Refraction-reflection type medium wave infrared lens

Abstract

The utility model provides a catadioptric medium-wave infrared lens, and belongs to the field of optical infrared imaging. Comprising a lens barrel and an optical system. The optical system comprises a plane mirror, a field diaphragm, an aspheric biconvex lens, a biconcave lens, an aspheric negative meniscus lens, a positive meniscus lens, a positive meniscus lens with a diffraction surface, a detector window, a cold diaphragm, a light transmitting sheet and a target surface which are coaxially and sequentially arranged in a lens barrel; the main reflection even-order aspherical mirrors are located on the two sides of the junction of the biconcave lens and the aspherical negative meniscus lens; and a part of the inner diameter of the main reflection even-order aspherical mirror is coated outside the aspherical negative meniscus lens. According to the utility model, the number of the aspherical mirrors of the catadioptric structure is small, the cost is low, and the stray light suppression capability of the optical system is greatly improved by the pure reflective optical elements of the front group and the field diaphragm added on the middle image plane.

Description

Technical Field

[0001] The utility model relates to the field of optical infrared imaging, in particular to a catadioptric medium-wave infrared lens. Background Art

[0002] Infrared optical system is a kind of passive detection optical system with obvious functions. Such system can detect, locate and continuously track objects and targets that emit infrared rays under infrared background radiation and other interference. Therefore, it has broad application prospects in target search, early warning detection, forest fire prevention and other fields.

[0003] Chinese Patent No. 202010739554.9, Publication No. CN111897107B, discloses "A Medium-wave Infrared Athermal Lens", which includes a lens barrel and an optical system arranged in the lens barrel, and the lens barrel is made of aluminum; the first lens is a meniscus Si lens with positive focal length bent toward the image side; the second lens is a meniscus Ge lens with negative focal length bent toward the object side; the third lens is a meniscus ZnS lens with positive focal length bent toward the object side; the fourth lens is a meniscus IG6 lens with positive focal length bent toward the image side. The design uses 5 aspherical mirrors in the entire system, which makes the cost too high, and the aperture of the design system for one-time imaging is also large, which does not meet the requirements of miniaturization; at the same time, the multiple aspherical surfaces in the design have only one symmetry axis, which is symmetrical relative to the spherical axis of the spherical mirror, and its processing, detection and assembly are relatively difficult. The above limitations make the optical system processing and detection more difficult, resulting in a high cost for the entire system. Utility Model Content

[0004] In order to overcome the shortcomings of the prior art and reduce costs as much as possible, the utility model provides a folding reflective medium-wave infrared lens, which uses only 7 lenses with smaller apertures, solves the problem of 100% cold aperture efficiency and has better imaging quality within the temperature range of -40° to 60°.

[0005] In order to achieve the above purpose, the technical solution provided by the utility model is:

[0006] A catadioptric medium-wave infrared lens comprises a lens barrel and an optical system; the optical system comprises a plane reflector 2, a field stop, an aspheric biconvex lens 3, a biconcave lens 4, an aspheric negative meniscus lens 5, a positive meniscus lens 6, a diffraction positive meniscus lens 7, a detector window 8, a cold stop 9, a light-transmitting sheet and a target surface 10, which are coaxially arranged in sequence in the lens barrel, and a main reflection even-order aspheric mirror 1 located on both sides of the junction of the biconcave lens 4 and the aspheric negative meniscus lens 5; a part of the inner diameter of the main reflection even-order aspheric mirror 1 is covered outside the aspheric negative meniscus lens 5.

[0007] Furthermore, the materials of the aspherical biconvex lens 3 and the aspherical negative meniscus lens 5 are both chalcogenide materials; the material of the biconcave lens 4 is MgF2 The material of the meniscus positive lens 6 is chalcogenide material; the material of the meniscus positive lens 7 on the diffraction surface is ZnSe crystal. The material of the detector window 8 is protective glass.

[0008] Parallel light from an infinitely distant object plane is incident on a primary reflective even-order aspheric mirror (1), forms a convergent light beam after reflection, and is converted into divergent light through a plane reflector (2), enters an aspheric double convex lens (3) and a double concave lens (4), and then converges through an aspheric negative meniscus lens (5) and a positive meniscus lens (6), as well as a positive meniscus lens (7), enters a detector window (8), passes through a cold aperture (9), and finally forms an image on a detector target surface (10), completing the entire imaging process.

[0009] Furthermore, various optical lenses are connected to the lens barrel through a lens frame, and the lens frame and the lens barrel are made of Invar material.

[0010] Furthermore, the material of the primary reflection even-order aspherical mirror (1) is aluminum alloy.

[0011] Furthermore, the outer circumference of the aspherical meniscus negative lens (5) has a step surface that is matched and connected to the inner aperture of the main reflection even-order aspherical mirror (1).

[0012] Furthermore, both the biconcave lens (4) and the meniscus positive lens (6) are pure spherical surfaces.

[0013] Compared with the prior art, the advantages of the utility model are:

[0014] (1) The outer circle of the aspheric meniscus negative lens described in the present invention has a step surface that matches the inner aperture of the main reflection even-order aspheric mirror. Preferably, the expansion coefficients of the materials used for the two are relatively close. This matching method can improve the installation accuracy of the lens.

[0015] (2) The utility model sets a field stop at the intermediate image plane, and the front group adopts a pure reflective optical element, which greatly avoids the ghost image effect that cannot be eliminated by the front group using a transmissive element, and improves the stray light suppression capability of the optical system.

[0016] (3) The utility model adopts a secondary imaging structure, which makes the optical element small in diameter, with a small number of aspheric surfaces, better imaging quality, simple structure and high stability. Preferably, the largest optical element uses an aluminum reflector, avoiding the use of large-sized infrared lenses, which can significantly reduce the cost of optical elements. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a light path structure diagram of the utility model's catadioptric medium-wave infrared lens (the lens barrel is not shown);

[0018] Figure 2This is the MTF curve of the catadioptric medium-wave infrared lens of the utility model when the spatial frequency is 42lp / mm and the temperature is +20°C;

[0019] Figure 3 This is the MTF curve of the catadioptric medium-wave infrared lens of the utility model when the spatial frequency is 42lp / mm and the temperature is -40°C;

[0020] Figure 4 This is the MTF curve of the catadioptric medium-wave infrared lens of the utility model when the spatial frequency is 42lp / mm and the temperature is +60°C;

[0021] Figure 5 This is a distortion curve diagram of the folding medium-wave infrared lens of the utility model at +20°C;

[0022] Figure 6 This is a distortion curve diagram of the folding medium-wave infrared lens of the utility model at -40°C;

[0023] Figure 7 This is a distortion curve diagram of the utility model's catadioptric medium-wave infrared lens at +60°C;

[0024] Figure 8 It is the PST diagram of the folding reflective medium wave infrared lens of the utility model;

[0025] The reference numerals are as follows:

[0026] 1-primary reflector even-order aspheric mirror; 2-plane reflector; 3-aspheric biconvex lens; 4-biconcave lens; 5-aspheric negative meniscus lens; 6-meniscus positive lens; 7-diffraction surface meniscus positive lens; 8-detector window; 9-cold aperture; 10-target surface. DETAILED DESCRIPTION

[0027] The content of the utility model is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0028] like Figure 1 As shown, a folding reflective medium-wave infrared lens based on passive athermalization technology includes a lens barrel and an optical system arranged in the lens barrel, the optical system is a 165.89mm / F1.65 cooled medium-wave infrared athermalized optical system, with a full field of view of 6°, a total length of 180mm, an operating band of 3μm to 5μm, and a secondary imaging structure and a seven-lens composition design to achieve an athermal design of -40℃ to +60℃ and 100% cold aperture efficiency.

[0029] The optical system includes a plane reflector 2, a field stop, an aspheric biconvex lens 3, a biconcave lens 4, an aspheric negative meniscus lens 5, a positive meniscus lens 6, a diffraction positive meniscus lens 7, a detector window 8, a cold stop 9 and a target surface 10, which are coaxially arranged in sequence in the lens barrel, and a main reflection even-order aspheric mirror 1 located on both sides of the junction of the biconcave lens 4 and the negative meniscus lens 5; part of the inner diameter of the main reflection even-order aspheric mirror 1 is covered outside the negative meniscus lens 5.

[0030] The aspheric biconvex lens 3 and the aspheric negative meniscus lens 5 are made of chalcogenide materials; the biconcave lens 4 is made of MgF 2 Crystal; the material of the meniscus positive lens 6 is chalcogenide material; the material of the meniscus positive lens 7 on the diffraction surface is ZnSe crystal. The material of the detector window 8 is protective glass; the lens material is Invar material. In this example, the aspheric biconvex lens 3 adopts chalcogenide glass of model IRG205, the biconcave lens 4 adopts MgF2 crystal of model MGF2_HP, the aspheric meniscus negative lens 5 adopts chalcogenide glass of model IRG209, the meniscus positive lens 6 adopts chalcogenide glass of model ZNS_BROAD, and the meniscus positive lens 7 on the diffraction surface adopts ZnSe crystal of model ZNSE. Parallel light is incident on the primary reflective even-order aspheric mirror 1, and after reflection, it forms a converging light beam, and becomes divergent light through the plane reflector 2, and enters the aspheric double convex lens 3 and double concave lens 4, and then converges through the aspheric negative meniscus lens 5 and positive meniscus lens 6, as well as the diffraction surface positive meniscus lens 7, enters the detector window 8, and passes through the cold aperture 9, and finally forms an image on the detector target surface 10.

[0031] The specific parameters of each lens of the optical system of this embodiment are shown in Table 1;

[0032] Table 1 Specific parameters of each lens in the optical system of this embodiment

[0033]

[0034] At the same time, the even-order aspheric mirror formula satisfies:

[0035]

[0036] Where r represents the vector height of each point on the lens surface; c is the inverse of the radius of curvature of the lens surface; k is the cone coefficient; α 1 , α 2 , α 3 ...are high-order aspheric coefficients, n is a natural number that can be infinite; Z is the distance from the aspheric surface vertex when the aspheric surface is at a height of r along the optical axis.

[0037] The aspheric coefficients of each mirror surface are shown in Table 2.

[0038] Table 2 Aspheric coefficients of each mirror surface of the optical system of this embodiment

[0039]

[0040]

[0041] In addition, the two-variable face polynomial expansion is:

[0042]

[0043] Where N is the number of polynomial coefficients in the series. i is the coefficient of ρ raised to the 2ith power, ρ is the normalized radial aperture coordinate. M is the diffraction order.

[0044] The diffraction surface of the meniscus positive lens 7, where M = 1, A 1 =-243.758.

[0045] The lens of this embodiment has a focal length of 165.89 mm and an F number of 1.65, and is suitable for medium-wave infrared thermal imagers with a high resolution of 1024×1024 and a pixel size of 12um×12um. By matching the materials of the optical elements (lenses) of the optical system with the optical focal length, an athermal design of the optical system can be achieved.

[0046] like Figures 2 to 4 As shown in FIG. 1 , the MTF curve values ​​of the optical system of this embodiment when the spatial frequency is 42lp / mm at +20°C, -40°C, and +60°C, from which it can be seen that the optical system has good imaging quality in the entire operating temperature range; Figures 5 to 7 As shown in FIG. 1 , the optical system distortion curves of the optical system of this embodiment at +20°C, -40°C, and +60°C are shown. It can be seen that the distortion of the entire field of view is small within the entire operating temperature range, and can meet the infrared target measurement accuracy requirements. Figure 8 As shown in the figure, thanks to the field stop set at the middle image plane and the pure reflective optics used in the front group, the ghost image effect that cannot be eliminated by the transmission element used in the front group is greatly avoided, and the stray light suppression ability of the optical system is greatly improved. There is basically no obvious stray light outside 6°, and the PST is also sufficiently small within 6° to ensure good image quality.

[0047] In this embodiment, the aperture of all optical elements is less than 55mm, the number of aspherical surfaces is small, the imaging quality is good, the structure is simple, and the stability is high. At the same time, the largest optical element uses an aluminum reflector, avoiding the use of large-sized infrared lenses, greatly reducing the cost of optical elements.

[0048] The above is only a description of the preferred implementation mode of the utility model, and the technical solution of the utility model is not limited thereto. Any known deformation made by technical personnel in this field on the basis of the main technical concept of the utility model belongs to the technical scope to be protected by the utility model.

Claims

1. A catadioptric medium-wave infrared lens, characterized in that: The invention comprises a lens barrel and an optical system; the optical system comprises a plane reflector (2), a field stop, an aspheric biconvex lens (3), a biconcave lens (4), an aspheric negative meniscus lens (5), a positive meniscus lens (6), a diffraction positive meniscus lens (7), a detector window (8), a cold stop (9), a light-transmitting sheet and a target surface (10), and a main reflection even-order aspheric mirror (1) located on both sides of the junction of the biconcave lens (4) and the aspheric negative meniscus lens (5); a part of the inner diameter of the main reflection even-order aspheric mirror (1) is covered outside the aspheric negative meniscus lens (5).

2. The catadioptric medium-wave infrared lens according to claim 1, characterized in that: The materials of the non-spherical biconvex lens (3) and the non-spherical meniscus negative lens (5) are both chalcogenide materials; the material of the biconcave lens (4) is MgF2 crystal; the material of the meniscus positive lens (6) is chalcogenide material; the material of the diffraction surface meniscus positive lens (7) is ZnSe crystal; and the material of the detector window (8) is protective glass.

3. The catadioptric medium-wave infrared lens according to claim 1, characterized in that: Various optical lenses are connected to the lens barrel through a lens frame, and the material of the lens frame and the lens barrel is Invar.

4. The catadioptric medium-wave infrared lens according to claim 1, characterized in that: The material of the primary reflection even-order aspherical mirror (1) is aluminum alloy.

5. The catadioptric medium-wave infrared lens according to claim 1, characterized in that: The outer circle of the aspherical meniscus negative lens (5) has a step surface which is matched and connected with the inner aperture of the main reflection even-order aspherical mirror (1).

6. The catadioptric medium-wave infrared lens according to claim 1, characterized in that: Both the double concave lens (4) and the meniscus positive lens (6) are pure spherical surfaces.

7. The catadioptric medium-wave infrared lens according to claim 1, characterized in that: The optical system is a 165.89mm / F1.65 refrigerated medium-wave infrared athermal optical system with a full field of view of 6°, a total length of 180mm, and an operating band of 3μm to 5μm; The even-order aspheric surface radius of the primary reflection even-order aspheric mirror (1) is -172.797 mm; The standard surface radius of the plane reflector (2) is infinite; The radius of the even-order aspheric surface of the aspheric biconvex lens (3) is 174.034 mm, the radius of the standard surface of the aspheric biconvex lens (3) is -296.830 mm, and the thickness of the aspheric biconvex lens (3) is 6.176 mm; The radius of one standard surface of the biconcave lens (4) is -210.271 mm, the radius of the other standard surface is 104.756 mm, the thickness of the biconcave lens (4) is 4.191 mm, the refractive index is 1.3835, and the Abbe constant is 54.706; The radius of the even aspheric surface of the aspheric meniscus negative lens (5) is -44.399 mm, the radius of the standard surface is -61.702 mm, and the thickness of the aspheric meniscus negative lens (5) is 11.244 mm; The radius of one standard surface of the meniscus positive lens (6) is -117.562 mm, the radius of the other standard surface is -39.273 mm, the thickness of the meniscus positive lens (6) is 11.462 mm, the refractive index is 2.3672, and the Abbe constant is 15.305; The radius of the binary surface of the diffractive surface meniscus positive lens (7) is 50.507 mm, the radius of the standard surface is 79.361 mm, and the thickness of the diffractive surface meniscus positive lens (7) is 8.187 mm; The thickness of the detector window (8) is 82.500 mm; The thickness of the filter is 0.800mm; The cone coefficients k of the front surface of the primary reflective even-order aspheric mirror (1), the front surface of the aspheric biconvex lens (3), and the front surface of the aspheric negative meniscus lens (5) are -1.713, 0, and 0, respectively; the aspheric coefficients α2 are 0, 2.727×10 -6 , -8.793×10 -6 ; The aspheric coefficient α3 is 1.788×10 -12 , -1.921×10 -8 , 1.146×10 -9 ; The aspheric coefficient α4 is 1.964×10 -16 , -8.276×10 -11 , -1.602×10 -13 ; The aspheric coefficient α5 is 1.041×10 -20 , 1.700×10 -12 , -2.783×10 -14 .

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