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ТЕХНОСФЕРА_РИЦ
DOI: 10.22184/1993-7296.FRos.2024.18.2.152.158
The design of long-focal length catadioptric lenses for the visible and infrared ranges (VIS, SWIR, MWIR and LWIR) is proided. The front group of all lenses is similar and consists of 2 mirrors, with the first (prime) mirror having a diameter of 150 mm. The diameter of the mirror is selected based on the requirements for lens aperture and possible production of the mirror. In addition, the same diameter of the primary mirror, if necessary, allows to combine several channels (TV and SWIR, TV and LWIR) into one. The article describes a television and SWIR lenses with a focal point of 600 mm and a relative aperture of F/4, a MWIR lens with a focal point of 400 mm and a relative aperture of F/4, and a LWIR lens with a focal point of 250 mm and a relative aperture of F/1.6. The design options for a catadioptric zoom and multi-channel lenses with a focal point of 400–800 mm and a relative aperture F/5-F/10 are indicated.
The design of long-focal length catadioptric lenses for the visible and infrared ranges (VIS, SWIR, MWIR and LWIR) is proided. The front group of all lenses is similar and consists of 2 mirrors, with the first (prime) mirror having a diameter of 150 mm. The diameter of the mirror is selected based on the requirements for lens aperture and possible production of the mirror. In addition, the same diameter of the primary mirror, if necessary, allows to combine several channels (TV and SWIR, TV and LWIR) into one. The article describes a television and SWIR lenses with a focal point of 600 mm and a relative aperture of F/4, a MWIR lens with a focal point of 400 mm and a relative aperture of F/4, and a LWIR lens with a focal point of 250 mm and a relative aperture of F/1.6. The design options for a catadioptric zoom and multi-channel lenses with a focal point of 400–800 mm and a relative aperture F/5-F/10 are indicated.
Теги: long-focal length catadioptric lens multi-channel lens mwir and lwir mwir и lwir observation devices swir vis zoom lens длиннофокусный зеркально-линзовый объектив зум объектив мультиканальный объектив приборы наблюдения
Long-Focal Length Catadioptric Lenses
I. P. Shishkin, A. P. Shkadarevich
LEMT Scientific and Technical Center of BelOMO, Minsk, Republic of Belarus
The design of long-focal length catadioptric lenses for the visible and infrared ranges (VIS, SWIR, MWIR and LWIR) is proided. The front group of all lenses is similar and consists of 2 mirrors, with the first (prime) mirror having a diameter of 150 mm. The diameter of the mirror is selected based on the requirements for lens aperture and possible production of the mirror. In addition, the same diameter of the primary mirror, if necessary, allows to combine several channels (TV and SWIR, TV and LWIR) into one. The article describes a television and SWIR lenses with a focal point of 600 mm and a relative aperture of F/4, a MWIR lens with a focal point of 400 mm and a relative aperture of F/4, and a LWIR lens with a focal point of 250 mm and a relative aperture of F/1.6. The design options for a catadioptric zoom and multi-channel lenses with a focal point of 400–800 mm and a relative aperture F/5-F/10 are indicated.
Keywords: long-focal length catadioptric lens, zoom lens, multi-channel lens, VIS, SWIR, MWIR and LWIR, observation devices
Article received:13.02.2024
Article accepted:11.03.2024
Introduction
The use of long-focal length lenses in the observation devices makes it possible to bring an object as close as possible, identify it and determine the exact range. On the other part, the lens complexity determines the overall dimensions and control parameters of the entire optical-electronic observation complex. It is known that the aperture significantly affects the lens resolution designed for a particular spectral range.
Figure 1a shows the dependence of the lens aperture on the spectral range and pixel size. Obviously, when moving to the long-wavelength region of the spectrum, it is necessary to increase the aperture ratio in order to obtain an acceptable lens resolution. However, an increase in the relative aperture in a long-focal length lens automatically leads to an increase in the lens diameters and dimensions. Moreover, the production accuracy of lenses and mechanical parts shall be high enough to ensure the rated resolution in the assembled lens. The purpose of this paper is to develop an optimal design of long-focal length catadioptric lenses for wide application.
TV lens
The lens plays a key role in the television channel. In combination with a selected sensor, it provides the required detection range. The detection range depends on both the focal length of the lens and the aperture ratio.
The pixel size and sensor diagonal also affect the detection range, since for a larger pixel and a limited field of view, a lens with the performance close to the theoretical limit can be designed. In practice, a long-focal length high-aperture lens with the high image contrast allows to obtain a more detailed image of a distant object.
Figure 1b‑1c shows a view of a catadioptric television lens and an optical transfer function chart. The first mirror with a diameter of 150 mm is aspherical, the second mirror is a flat one installed at an angle of 45°, followed by a group of 3 lenses that adjust any chromatic and field aberrations. The lens with a focal length of 600 mm and a relative aperture of 1:4 is designed for a 1″ sensor (with the diagonal of 16 mm) and has an angular field of view of 1.5°. As it can be seen from the chart, the lens resolution is 80 lines/mm with a contrast of 0.5.
SWIR lens
Due to the occurrence of sensors operating in a wide spectral range of 0.8–1.6 µm, a new category of lenses has been developed. An example of a catadioptric lens with a focal length of 600 mm and a relative aperture of 1:4 is shown in Fig. 2. Structurally, the lens is made according to the well-known Maksutov (Cassegrain) circuit and is distinguished by the fact that the first mirror with a diameter of 150 mm has a spherical shape, the second mirror is flat, and the third mirror is installed at an angle of 45° that significantly reduces the lens dimensions. With due regard to the wide spectral range, the use of mirrors in the lens makes it easy to achromatize it that would not be easy to do with an option consisting of the lenses only.
LWIR lens
The lenses operating in the long-wave infrared range of 8–12 µm are actively used in the up-to-date observation devices. Figure 3a shows a type of a long-focal length lens with a focal point of 250mm and an aperture ratio of 1:1.6. The first mirror with a diameter of 150 mm is aspherical, and the second mirror is flat.
The chart in Fig. 3c is shown for a spatial frequency of 30 lines/mm that corresponds to a pixel resolution of 12–17 µm.
Another option of the lens with a 2x mirror extender is shown in Fig. 3b. In the extender, the first mirror with a diameter of 150 mm is aspherical, and the second mirror is a sphere.
MWIR lens
An example of a long-focal length lens designed for the mid-wave infrared region of 3…5 µm is shown in Fig. 4. A design feature of this type of lens available on the market is the diffractive, aspherical elements and cooled bolometers (sensors). In addition, the lens aperture diaphragm shall be located behind the last lens, and its diameter shall be consistent with the operating aperture of the bolometer (cold stop). Figure 4a shows a view of a long-focal length lens with a focal point of 400mm and a relative aperture of 1:4. Both mirrors (the first one with a diameter of 150 mm) and two lenses are aspherical. The image quality of the lens is shown in Fig. 4b. Focusing at close range is achieved using the last lens. Figure 4c provides another option of the lens with the variable focus of 20–400 mm and a 2x mirror extender. In the extender, the first mirror with a diameter of 150 mm is aspherical, and the second mirror is a sphere.
Zoom and multi-channel lens
The two-mirror design can be used in the production of varifocal lenses and multi-channel lenses. Fig. 5a-b shows a zoom lens with a focus of 400–800 mm. Any changes in the focal length are performed by moving 2 groups of lenses. The front mirror is made with a diameter of 80 mm that leads to the optimal combination of main parameters of the lens: focal length range (400–800 mm), dimensions (<250 mm), field of view (2–1°), resolution (80 lines/mm). Figure 6a-c shows the combination of two spectral channels (TV and SWIR) using a beam splitting prism (x-cube) and with their optical specifications. The comparative features of the lenses are given in the table.
Conclusion
The given design of long-focal length lenses based on a two-mirror design is optimal in terms of the number of components, overall dimensions and production capabilities. All lenses have an acceptable image quality and can be used in the observation tools for various purposes.
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.
I. P. Shishkin, A. P. Shkadarevich
LEMT Scientific and Technical Center of BelOMO, Minsk, Republic of Belarus
The design of long-focal length catadioptric lenses for the visible and infrared ranges (VIS, SWIR, MWIR and LWIR) is proided. The front group of all lenses is similar and consists of 2 mirrors, with the first (prime) mirror having a diameter of 150 mm. The diameter of the mirror is selected based on the requirements for lens aperture and possible production of the mirror. In addition, the same diameter of the primary mirror, if necessary, allows to combine several channels (TV and SWIR, TV and LWIR) into one. The article describes a television and SWIR lenses with a focal point of 600 mm and a relative aperture of F/4, a MWIR lens with a focal point of 400 mm and a relative aperture of F/4, and a LWIR lens with a focal point of 250 mm and a relative aperture of F/1.6. The design options for a catadioptric zoom and multi-channel lenses with a focal point of 400–800 mm and a relative aperture F/5-F/10 are indicated.
Keywords: long-focal length catadioptric lens, zoom lens, multi-channel lens, VIS, SWIR, MWIR and LWIR, observation devices
Article received:13.02.2024
Article accepted:11.03.2024
Introduction
The use of long-focal length lenses in the observation devices makes it possible to bring an object as close as possible, identify it and determine the exact range. On the other part, the lens complexity determines the overall dimensions and control parameters of the entire optical-electronic observation complex. It is known that the aperture significantly affects the lens resolution designed for a particular spectral range.
Figure 1a shows the dependence of the lens aperture on the spectral range and pixel size. Obviously, when moving to the long-wavelength region of the spectrum, it is necessary to increase the aperture ratio in order to obtain an acceptable lens resolution. However, an increase in the relative aperture in a long-focal length lens automatically leads to an increase in the lens diameters and dimensions. Moreover, the production accuracy of lenses and mechanical parts shall be high enough to ensure the rated resolution in the assembled lens. The purpose of this paper is to develop an optimal design of long-focal length catadioptric lenses for wide application.
TV lens
The lens plays a key role in the television channel. In combination with a selected sensor, it provides the required detection range. The detection range depends on both the focal length of the lens and the aperture ratio.
The pixel size and sensor diagonal also affect the detection range, since for a larger pixel and a limited field of view, a lens with the performance close to the theoretical limit can be designed. In practice, a long-focal length high-aperture lens with the high image contrast allows to obtain a more detailed image of a distant object.
Figure 1b‑1c shows a view of a catadioptric television lens and an optical transfer function chart. The first mirror with a diameter of 150 mm is aspherical, the second mirror is a flat one installed at an angle of 45°, followed by a group of 3 lenses that adjust any chromatic and field aberrations. The lens with a focal length of 600 mm and a relative aperture of 1:4 is designed for a 1″ sensor (with the diagonal of 16 mm) and has an angular field of view of 1.5°. As it can be seen from the chart, the lens resolution is 80 lines/mm with a contrast of 0.5.
SWIR lens
Due to the occurrence of sensors operating in a wide spectral range of 0.8–1.6 µm, a new category of lenses has been developed. An example of a catadioptric lens with a focal length of 600 mm and a relative aperture of 1:4 is shown in Fig. 2. Structurally, the lens is made according to the well-known Maksutov (Cassegrain) circuit and is distinguished by the fact that the first mirror with a diameter of 150 mm has a spherical shape, the second mirror is flat, and the third mirror is installed at an angle of 45° that significantly reduces the lens dimensions. With due regard to the wide spectral range, the use of mirrors in the lens makes it easy to achromatize it that would not be easy to do with an option consisting of the lenses only.
LWIR lens
The lenses operating in the long-wave infrared range of 8–12 µm are actively used in the up-to-date observation devices. Figure 3a shows a type of a long-focal length lens with a focal point of 250mm and an aperture ratio of 1:1.6. The first mirror with a diameter of 150 mm is aspherical, and the second mirror is flat.
The chart in Fig. 3c is shown for a spatial frequency of 30 lines/mm that corresponds to a pixel resolution of 12–17 µm.
Another option of the lens with a 2x mirror extender is shown in Fig. 3b. In the extender, the first mirror with a diameter of 150 mm is aspherical, and the second mirror is a sphere.
MWIR lens
An example of a long-focal length lens designed for the mid-wave infrared region of 3…5 µm is shown in Fig. 4. A design feature of this type of lens available on the market is the diffractive, aspherical elements and cooled bolometers (sensors). In addition, the lens aperture diaphragm shall be located behind the last lens, and its diameter shall be consistent with the operating aperture of the bolometer (cold stop). Figure 4a shows a view of a long-focal length lens with a focal point of 400mm and a relative aperture of 1:4. Both mirrors (the first one with a diameter of 150 mm) and two lenses are aspherical. The image quality of the lens is shown in Fig. 4b. Focusing at close range is achieved using the last lens. Figure 4c provides another option of the lens with the variable focus of 20–400 mm and a 2x mirror extender. In the extender, the first mirror with a diameter of 150 mm is aspherical, and the second mirror is a sphere.
Zoom and multi-channel lens
The two-mirror design can be used in the production of varifocal lenses and multi-channel lenses. Fig. 5a-b shows a zoom lens with a focus of 400–800 mm. Any changes in the focal length are performed by moving 2 groups of lenses. The front mirror is made with a diameter of 80 mm that leads to the optimal combination of main parameters of the lens: focal length range (400–800 mm), dimensions (<250 mm), field of view (2–1°), resolution (80 lines/mm). Figure 6a-c shows the combination of two spectral channels (TV and SWIR) using a beam splitting prism (x-cube) and with their optical specifications. The comparative features of the lenses are given in the table.
Conclusion
The given design of long-focal length lenses based on a two-mirror design is optimal in terms of the number of components, overall dimensions and production capabilities. All lenses have an acceptable image quality and can be used in the observation tools for various purposes.
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.
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