DEVELOPMENT OF THE INFRARED INSTRUMENT FOR GAS DETECTION Ching-Wei Chen 1, Chia-Ray Chen 1 1 National Space Organization, National Applied Research Laboratories ABSTRACT MWIR (Mid-Wave Infrared) spectroscopy shows a large potential in the current IR devices market, due to its multiple applications, such as gas detection, chemical analysis, industrial monitoring, combustion and flame characterization. It opens this technique to the fields of application, such as industrial monitoring and control, agriculture and environmental monitoring. However, a major barrier, which is the lack of affordable specific key elements such a MWIR light sources and low cost uncooled detectors, have held it back from its widespread use. In this paper an uncooled MWIR detector combined with image enhancement technique is reported. This investigation shows good results in gas leakage detection test. It also verify the functions of self-developed MWIR lens and optics. A good agreement in theoretical design and experiment give us the lessons learned for the potential application in infrared satellite technology. A brief discussions will also be presented in this paper. KEYWORDS : MWIR, uncooled detector, image enhancement, gas leakage 1. INTRODUCTION The goal of this research is to develop the optical imaging system, which is the most critical core component of infrared image detection equipment for gas leakage, and to design and build an MWIR optical prototype system. It is complemented by the actual volatile organic compounds (VOC) experiment to validate the test. Time - domain and spatial - domain image enhancement techniques reduce image noise. The main development contents are as follows: The optical lens of MWIR band is set up, the lens group is made of CaF 2 material. The optical design is based on the center wavelength of 3.3-3.5μm. The focal length of lens is 100mm. The field of viewing angle is 5.5 degree, which corresponds f-number 2.2. The self-developed lens combined with the commercial MWIR image sensor, a well funcional imaging system is accomplished. The overall work includes optical lens design, system integration, gas detection test and verification work is accomplished in National Space Organization (NSPO). 2. EXPERIMENT 2.1 Design This project is a feasibility study, which display the design, integration and verification ability of the uncooled MWIR imaging system. Therefore, an uncooled, low-cost and low-resolution 80x80 array of image sensors, TACHYON 6400 CORE module, is used in this study [1]. This module, shown in Fig. 1, has a response wavelength in the range of 1.0 to 5.0 μm. A1
Figure 2 Optical design of lens Figure 1 TACHYON 6400 CORE module In this research, the optical lens, suitable for MWIR band, is developed by National Space Organization (NSPO) and manufactured by domestic optical lens manufacturers. The lens group is made by CaF 2 material, having high transmittance properties in our interested wavelength range. The optical design is based on the central wavelength of 3.3-3.5μm, the fixed effective focal length 100mm. The field of viewing angle is 5.5 degrees, which correspond the F-number of F / 2.2. Optical lens design is low cost and with good optical performance. The architecture of the optical design is shown in Fig. 2. This lens group contains four spherical mirrors, which reduce the manufacturing difficulty to the domestic optical lens manufacturers. Besides, spherical lens is easy to manufacture and testing, and relatively low cost. The lens size is about 2 inches in diameter. The first surface to the last surface of the lens group correspond to the total length of about 95mm. According to the Nyquist sampling theorem, to make the signal independent of each other, it is necessary to make the spatial sampling frequency of the system more than 1/2. The pixel size determine the space frequency of the MTF, so 30μm corresponding to spatial frequency of 16.7 lp / mm for this system. The optical system, which is for human eye or automatic identification, required to achieve the MTF greater than 0.3 at any viewing angle of 16.7 lp / mm space frequency. The MTF of this optical system design is shown in Fig.3. The MTF at 16.7 lp / mm at each field angle is greater than 0.31. The aluminum mechanical tube is also manufactured to mount and assembly the four CaF 2 spherical lens group. The tube is quite stable and also has the function to adjust the focus position between the sensor and the lens group in order to obtain the clear image. A mid-infrared band gas leak detector system can be considered completed now, its photo is shown in Fig. 4. A2
improved. Figure 6 shows the filter components and holder fixture. Figure 3 MTF value of the lens group Figure 5 Transmission spectrum of the filter Figure 4 MWIR gas leakage detection system Due to the difficulties and limitations of the large-band width anti-reflective coating technology, the system is introduced a mid-infrared band filter to block the noise in the visible light. As shown in Fig. 5, the cutting wavelength of this filter is about 2.4μm. It shows that the transmittance of this filter is close to 0 before the wavelength of 2.4μm, and the transmittance is as high as 95% after the wavelength of 2.4μm. Therefore, the interested wavelength range 3.3-3.5μm in our experiment is with quite high transparency. The size of the infrared band filter is about 1 inch in diameter. A holder is designed to put it inside, and be fixed between the lens tube and the sensor. The filter not only achieve the effective filtering, the signal to noise ratio (S / N ratio) of this experiment can also be Figure 6 Filter holder and the lens 2.2 System architecture The infrared band gas leak detector system is self-developed from optical design, manufacturing, assembly and integration. The next work will be carried out volatility gas detection test verification. The experimental system architecture shown in Fig. 7. The optical system is connected with the computer by the USB interface, and the commercial software is used to data acquisition. The stored raw data is the experiment images. Between the lens and the radiative background source is placed a volatile alcohol. The alcohol in the experimental process continuously release the volatile alcohol vapor, which provide the gas leak target in this image capture test. A3
Figure 7 Experimental setup In this experiment, we prepared a high-temperature furnace to provide a stable mid-infrared band of radiative source emittor, as shown in Fig 8. The furnace can provide a very stable background value for mid-infrared band detection test. According to the data of the relationship between the wavelength and intensity of black body radiation at different temperatures shown in Fig. 9, the peak valure of the wavelength in blackbody radiation located at about 3.2μm is the temperature around 900K. It corresponds the temperature of the furnace is about 600 C. It is found that the background value of the mid-infrared band is stable and good under the temperature operation. It is helpful for the experiment of the detection of volatile gas leakage in the mid-infrared band. The alcohol vapor gas molecules vibration absorption energy level is located at the mid-infrared wavelength range. Therefore, there will be a dark region shown in the computer software when the gas passed through the background infrared light source. Therefore, according to this principle and experimental methods, it can achieve the purpose of volatile gas leak detection. Figure 8 High temperature furnace as the infrared emittor Figure 9 Black body radiation relation 2.3 Results The left-side in Fig. 10 shows the data of original capture image, is not through the post-processing. While the right-side of the experimental data, through the gas leakage enhancement algorithm to improve the image results of gas leakage. The figure clearly shows that the image of the gas leak in the picture showing a black region of smoke. According to the experimental results, it is confirmed that the design and manufacture of A4
this system can meet the original design specifications, and the experimental data and results can meet the purpose of this project. 4. FUTURE WORK This project is to provide a stable high-temperature furnace in the infrared band of radioactive sources to confirm the practicality of this system and the feasibility of the design, but the actual application to consider, this radioactive source is not mobile use, so In the future, R & D will focus on the development of small-sized, high-intensity mobile handheld mid-infrared light sources, in addition to the role of auxiliary light source to provide the background of the mid-infrared radiation source to enhance the practical value of this gas leak detection system. Figure 10 Experimental results In addition, the zoom optical lens is also the focus of future development, according to the different sources of gas leakage to change the focal length of the lens can effectively enhance the overall image resolution, and in actual use can not replace the lens, can increase Its use of convenience. 5. REFERENCE 3. CONCLUSIONS An uncooled MWIR detector combined with image enhancement technique is presented. Gas leakage images can be detected and enhanced in this research. The sensitivity and response time of sensor could be improved for the widespread further applications. Besides, zoom lens design and assisted external infrared light source are also the key components for the practical use. It is suggested to upgrade the frame rate of detector and calculation speed of algorithm for future satellite applications. [1] R. Linares-Herrero, G. Vergara, R. Gutiérrez Álvarez, C. Fernández Montojo, L. J. Gómez, V. Villamayor, A. Baldasano Ramírez, M. T. Montojo, Variable filter array spectrometer of VPD PbSe, Proc. SPIE 8354, Thermosense: Thermal Infrared Applications XXXIV, 835412 (18 May 2012). A5