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DOI: 10.22184/1993-7296.FRos.2024.18.4.314.319
LEMT Scientific and Technical Center of BelOMO, Minsk, Republic of Belarus
Night vision lenses is presented: lenses with a focal length of 26 mm, 80 mm, 150 mm, catadioptric lens with 200 mm focal length and zoom lens with variable focal length of 24–80 mm. The use of low dispersion glass in lenses significantly increases their resolution. The design is optimal in terms of sensitivity of lens tolerances and can be successfully used in mass production.
LEMT Scientific and Technical Center of BelOMO, Minsk, Republic of Belarus
Night vision lenses is presented: lenses with a focal length of 26 mm, 80 mm, 150 mm, catadioptric lens with 200 mm focal length and zoom lens with variable focal length of 24–80 mm. The use of low dispersion glass in lenses significantly increases their resolution. The design is optimal in terms of sensitivity of lens tolerances and can be successfully used in mass production.
Теги: lemt scientific and technical center of belomo minsk republic of belarus зум объектив объективы ночных приборов
Night Vision Lens
I. P. Shishkin, A. P. Shkadarevich
LEMT Scientific and Technical Center of BelOMO, Minsk, Republic of Belarus
Night vision lenses is presented: lenses with a focal length of 26 mm, 80 mm, 150 mm, catadioptric lens with 200 mm focal length and zoom lens with variable focal length of 24–80 mm. The use of low dispersion glass in lenses significantly increases their resolution. The design is optimal in terms of sensitivity of lens tolerances and can be successfully used in mass production.
Keywords: NIR lens, night vision lens, zoom lens
Received on: 17.05.2024
Accepted on: 06.06.2024
Introduction
The night vision (near infrared) lenses are designed to operate in a wide spectral range (400–900 nm). Figure 1 shows the photocathode sensitivity plots of the intensifier for 2+ and 3+ generations. Ideally, the lens should be adapted for both generations.
One of the main parameters of the lens is Fnumber. The higher Fnumber, the higher the resolution can be achieved in optical design, and a high-contrast of high-power lens allows to get a more detailed image of a distant object.
All the presented lenses use low dispersion (LD) glasses, which allows you to achieve the highest image quality (resolution) with a minimum number of lens elements.
On the other hand, the design of lenses for mass production should be optimal in terms of the sensitivity of design parameters on the output performance of the lens during assembly. The most sensitive parameters are tolerances for lens thickness and air gaps, decentering and tilt, as well as refractive index and dispersion. The shape of the lenses also has a significant effect on sensitivity. For example, if the front lens has a biconvex shape, then the tolerance for the thickness of such a lens may be 2–3 times tougher than for a lens made in the shape of a meniscus.
Design
Figures 2–5 show the lens optical diagrams and MTF plots.
The 7 elements a wide-angle lens with a focal length f' = 26 mm is a well-known “Helios” type, complemented by a single lens from the image side. The STOP diaphragm is located in the air gap between the cemented lenses. A feature of the lens design is that both cemented lens can be made the same, which is very convenient for production. The field of view angle of the lens is 40 degree, and the image diameter is 18 mm.
Figure 3 shows a lens with a focal length f' = 80 mm. Its design is also optimal in terms of the number of lenses – there are only 5 elements. The lens has an even image quality within the field of view of 13 degree at an aperture of F / 1.6, and its length is 20% longer than the focal length.
Figure 4 shows a lens with a focal length f' = 150 mm. In addition to the simplicity of the design, a distinctive feature of the lens is the internal focusing at a short distance using the movement of the 2nd component. The lens is compact (length 175 mm) and thermally stabilized (thanks to the use of a special combination of glasses). The image quality of the lens practically does not change when refocusing from infinity to 25 m. The range of focus movement is ~3 mm.
When creating devices with high magnification (5–8 times), long-focus lenses are used, and this is associated with an increase in size and weight. In such cases, it is advisable to consider a catadioptric design.
Figure 5 shows an example of a catadioptric lens f' = 200 mm, the distinctive feature of which is its compactness and high resolution (160 Lp / mm). The movement of the last element provides internal focusing at a short distance.
The disadvantages of the mirror-lens design include vignetting of the central zone of the entrance pupil. The effective diameter of the entrance pupil is calculated by the formula:
Deff. = Ddesign × — ,
where Ddesign and Deff. – design and effective diameter of entrance pupil.
Zoom lens
Design of zoom lens for night vision application is not an easy task. This is due to the need to simultaneously perform a number of parameters: a focal length range of 24–80 mm, a field of view angle of 40–12° and a large aperture of f / 1.4. The extreme values of the focal lengths of the lens are traditionally used in night devices: 24 mm – in night vision goggles (pseudobinoculars) with a magnification of 1×, and 80 mm – in sights with 3‑multiple magnification. The angular field of view is determined by the 18 mm diameter of the sensitive area of the intensifier 2+ and 3+ generartion. The view of the lens for the extreme focus positions is shown in Fig. 6. The focal length is changed by moving two groups of lenses. The diameter of the front lens is 70 mm, and the lens length is 195 mm. The resolution of the lens is at least 40 Lp / mm over the entire focal length range.
The data of described lenses are shown in Table1.Table 2 shows the tolerance values for the design parameters. As you can see from the table, the tolerances are quite acceptable for mass production when special methods and equipment are not required in the assembly process.
For comparison: in a highly sensitive design (tight tolerances) of lenses, the lens centering tolerance is about 5–10 µm, the tilt is 0.3~0.5 arc minutes, the thicknesses of some lenses and air gaps must be maintained with an accuracy of ±0.01 mm, the refractive index Δnd is 0.0001~0.0002, the dispersion ΔVd is 0.1~0.2%. In such cases, a rather laborious autocollimation method is used to assemble and mounting, and a special measuring tool is used to control the thickness of the lenses.
Conclusion
The use low dispersion glasses makes it possible to significantly increase optical performance of lenses. Based on the sensitivity analysis of various optical diagrams, it is possible to select the most suitable design for mass production.
About authors
Shishkin Igor Petrovich, Candidate of Technical Sciences, RTC “LEMT” BelOMO, Minsk, Republic of Belarus.
ORCID ID: 0000-0002-4592-1060
Shkadarevich Alexey Petrovich, Doctor of Technical Sciences, RTC «LEMT» BelOMO, Minsk, Republic of Belarus.
Contribution by the members
of the team of authors
The article was prepared on the basis of many years of work by all members of the team of authors. Development and research are carried out at the expense of RTC “LEMT” BELOMO.
Conflict of interest
The authors claim that they have no conflict of interest.
I. P. Shishkin, A. P. Shkadarevich
LEMT Scientific and Technical Center of BelOMO, Minsk, Republic of Belarus
Night vision lenses is presented: lenses with a focal length of 26 mm, 80 mm, 150 mm, catadioptric lens with 200 mm focal length and zoom lens with variable focal length of 24–80 mm. The use of low dispersion glass in lenses significantly increases their resolution. The design is optimal in terms of sensitivity of lens tolerances and can be successfully used in mass production.
Keywords: NIR lens, night vision lens, zoom lens
Received on: 17.05.2024
Accepted on: 06.06.2024
Introduction
The night vision (near infrared) lenses are designed to operate in a wide spectral range (400–900 nm). Figure 1 shows the photocathode sensitivity plots of the intensifier for 2+ and 3+ generations. Ideally, the lens should be adapted for both generations.
One of the main parameters of the lens is Fnumber. The higher Fnumber, the higher the resolution can be achieved in optical design, and a high-contrast of high-power lens allows to get a more detailed image of a distant object.
All the presented lenses use low dispersion (LD) glasses, which allows you to achieve the highest image quality (resolution) with a minimum number of lens elements.
On the other hand, the design of lenses for mass production should be optimal in terms of the sensitivity of design parameters on the output performance of the lens during assembly. The most sensitive parameters are tolerances for lens thickness and air gaps, decentering and tilt, as well as refractive index and dispersion. The shape of the lenses also has a significant effect on sensitivity. For example, if the front lens has a biconvex shape, then the tolerance for the thickness of such a lens may be 2–3 times tougher than for a lens made in the shape of a meniscus.
Design
Figures 2–5 show the lens optical diagrams and MTF plots.
The 7 elements a wide-angle lens with a focal length f' = 26 mm is a well-known “Helios” type, complemented by a single lens from the image side. The STOP diaphragm is located in the air gap between the cemented lenses. A feature of the lens design is that both cemented lens can be made the same, which is very convenient for production. The field of view angle of the lens is 40 degree, and the image diameter is 18 mm.
Figure 3 shows a lens with a focal length f' = 80 mm. Its design is also optimal in terms of the number of lenses – there are only 5 elements. The lens has an even image quality within the field of view of 13 degree at an aperture of F / 1.6, and its length is 20% longer than the focal length.
Figure 4 shows a lens with a focal length f' = 150 mm. In addition to the simplicity of the design, a distinctive feature of the lens is the internal focusing at a short distance using the movement of the 2nd component. The lens is compact (length 175 mm) and thermally stabilized (thanks to the use of a special combination of glasses). The image quality of the lens practically does not change when refocusing from infinity to 25 m. The range of focus movement is ~3 mm.
When creating devices with high magnification (5–8 times), long-focus lenses are used, and this is associated with an increase in size and weight. In such cases, it is advisable to consider a catadioptric design.
Figure 5 shows an example of a catadioptric lens f' = 200 mm, the distinctive feature of which is its compactness and high resolution (160 Lp / mm). The movement of the last element provides internal focusing at a short distance.
The disadvantages of the mirror-lens design include vignetting of the central zone of the entrance pupil. The effective diameter of the entrance pupil is calculated by the formula:
Deff. = Ddesign × — ,
where Ddesign and Deff. – design and effective diameter of entrance pupil.
Zoom lens
Design of zoom lens for night vision application is not an easy task. This is due to the need to simultaneously perform a number of parameters: a focal length range of 24–80 mm, a field of view angle of 40–12° and a large aperture of f / 1.4. The extreme values of the focal lengths of the lens are traditionally used in night devices: 24 mm – in night vision goggles (pseudobinoculars) with a magnification of 1×, and 80 mm – in sights with 3‑multiple magnification. The angular field of view is determined by the 18 mm diameter of the sensitive area of the intensifier 2+ and 3+ generartion. The view of the lens for the extreme focus positions is shown in Fig. 6. The focal length is changed by moving two groups of lenses. The diameter of the front lens is 70 mm, and the lens length is 195 mm. The resolution of the lens is at least 40 Lp / mm over the entire focal length range.
The data of described lenses are shown in Table1.Table 2 shows the tolerance values for the design parameters. As you can see from the table, the tolerances are quite acceptable for mass production when special methods and equipment are not required in the assembly process.
For comparison: in a highly sensitive design (tight tolerances) of lenses, the lens centering tolerance is about 5–10 µm, the tilt is 0.3~0.5 arc minutes, the thicknesses of some lenses and air gaps must be maintained with an accuracy of ±0.01 mm, the refractive index Δnd is 0.0001~0.0002, the dispersion ΔVd is 0.1~0.2%. In such cases, a rather laborious autocollimation method is used to assemble and mounting, and a special measuring tool is used to control the thickness of the lenses.
Conclusion
The use low dispersion glasses makes it possible to significantly increase optical performance of lenses. Based on the sensitivity analysis of various optical diagrams, it is possible to select the most suitable design for mass production.
About authors
Shishkin Igor Petrovich, Candidate of Technical Sciences, RTC “LEMT” BelOMO, Minsk, Republic of Belarus.
ORCID ID: 0000-0002-4592-1060
Shkadarevich Alexey Petrovich, Doctor of Technical Sciences, RTC «LEMT» BelOMO, Minsk, Republic of Belarus.
Contribution by the members
of the team of authors
The article was prepared on the basis of many years of work by all members of the team of authors. Development and research are carried out at the expense of RTC “LEMT” BELOMO.
Conflict of interest
The authors claim that they have no conflict of interest.
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