Cassegrain infrared lens and optical system

Abstract

The application discloses an optical system, which comprises a Cassegrain infrared lens and a detector for receiving an image formed by the infrared lens. The effective focal length of the Cassegrain infrared lens is 1200 mm, the relative aperture is 4.0, the working waveband is 3.7-4.8 mm, and the infrared lens is composed of a primary mirror, a secondary mirror, a first lens, a second lens, a third lens and a fourth lens arranged in sequence along the optical axis. The detector is a refrigeration type infrared detector, and the number of pixels is 640*512, and the size of the pixel is 15 microns. By adopting the Cassegrain infrared lens, the total optical length of the optical system is reduced to 860 mm while achieving super long focus, so that the volume of the lens is reduced.

Description

Technical Field

[0001] This application belongs to the field of infrared lens technology, specifically relating to a Cassegrain infrared lens and optical system. Background Technology

[0002] Infrared detection possesses a certain ability to penetrate smoke, fog, haze, and snow, as well as the ability to identify camouflage, and is unaffected by strong light or flashes. Ultra-long telephoto infrared lenses, in particular, have wide applications in long-range surveillance, border reconnaissance, and photoelectric tracking. To meet the demands of long-range detection, the effective focal length of ultra-long telephoto infrared lenses typically reaches hundreds of millimeters or even thousands of millimeters. Existing ultra-long telephoto infrared lenses usually employ a transmissive coaxial structure, and due to limitations imposed by Gaussian optics and aberration correction laws, their total optical length is strongly positively correlated with their focal length. When the effective focal length exceeds 800mm, the total optical length easily exceeds 1000mm. A longer total optical length results in an excessively large lens size. Summary of the Invention

[0003] The technical problem to be solved by this application is that the total optical length of existing ultra-telephoto infrared lenses tends to exceed 1000mm, resulting in an excessively large lens size. To solve this technical problem, a Cassegrain infrared lens and optical system that can shorten the total optical length and meet the ultra-telephoto requirements is provided.

[0004] The technical solution proposed in this application is as follows: A Cassegrain infrared lens has an effective focal length of 1200mm and a relative aperture of 4.0. The infrared lens is composed of a primary mirror, a secondary mirror, a first lens, a second lens, a third lens, and a fourth lens arranged sequentially along the optical axis. The first lens is a meniscus negative lens with its convex surface facing the image side, the second lens is a meniscus positive lens with its convex surface facing the object side, the third lens is a meniscus negative lens with its convex surface facing the object side, and the fourth lens is a meniscus negative lens with its convex surface facing the image side. The air gap between the primary lens and the secondary lens is -550mm, the air gap between the secondary lens and the first lens is 734.68mm, the air gap between the first lens and the second lens is 33.835mm, the air gap between the second lens and the third lens is 2.112mm, and the air gap between the third lens and the fourth lens is 11.363mm.

[0005] Furthermore, the reflecting surfaces of both the primary mirror and the secondary mirror are quadratic surfaces, with the radius of curvature of the primary mirror reflecting surface being -1500mm and the quadratic surface coefficient being -1.062, and the radius of curvature of the secondary mirror reflecting surface being -560mm and the quadratic surface coefficient being -3.956.

[0006] Furthermore, the first lens has a center thickness of 10 mm, an object-side radius of curvature of -84.558 mm, and an image-side radius of curvature of -60.861 mm; the second lens has a center thickness of 9.99 mm, an object-side radius of curvature of 36.294 mm, and an image-side radius of curvature of 15.183 mm; the third lens has a center thickness of 9.768 mm, an object-side radius of curvature of 31.312 mm, and an image-side radius of curvature of 53.649 mm; and the fourth lens has a center thickness of 9.658 mm, an object-side radius of curvature of -63.521 mm, and an image-side radius of curvature of -30.922 mm.

[0007] Furthermore, the primary mirror and the secondary mirror are both made of microcrystalline glass, the first lens, the third lens and the fourth lens are all made of silicon glass, and the second lens is made of zinc sulfide glass.

[0008] Furthermore, the image-side surface of the fourth lens is aspherical and satisfies the aspherical formula: Where Z is the distance vector from the vertex of the aspherical surface at a height Y along the optical axis; R is the paraxial curvature fitting radius of the mirror; K is the conic coefficient; and A, B, C, D, and E are higher-order aspherical coefficients.

[0009] An optical system comprising the aforementioned Cassegrain infrared lens and a detector for receiving images from the infrared lens.

[0010] Furthermore, the total optical length is 860mm.

[0011] Furthermore, the detector has 640×512 pixels and a pixel size of 15μm.

[0012] Furthermore, the detector is a cooled infrared detector.

[0013] In summary, the optical system provided in this application includes a Cassegrain infrared lens and a detector. The infrared lens has a focal length of 1200mm, a relative aperture of 4.0, and an operating wavelength of 3.7~4.8μm, and is compatible with a cooled detector with a pixel count of 640×512 and a pixel size of 15μm. By employing a Cassegrain infrared lens, while achieving ultra-long focal length, the total optical length of the optical system is reduced to 860mm, thereby reducing the size of the lens. Attached Figure Description

[0014] The accompanying drawings are provided to further understand this application and form part of the specification. They are used together with the embodiments of this application to explain this application and do not constitute a limitation thereof.

[0015] Figure 1 This is a schematic diagram of the optical path structure of an optical system provided in an embodiment of this application; Figure 2 MTF diagram of a Cassegrain infrared lens provided in an embodiment of this application; Figure 3 A dot diagram of a Cassegrain infrared lens provided in an embodiment of this application; Figure 4 Field curvature distortion diagram of a Cassegrain infrared lens provided in an embodiment of this application.

[0016] Label Explanation: 11. Primary lens; 12. Secondary lens; 13. First lens; 14. Second lens; 15. Third lens; 16. Fourth lens. Detailed Implementation

[0017] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application are described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0018] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0019] This application provides an optical system including a Cassegrain infrared lens and a detector for receiving images from the infrared lens. The Cassegrain infrared lens has an effective focal length of 1200 mm, a relative aperture of 4.0, and an operating wavelength of 3.7–4.8 mm. The detector is a cooled infrared detector with 640 × 512 pixels and a pixel size of 15 μm.

[0020] like Figure 1 As shown, in one embodiment, the infrared lens is composed of a primary mirror 11, a secondary mirror 12, a first lens 13, a second lens 14, a third lens 15, and a fourth lens 16 arranged sequentially along the optical axis; the first lens 13 is a meniscus negative lens with its convex surface facing the image side, the second lens 14 is a meniscus positive lens with its convex surface facing the object side, the third lens 15 is a meniscus negative lens with its convex surface facing the object side, and the fourth lens 16 is a meniscus negative lens with its convex surface facing the image side.

[0021] Please also refer to Table 1. In one embodiment, the air gap between the primary mirror 11 and the secondary mirror 12 is -550mm (with the optical axis direction as the positive direction), the air gap between the secondary mirror 12 and the first lens 13 is 734.68mm, the air gap between the first lens 13 and the second lens 14 is 33.835mm, the air gap between the second lens 14 and the third lens 15 is 2.112mm, and the air gap between the third lens 15 and the fourth lens 16 is 11.363mm.

[0022] Furthermore, the first lens 13 has a center thickness of 10 mm, an object-side radius of curvature of -84.558 mm, and an image-side radius of curvature of -60.861 mm; the second lens 14 has a center thickness of 9.99 mm, an object-side radius of curvature of 36.294 mm, and an image-side radius of curvature of 15.183 mm; the third lens 15 has a center thickness of 9.768 mm, an object-side radius of curvature of 31.312 mm, and an image-side radius of curvature of 53.649 mm; and the fourth lens 16 has a center thickness of 9.658 mm, an object-side radius of curvature of -63.521 mm, and an image-side radius of curvature of -30.922 mm. In summary, it should be noted that in this embodiment, the total optical length of the optical system is 860 mm.

[0023] Please also refer to Table 2. Specifically, the focal length of the first lens 13 is -60.98mm and the optical power is -0.016; the focal length of the second lens 14 is 38.67mm and the optical power is 0.026; the focal length of the third lens 15 is -26.75mm and the optical power is -0.037; and the focal length of the fourth lens 16 is -16.01mm and the optical power is -0.062.

[0024] In one embodiment, the reflecting surfaces of the primary mirror 11 and the secondary mirror 12 are both quadratic surfaces, and the radius of curvature of the reflecting surface of the primary mirror 11 is -1500mm and the quadratic surface coefficient is -1.062, while the radius of curvature of the reflecting surface of the secondary mirror 12 is -560mm and the quadratic surface coefficient is -3.956.

[0025] In one embodiment, the primary lens 11 and the secondary lens 12 are both made of microcrystalline glass, the first lens 13, the third lens 15 and the fourth lens 16 are all made of silicon glass, and the second lens 14 is made of zinc sulfide glass.

[0026] Table 1 Parameters of each lens It should be noted that, with Figure 1For example, the optical axis transmission direction is from left to right, with the left side being the object side and the right side being the image side. For lenses, such as the first lens 13, surface S3 is the object side and surface S4 is the image side. The same applies to other lenses, which will not be elaborated here.

[0027] Table 2 Lens Optical Power In one embodiment, all surfaces of the first lens 13, the second lens 14, the third lens 15, and the object-side surface of the fourth lens 16 are spherical. The image-side surface of the fourth lens 16 is aspherical and satisfies the aspherical formula: Where Z is the distance vector from the vertex of the aspherical surface at height Y along the optical axis; R is the paraxial curvature fitting radius of the mirror; K is the conic coefficient; and A, B, C, D, and E are higher-order aspherical coefficients. The aspherical data are shown in Table 3.

[0028] Table 3 Aspherical Data Please see Figures 2 to 4 , Figure 2 This is the MTF chart of the infrared lens at room temperature (20℃). Figure 3 This is a dot plot of the infrared lens at room temperature. Figure 4 The image shows the field curvature distortion of the infrared lens. It can be seen from the image that at 30 lp / mm, the MTF is greater than 0.2 across the entire field of view; the dot plot size is smaller than the Airy disk, and the distortion is <2%. Therefore, the Cassegrain infrared lens provided in this application exhibits good image quality.

[0029] In summary, the optical system provided in this application includes a Cassegrain infrared lens and a detector. The infrared lens has a focal length of 1200mm, a relative aperture of 4.0, and an operating wavelength of 3.7~4.8μm, and is compatible with a cooled detector with a pixel count of 640×512 and a pixel size of 15μm. By employing a Cassegrain infrared lens, while achieving ultra-long focal length, the total optical length of the optical system is reduced to 860mm, thereby reducing the size of the lens.

[0030] Although embodiments of this application have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of this application, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A Cassegrain-type infrared lens, characterized in that, With an effective focal length of 1200mm and a relative aperture of 4.0, the infrared lens consists of a primary mirror, a secondary mirror, a first lens, a second lens, a third lens, and a fourth lens arranged sequentially along the optical axis. The first lens is a meniscus negative lens with its convex surface facing the image side, the second lens is a meniscus positive lens with its convex surface facing the object side, the third lens is a meniscus negative lens with its convex surface facing the object side, and the fourth lens is a meniscus negative lens with its convex surface facing the image side. The air gap between the primary lens and the secondary lens is -550mm, the air gap between the secondary lens and the first lens is 734.68mm, the air gap between the first lens and the second lens is 33.835mm, the air gap between the second lens and the third lens is 2.112mm, and the air gap between the third lens and the fourth lens is 11.363mm.

2. The Cassegrain infrared lens according to claim 1, characterized in that, Both the primary mirror and the secondary mirror have quadratic surfaces as their reflecting surfaces. The primary mirror has a radius of curvature of -1500 mm and a quadratic surface coefficient of -1.062, while the secondary mirror has a radius of curvature of -560 mm and a quadratic surface coefficient of -3.

956.

3. The Cassegrain infrared lens according to claim 1, characterized in that, The first lens has a center thickness of 10 mm, an object-side radius of curvature of -84.558 mm, and an image-side radius of curvature of -60.861 mm; the second lens has a center thickness of 9.99 mm, an object-side radius of curvature of 36.294 mm, and an image-side radius of curvature of 15.183 mm; the third lens has a center thickness of 9.768 mm, an object-side radius of curvature of 31.312 mm, and an image-side radius of curvature of 53.649 mm; and the fourth lens has a center thickness of 9.658 mm, an object-side radius of curvature of -63.521 mm, and an image-side radius of curvature of -30.922 mm.

4. The Cassegrain infrared lens according to claim 1, characterized in that, The primary mirror and the secondary mirror are both made of microcrystalline glass, the first lens, the third lens and the fourth lens are all made of silicon glass, and the second lens is made of zinc sulfide glass.

5. The Cassegrain infrared lens according to claim 1, characterized in that, The image-side surface of the fourth lens is aspherical and satisfies the aspherical formula: Where Z is the distance vector from the vertex of the aspherical surface at a height Y along the optical axis; R is the paraxial curvature fitting radius of the mirror; K is the conic coefficient; and A, B, C, D, and E are higher-order aspherical coefficients.

6. An optical system, characterized in that, It includes the Cassegrain infrared lens as described in any one of claims 1-5 and the detector that receives the image from the infrared lens.

7. The optical system according to claim 6, characterized in that, The total optical length is 860mm.

8. The optical system according to claim 6, characterized in that, The detector has 640×512 pixels and a pixel size of 15μm.

9. The optical system according to claim 6, characterized in that, The detector is a cooled infrared detector.

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