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DOI: 10.22184/1993-7296.FRos.2021.15.2.154.159
Thermally Stabilized Thermal Imaging Lenses
I.P. Shishkin, A.P. Shkadarevich
STC “LEMT”, BelOMO Minsk, Republic of Belarus
The design of thermally stabilized lenses operating in the IR spectral range of 8–12 µm is presented. The use of diffractive and aspherical elements in the construction with a certain combination of types of glasses ensures image plane stabilization and high resolution. A description is given of a 2- and 3‑element lens with a fixed focal length of 35 mm and a 4‑element lens with a variable focal length of 30–75 mm.
Keywords: thermal imaging lenses, kinoform and aspherical elements, achromatization, thermal stabilization
Received: 15.02.2021
Accepted: 29.03.2021
INTRODUCTION
Modern devices operate in a wide temperature range from –50 °C to 50 °C. Fluctuations in the operating temperature of lenses lead to changes in frame length, air gaps, and lens optical powers. And this, in turn, leads to a displacement of the image plane. Therefore, the task of simultaneously fulfilling the conditions of thermal stabilization and high resolution is relevant for the lenses of thermal imagers.
THERMAL STABILIZATION
There are two methods of thermal stabilization of lenses. The first is passive stabilization, when a combination of optical materials with different coefficients of thermal expansion is used in the lens design. Due to the method of passive stabilization, when the temperature changes, the defocusing of the image does not exceed a few micrometers. The second method involves moving a specific group of lenses to stabilize the image plane. This method requires a more complex design due to the need to thermally stabilize a larger number of lenses and create a focusing mechanism.
RESOLUTION
Achromatization is an effective method of increasing the resolution in a lens. Achromatization consists in the selection of a combination of glasses with different dispersion coefficients. Achromatization can significantly reduce chromatic aberration and provide the required image quality. Despite the fact that the list of optical materials transparent in the spectral range of 8–12 µm is very limited, and the most common material is germanium, it is possible to choose a grade of material with a lower refractive index. This glass paired with germanium will allow the lens to be achromatized. If a diffractive microstructure (kinoform) is applied to one of the lens lenses, and an aspherical profile is given to the other lens, it is possible to achieve thermal stabilization of the image plane. In [1], it was proposed to use diffractive optical elements deposited on one of the surfaces of the lens element for the dual infrared range to simultaneously fulfil the requirements for correcting chromatic and monochromatic aberrations.
The authors relied on works [2, 3], the results of which proved that the unique aberration properties of diffractive optical structures give the greatest effect in the IR range. And taking into account the strict requirements for the transmittance of thermal imaging lenses, the most optimal system is a design of 2–4 lens elements [4, 5].
PASSIVE-STABILIZATION LENS CONSTRUCTION
The main parameters of a 2- and 3‑element lens with passive image plane stabilization, obtained after dimensional and aberration calculations, are given in Table 1. Diagrams of these lenses and graphs of the optical transfer function for the operating temperature range are shown in Fig. 1.
VARIABLE-FOCUS LENS
Fig. 2 shows the view and graphs of the optical transfer function of an achromatized lens with a variable focal length of 30–75 mm and a relative aperture of F / 1. The thermal compensation in the lens is combined with the zoom function, which is carried out by moving the 2nd and 3rd elements. The zoom range of the lens is 2.5x and is subject to the following restrictions:
The main parameters of the calculated design of the thermal imaging lens are given in table. 2. All lenses in the lens are spherical. In the first embodiment, the second lens is made of germanium with a diffraction profile, in the second embodiment, the second lens is made of an optical crystal.
CONCLUSION
Analysis of the functions of the optical transfer function showed that the use of kinoform, aspherical elements and a combination of optical materials makes it possible to create thermal imaging lenses operating in the IR range with high resolution and thermal stabilization of the image plane.
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
STC “LEMT”, BelOMO Minsk, Republic of Belarus
The design of thermally stabilized lenses operating in the IR spectral range of 8–12 µm is presented. The use of diffractive and aspherical elements in the construction with a certain combination of types of glasses ensures image plane stabilization and high resolution. A description is given of a 2- and 3‑element lens with a fixed focal length of 35 mm and a 4‑element lens with a variable focal length of 30–75 mm.
Keywords: thermal imaging lenses, kinoform and aspherical elements, achromatization, thermal stabilization
Received: 15.02.2021
Accepted: 29.03.2021
INTRODUCTION
Modern devices operate in a wide temperature range from –50 °C to 50 °C. Fluctuations in the operating temperature of lenses lead to changes in frame length, air gaps, and lens optical powers. And this, in turn, leads to a displacement of the image plane. Therefore, the task of simultaneously fulfilling the conditions of thermal stabilization and high resolution is relevant for the lenses of thermal imagers.
THERMAL STABILIZATION
There are two methods of thermal stabilization of lenses. The first is passive stabilization, when a combination of optical materials with different coefficients of thermal expansion is used in the lens design. Due to the method of passive stabilization, when the temperature changes, the defocusing of the image does not exceed a few micrometers. The second method involves moving a specific group of lenses to stabilize the image plane. This method requires a more complex design due to the need to thermally stabilize a larger number of lenses and create a focusing mechanism.
RESOLUTION
Achromatization is an effective method of increasing the resolution in a lens. Achromatization consists in the selection of a combination of glasses with different dispersion coefficients. Achromatization can significantly reduce chromatic aberration and provide the required image quality. Despite the fact that the list of optical materials transparent in the spectral range of 8–12 µm is very limited, and the most common material is germanium, it is possible to choose a grade of material with a lower refractive index. This glass paired with germanium will allow the lens to be achromatized. If a diffractive microstructure (kinoform) is applied to one of the lens lenses, and an aspherical profile is given to the other lens, it is possible to achieve thermal stabilization of the image plane. In [1], it was proposed to use diffractive optical elements deposited on one of the surfaces of the lens element for the dual infrared range to simultaneously fulfil the requirements for correcting chromatic and monochromatic aberrations.
The authors relied on works [2, 3], the results of which proved that the unique aberration properties of diffractive optical structures give the greatest effect in the IR range. And taking into account the strict requirements for the transmittance of thermal imaging lenses, the most optimal system is a design of 2–4 lens elements [4, 5].
PASSIVE-STABILIZATION LENS CONSTRUCTION
The main parameters of a 2- and 3‑element lens with passive image plane stabilization, obtained after dimensional and aberration calculations, are given in Table 1. Diagrams of these lenses and graphs of the optical transfer function for the operating temperature range are shown in Fig. 1.
VARIABLE-FOCUS LENS
Fig. 2 shows the view and graphs of the optical transfer function of an achromatized lens with a variable focal length of 30–75 mm and a relative aperture of F / 1. The thermal compensation in the lens is combined with the zoom function, which is carried out by moving the 2nd and 3rd elements. The zoom range of the lens is 2.5x and is subject to the following restrictions:
- weight and size characteristics impose a limit on the number of lenses used in the design (the minimum is 4 lens elements);
- diameter of the front lens element should not exceed the specified value (accepted 70 mm);
- distortion should not exceed 10%.
The main parameters of the calculated design of the thermal imaging lens are given in table. 2. All lenses in the lens are spherical. In the first embodiment, the second lens is made of germanium with a diffraction profile, in the second embodiment, the second lens is made of an optical crystal.
CONCLUSION
Analysis of the functions of the optical transfer function showed that the use of kinoform, aspherical elements and a combination of optical materials makes it possible to create thermal imaging lenses operating in the IR range with high resolution and thermal stabilization of the image plane.
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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