Series of wide-angle high-aperture lenses for thermal imaging devices Текст научной статьи по специальности «Медицинские технологии»

УДК 681.7 : 535.31

РАЗРАБОТКА СЕРИИ ШИРОКОУГОЛЬНЫХ СВЕТОСИЛЬНЫХ ОБЪЕКТИВОВ ДЛЯ ТЕПЛОВИЗИОННЫХ ПРИБОРОВ

Михаил Алексеевич Михалюта

Сибирский государственный университет геосистем и технологий, 630108, Россия, г. Новосибирск, ул. Плахотного, 10, магистрант кафедры фотоники и приборостроения, тел. (913)955-04-42, e-mail: www.dark94m@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

Евгений Витальевич Дружкин

ООО «ЛУГГАР», 630136, Россия, г. Новосибирск, 18-175, ул. Троллейная, 200, оф. 306, генеральный директор, тел. (962)829-63-39, e-mail: 2496339@mail.ru

В статье рассмотрены особенности и результаты разработки широкоугольных светосильных объективов в инфракрасной длинноволновой области спектра для малогабаритных камер.

Ключевые слова: тепловизионный прибор, широкоугольный объектив, инфракрасный диапазон.

SERIES OF WIDE-ANGLE HIGH-APERTURE LENSES FOR THERMAL IMAGING DEVICES

Michael A. Mikhaluta

Siberian State University of Geosystems and Technologies, 10, Plakhotnogo St., Novosibirsk, 630108, Russia, Graduate, Department of Photonics and Device Engineering, phone: (913)955-04-42, e-mail: www.dark94m@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

Yevgeny V. Druzhkin

LUGGAR Co Ltd, 18-175, Trollejnaja St., Novosibirsk, 630136, Russia, Director General, phone: (962)829-63-39, e-mail: 2496339@gmail.com

In the article, the peculiarities of methods and results of designing compact LWIR lenses with a wide angle and high aperture are considered. The presented results will serve as the beginning for the development of a series of thermo-tunable infrared lenses with passive atermalization.

Key words: thermal imager, wide angle lens, infrared range.

Introduction

The efficiency of using thermal imaging devices in the long-wave spectral range (LWIR, from 8 to 14 ^m) is determined in part by the fact that the maximum thermal radiation of a human body as an object of observation falls precisely on this range [1]. With a large number of thermal imaging devices of the LWIR range, presented in various sources (such as literature, websites of companies producing and selling devices, brochures at exhibitions, etc.), only a small part of them is produced on the domestic element base. The development of thermal imaging devices on uncooled matrix receivers is a promising area for both military and civilian applications [2]. The main modules include a lens, a microbolometric matrix radiation receiver, an image processing module, a control module, a microdisplay, an eyepiece, an interface connector for transmitting information, a power source, while some modules can be removed, combined or supplemented depending on the particular model of the device [3].

The most important element of the device is a high-quality optical system of the lens, which forms an image in the plane of the photodetector device. For a number of applications, such as, for example, security systems, thermal imaging cameras with a large value of the angular field in the space of objects are required.

The aim of the research work is to develop a series of optical systems of wide-angle high-speed lenses, the optical characteristics and quality of aberration correction of which are oriented to the most commonly used microbolometric radiation receivers. This will allow us to formulate proposals for manufacturers to expand the element base of domestic infrared lenses with competitive characteristics [4].

This article presents the results of research and development carried out by the graduate student with a scientific guidance from the supervisor during the first semester of master's studies in the development of the first lens series.

Methods of study

Research method: computer simulation of optical elements and methods of automatic optimization of optical systems.

When developing an optical system of a wide-angle high-speed lens for the LWIR band, it is required to match the characteristics of the lens with the characteristics of the receiver. Characteristic correlation is based on the consideration of geometric, spectral, energy, and aberration factors.

Geometric matching is based on the relationship between the geometric dimensions of the receiving matrix, the focal length, and the angular field. It is based on a direct relationship between the size of the image, the value of the focal length f' and the magnitude of the angle w in the space of objects:

y = f'tga).

Since, in a wide-angle lens, the presence of distortion can change the relationship between these characteristics, lenses with the same focal length and different magnitude of the distortion may have slightly different values of the angular fields in the space of objects when they are conjugated to the same receiver. Therefore, the geometric matching includes the requirement of orthoscopy.

Spectral matching is based on the use of materials having a high transmittance for the spectral range corresponding to the spectral sensitivity of the receiver. When developing an optical system, the relative spectral efficiency of radiation is determined for the wavelengths of the working spectral range, taking into account the spectral sensitivity of the receiver used, the spectral transmission of optical materials and the atmosphere. This allows us to take into account the influence of these factors at the design stage of the optical system [5].

The coordination of the objective and the receiver from the energy standpoint, in our opinion, includes two aspects: ensuring a high relative opening of the optical system, and ensuring the same conditions for irradiating the receiver's pick-offs located at different parts of the array (in the center, at the corners and at all other points). Hence follows the requirement to ensure the telecentric path of the main rays in the image space of the object. The aberration matching is based on the fact that a level of residual aberrations is provided in the optical system that, taking into account the diffraction, will provide a high level of energy concentration at the pixel of the receiver in the image of the point object within the angular field of the camera. Thus, the synthesis of the optical system of the infrared wide-angle light-fast telecentric orthoscop-ic lens should be performed on the basis of rational use of the properties of individual optical surfaces and elements.

It was found by methods of computer experiment, using the example of a germanium lens with a relative aperture 1:1 and a focal length of 10 mm, that the lens with the smallest value of spherochromatic aberration in the spectral range from 8 to 12 ^m has a ratio between the radii of curvature of the first and second refractive surfaces equal to R2 = 1,27 R1, and faces the image plane with its concave surface. The resulting value of the proportionality coefficient between the radii is different from the one known from the relation R2 = 1,45 R1 [6] by about 20 %, and determines the shape of the high-aperture germanium lens with minimal spherical and chromatic aberrations in the LWIR range of the spectrum.

Results

Computer modeling of a lens with a similar focal length and relative aperture has shown that in order to ensure minimal astigmatism in the outgoing pupil, the lens of germanium should have the shape of a meniscus, which is turned by its concave surface to the space of objects. The results of computer modeling explain why systems with different meniscus orientations are obtained with the synthesis of optical systems of infra-red wide-angle high-speed co-optic lenses, using computer optimization methods: the first and last menisci face each other with their concave surfaces -

unlike from optical systems with small angular fields, in which the menisci are oriented by concave surfaces to the plane of the images [7].

The obtained results are the basis for the development of an optical system of infrared wide-angle light-fast telecentric orthoscopic lens with an angular field of not less than 50°. To reduce geometric aberrations, two meniscus were used, oriented with their concave surfaces to the plane of the receiver, and two meniscus of the opposite orientation, while the third in the course of the rays. In the case of a second optical system, an aspheric is added at the second lens. Fig. 1 represents the developed optical scheme of the IR14 / 1-40x30 objective with a focal length of 14.5 mm, a relative aperture of 1:1, angular fields in the horizontal and vertical directions, respectively 40° and 30° (the field along the diagonal of the frame is 50°). In Fig. 2 the optical scheme of the IR11 / 1-49x37 lens with a focal length of 11.7 mm and relative aperture 1:1 is presented, angular fields in the horizontal and vertical directions, respectively, are 49° and 37° (the diagonal field of the frame is 60°).

Optical systems of lenses are oriented towards the use of the Pico640 Gen2 microbolometer matrix receiver.

Fig. 1. Optical scheme of the IR14 / 1-40x30 lens

The IR14 / 1-40x30 and the IC11 / 1-49x37 lenses consist of four components arranged in the direction of the rays. Components 1, 4 of the optical system, made of germanium, face each other with concave surfaces, and similarly the components 2, 3, made of chalcogenide glass IRG25.

Fig. 2. Optical scheme of the IR14 / 1-49x37 lens

The aspherical surfaces of the IR14 / 1-40x30 objective 3 component and the IR11 / 1-49x37 lens components 2 and 3 correspond to the equation:

z =-. Cr + a1r2 + a 2 r4 + a3r6 + a 4 r8,

1 + yj 1 -(1 + k )c 2r2

where z - the coordinate of the aspherical surface; c = 1/ R - curvature of the surface; r - radial coordinate; k - conic constant; a - aspheric coefficients.

For the first surface of the lens 3 of the lens IR14 / 1-40x30:

a1 =-7,796 x 10-3 mm-1; a 2 = 4,751x 10-4 mm-4; a3 = 1,797 x 10-8 mm -5; a4 =-1,544 x 10-7. For the second surface of the objective lens 3 of the IR14 / 1-40x30 lens: a1 = 5,556 x 10-3 mm-1; a 2 = -3,214 x 10-3 mm-4; a3 =-2,603 x 10-6 mm-5; a 4 = 3,11 x 10-8. For the second surface of the lens 2 of the IR 11 / 1-49x37 lens: a1 =-8,987 x 10-3 mm-1; a 2 = 2,178 x 10-4 mm-4; a3 = -2,362 x 10-6 mm-5; a 4 = -1,39 x 10-7.

For the first surface of the lens 3 of the objective IR11 / 1-49x37:

a1 = -0,024mm-1; a2 = -2,245 x 10-4 mm -4; a3 = 1,55 x 10-6 mm-5; a4 = -5,584 x 10-7.

For the second surface of the objective lens 3 of the IR11 / 1-49x37 lens: a1 =-1,978 x10-4 mm-1; a2 = 1,299 x 10-4 mm-4; a3 =-2,168 x 10-6 mm-5; a4 = 9,667 x 10-9.

For each of the aspherical surfaces k = 0.

Technological possibilities of manufacturing of aspherical surfaces of the applied profile are available by the diamond turning method.

The distances between components are chosen so that the position of the front focal surface of components 3 and 4 is aligned with the center of the concave surface of the lens 2, which acts as an aperture diaphragm, providing a telecentric path of the main rays in the image space.

As a result of the balancing of the aberrations, it is achieved that the rms value of the transverse geometric aberration for all image points is less than the diffraction scattering spot (Fig. 3, 4).

Fig. 3. Results of the calculation of the scattering spots for different points of the field for the IR14/1-40x30 lens

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Fig. 4. Results of the calculation of the scattering spots for different points of the field for the IR11/1-49x37 lens

The results of the calculation of the frequency-contrast characteristic, presented graphically in Fig. 5-6, indicate that at a frequency of 30 lines per mm, the contrast transfer coefficient for all points in the field has a value of at least 0.5.

Fig. 5. LCS plots for different points of the field for the IR14 / 1-40x30 lens

Fig. 6. LCS plots for different points of the field for the IR11/1-49x37 lens

The results of the analysis, presented graphically in Fig. 7-8, show that the value of the energy concentration for all points of the field in a spot 0,017 mm in diameter, the size of which corresponds to the size of the pixel of the selected matrix radiation receiver, is not less than 80 %.

Fig. 7. Graph of the function of energy concentration in the image of points for the IR14 / 1-40x30 lens

Fig. 8. Graph of the function of energy concentration in the image of points for the IR11/1-49x37 lens

Comparative analysis of the image quality achieved in the optical systems of the IR14 / 1-40x30 lens and the IR11 / 1-49x37 lens with the image quality of infrared lenses used in competitive thermal imaging devices [3] allows to conclude a high level of image quality in the developed optical systems.

The IR14 / 1-40x30 lens has a mass of optical parts 11 g, overall dimensions 25x22x22 mm. The IR11 / 1-49x37 lens has a mass of optical parts 8 g, overall dimensions 22x18x18 mm.

Conclusion

In conclusion, it is noted that within the framework of the research and optical design, optical schemes for small-size wide-angle telecentric orthoscopic telescene lenses IR14 / 1-40x30 and IR11 / 1-49x37 were developed for thermal imaging cameras based on microbolometric matrix receivers. The results obtained will be used as the basis for the development of a series of thermo-tunable infrared lenses with passive atermalization.

Thanks

The work was carried out with the support of LLC "Optical Design Bureau", Novosibirsk, which provided the possibility of performing work using the Zemax13 Professional software.

REFERENCES

1. E. V. Druzhkin, T. N. Khatsevich. Realization of Technical and Special Requirements at the Development of Compact Infrared Devices // Pribory ["Instruments" Journal]. - 2018. -№ 1. -pp. 43-50. [in Russian].

2. Tarasov, V. V. and Yakushenkov Yu. 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.

3. Druzhkin, E. V., & Khatsevich, T. N. Small-sized thermal imaging devices // Optical log -2013. - T. 80. № 6. - pp. 20-27.

4. Druzhkin, E. V., Thermal imaging device for medical purposes^.en // Druzhkin, E. V., & & Khatsevich, T. N., & Brovka N, V. // Optical log. - 2015. - T. 82, № 7. 1. - pp. 15-18. - 1

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 GE0-Siberia-2011: International Scientific Conference: Vol. 1. Sib0ptics-2011] (pp. 69-72). Novosibirsk: SSGA [in Russian].

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© M. A. Muxanwma, T. H. Xa^eun, E. B. flpywKUH, 2018