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DOI: 10.22184/1993-7296.FRos.2022.16.1.38.43
The design of athermalized television lenses is presented for systems observations: 3‑lens F / 5.6 telephoto lens, 7‑lens F / 4 apochromat and 9‑lens F / 2.8 aperture. At calculation lenses have been read factors affecting on the value lens design resolution: size sensor, focal distance, relative hole. However, in real production, when assembling a lens structure, it is not always possible to achieve the calculated image quality. The sensitivity of optical schemes to design tolerances is considered.
The design of athermalized television lenses is presented for systems observations: 3‑lens F / 5.6 telephoto lens, 7‑lens F / 4 apochromat and 9‑lens F / 2.8 aperture. At calculation lenses have been read factors affecting on the value lens design resolution: size sensor, focal distance, relative hole. However, in real production, when assembling a lens structure, it is not always possible to achieve the calculated image quality. The sensitivity of optical schemes to design tolerances is considered.
Теги: athermalization optical transfer function television lenses атермализация оптическая передаточная функция телевизионные объективы
Athermalized Television Lenses
I. P. Shishkin, A. P. Shkadarevich
RTC “LEMT” BelOMO, Minsk, Republic of Belarus
The design of athermalized television lenses is presented for systems observations: 3‑lens F / 5.6 telephoto lens, 7‑lens F / 4 apochromat and 9‑lens F / 2.8 aperture. At calculation lenses have been read factors affecting on the value lens design resolution: size sensor, focal distance, relative hole. However, in real production, when assembling a lens structure, it is not always possible to achieve the calculated image quality. The sensitivity of optical schemes to design tolerances is considered.
Key words: television lenses, athermalization, optical transfer function
Received on: 15.10.2021
Accepted on: 29.11.2021
INTRODUCTION
Lenses used in modern devices operate in a wide temperature range from –50° to 50°. In long-focus lenses, the components are separated by a large air gap, and temperature fluctuations cause a change in the length of the frames, air gaps, and optical powers of the lenses, which leads to a shift in the image plane [1, 2]. For many optical systems that are exposed to a wide range of temperatures, temperature change has a major effect on the change in the refractive index of the optical glass and the geometry of the lens elements. This affects the image properties of the lens and causes additional aberrations due to the deviation of the design parameters from the calculated values of the system. Therefore, athermalization of lenses is one of the key requirements for the lenses of surveillance systems [3].
There are two methods for athermalizing lenses. The first method, the passive athermalization method, consists in using a combination of glasses with different thermal expansion coefficients in the lens design, due to which the defocusing of the image does not exceed a few micrometers when the temperature changes. The second method involves using the movement of a certain group of lenses to stabilize the image plane, which requires a more complex lens design due to the need to include more lenses and a focusing mechanism. The article deals with the application of the passive athermalization method.
One of the parameters that determine the image quality of a lens is aperture ratio. The higher it is, the higher resolution can be achieved by optical calculation. In practice, a high-aperture, high-contrast lens allows you to get a more detailed image of a distant object.
On the other hand, an increase in the relative aperture in a long-focus lens automatically leads to an increase in lens diameters and dimensions. In this case, the accuracy of lens manufacturing should be high enough to provide the calculated resolution in the assembled lens [4].
DESIGN
Figure 1 shows the layout and optical characteristics of athermalized lenses with a focal length of 300 mm. The presented optical transfer function graphs show that the image contrast obtained with a 7‑element lens is noticeably higher than with a 3‑element lens. The main design parameters of the lenses under consideration are given in Table 1.
APOCHROMATIZATION
A diffractive microstructure in the form of a diffractive optical element (DOE) placed on one of the surfaces of a 3‑element lens makes it possible to perform its apochromatization [5–8]. As a result, the resolution of the lens will be almost the same as in the 7‑lens design. The influence of apochromatization on the value of the contrast of the lenses is shown in Table 2.
SENSITIVITY
The output characteristics of the assembled lens are highly dependent on the design. For this reason, the image quality achieved in the calculation cannot always be ensured in real production. Let’s compare the two lens designs. Both designs have different sensitivity to tolerances (de- centering and lens tilt):
In table. Figure 3 compares two lens options in terms of sensitivity to decentering and tilt of the lenses, taking into account the calculated parameters for resolution and contrast. Table 3 shows that the 2‑element lens is less sensitive to lens decentration and tilt (about 5 times). This means that the 2‑element lens design is more preferable for production, despite the fact that the calculated contrast of the lens is lower.
Athermalization
Figure 4 shows the brands of glasses from the Ohara catalog [9], which were used to create athermalized lenses with a focal length of 300 mm. Table 4 shows: nD is the refractive index, νD is the Abbe number and αD is the coefficient of thermal expansion of the optical glass.
The result of stabilization of the position of the image plane of the 3‑element and 7‑element lenses with a focal length of 300 mm when operating temperatures change from –50° to 50° is illustrated by the graphs in Fig. 3. The graphs show the appearance of a focus shift at a spatial frequency of 80 lines / mm. The defocusing of the image when the temperature changes does not exceed a few units of micrometers.
Lens with a focal length of 100 mm
The appearance and characteristics of an athermalized lens with a fixed focus of 100 mm and a relative aperture of F / 2.8 are shown in Fig. 4. The lens is designed for a 4 / 3" sensor and has a resolution of 160 lines / mm. For a 3 micron pixel, the camera resolution with this lens will be 24 megapixels.
Conclusion
A certain combination of glasses and the use of a DOE makes it possible to create an athermalized television lens with high resolution and stabilization of the image plane. The length of the focal length of the lens and the size of the relative aperture depend on the technological capabilities of the production of large-diameter lenses and equipment that provides high accuracy when assembling the lens and controlling its output characteristics.
About authors
Shishkin Igor Petrovich, Cand. of Science (Eng.), shipoflens@mail.ru, RTC «LEMT» BelOMO, Minsk, Republic of Belarus.
ORCID ID: 0000-0002-4592-1060
Shkadarevich Alexey Petrovich, Dr. of of Science (Phys.&Math.), 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
The design of athermalized television lenses is presented for systems observations: 3‑lens F / 5.6 telephoto lens, 7‑lens F / 4 apochromat and 9‑lens F / 2.8 aperture. At calculation lenses have been read factors affecting on the value lens design resolution: size sensor, focal distance, relative hole. However, in real production, when assembling a lens structure, it is not always possible to achieve the calculated image quality. The sensitivity of optical schemes to design tolerances is considered.
Key words: television lenses, athermalization, optical transfer function
Received on: 15.10.2021
Accepted on: 29.11.2021
INTRODUCTION
Lenses used in modern devices operate in a wide temperature range from –50° to 50°. In long-focus lenses, the components are separated by a large air gap, and temperature fluctuations cause a change in the length of the frames, air gaps, and optical powers of the lenses, which leads to a shift in the image plane [1, 2]. For many optical systems that are exposed to a wide range of temperatures, temperature change has a major effect on the change in the refractive index of the optical glass and the geometry of the lens elements. This affects the image properties of the lens and causes additional aberrations due to the deviation of the design parameters from the calculated values of the system. Therefore, athermalization of lenses is one of the key requirements for the lenses of surveillance systems [3].
There are two methods for athermalizing lenses. The first method, the passive athermalization method, consists in using a combination of glasses with different thermal expansion coefficients in the lens design, due to which the defocusing of the image does not exceed a few micrometers when the temperature changes. The second method involves using the movement of a certain group of lenses to stabilize the image plane, which requires a more complex lens design due to the need to include more lenses and a focusing mechanism. The article deals with the application of the passive athermalization method.
One of the parameters that determine the image quality of a lens is aperture ratio. The higher it is, the higher resolution can be achieved by optical calculation. In practice, a high-aperture, high-contrast lens allows you to get a more detailed image of a distant object.
On the other hand, an increase in the relative aperture in a long-focus lens automatically leads to an increase in lens diameters and dimensions. In this case, the accuracy of lens manufacturing should be high enough to provide the calculated resolution in the assembled lens [4].
DESIGN
Figure 1 shows the layout and optical characteristics of athermalized lenses with a focal length of 300 mm. The presented optical transfer function graphs show that the image contrast obtained with a 7‑element lens is noticeably higher than with a 3‑element lens. The main design parameters of the lenses under consideration are given in Table 1.
APOCHROMATIZATION
A diffractive microstructure in the form of a diffractive optical element (DOE) placed on one of the surfaces of a 3‑element lens makes it possible to perform its apochromatization [5–8]. As a result, the resolution of the lens will be almost the same as in the 7‑lens design. The influence of apochromatization on the value of the contrast of the lenses is shown in Table 2.
SENSITIVITY
The output characteristics of the assembled lens are highly dependent on the design. For this reason, the image quality achieved in the calculation cannot always be ensured in real production. Let’s compare the two lens designs. Both designs have different sensitivity to tolerances (de- centering and lens tilt):
- 3‑element telephoto lens (Fig. 1 a, left) – 1 and 2 lenses are separated by a small air gap (0.3–0.5 mm);
- 2‑element lens (Fig. 2) – two glued lenses are separated by a large air gap (70–80mm).
In table. Figure 3 compares two lens options in terms of sensitivity to decentering and tilt of the lenses, taking into account the calculated parameters for resolution and contrast. Table 3 shows that the 2‑element lens is less sensitive to lens decentration and tilt (about 5 times). This means that the 2‑element lens design is more preferable for production, despite the fact that the calculated contrast of the lens is lower.
Athermalization
Figure 4 shows the brands of glasses from the Ohara catalog [9], which were used to create athermalized lenses with a focal length of 300 mm. Table 4 shows: nD is the refractive index, νD is the Abbe number and αD is the coefficient of thermal expansion of the optical glass.
The result of stabilization of the position of the image plane of the 3‑element and 7‑element lenses with a focal length of 300 mm when operating temperatures change from –50° to 50° is illustrated by the graphs in Fig. 3. The graphs show the appearance of a focus shift at a spatial frequency of 80 lines / mm. The defocusing of the image when the temperature changes does not exceed a few units of micrometers.
Lens with a focal length of 100 mm
The appearance and characteristics of an athermalized lens with a fixed focus of 100 mm and a relative aperture of F / 2.8 are shown in Fig. 4. The lens is designed for a 4 / 3" sensor and has a resolution of 160 lines / mm. For a 3 micron pixel, the camera resolution with this lens will be 24 megapixels.
Conclusion
A certain combination of glasses and the use of a DOE makes it possible to create an athermalized television lens with high resolution and stabilization of the image plane. The length of the focal length of the lens and the size of the relative aperture depend on the technological capabilities of the production of large-diameter lenses and equipment that provides high accuracy when assembling the lens and controlling its output characteristics.
About authors
Shishkin Igor Petrovich, Cand. of Science (Eng.), shipoflens@mail.ru, RTC «LEMT» BelOMO, Minsk, Republic of Belarus.
ORCID ID: 0000-0002-4592-1060
Shkadarevich Alexey Petrovich, Dr. of of Science (Phys.&Math.), 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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