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УДК 681.7 : 535.31
НАПРАВЛЕНИЯ РАЗРАБОТКИ ОБЪЕКТИВОВ ОПТИКО-ЭЛЕКТРОННОЙ АППАРАТУРЫ ДЛЯ SWIR ДИАПАЗОНА СПЕКТРА
Александр Сергеевич Красных
Сибирский государственный университет геосистем и технологий, 630108, Россия, г. Новосибирск, ул. Плахотного, 10, магистрант кафедры фотоники и приборостроения, тел. (913)910-28-93, e-mail: krasnyx2015@mail.ru
Татьяна Николаевна Хацевич
Сибирский государственный университет геосистем и технологий, 630108, Россия, г. Новосибирск, ул. Плахотного, 10, кандидат технических наук, профессор кафедры фотоники и приборостроения, тел. (383)344-29-29, e-mail: khatsevich@rambler.ru
Николай Николаевич Мордвин
ООО «КБ "ЛУГГАР"», 630049, Россия, г. Новосибирск, Красный пр., 200, оф. 305, генеральный директор, тел. (903)900-04-08, e-mail: mordvin_nn@mail.ru
Проведен анализ характеристик объективов для SWIR диапазона спектра. Выявлены диапазоны и наиболее часто встречающиеся значения основных характеристик: фокусного расстояния, относительного отверстия, угловых полей и др.
Ключевые слова: SWIR, камера, объектив, относительное отверстие, фокусное расстояние, угол обзора, тенденция развития.
THE TRENDS IN THE DEVELOPMENT OF LENSES FOR SWIR SPECTRAL RANGE
Alexander S. Krasnykh
Siberian State University of Geosystems and Technologies, 10, Plakhotnogo St., Novosibirsk, 630108, Russia, Graduate, Department of Photonics and Device Engineering, phone: (913)910-28-93, e-mail: krasnyx2015@mail.ru
Tatiana N. Khatsevich
Siberian State University of Geosystems and Technologies, 10, Plakhotnogo St., Novosibirsk, 630108, Russia, Ph. D., Professor, Department of Photonics and Device Engineering, phone: (383)344-29-29, e-mail: khatsevich@rambler.ru
Nikolay N. Mordwin
CB "LUGGAR" Co Ltd, 200, Krasny Prospect St., of. 305, Novosibirsk, 630049, Russia, Director General, phone: (903)900-04-08, e-mail: mordvin_nn@mail.ru
The analysis of the characteristics of the objectives for the SWIR spectral range is carried out. The ranges and most frequently encountered values of the main characteristics are determined: focal length, relative aperture, angular fields, etc. The results of the optical design of the objective are shown whose optical characteristics correspond to: a relative aperture of 1:1,4; focal length 50 mm.
Key words: SWIR, camera, lens, aperture, focal length, viewing angle, development trends.
Introduction
Currently, the possibilities of observing objects outside the visible range of the spectrum of electromagnetic radiation are actively realized in the infrared short-wave (SWIR) spectral range with wavelengths from 0,9 to 3,0 ^m [1]. Thus, uncooled matrix receivers based on InGaAs [2], operating in the spectral range from 0,9 to 1,7 ^m, are the basis for the development of small-sized SWIR cameras. Unlike classical thermal imaging devices recording their own radiation of objects and backgrounds of medium-wave IR (MWIR) and (or) long-wave IR (LWIR) ranges, devices of the SWIR range record the radiation of natural and artificial external sources reflected from the observed objects and backgrounds, which is the basis for providing a wide dynamic range, necessary for the formation of quality images of objects. The advantages that determine the relevance of the development of SWIR range devices usually are: the ability to create an image in the night conditions; the possibility of round-the-clock application; the possibility of observation in dusty, fog or smoke conditions; the possibility of using the hidden illumination of the observed objects; the ability to detect hidden laser signals; no requirements for deep cooling; compactness; low energy consumption, etc. Realization of the advantages opened by receivers that are sensitive in the SWIR spectral range can be provided together with high-quality optical systems specially tuned for this range.
The analysis of the market of SWIR lenses showed that the number of firms offering lenses for the SWIR range is small: there are several foreign companies, and there are no mass-produced domestic SWIR lenses. The foregoing confirms the relevance of the development of domestic lenses for the SWIR spectral range.
The purpose of this study is to identify competitive values of technical characteristics for new developments of SWIR lenses and to create an optical lens circuit with characteristics corresponding to the most frequently encountered foreign lenses.
Methods of study
Methods of research: statistical analysis of scientific and technical information on the characteristics of foreign objectives of a certain type, computer methods for designing and calculating optical systems.
The optical system of any lens is represented by external and internal models. The task of designing a new optical system can be considered as the creation of such an internal model (design parameters, design of individual lenses, aberration characteristics, etc.), which guarantees the provision of competitive advantages for the external model.
In the course of the research, information on external characteristics of 34 SWIR range lenses offered by three foreign companies was systematized and analyzed: "Kowa", Japan; "Xenics", Belgium; «Edmund optics», USA. The information is tabulated, including designations of the lenses, image format, focal length, diaphragm number, angular field along the diagonal of the frame, resolving power, mass, overall dimensions, price, etc. Fragmentally analyzed information is given in Table 1.
The results of the analysis on the most important characteristics included in the external model of lenses are shown in Fig. 1 - 5:
- the distribution of SWIR lenses by the focal length range;
- the ratio between adjustable and unregulated relative apertures of SWIR lenses;
- the distribution of lenses by relative opening;
- the distribution of lenses over the angular field (corresponding to the diagonal of the receiver);
- the distribution of SWIR lenses on the minimum focusing distance.
Analysis results
Based on the analysis of the technical characteristics of the investigated sample of lenses, the following trends in the development of lenses for the SWIR spectral range are revealed:
- lenses have a fixed focal length: the focal length range is from 8 to 200 mm, with most (29%) models having a focal length of 50 mm (Fig. 1);
- in the designs of more than a half of models (59%), an adjustable aperture diaphragm is provided (Fig. 2), which allows to reduce the illumination level of the image in comparison with the illumination obtained at the maximum relative aperture of the lens, and also increases the depth of the sharply portrayed space as the relative aperture decreases;
Table 1
Model Focal length, mm Diaphragm number Angular field diagonally, degree Minimum focusing distance, m Weight, g Length; diameter, mm Source
LM8HC-SW 8 1,4 - 16 92,4 0,1 200 58; 57
LM16HC-SW 16 1,4 - 16 54,4 0,3 140 52,9; 43
LM25HC-SW 25 1,4 - 16 36,6 0,3 125 43; 43 [21
LM35HC-SW 35 1,4 - 16 26,1 0,3 130 47; 42
LM50HC-SW 50 1,4 - 16 18 0,3 200 52; 47,5
ОРТ-000107 16 1,4 27 0,65 140 70,4; 42
ASY-000658 200 2,4 4 4,5 - 225;104
ОРТ-000194 25 1,8 36,6 0,4 92,5 49; 32 [31
ОРТ-000195 50 1,5 29,8 1,5 319 94; 54
ОРТ-000089 8 1,4 92,4 0,3 200 75,5; 57
83-170 100 2,25 - 22 14,6 0,4 1800 180,1; 84
83-160 25 2,1 - 16 55,8 0,2 180 63,5; 40
68-689 35 1,4 - 16 14,4 0,3 130 43; 42 [41
83-167 50 2,15 - 16 29,8 0,275 574 124; 56
83-815 8 1,4 - 16 79,7 0,1 200 58; 57
Characteristics of SWIR range lenses
- when analyzing the distribution of SWIR lenses on a relative aperture (Fig. 3 shows the values of the corresponding aperture numbers - fixed or maximum (for lenses with adjustable apertures)), the largest number of models has a relative aperture of 1:1,4. The maximum focal length corresponding to the indicated relative aperture is 50 mm. Only more luminous lens (1:0,95) also has a focal length of 50 mm. The longest focus of the sample under study has a relative aperture of 1:2,4 at a focal length of 200 mm. The shortest-focal lenses have a relative aperture of 1:1,4 at a focal length of 8 mm;
- the distribution of the lenses by the angular fields (in Fig. 4 the values of the angular fields corresponding to the diagonal of the image receiver are shown) shows that the maximum number of lens models has an angular field close to 30°, namely: 23% of models have diagonal viewing angle of 29,8°; 12% have the field of 36,6°; 6% of models have fields of 92,4°; 27°, 20,5° and 26,1°;
- the minimum focusing distance is a characteristic showing the shortest distance to the object of observation, to which the lens can be refocused. Two-thirds of the lenses in question have a minimum focusing distance of 0,25 to 0,5 m (Fig. 5).
■ regulated ■ unregulated
59%
Fig. 2. The ratio between regulated and unregulated relative apertures of SWIR lenses
F/2,4 F/2,25 F/2,15 F/2,1 F/2 F/1,8 F/1,5 F/1,4 F/0,95
0% 10% 20% 30% 40% 50% 60% 70%
Fig. 1. Distribution of SWIR lenses by focal lengths
Fig. 3. Distribution of SWIR lenses by aperture
249
The results of design of the optical system of the lens
On the basis of the study, a lens was designed, with characteristics which can often be found at SWIR lenses. The most number of models has a relative aperture of 1:1,4. The maximum focal length corresponding to the indicated relative opening is 50 mm. The lens was designed for use with a receiver 640x512 and with a step of 20 microns. With angular fields in the horizontal and vertical directions, respectively, 14° and 11° (the field along the diagonal of the frame is 18°).
The two-component system is adopted as the initial scheme: the first component is assigned to eliminate position chromaticity and corrected aberrations of broad beams (spherical aberration and coma); the second is corrected aberrations of narrow inclined beams (astigmatism, image curvature and distortion). As a basic component of the first component, a two-lens gluing is adopted.
Fig.4. Distribution of SWIR lenses, the angular field
Fig.5. Minimum focusing distance of SWIR range lenses
The optical scheme of an objective with a focal length of 50 mm and a relative aperture of 1:1,4 is constructed according to a two-component scheme. The first component of the lens is made in the form of two lens joints spaced along the optical axis. The second component of the lens, a three-lens corrector, is located near the focal plane and serves to reduce field aberrations. The optical scheme of the objective is shown in Fig. 6.
Fig. 6. Optical scheme of the lens
When optimizing the design parameters of the objective, the radii of the refractive surfaces, the thicknesses of the lenses and the size of the air gaps are selected as variables, and restrictions on the focal length and length of the system are indicated. The choice of materials is based on the results of the study [6].
The creation of the optimization function was carried out on the basis of minimization of wave aberrations of the image in various points in the field. This method proved to be effective and allowed to obtain high image quality for several optimization iterations.
Thus, Fig. 7 shows graphs of astigmatic segments confirming that in the lens the magnitude of astigmatism for the main wavelength does not exceed 0,01 mm, the curvature of the field is 0,003. The amount of distortion does not exceed 2%.
It follows from Fig. 8, that for all points of the field, both the mean-square and the largest geometric dimensions of the scattering spots do not exceed the dimensions of the diffraction scattering spots. In other words, the resulting lens is diffraction limited, as evidenced by the frequency contrast characteristic (MTF) and the spot energy concentration function (PSF).
Next, the modulation transfer function is presented in Fig. 8 and the graph of the energy concentration function is in Fig. 9.
To analyze the size of the aberration scattering spots for different points of the field, a scatter diagram of the light beam scattering is plotted in Fig. 8. This diagram qualitatively illustrates the distribution of light energy and quantitatively determines the dimensions of the spots.
Fig. 7. Graph of astigmatic segments and distortion
Fig. 8. The result of calculations of scattering spots for different
points of the field
Fig.9. PSF for different points of the field
Fig.10. Graph of energy concentration function
The results of the calculation of the frequency-contrast characteristic, presented graphically in Fig. 9, show that at a frequency of 33 lines/mm, the contrast transfer coefficient for all points of the field has a value of at least 0,7. From graph 10 it follows, that in a spot with a diameter of 0,02 mm, equal to the step of the receiver, not less than 80% of energy is concentrated for all points of the field
Conclusion
In conclusion, it is noted that the use of results of this study allows us to lay down on the circuit design level of the device such technical requirements for the development of lens optics that will ensure the creation of products with characteristics not inferior to foreign analogues.
On the other hand, if the objective of the enterprise is to develop the optical element base of SWIR devices, then, when creating new lenses, scientific and technical interest is represented not only by those values of characteristics that correspond to most models on the market, but also values that are rare: shorter-focus, with large fields, more luminous, longer focal lengths, lenses with a discrete or pancratic focal length change, etc.
Based on the results of the analysis of SWIR lenses of a range of foreign manufacturers, the calculation was carried out with the most competitively capable characteristics and the development of the optical scheme of the lens.
REFERENCES
1. Tarasov, V. V. and Yakushenkov Y. G. Modern state and prospects of development of foreign thermal imaging systems // Scientific and technical journal of information technologies, mechanics and optics. - 2013. - No. 3 (85). - Pp. 1-13.
2. Kowa Company [Electronic resource] - Access mode http://www.kowa-eu-rope.com/lenses/en/index.php. - Ver. from the screen.
3. Company Xenics [Electronic resource] - Access mode http://www.xenics.com/en. - Ver. from the screen.
4. Edmund optics [Electronic resource] - Access mode https://www.edmundoptics.com/. -Ver. from the screen.
5. Khatsevich, T. N., & Parfenova T.V. Two-band lenses for the infrared region of the spectrum In Sbornik materialov Interekspo Geo-Sibir'-2011: Mezhdunarodnoy nauchnoy konferentsii: T. 1. Sib0ptika-2011 [Proceedings of Interexpo GEO-Siberia-2011: International Scientific Conference: Vol. 1. Sib0ptics-2011] (pp. 69-72). Novosibirsk: SSGA [in Russian].
6. Krasnykh А.С. Khatsevich Т.Н. Investigation of materials for the development of optical systems of the SWIR range [Text] / А.С. Krasnykh, Т. Н. Khatsevich - LXIV student scientific conference SGUGiT, 2016.- Pp.103-104 [in Russian].
© А. С. Красных, Т. Н. Хацевич, Н. Н. Мордвин, 2018
