ИССЛЕДОВАНИЕ АБЕРРАЦИОННЫХ СВОЙСТВ И ВОЗМОЖНОСТИ ПАССИВНОЙ АТЕРМАЛИЗАЦИИ ОПТИЧЕСКИХ СИСТЕМ, РАБОТАЮЩИХ В ОБЛАСТИ 8 12 МКМ СИСТЕМ

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1 УДК ИССЛЕДОВАНИЕ АБЕРРАЦИОННЫХ СВОЙСТВ И ВОЗМОЖНОСТИ ПАССИВНОЙ АТЕРМАЛИЗАЦИИ ОПТИЧЕСКИХ СИСТЕМ, РАБОТАЮЩИХ В ОБЛАСТИ 8 12 МКМ СИСТЕМ Галина Эдуардовна Романова Университет ИТМО, , Россия, г. Санкт-Петербург, Кронверкский пр., 49, кандидат технических наук, доцент кафедры прикладной и компьютерной оптики, тел. (812) , romanova_g_e@mail.ru Гжегож Пысь Варшавский политехнический университет, , Польша, Варшава, пл. Политехники, 1, кафедра инженерной фотоники, студент, grzegorzpys@gmail.com Работа посвящена исследованию возможности создания ИК оптических систем с пассивной компенсацией терморасфокусировки. Представлены некоторые результаты для двухи трехлинзовых систем. Ключевые слова: инфракрасная оптика, расчет оптических систем, термоаберрации, атермализация, ахроматизм. RESEARCH OF ABERRATION PROPERTIES AND PASSIVE ATHERMALIZATION OF LWIR OPTICAL SYSTEMS WORKING WITHIN SPECTRAL RANGE 8-12µm. Galina E. Romanova ITMO University, , Russia, Saint-Petersburg, Kronverksky pr., 49, Ph. D., associate professor of Applied and Computer Optics Department, tel. (812) , romanova_g_e@mail.ru Grzegorz Pyś Warsaw University of Technology, , Poland, Warsaw, Pl. Politechniki 1, Department of Photonics Engineering, student, grzegorzpys@gmail.com The work is aimed to research of possibility of designing and creating of the IR systems with thermal defocus compensation. Some results for two- and three-lens system are given. Key words: infrared optics, optical system design, thermoaberrations, athermalization, achromatism. 1. Introduction Thermal imaging camera was initially developed for military applications and for many years this technology was restricted to military and defense, academia and narrow niche markets. However, due to developments in uncooled IR imaging technology and availability of affordable IR cameras, the IR imaging market has witnessed a shift in focus towards the applications related to commercial segment [1, 2]. Now, thermal imaging cameras are widely used around the world for applications as diverse as microelectronics, security, surveillance, firefighting, paper processing, automotive, plastics, assessment of materials, target heat signatures, mechanical testing 10

2 and much more. Value of global infrared optics market is substantially increasing every year. Usual working temperature of IR systems is situated in range from -40 up to +60 Celsius degrees. Image quality of such systems should be stable over the whole working temperature range. Due to temperature dependence of properties of optical materials, lens housing and other mechanical parts it is a very difficult task to keep the required system stability. There are several compensation methods of the most considerable thermal aberration - thermal defocus [3, 4]. Among these methods are active and passive ways of thermal compensation. The passive compensation ways are the most attractive because complicated mechanical parts or devices are not required. One of the ways is to find such a combination of optical materials for particular housing materials that provides passive correction of thermal defocus. The method of designing of IR systems based on simple ideas was proposed a few years ago [5]. During designing we can choose a set of optical powers that eliminates field curvature in the system. The next step is a proper designing way of components to correct spherical aberration and coma in each component separately. For materials in IR these aberrations are not significant, that is why components could be rather simple. Astigmatism is corrected in the whole system and as a result the system is free of all monochromatic aberrations except distortion. This method does not include chromatic aberration correction and analysis of thermoaberrations. The work is aimed at developing and improving of LWIR (spectral range 8-12µm) system design methods. The analysis of thermoaberrations starts with analysis of possibilities of chromatic and thermal defocus correction in two- and three-component systems. These components might be applied in previously developed method. Formulas for calculations of components with chromatic and thermal defocus correction are presented. Also research results of thermal defocus correction possibilities and examples of designed systems with different characteristics are given. Results of the work can be helpful for designers to find optimal material combination for further designing of thermostabilized systems working in IR region. 2. Material review We consider several manufactures for IR materials that could be used for IR systems: Schott chalcogenide glasses IRG22-IRG26 [6] Infrared crystals (Ge, GaAs, ZnSe, ZnS, ZnS MS) [7] BD-2 glass [8] Main optical and thermooptical properties are summarized in table

3 Material General specifications of optical materials Table 1 Refractive index dn/dt TCE Transmission [%] Abbe number 7µm 8µm 9µm 10µm 11µm 12µm for 10.6µm at 20 C 8µm 9µm 10µm 11µm 12µm 8-12µm IRG E E IRG E E IRG E E IRG E E IRG E E BD E E Ge E E GaAs E E ZnSe E E ZnS E E ZnS MS E E In the table 1: dn/dt Temperature Coefficient of Refractive Index, TCE Thermal Expansion Coefficient. Refractive index for 7 µm column is required for glass fitting method in Zemax which demands six wavelengths to define a new optical material. 3. Microbolometer matrices review. For commercial devices uncooled matrices can be applied. They have not as high temperature resolution as cooled matrices but it s usually enough for many purposes. Main features of matrices [1, 2]: Resolution range : from 160x120 up to 1024x768 [pixel] Spectrum response : 8-14μm Pixel size: 17-25μm Material: resistive amorphous silicon Working temperature range: from -40 C to +60 C Producer ULIS ( SCD ( Microbolometer matrices. Table 2 Resolution / Pixel Spectral Model Weight [g] working area size [µm] response [µm] UL x <30 UL x <6.5 UL x <6.5 PIC0384O x <7 UL x <6.5 UL x <5 Bird XGA 1024x Bird x Bird x Bird x

4 4. Creating model of materials in Zemax. Glass fitting method. It is necessary to create an appropriate material model in optical design software to be able of evaluating thermooptical properties [9 11]. Refractive indices of each given material for consecutive temperatures should be calculated by using following formula : 13, (1) where T temperature, n T refractive index at temperature T, temperature change of refractive index. Temperature range: from -40 C to +55 C. All materials in Zemax catalog are defined by coefficients of "Sellmeier 1" formula [9, 10] 5. Ways of compensation of thermal aberrations. Among the ways of compensation there are two group: 1. Mechanical passive mechanical athermalization active mechanical athermalization 2. Optical The focus position of optical system changes along with temperature. Mechanical athermalization is simply based on passive or active refocusing. Both methods are automatic. Passive ways of refocusing often involve materials with abnormal thermal expansion coefficients to maintain focus without any powered drive mechanism being required. This method is not sufficient in infrared bandwidth. Active compensation of thermal defocus involves the computation of focus compensation algorithms that are stored and implemented by a powered device. Athermalization of the focus position of an optical system by choice of proper optical materials can be obtained if the following conditions are satisfied [3,4]: 1. Power:, (2) 2. Achromatism: where 3. Athermalism:, (3), (4), (5) thermal glass constant (normalized thermal change of optical power), number of thin lens elements, optical power (reciprocal of focal length), Abbe number, temperature change of refractive index, thermal expansion coefficient of lens material, thermal expansion coefficient of housing material. These three conditions (Eq. 2-4) require three different materials.

5 6. Achromatic and athermal doublet (small air gap). Optical powers of thin elements in achromatic two-lens system can be calculated by using following formulas:, (6), (7) where optical power of thin elements, optical power of system (for clarity of calculations assumed,, (8) V Abbe number, refractive index for wavelength, refractive index for wavelength, refractive index for wavelength. Thermal expansion coefficient of housing can be calculated by using the third condition for athermalization (Eq. 4): Only few obtained values of the coefficient correspond to real material (positive) values. Table 3 Optical material combinations corresponding with positive value of thermal expansion coefficient of housing ( ). Optical power of the system Material A Material B IRG24 ZnSe IRG24 ZnS IRG24 ZnS MS IRG26 ZnSe IRG26 ZnS IRG26 ZnS MS Achromatic and athermal triplet Optical powers of thin elements in three-lens system which satisfies all three conditions (Eq. 2-4) can be calculated using formulas: (9), (10), (11), (12) where optical power, thermal glass constant, Abbe number (Eq. 8). 14

6 Sample data table. Optical powers of lens elements. Housing material - Invar Material A Material B Material C Power A Power B Power C Ge IRG22 IRG23 IRG IRG IRG IRG BD GaAs ZnSe ZnS ZnS MS IRG IRG IRG IRG BD GaAs ZnSe ZnS ZnS MS Sample data table. Optical powers of lens elements. Housing material Steel 410 SS Material A Material B Material C Power A Power B Power C Ge IRG22 IRG23 IRG IRG IRG IRG BD GaAs ZnSe ZnS ZnS MS IRG IRG IRG IRG BD GaAs ZnSe ZnS ZnS MS Table 4 Table 5 15

7 Sample table. Optical powers of lens elements. Housing material - Aluminum Table 6 Material A Material B Material C Power A Power B Power C IRG IRG IRG IRG22 BD GaAs ZnSe ZnS ZnS MS Ge IRG IRG IRG IRG IRG23 BD GaAs ZnSe ZnS ZnS MS IRG IRG IRG IRG GaAs IRG BD ZnSe ZnS Ge ZnS MS IRG IRG IRG IRG ZnSe IRG BD GaAs ZnS ZnS MS Conclusion Passive athermalization is a very attractive way to construct IR systems working in a wide temperature range. It s possible to compensate thermal defocus if proper materials for lens and its mount are used. In that case it s necessary to find material 16

8 combination that allows not only athermalize the system but to correct chromatic aberrations also. In the work research results for two- and three-lens systems are given that can be useful for designing IR systems. REFERENCES: 1. Infrared imaging sensors for IR cameras [electronic resource] Semiconductor Devices. Application [electronic recourse] Handbook of optics. Volume I: Fundamentals, techniques and design. McGraw-Hill Professional, Riedl J. Max Optical Design Fundamentals for Infrared Systems. Second Edition, The Society of Photo-Optical Instrumentation Engineers SPIE, A.P.Grammatin, K.T.Chan Technique for constructing starting systems for thermal-viewer lens objectives // Journal of Optical Technology, Vol. 75, Issue 7, pp , IR materials [electronic resource] 7. Optical materials [ electronic resource] 8. Infrared optics [ electronic resource] 9. Sanjay Gangadhara. Fitting Index Data in Zemax [ electronic resource] Sanjay Gangadhara. How Zemax Calculates Refractive Index at Arbitrary Temperatures and Pressures [ electronic resource] Mark Nicholson. How to Create a New Glass Catalog [ electronic resource] Г. Э. Романова, Г. Пысь,