Study on Design and Fabrication of Blackbody Simulator for Image Non - Uniformity Correction of Long-Wave Infrared (8 - 12 m) Thermal Cameras

MINISTRY OF EDUCATION AND TRAINING VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY ...*** Nguyen Quang Minh Study on Design and Fabrication of Blackbody Simulator for Image Non-Uniformity Correction of Long-Wave Infrared (8-12 m) Thermal Cameras Major: Optics Code: 9440110 SUMMARY OF DOCTORAL THESIS IN PHYSICS Hanoi – 2018 The doctoral thesis was completed at Institute of Physics, Graduate University of Science and Technology, Vietnam Academy of Science and Technology Supervisors: 1. Prof. Dr. Nguyen Dai Hung 2. Dr. Ta Van Tuan Reviewer 1: .......................................................................... Reviewer 2: .......................................................................... Reviewer 3: .......................................................................... This doctoral thesis will be defensed at Graduate University of Science and Technology, Vietnam Academy of Science and Technology on .....hour....., date .....month.....year..... This doctoral thesis can be found at: - Library of the Graduate University of Science and Technology - National Library of Vietnam 25 LIST OF PUBLICATIONS 1. Nguyen Quang Minh, Nguyen Van Thanh, and Nguyen Ba Thi, "Non-Uniformity of Infrared Imaging Systems using FPA and some Its Correction Techniques," in Hội nghị Hội nghị Quang học, Quang phổ Toàn quốc lần thứ VII, Session C: Optics, Laser and Applications, C-24, HCMC, Vietnam, 2012. 2. Nguyen Quang Minh, Ta Van Tuan, and Nguyen Van Binh, "Design Considerations of a Simple Optical LWIR Imaging System," in Hội nghị Quang học, Quang phổ Toàn quốc lần thứ VII, Session C: Lasers, Optics and Applications, C-32, HCMC, Vietnam, 2012. 3. Nguyễn Quang Minh and Tạ Văn Tuân, "Thiết kế ống kính tạo ảnh hồng ngoại xa cho một camera ảnh nhiệt không làm lạnh," Tạp chí Nghiên cứu khoa học và công nghệ quân sự, ISSN 1859-1043, (2013) pp. 104-112. 4. Tạ Văn Tuân and Nguyễn Quang Minh, "Phân tích một hệ quang vô tiêu vùng hồng ngoại xa," Tạp chí Nghiên cứu khoa học và công nghệ quân sự, ISSN 1859- 1403, (2013) pp. 96-103. 5. Nguyen Quang Minh and Ta Van Tuan, "Evaluation of the Emissivity of an Isothermal Diffuse Cylindro-Inner-Cone Blackbody Simulator Cavity" in Proceedings of The 3rd Academic Conference on Natural Science for Master and PhD Students from ASEAN Countries, CASEAN, Phnompenh, Cambodia, (2014) pp. 397-405. ISBN 978-604-913-088-5. 6. Nguyen Quang Minh and Ta Van Tuan, "Design of a Cylinder-Inner-Cone Blackbody Simulator Cavity based on Absorption of Reflected Radiation Model," in Proceedings of The 3rd Academic Conference on Natural Science for Master and PhD Students from Asean Countries, CASEAN, Phnompenh, Cambodia, (2014), pp.111-121. ISBN 978-604-913-088-5. 7. Ta Van Tuan and Nguyen Quang Minh, "Calculation of Effective Emissivity of the Conical Base of Isotherrmal Diffuse Cylindrical-Inner-Cone Cavity using Polynomial Interpolation Technique" Communications in Physics, vol. 26, no. 4, pp. 335-343, (2016). ISSN 0868-3166, Viện Hàn lâm KH&CN VN. 8. Nguyen Quang Minh and Nguyen Van Binh, "Evaluation of Average Directional Effective Emissivity of Isotherrmal Cylindrical-inner-cone Cavities Using Monte- Carlo Method", Communications in Physics, vol.27, no.4, pp.357-367, (2017). ISSN 0868-3166, Viện Hàn lâm KH&CN VN. 24 CONCLUSIONS From the requirements arising in practice of thermal imaging cameras research and development in Vietnam, we have chosen the topic " Study on design and fabrication of blackbody simulator for image non-uniformity correction of long - wave infrared (8-12 m) thermal cameras". The main results and new points of this thesis are: - The effective emissivity of the diffuse and isothermal cylindrical - inner - cone cavity has been calculated using the polynomial interpolation technique for the angle factor integrals describing the radiation exchange inside the cavity. The interpolation - calculated results are approximately accurate in comparison with those obtained by the analytical methods. This approach is a rather new in the practice of cavity effective emissivity calculation. - The Monte Carlo radiation absorption simulation algorithm using the 2 - dimentional, directional - diffuse surface reflection model has been developed for the system design of the cylindrical - inner - cone blackbody cavity. It can calculate the normal effective emissivity of the isothermal cavity with any system parameters. The developed algorithm is light, simple in computation and helpful in practice of radiation cavity design. - The research on system design of the cylindrical - inner - cone cavity has been implemented using the developed Monte Carlo algorithm. The system parameters of the cavity have been determined through the simulation - based optimization method. The simulation - calculated values have been verified by the polynomial interpolation technique to prove their reliability. - The blackbody simulator based on the cylindrical - inner- cone cavity with determined system design has been fabricated. It has been experimentally characterized to meet all the requirements. This blackbody simulator has been used in two-point calibration - based image non-uniformity correction (NUC) for thermal cameras in the room and field conditions. Further research direction - Study of design and fabrication of blackbody simulators for image NUC of Mid-Wave Infrared (MWIR) thermal cameras. - Research on development of efficient 2-point calibration NUC algorithm for thermal cameras developed in Nacentech. 1 INTRODUCTION Thermal imaging cameras based on infrared focal plane arrays (IR FPA) are increasingly used for day/night electro-optical observation systems. Thermal images captured by such cameras are generally degraded by fixed pattern noises (FPN). The most used Non-Uniformity Correction (NUC) technique to minimize the influence of FPN and improve the infrared image quality of thermal cameras is the linear calibration using the radiation sources such as blackbody simulators. The image NUC should be implemented regularly or instantly in field conditions when required. The blackbody simulators for this purpose are not popular and generally customized by specific needs. Thus, the topic "Study on design and fabrication of blackbody simulator for image non-uniformity correction of long-wave infrared (8-12 m) thermal cameras" is chosen and performed in this thesis to contribute an effort in solving such practical need. It is a new problem in the research and development activity of Vietnam. Purpose of thesis is to research on the efficient calculation methods and the computational tools usable for designing and fabricating the compact and portable blackbody simulator based on cylindrical-inner-cone cavity for NUC technique of LWIR (8-12 m spectral band) thermal cameras in the field conditions. Research scope of thesis: - Study on processes of thermal radiation exchange inside real cavity and cavity radiation characteristics. - Study on methods of cavity effective emissivity calculation and blackbody radiation sources characterization. - Research in development of computational tools and techniques for calculation of effective emissivity of cylindrical-inner-cone cavity. - Design and fabrication of blackbody simulator based on cylindrical-inner- cone cavity. Practical applications of created blackbody in image NUC of thermal cameras. Structure of thesis: Except the introduction and the conclusion parts, the thesis contents of 4 chapters as following: Chapter 1: Theoretical basics of blackbody radiation. Chapter 2: Methods of determination of blackbody cavity radiation characteristics. Chapter 3: Study of calculation of directional effective emissivity of cylindrical- inner-cone cavity. Chapter 4: Research in design, fabrication and characterization of blackbody simulator based on cylindrical-inner-cone cavity for image non-uniformity correction of thermal cameras. Methodology of research: the research in thesis is carried out by theoretical calculation combined with experimental methods. The main scientific and practical contributions of thesis are: 2 - Calculation of the effective emissivity of the isothermal diffuse cylindrical- inner-cone cavity using polynomial interpolation technique for the integral equations describing radiation exchanges inside cavity. This approach is almost not found in published scientific literature concerning blackbody cavity calculation till 2016. - Calculation of the normal effective emissivity of the isothermal cylindrical- inner-cone cavity using self - developed algorithm based on Monte Carlo simulation of cavity radiation. In this algorithm the interaction of radiation is modelled by a 2 - dimensional, directional - diffuse reflectance distribution function of surface. Thus, it is considerably new contribution in Monte Carlo simulation methods applied in blackbody cavity system designing. - Design and fabrication of the blackbody simulator based on cylindrical-inner- cone cavity working in 8-12 m spectral band. Achievements in this thesis are useful for image NUC of thermal cameras in room and field conditions and have meaningful contributions in practice of R&D activity, application and technical service of thermal cameras developed for special uses in Vietnam. - The research results of thesis were presented and published in scientific journals /periodicals and in proceedings of Vietnam and international conferences. CHAPTER 1: THEORETICAL BASICS OF BLACKBODY RADIATION 1.1. Radiometric quantities The therrmal radiation emitting by a surface has continuous spectrum and its energy distribution depends on radiation wavelength and direction [26,28,43]. The thermal radiation travels in space and interacts with the optical materials in compliance with the optical laws. The characteristic radiometric quantities such as radiant power (flux) , radiance L, exitance M, radiant intensity I and irradiance E are introduced. Among them, the spectral radiance in spherical coordinate system is defined as follows [26,43-45,47]: (1.3) where is the power emitted by a surface area unit into a solid angle unit around the direction ,  is the radiation wavelength, and are the angular coordinates in the spherical coordinate system. 1.2. Radiation absorption, reflection and transmission Assume that the radiation interacts with the optical material in the thermal equilibrium conditions. According to the energy conservation law, we have [44,45]: (1.12) where , , and are the radiant powers of irradiation, reflection, absorption and transmission, respectively; are the spectral reflectivity, absorptivity and transmissivity of material , respectively. 1.3. Absolute blackbody radiation 23 simulator. Suppose that at the temperatures T1  T2 the source emits the radiations and . If were the calibrated grey values of image pixels, than and can be found by solving the system of equations: (4.13) The image affected by FPN at 20C and its grey level histogram are presented in Fig. 4.29(a) and Fig. 4.30(a). The NUC results are shown in Fig. 4.29(b), Fig.4.30(b) and in the Table 4.10. The fabricated blackbody simulator also has been used to perform NUC for the thermal cameras in the field operation, independent of the weather conditions. Table 4.10: Evaluation of image non-uniformity (NU) No Blackbody temperature TPV (C) NU(/mean),(%) Before NUC After NUC 1 27 28,6 1,9 2 25 29,1 1,9 3 22 29,8 1,7 4 20 30,3 1,5 5 18 30,9 1,9 6 15 31,7 1,8 7 12 32,6 1,9 Average NU 30,4 1,8 4.6. Conclusions for Chapter 4 The system design parameters of the cavity are determined by the simulation - based optimization method through evaluating the distribution of of the cavity depending on those parameters. The results obtained by the simulation algorithm are then evaluated by the polynomial interpolation technique, which shows that their reliability is satisfactory. The fabricated blackbody simulator consists of the designed cavity, the TE heat source AC-027 which is controlled by the Yamatake SDC15 temperature controller with the Omron E52-CA1DY temperature sensor. The experimental results show that the designed and fabricated blackbody simulator meets all the technical and user requirements. It has been used to perform NUC for the LWIR thermal cameras in the room conditions with the image NU after NUC is 1,8% or is 17 times lower than those before NUC. This blackbody simulator also has been used to perform NUC for thermal cameras in the field, independent of the weather conditions. 22 4.5. Image non-uniformity correction for thermal cameras The digitalized image pixel value of the thermal camera can be represented by the linear expression [5,18,20,29,122,123]: (4.10) where is the data of position (r,c) of the input image, are the multiplicative and additive coefficients, respectively. The image non-uniformity correction includes the update of the coefficients in the Eq. (4.10) to calibrate the value of the output image. Fig. 4.29: The blackbody radiation images at 20C before (a) and after (b) NUC. Fig.4.30: The grey level histograms of the blackbody radiation images at 20C before (a) and after (b) NUC. We have set up a model of thermal camera that consists of the IR118 uncooled module based on 384x288 a-Si microbolometer FPA, the unfocal IR lens [35], the iris (aperture from 1,0...41,3 mm), and the image-forming IR lens [36]. The image uniformity of this camera is evaluated by the NU criteria. The video image of IR118 module is captured by the PX610 (Cyber Optics) frame grabber and the grey value of image pixels can be represented by the linear expression: (4.12) The image non-uniformity correction based on two-point calibration technique for this thermal camera is implemented by exposing the camera to the blackbody (a) (b) (a) (b) 3 Absolute (perfect) blackbody can absorb all incident electromagnetic radiation at any temperature, regardless of its wavelength or direction (angle of incidence). The blackbody radiation is described according to the Plank's law and its spectrum is determined by the temperature only [26,50]: (1.15) where c1 and c2 are the radiation coefficients, and are the blackbody spectral exitance and radiance at the temperature T. Blackbody radiation also is described by the Stefan-Boltzmann's and the Wien's laws. 1.4. Blackbody simulator radiation theory 1.4.1. Real body radiation The radiation capability of real body is characterized by a physical quantity - emissivity . It is defined as the ratio between radiation quantities of real body at temperature T and those of absolute blackbody at same temperature describing "blackness" of real body in comparison with absolute one [26,28,47]: (1.20) The radiation characteristics of the real body are just approximate of those of the perfect blackbody at certain temperatures and spectral ranges [51,52]. 1.4.2. Blackbody simulator cavity In practice, there are 2 kind of popular radiation sources: (i) Blackbody simulators based on cavities, and (ii) Flat plate radiation sources [26,28,30,43,50]. 1.4.2.1. Cavity shapes The radiation of isothermal cavity has the characteristics nearly like those of the perfect blackbody [26,30,47]. The radiation flux at aperture of the cylindrical-inner-cone cavity is relatively collimated and distributed similarly to those of the cylindrical one, but with smaller divergence and higher emissivity. Its uniformity is better than that of the conical shaped cavity. Even more, the cylindrical-inner-cone cavity can be fabricated in affordable, lightweight and compact forms, with large aperture and shorten cylinder length [26,41,53].., that satisfy requirements stated in this thesis. 1.4.2.2. Radiant flux from cavity surface The outgoing radiant flux from a surface in the direction (Fig.1.6) has the spectral radiance , which can be represented as the sum of the intrinsic surface radiance and the radiance of surface reflection portion [26]: (1.21) (1.22) (1.23) where is the intrinsic surface emissivity, is the surface Bi- directional Reflectance Distribution Function (BRDF) [26,28,54-56], is the 4 perfect blackbody spectral radiance at temperature T, is the spectral irradiance, and are the incident angle and solid angle, respectively. If the cavity surfaces were diffuse, the irradiation onto the surface can be represented by the angle factors describing the solid angles, under which this surface is "seeing" other ones inside the cavity [26,28,39,40,45,50]. Evidently, radiant flux of cavity surface is always greater than that of flat radiation source at same conditions (cavity effect) [26,28]. Fig.1.6: Radiant flux of blackbody cavity surface. 1.4.2.3. Effective emissivity of cavity A blackbody simulator based on cavity is characterized by the effective emissivity, , that is disimilar to the emissivity of the material, . The local spectral directional effective emissivity is primary radiation characteristic of the blackbody simulator that can be defined as [26,28,47]: (1.25) where is the local spectral radiance of surface area unit of cavity at coordinate in direction ; is the spectral radiance of absolute blackbody at reference temperature . Other effective quantity such as the total local directional , local spectral hemispherical , and total hemispherical effective emissivity can be also defined from Eq.(1.25). 1.4.2.4. Radiation temperature The cavity radiance temperature is defined as [28]: (1.30) Commonly, the term radiation temperature rather than radiance temperature is used and is defined as follows [28]: (1.31) A1 21 The IT-545 (Horiba) portable infrared thermometer is used to measure the temperature distribution on 3 areas of the conical surface: around the apex of the cone (P1), in the middle of the cone (P2) and nearby the base of the cone (P3). As presented in Table 4.7. the temperature differences between areas are in the range of 0,1C...0,3C and the temperature distribution on the conical surface can be considered quite uniform. The values TTB are a bit higher than TSV due to the temperature gradient depending on the thermal conductivity density of the cone. The differences between them become larger as the temperature offsets of the opposite surfaces increase. However, these deviations are within the acceptable range ((1K [16]). As the cylinder of cavity is short enough, so the contribution of its radiation in the normal directional radiation of the cavity is negligible. Fig.4.22: The spectral radiance of blackbody simulator measured experimentally. The radiation characteristics of the fabricated blackbody simulator are evaluated by using the SR-5000 (CI Systems) spectroradiometer. The output data of SR-5000 are the values of the spectral radiance of the measured sample source (Fig 4.22) at TSV =16, maximum wavelength  =10,2 m, corresponding to the reference temperature of the perfect blackbody T = 290K, max = 10 m. In the spectral ranges of 5,5m    8,0 m and   12,0 m, the experimental spectral radiance decreases sharply, possibly related to the absorption caused of water vapor during the measurements. The average normal effective emissivity of the cavity is defined as: (4.8) Around the wavelength =10m the effective emissivity is up to 0,999 that matched with the theoretical calculation result. In the spectral range of , is 0,973 that sati