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DOI: 10.22184/1993-7296.FRos.2020.14.4.360.367
A description is given of the design of achromatized lenses operating in the infrared region of the spectrum (8–12 μm). When designing thermal imaging lenses, it is necessary to take into account the fact that the pixel size of modern microbolometers is 10–12 μm, and the pixel density has reached 1280 × 1024 format. These parameters determine higher requirements for image quality, resolution and field of view of the developed lens. In this case, the lens should be fast, thermally stabilized and have focus. The following lenses are considered: 4-lens with a focus of 50 mm, 3-lens with a focus of 150 mm and 7-lens with a variable focus of 30–150 mm.
A description is given of the design of achromatized lenses operating in the infrared region of the spectrum (8–12 μm). When designing thermal imaging lenses, it is necessary to take into account the fact that the pixel size of modern microbolometers is 10–12 μm, and the pixel density has reached 1280 × 1024 format. These parameters determine higher requirements for image quality, resolution and field of view of the developed lens. In this case, the lens should be fast, thermally stabilized and have focus. The following lenses are considered: 4-lens with a focus of 50 mm, 3-lens with a focus of 150 mm and 7-lens with a variable focus of 30–150 mm.
Теги: achromatized lens focusing thermal stabilization zoom ахроматизированный объектив зум термостабилизация фокусировка
Achromatized Lenses of Thermal Imagers
I. P. Shishkin, A. P. Shkadarevich
RTC “LEMT” BelOMO, Minsk, Republic of Belarus
A description is given of the design of achromatized lenses operating in the infrared region of the spectrum (8–12 μm). When designing thermal imaging lenses, it is necessary to take into account the fact that the pixel size of modern microbolometers is 10–12 μm, and the pixel density has reached 1280 × 1024 format. These parameters determine higher requirements for image quality, resolution and field of view of the developed lens. In this case, the lens should be fast, thermally stabilized and have focus. The following lenses are considered: 4-lens with a focus of 50 mm, 3-lens with a focus of 150 mm and 7-lens with a variable focus of 30–150 mm.
Keywords: achromatized lens, thermal stabilization, focusing, zoom
Received: 22.05.2020
Accepted: 16.06.2020
INTRODUCTION
The list of available optical materials that are transparent in the spectral range of 8–12 μm is very limited. The most common material is germanium. It has a high refractive index and low toxicity, which is important in the manufacture and operation of the lens. Almost all optical materials used in thermal imaging devices have a strong temperature dependence of the refractive index. Therefore, when developing a lens, it is necessary to provide for the possibility of thermal compensation in its design. Usually for this purpose, compensation is used in the back focus of the lens, moving the lens or microbolometric matrix with temperature fluctuations, which greatly complicates the mechanics of the lens.
Most modern fixed-focus lenses for thermal imaging cameras consist of 3–4 single lenses. This is primarily due to the need to combine the maximum achievable aperture and budget cost. At the same time, with a limited choice of glass brands, it is impossible to significantly improve the image quality of the lens by increasing the number of lenses or the length of the lens. Therefore, many developers [1, 2] actively use aspherical lenses or diffractive elements in IR lenses. With this approach, you can reduce the number of lenses, but to achieve a qualitative improvement in the resolution of the lens fails. In addition, the manufacture of large diameter aspherical menisci for lenses with a 1:1 relative aperture is a rather laborious process due to the high demands on the quality of manufacture of optical surfaces. Their creation often reveals the inappropriateness, in terms of maintaining a balance between image quality and the cost of the lens.
An alternative method to achieve the theoretical limit of resolution in the lens can be its achromatization [3]. This method consists in selecting a combination of glasses having different dispersion coefficients. The right choice makes it possible to compensate for chromatic aberration and thereby significantly improve the image quality of the lens.
FOCAL DISTANCE AND RELATIVE HOLE
It is known that the detection range of an object depends on the focal length of the lens and its aperture. The market offers a wide selection of thermal imaging lenses in the range of focal lengths of 30 ~ 300 mm and relative apertures F / 1 ~ F / 1.5. But most of them are designed for 640 × 480 format with a pixel size of 17–25 microns. On the other hand, an increase in the focal length of a fast lens leads to a significant increase in its size and weight. Table 1 shows the dependence of the weight of the lens on the focal length, aperture, number of lenses and length. But it must be borne in mind that it is the technological capabilities of manufacturing lenses of maximum diameter with the required accuracy that determine the image quality of the created lenses.
LENS DESIGN
Figure 1 shows the type and characteristics of the lenses. On the left is a 4-lens lens with a focal length of 50 mm and a field of view of 22°, on the right is a 3-lens lens with a focal length of 150 mm and a field of view of 7.5°. Both lenses are designed for 1280x1024 format with a diagonal of 20 mm. The lenses are thermally stabilized and have the function of focusing at close range.
In a 4-lens lens, thermal stabilization is passive. It is provided by a special combination of glasses: the outer lenses are made of germanium, and the inner ones are made of a material with a low refractive index. All lenses are spherical. The result of passive stabilization is illustrated by graphs of the optical transfer function. A certain combination of lenses ensures a constant position of the image plane when the operating temperature changes from –50 °C to 50 °C. Moving the last lens allows for internal focusing at close range.
In the 3-lens lens, the thermal stabilization function is combined with the internal focusing function. and performed. Depending on the chosen constructive solution, these functions are performed by the movements of the 2nd or 3rd lens. In the lens, the outer lenses are made of Germany, and the middle lens is made of a material with a low refractive index.
ACHROMATIZATION
Fig. 2 presents graphs that illustrate the level of achromatization of lenses. The focus shift within the working spectrum (8–12 μm) in a lens with a focal length of 50 mm is 12 μm, and in a lens with a focal length of 150 mm it reaches approximately 35 μm.
FOCUSING
Fig. 3 shows the characteristics of the lenses when focusing on infinity and at close range: 2.5 m and 15 m, respectively.
TOLERANCE ANALYSIS
In the table. Figure 2 shows the result of the influence of tolerances on the contrast of the image of a 4-lens (left) and a 3-lens (right) lens. A change in contrast is shown for five points of the angular field at a spatial frequency of 30 lines / mm.
Table 3 shows the tolerances for decentration and inclination. Table 4 contains the tolerance values for the shape and thickness of the lenses, air gaps and glasses.
The calculation shows that the tolerances on the lenses of the 4-lens are stricter. This means that the design of the 3-lens lens is less sensitive to tolerances.
VARIABLE FOCUS DISTANCE LENS
An achromatized 7-lens lens with a variable focal length of 30–150 mm, a relative aperture of 1:1.2, and a field of view of 33.7–7.6° is shown in Fig. 4. All lenses are spherical, three lenses are made of a material with a lower refractive index than germanium. The focus shift within the working spectrum (8–12 μm) in the focal position of 30 mm is approximately 20 μm, and in the focal position of 150 mm it reaches about 30 μm. Temperature stabilization is performed using the zoom function. The lens length is 240 mm, the back focus is 32 mm, the weight of the lenses is 1 kg. Distortion at a focal length of 30 mm does not exceed 5%.
CONCLUSION
Achromatization can improve the quality of the image of the lens, provide thermal stabilization, focusing and zoom without performing technologically complex aspherical surfaces.
About authors
Shishkin Igor Petrovich, Candidate of Technical Sciences, shipoflens@mail.ru, 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
RTC “LEMT” BelOMO, Minsk, Republic of Belarus
A description is given of the design of achromatized lenses operating in the infrared region of the spectrum (8–12 μm). When designing thermal imaging lenses, it is necessary to take into account the fact that the pixel size of modern microbolometers is 10–12 μm, and the pixel density has reached 1280 × 1024 format. These parameters determine higher requirements for image quality, resolution and field of view of the developed lens. In this case, the lens should be fast, thermally stabilized and have focus. The following lenses are considered: 4-lens with a focus of 50 mm, 3-lens with a focus of 150 mm and 7-lens with a variable focus of 30–150 mm.
Keywords: achromatized lens, thermal stabilization, focusing, zoom
Received: 22.05.2020
Accepted: 16.06.2020
INTRODUCTION
The list of available optical materials that are transparent in the spectral range of 8–12 μm is very limited. The most common material is germanium. It has a high refractive index and low toxicity, which is important in the manufacture and operation of the lens. Almost all optical materials used in thermal imaging devices have a strong temperature dependence of the refractive index. Therefore, when developing a lens, it is necessary to provide for the possibility of thermal compensation in its design. Usually for this purpose, compensation is used in the back focus of the lens, moving the lens or microbolometric matrix with temperature fluctuations, which greatly complicates the mechanics of the lens.
Most modern fixed-focus lenses for thermal imaging cameras consist of 3–4 single lenses. This is primarily due to the need to combine the maximum achievable aperture and budget cost. At the same time, with a limited choice of glass brands, it is impossible to significantly improve the image quality of the lens by increasing the number of lenses or the length of the lens. Therefore, many developers [1, 2] actively use aspherical lenses or diffractive elements in IR lenses. With this approach, you can reduce the number of lenses, but to achieve a qualitative improvement in the resolution of the lens fails. In addition, the manufacture of large diameter aspherical menisci for lenses with a 1:1 relative aperture is a rather laborious process due to the high demands on the quality of manufacture of optical surfaces. Their creation often reveals the inappropriateness, in terms of maintaining a balance between image quality and the cost of the lens.
An alternative method to achieve the theoretical limit of resolution in the lens can be its achromatization [3]. This method consists in selecting a combination of glasses having different dispersion coefficients. The right choice makes it possible to compensate for chromatic aberration and thereby significantly improve the image quality of the lens.
FOCAL DISTANCE AND RELATIVE HOLE
It is known that the detection range of an object depends on the focal length of the lens and its aperture. The market offers a wide selection of thermal imaging lenses in the range of focal lengths of 30 ~ 300 mm and relative apertures F / 1 ~ F / 1.5. But most of them are designed for 640 × 480 format with a pixel size of 17–25 microns. On the other hand, an increase in the focal length of a fast lens leads to a significant increase in its size and weight. Table 1 shows the dependence of the weight of the lens on the focal length, aperture, number of lenses and length. But it must be borne in mind that it is the technological capabilities of manufacturing lenses of maximum diameter with the required accuracy that determine the image quality of the created lenses.
LENS DESIGN
Figure 1 shows the type and characteristics of the lenses. On the left is a 4-lens lens with a focal length of 50 mm and a field of view of 22°, on the right is a 3-lens lens with a focal length of 150 mm and a field of view of 7.5°. Both lenses are designed for 1280x1024 format with a diagonal of 20 mm. The lenses are thermally stabilized and have the function of focusing at close range.
In a 4-lens lens, thermal stabilization is passive. It is provided by a special combination of glasses: the outer lenses are made of germanium, and the inner ones are made of a material with a low refractive index. All lenses are spherical. The result of passive stabilization is illustrated by graphs of the optical transfer function. A certain combination of lenses ensures a constant position of the image plane when the operating temperature changes from –50 °C to 50 °C. Moving the last lens allows for internal focusing at close range.
In the 3-lens lens, the thermal stabilization function is combined with the internal focusing function. and performed. Depending on the chosen constructive solution, these functions are performed by the movements of the 2nd or 3rd lens. In the lens, the outer lenses are made of Germany, and the middle lens is made of a material with a low refractive index.
ACHROMATIZATION
Fig. 2 presents graphs that illustrate the level of achromatization of lenses. The focus shift within the working spectrum (8–12 μm) in a lens with a focal length of 50 mm is 12 μm, and in a lens with a focal length of 150 mm it reaches approximately 35 μm.
FOCUSING
Fig. 3 shows the characteristics of the lenses when focusing on infinity and at close range: 2.5 m and 15 m, respectively.
TOLERANCE ANALYSIS
In the table. Figure 2 shows the result of the influence of tolerances on the contrast of the image of a 4-lens (left) and a 3-lens (right) lens. A change in contrast is shown for five points of the angular field at a spatial frequency of 30 lines / mm.
Table 3 shows the tolerances for decentration and inclination. Table 4 contains the tolerance values for the shape and thickness of the lenses, air gaps and glasses.
The calculation shows that the tolerances on the lenses of the 4-lens are stricter. This means that the design of the 3-lens lens is less sensitive to tolerances.
VARIABLE FOCUS DISTANCE LENS
An achromatized 7-lens lens with a variable focal length of 30–150 mm, a relative aperture of 1:1.2, and a field of view of 33.7–7.6° is shown in Fig. 4. All lenses are spherical, three lenses are made of a material with a lower refractive index than germanium. The focus shift within the working spectrum (8–12 μm) in the focal position of 30 mm is approximately 20 μm, and in the focal position of 150 mm it reaches about 30 μm. Temperature stabilization is performed using the zoom function. The lens length is 240 mm, the back focus is 32 mm, the weight of the lenses is 1 kg. Distortion at a focal length of 30 mm does not exceed 5%.
CONCLUSION
Achromatization can improve the quality of the image of the lens, provide thermal stabilization, focusing and zoom without performing technologically complex aspherical surfaces.
About authors
Shishkin Igor Petrovich, Candidate of Technical Sciences, shipoflens@mail.ru, 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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