the solid line (DS) represent astigmatism on the meridional surface and the sagittal surface, respectively. In addition, ω represents a half angle of view.

Conditional expression (1) in Examples 1 to 5
To the value of (3)

It shows in [Table 2].

[0038]

[Table 2]

[0039]

According to the structure of the present invention, a sufficient amount of back focus can be obtained even at a wide angle of view by the retrofocus structure composed of the negative first component and the positive second component, and mainly on the most object side. Proper definition of the object-side shape of the lens reduces the amount of ghost light that is likely to occur in retrofocus configurations. Therefore, it is possible to provide a high-performance and low-cost infrared lens having a sufficient back focus even in a wide field angle of 40 ° or more.

[Brief description of drawings]

FIG. 1 is a schematic diagram for explaining a relationship between a position of a diaphragm and an off-axis ray.

FIG. 2 is a schematic diagram for explaining reflection of ghost light on the first surface of the first lens.

FIG. 3 is a lens configuration diagram of Example 1 of the present invention.

FIG. 4 is a lens configuration diagram of a second embodiment of the present invention.

FIG. 5 is a lens configuration diagram of a third embodiment of the present invention.

FIG. 6 is a lens configuration diagram of a fourth embodiment of the present invention.

FIG. 7 is a lens configuration diagram of a fifth embodiment of the present invention.

FIG. 8 is an aberration diagram of Example 1 of the present invention.

FIG. 9 is an aberration diagram of Example 2 of the present invention.

FIG. 10 is an aberration diagram of Example 3 of the present invention.

FIG. 11 is an aberration diagram of Example 4 of the present invention.

FIG. 12 is an aberration diagram of Example 5 of the present invention.

Claims (2)

[Claims] 1. A first component having a negative refracting power and a second component having a positive refracting power, which are made of an optical material that transmits infrared rays, are arranged in order from the object side, and a cooled aperture stop is provided. An infrared lens arranged behind the second component and further satisfying the following conditions: 4.5 <d / (f · tan ω) <7.5 0.021 <f / (n1 · FNO -R1) <0.060 However, d: back focus (air conversion length), f: focal length of the whole system, ω: half angle of view, n1: refractive index of lens on the most object side, r1: most object side The radius of curvature of the object side surface of the lens is FNO: F number. 2. The infrared lens according to claim 1, wherein the refractive power of the first component and the refractive power of the second component satisfy the following condition: −1.3 <(φ1 / Φ2) tan ω <−0.25 where φ1 is the refractive power of the first component, and φ2 is the refractive power of the second component.