Lightweight objective optical system for head-mounted low-light-level night vision devices

Abstract

The present invention discloses a lightweight objective optical system for head-mounted low-light-level night vision devices. The system is composed of four lens groups along the optical axis from the direction of incident light to the image plane. The first lens group is composed of a first objective lens with positive refractive power; the second lens group is composed of a cemented lens consisting of a biconvex second objective lens with positive refractive power and a biconcave third objective lens with negative refractive power; the third lens group is composed of a single aspherical objective lens with positive refractive power or two separated lenses; and the fourth lens group is composed of a single lens with negative refractive power. At night, the low-light-level objective lens manufactured using this optical system is used to image the target and scene images onto the image plane of a low-light-level image intensifier, thereby obtaining clear nighttime images of the target and scene. The objective optical system disclosed by the present invention includes an aspherical objective lens and has the advantages of good imaging quality, simple structure, light weight, good manufacturability, etc., and the aberrations of the optical system are well corrected.

Description

Technical Field

[0001] The present invention relates to a high-performance lightweight objective optical system based on an aspherical surface design and suitable for a low-light-level night vision device, in particular to a lightweight objective optical system for a head-mounted low-light-level night vision device. Background Art

[0002] The human eye is a highly precise optical system, but its spectral sensitivity is limited, and its resolution is also limited. As light intensity decreases, the eye's ability to discern objects gradually deteriorates, ultimately rendering it incapable of discerning objects. At night, both spectral range and light intensity limit the eye's ability to discern objects. Low-light-level night vision devices overcome these limitations, enabling observation in low-light conditions.

[0003] Today, low-light-level night vision technology is a mature, high-tech technology. Low-light-level night vision devices are the most widely produced and deployed night vision equipment in developed countries. As demand for low-light-level night vision continues to increase, the need for continuous technological improvements in low-light-level devices and optical structures is increasing, enabling them to achieve higher sensitivity, resolution, longer range, and a wider field of view.

[0004] Helmet-mounted low-light-level night vision goggles are a common type of low-light-level night vision goggles. Unlike general night vision goggles, users need to wear them on their heads, so improvements must be made to reduce weight and ease the burden on users. Most traditional night vision goggles use a spherical design. To correct for various aberrations caused by the relatively large aperture, the structure is relatively complex and the number of lenses is large, resulting in a heavy weight. Improvement methods mainly include the use of optical diffraction elements, aspheric surfaces, and a hybrid design of diffraction elements and aspheric surfaces. In addition to achieving the functions of traditional optical elements, these methods can also achieve special functions such as chromatic aberration correction and athermalization, increasing the freedom of optical design and having unique advantages in improving image quality and reducing volume and weight.

[0005] Optical diffraction elements offer excellent performance, but they place high demands on their manufacturing. Errors caused by processing and assembly can significantly impact performance, making them challenging to implement in practical applications and resulting in high manufacturing costs. Thanks to advances in domestic processing and testing technologies, the manufacturing difficulty of aspheric surfaces has been reduced. Incorporating aspheric surfaces into optical design expands design freedom and allows for more efficient aberration correction. This reduces lens mass while still meeting optical performance requirements, simplifying the overall structure and ultimately reducing size and weight.

[0006] Currently, most low-light-level night vision goggles use double-Gauss lenses with 7-10 elements. For example, patent CN107991767B, "Lightweight Low-Light-Level Night Vision Optical System," uses a design with 8 elements in 4 groups. Patent CN02268250.3, "Low-Light-Level Night Vision Rifle Objective," uses 8 elements in 6 groups. To make low-light-level night vision goggles lighter and more compact, a 4-group, 5-element objective lens using aspherical technology is needed. This simplifies the structure while maintaining optical performance, improving the performance of low-light-level night vision goggles. Summary of the Invention

[0007] The technical problems to be solved by the present invention are:

[0008] The current head-mounted low-light-level night vision device objective lens adopts traditional spherical lens design, which usually adopts 7-8 double-Gaussian lenses. Due to the size limitation, there are problems such as aberrations that are difficult to correct and heavy weight. How to effectively reduce the volume and weight while ensuring image quality?

[0009] The technical solution of the present invention is:

[0010] To address the above technical issues, this invention addresses the requirements of head-mounted low-light-level night vision devices by utilizing environmentally friendly materials and aspherical components, reducing system size and weight while ensuring image quality. The objective lens is designed with a focal length of 24.74mm and a relative aperture of 1:1.2. It utilizes either four groups of five elements or four groups of six elements. The objective lens' on-axis transfer function achieves a value exceeding 0.7 at 40 lp / mm. The total optical system length is no greater than 35mm, and the optical components weigh no more than 17g.

[0011] Specifically, the technical solution of the present invention may be a technical solution using 4 groups of 5 pieces or 4 groups of 6 pieces.

[0012] (1) The technical solution of using 4 groups of 5 pieces is:

[0013] A lightweight objective optical system for a low-light-level night vision device consists of four lens groups along the optical axis from the direction of incident light to the image plane, which include: a first lens group is an objective lens with a positive refractive power using an aspheric surface; a second lens group is a cemented lens, consisting of a biconvex objective lens with a positive refractive power and a biconcave objective lens with a negative refractive power, which has the function of reducing spherical aberration and chromatic aberration; a third lens group is an objective lens with a positive refractive power using an aspheric surface; and a fourth lens group is an objective lens with a negative refractive power using an aspheric surface and has the function of reducing field curvature.

[0014] Preferably, the first lens of the objective lens is a meniscus aspheric positive lens, and the first lens of the objective lens satisfies the following formula:

[0015] 0.5<f1 / f<3.5

[0016] Where f1 is the focal length of the cemented lens; f represents the focal length of the entire objective lens group.

[0017] Preferably, the cemented lens composed of the second lens and the third lens of the objective lens satisfies the following expression:

[0018] 4<f2 / f<6

[0019] Where f2 is the focal length of the cemented lens; f represents the focal length of the entire objective lens group.

[0020] Preferably, the fourth lens of the objective lens is a biconvex aspheric positive lens, satisfying the following expression:

[0021] 0.5<f4 / f<1.5

[0022] Where f4 is the focal length of the fourth lens of the objective lens; f represents the focal length of the entire objective lens group.

[0023] Preferably, the fifth lens of the objective lens is a meniscus aspheric negative lens, and the fifth lens of the objective lens satisfies the following expression:

[0024] -1.5<f5 / f<-0.5

[0025] Where f5 is the focal length of the fifth lens of the objective lens; f represents the focal length of the entire lens group.

[0026] (2) The technical solution using 4 groups of 6 pieces is:

[0027] A lightweight objective optical system for a low-light-level night vision device consists of four lens groups along the optical axis from the direction of incident light to the image plane, which include: a first lens group is an objective lens with a positive refractive power using an aspheric surface; a second lens group is a cemented lens, consisting of a biconvex objective lens second lens with positive refractive power and a biconcave objective lens third lens with negative refractive power, which has the function of reducing spherical aberration and chromatic aberration; a third lens group includes two separate objective lens fourth lenses and objective lens fifth lenses, the objective lens fourth lens is a meniscus negative lens, and the objective lens fifth lens is a biconvex positive lens; the fourth lens group includes a meniscus negative objective lens sixth lens with a concave surface facing the object side, which has the function of reducing field curvature.

[0028] Preferably, the first lens of the objective lens satisfies the following expression:

[0029] 0.5<f1 / f<1.5

[0030] Where f1 is the focal length of the first lens of the objective lens; f represents the focal length of the entire lens group.

[0031] Preferably, the cemented lens composed of the second lens and the third lens of the objective lens satisfies the following expression:

[0032] -2<f2 / f<-0.5

[0033] Where f2 is the focal length of the cemented lens; f represents the focal length of the entire objective lens group.

[0034] Preferably, the third lens group consisting of the fourth lens and the fifth lens of the objective lens is composed of a meniscus lens and a biconvex positive lens, and satisfies the following expression:

[0035] 0.5<f4 / f<1.5

[0036] Where f4 is the focal length of the third lens group; f represents the focal length of the entire objective lens group.

[0037] Preferably, the sixth lens of the objective lens satisfies the following expression:

[0038] -1.5<f6 / f<-0.5

[0039] Where f6 is the focal length of the sixth lens of the objective lens; f represents the focal length of the entire lens group.

[0040] Preferably, no matter whether the technical solution of 4 groups of 5 lenses or 5 groups of 6 lenses is adopted, the spectral response band of the objective lens ranges from 550nm to 900nm; and the four groups of lenses are all made of lightweight and environmentally friendly materials.

[0041] The beneficial effects of the present invention are:

[0042] The objective lens of the present invention is designed to have a field of view of 40° (circular field of view or diagonal line), which can meet the image quality and field of view requirements of head-mounted night vision devices, reduce the system weight and size, and the objective lens optical system can be effectively used in head-mounted low-light-level night vision devices using image intensifiers. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Schematic diagram of the optical system composition of Example 1.

[0044] Figure 2 From left to right are the spherical aberration, field curvature and distortion diagrams of Example 1.

[0045] Figure 3 Schematic diagram of the optical system composition of Example 2.

[0046] Figure 4 From left to right are the spherical aberration, field curvature and distortion diagrams of Example 2.

[0047] In the figure: S1, S2...S13 are the 1st, 2nd...13th surfaces; Figure 1In the diagram, L1, L2, ..., L5 are the first lens, the second lens, ..., the fifth lens of the objective lens, and L6 is the photocathode protection window of the image intensifier; Figure 3 In the figure, L1, L2...L6 are the first lens, the second lens...the sixth lens of the objective lens, and L7 is the photocathode protection window of the image intensifier. DETAILED DESCRIPTION

[0048] In order to make the purpose, content, and advantages of the present invention more clear, the specific implementation methods of the present invention are further described in detail below with reference to the accompanying drawings and examples.

[0049] Low-light-level night vision devices are primarily used at night, and their primary operating environment is nighttime. Therefore, when designing the objective optical system, it is necessary to consider the device's light collection capabilities, nighttime skylight, and the device's spectral characteristics. This embodiment of the present invention is designed for an image intensifier with a φ18mm cathode surface. The designed objective lens has a focal length of 24.74mm, a field of view of 40°, a relative aperture of 1:1.2, a designed spectral response band of 550nm to 900nm, and a total lens weight of no more than 17g.

[0050] Example 1:

[0051] like Figure 1 As shown, the objective optical system is composed of four lens groups along the optical axis from the direction of incident light to the image plane, including: the first lens group is the first objective lens with positive refractive power, the second lens group is the objective lens cemented lens (the second objective lens and the third objective lens) with positive refractive power, the third lens group is the fourth objective lens with positive refractive power, and the fourth lens group is the fifth objective lens with negative refractive power.

[0052] Among them, the refractive power of the first lens of the objective lens is positive, and it is a meniscus positive lens. The S2 surface is aspherical, which can effectively shrink the light and reduce the size of the objective lens.

[0053] The first lens of the objective lens satisfies the following expression:

[0054] 1.5<f1 / f<3.5

[0055] Where f1 is the focal length of the objective doublet; f represents the focal length of the entire lens group.

[0056] The second group of lenses is the objective lens cemented lens, which consists of a double convex positive lens, namely the second lens of the objective lens, and a double concave negative lens, namely the third lens of the objective lens. It is designed with materials with different dispersion coefficients to effectively reduce chromatic aberration and spherical aberration.

[0057] The objective doublet lens satisfies the following expression:

[0058] 4<f2 / f<6

[0059] Where f2 is the focal length of the objective doublet; f represents the focal length of the entire lens group.

[0060] The third lens group, i.e. the fourth lens of the objective lens, is a biconvex positive lens with an aspherical S7 surface.

[0061] The fourth lens of the objective lens satisfies the following expression:

[0062] 0.5<f4 / f<1.5

[0063] Where f4 is the focal length of the fourth lens of the objective lens; f represents the focal length of the entire lens group.

[0064] The fourth lens group, i.e. the fifth lens of the objective lens, is a meniscus-shaped negative lens with an aspherical S8 surface. This lens is close to the image plane and acts as a field lens to reduce field curvature.

[0065] The fifth lens of the objective lens satisfies the following expression:

[0066] -1.5<f5 / f<-0.5

[0067] Where f5 is the focal length of the fifth lens of the objective lens; f represents the focal length of the entire lens group.

[0068] To reduce weight, all materials are made of lightweight, environmentally friendly glass. The total weight of the objective lens and optical components is no more than 17g.

[0069] The relative illumination of the edge field corresponding to Example 1 is 43%.

[0070] The total optical length of the objective optical system described in Example 1 is 34.5 mm.

[0071] Figure 1 L6 shown is the image intensifier cathode flat plate protection window with a thickness of 5.6 mm.

[0072] The design data of Example 1 are shown in Table 1 and Table 2.

[0073] Figure 2 This is the image quality evaluation analysis chart of Example 1.

[0074] Table 1 Design data table of embodiment 1

[0075]

[0076]

[0077] The surface marked with * in the remarks is an aspherical surface, and the aspherical surface equation is:

[0078]

[0079] In formula (1), Z is the arc height in the direction of the optical axis, with the vertex of the sphere as the coordinate origin; ρ is the vertex curvature, ρ = 1 / r, r is the radius; k is a quadratic constant; y is the distance between the coordinate point and the vertex of the lens sphere; A, B, C, and D are the deformation coefficients of the corresponding order.

[0080] The aspheric surface data of Example 1 are shown in Table 2.

[0081] Table 2 Aspheric surface data table

[0082]

[0083] The optical transfer function (MTF) of Example 1 is shown in Table 3.

[0084] Table 3 Optical Transfer Function (MTF)

[0085]

[0086] Example 2:

[0087] like Figure 3 As shown, the objective optical system is composed of four lens groups in sequence along the optical axis from the direction of incident light to the image plane, including: the first lens group is composed of an objective first lens with positive refractive power, the second lens group is composed of an objective second lens and an objective third lens with negative refractive power, the third lens group is composed of two separated lenses, an objective fourth lens and an objective fifth lens, and the fourth lens group is composed of an objective sixth lens with negative refractive power.

[0088] Among them, the refractive power of the first lens of the objective lens is positive, and it is a meniscus positive lens. The S1 surface is aspherical, which can effectively shrink the light, reduce the size of the objective lens, improve the aberration, and increase the relative illumination of the edge field of view.

[0089] The first lens of the objective lens satisfies the following expression

[0090] 0.5<f1 / f<1.5

[0091] Where f1 is the focal length of the first lens of the objective lens; f represents the focal length of the entire lens group.

[0092] The second group of lenses is the objective lens cemented lens, which consists of a double convex positive lens, namely the second lens of the objective lens, and a double concave negative lens, namely the third lens of the objective lens. It is designed with materials with different dispersion coefficients to effectively reduce chromatic aberration and spherical aberration.

[0093] The objective doublet lens satisfies the following expression:

[0094] -2<f2 / f<-0.5

[0095] Where f2 is the focal length of the objective doublet; f represents the focal length of the entire objective lens group.

[0096] The third lens group consists of two separated lenses, the fourth lens of the objective lens is a meniscus negative lens, and the fifth lens of the objective lens is a biconvex positive lens. The third lens group satisfies the following expression:

[0097] 0.5<f4 / f<1.5

[0098] Where f4 is the focal length of the third lens group; f represents the focal length of the entire objective lens group.

[0099] The fourth lens group, namely the sixth lens of the objective lens, is a meniscus-shaped negative lens with the concave surface facing the object, which has the effect of reducing field curvature.

[0100] The sixth lens of the objective lens satisfies the following expression:

[0101] -1.5<f6 / f<-0.5

[0102] Where f6 is the focal length of the sixth lens of the objective lens; f represents the focal length of the entire lens group.

[0103] To reduce weight, all materials are made of lightweight, environmentally friendly glass. The total weight of the objective lens optical components is no more than 17g. The relative illumination of the edge field of view corresponding to Example 2 is ≥58%.

[0104] The total optical length of the objective optical system described in Example 2 is 34 mm.

[0105] Figure 3 L7 shown is the image intensifier cathode flat plate protection window with a thickness of 5.6 mm.

[0106] The data of Example 2 are shown in Table 4.

[0107] Figure 4 This is the image quality evaluation analysis chart of Example 2.

[0108] Table 4 Example 2 data sheet

[0109]

[0110] The ones marked with * in the remarks are aspherical surfaces. The aspherical surface formula is the same as formula (1). The data table is shown in Table 5.

[0111] Table 5 Aspheric surface data table

[0112]

[0113] The optical transfer function (MTF) of Example 2 is shown in Table 6.

[0114] Table 6 Optical Transfer Function (MTF)

[0115]

[0116] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A lightweight objective optical system for low-light-level night vision devices, characterized in that: Along the optical axis from the direction of incident light to the image plane, it consists of four groups of lenses, including: The first lens group is an objective lens with positive refractive power using an aspherical surface; The second lens group is a cemented lens, which is composed of a biconvex positive refractive power objective lens second lens and a biconcave negative refractive power objective lens third lens; the cemented lens satisfies 4<f2 / f<6, where f2 is the focal length of the cemented lens and f represents the focal length of the entire objective lens group; The third lens group is an aspherical fourth lens with positive refractive power; The fourth lens group is an objective lens, the fifth lens, which is a negative meniscus lens with its concave side facing the object. The spectral response band of the four groups of lenses is in the range of 550nm to 900nm, and the four groups of lenses are all made of glass material; The aperture of the optical system is located after the cemented lens, the focal length of the objective lens is 24.74 mm, the field of view is 40°, and the relative aperture is 1:1.

2.

2. A lightweight objective optical system for low-light-level night vision devices, characterized in that: Along the optical axis from the direction of incident light to the image plane, it consists of four groups of lenses, including: The first lens group is an objective lens with positive refractive power using an aspherical surface; The second lens group is a cemented lens, which is composed of a biconvex positive refractive power objective lens second lens and a biconcave negative refractive power objective lens third lens; the cemented lens satisfies -2<f2 / f<-0.5, where f2 is the focal length of the cemented lens and f represents the focal length of the entire objective lens group; The third lens group includes two separate objective lens fourth lens and objective lens fifth lens, the objective lens fourth lens is a meniscus negative lens, and the objective lens fifth lens is a biconvex positive lens; The fourth lens group includes an objective sixth lens which is a meniscus-shaped negative lens with its concave surface facing the object. The spectral response band of the four groups of lenses is in the range of 550nm to 900nm, and the four groups of lenses are all made of glass material; The aperture of the optical system is located after the cemented lens, the focal length of the objective lens is 24.74 mm, the field of view is 40°, and the relative aperture is 1:1.

2.

3. The objective optical system according to claim 1, wherein: The first lens of the objective lens is a meniscus aspheric positive lens, and the first lens of the objective lens satisfies the following formula: 0.5<f1 / f<3.5 Where f1 is the focal length of the doublet lens.

4. The objective optical system according to claim 1, wherein: The fourth lens of the objective lens is a biconvex aspheric positive lens that satisfies the following expression: 0.5<f4 / f<1.5 Where f4 is the focal length of the fourth lens of the objective lens.

5. The objective optical system according to claim 1, wherein: The fifth lens of the objective lens is a meniscus aspheric negative lens, and the fifth lens of the objective lens satisfies the following expression: -1.5<f5 / f<-0.5 Where f5 is the focal length of the fifth lens of the objective lens.

6. The objective optical system according to claim 2, wherein: The first lens of the objective lens satisfies the following expression: 0.5<f1 / f<1.5 Where f1 is the focal length of the first lens of the objective lens.

7. The objective optical system according to claim 2, wherein: The third lens group consisting of the fourth lens and the fifth lens of the objective lens is composed of a meniscus lens and a biconvex positive lens, and satisfies the following expression: 0.5<f4 / f<1.5 Where, f ４ is the focal length of the third lens group.

8. The objective optical system according to claim 2, wherein: The sixth lens of the objective lens satisfies the following expression: -1.5<f6 / f<-0.5 Where f6 is the focal length of the sixth lens of the objective lens.

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