Design of Infrared Zoom Collimator in Focal Length Testing of IR Image Simulator

Transcription

1 Design of Infrared Zoom Collimator in Focal Length Testing of IR Image Simulator DER-CHIN CHEN 1, RUNG-SHENG CHEN 2,* AND YU-CHENG SU 1 1 Department of Electrical Engineering, Feng Chia University, Taiwan 2 Department of Photonics and Communication Engineering, Asia University, Taiwan ABSTRACT An infrared (IR) target simulator produces an IR scene that includes a static background, a moving/growing target and a moving/growing flare. The simulator is computer-controlled and includes all the algorithms to transform object space inputs to control commands. The IR collimator is the subsystem of the target simulator, and it works as the standard targets placed in optical infinity (very long distance) for the tested thermal cameras. A novel infrared zoom collimator design and illumination analysis are given in this paper. The zooming results give the dynamic simulation of the IR target by zoom in or zoom out, and show the real simulated image rather than the energy pattern. The illumination analysis of an IR source is presented to achieve an optimized image quality. Key words: infrared, target simulator, zoom lens design. 1. IR TARGET SIMULATOR The combination of the source, target, and collimator is sometimes called a target simulator or target projector. An IR target simulator has the characteristics of long working distance, anti-electric noise and is one of the test instruments to evaluate the performance of an IR imaging system. Figure 1 shows an example of the IR target simulation system (Cabib et al., 2006). Figure 1. An example of IR target simulation system. On the right are the control panels and computer. * Corresponding author. rschen@asia.edu.tw 143

2 As an IR collimator is a subsystem of the target simulator, it gives an optically infinite condition in the laboratory state of a real field test. So it supplies a more convenient test environment during a research procedure. The development of an IR collimator is progressive from a target simulator to an image simulator to meet the test requirement of a precise IR imaging system (Chen & Lin, 2006; Zhao et al., 2002). The basic requirements of designing an IR target simulator can be shown as follows. 1.1 Clear Aperture of IR Target Simulator Design Collimators are lens assemblies that optically place targets at infinity. Collimators may contain either refractive or reflective elements. The collimator clear aperture should be much greater than the system clear aperture, otherwise the system output will be reduced by the ratio of the aperture areas and the measured modulation transfer function (MTF) will be modified by the collimator MTF. With small aperture collimators, the unobstructed infrared imaging system aperture can sense radiation from elsewhere. This radiation is superimposed onto the source radiation. In principle, correction factors can be applied but this approach is not recommended because the correction factors are system dependent. 1.2 Aberrations of IR Target Simulator Design Collimator aberrations should be significantly less than the aberrations of the infrared imaging system. Since aberrations are inversely proportional to the f/# or to powers of the f/#, the collimator f/# should be larger than the infrared imaging system f/#. Equivalently, the focal length of the collimator should be much longer than the focal length of the infrared imaging system. As a general guideline, the collimator focal length should be at least 5 times that of the infrared imaging system (Watanabe & Fumio, 1998). 1.3 Zooming Lens Design of IR Target Simulator Design Suitable of CCD Detector In order to have a dynamic simulation of the IR target, infrared zoom lenses are designed over a range of required focal lengths. As a result of IR detector technology, a two dimension CCD was developed to meet the specification of IR target simulator, i.e., more compact and multifunctional. Following these ideas, high performance IR zoom lenses are required to have the advantage that no mechanical parts, such as a scanning mirror, are needed and high sensitivity is expected. 1.4 Focal Length Testing of IR Target Simulator One of the major tasks of an IR simulator is to test the focal length of an IR camera. The scheme of an IR camera test is shown in Figure

3 Figure 2. Illustrating the nodal slide (upper) and the focal collimator (lower) methods of measuring focal length on the optical bench. The aim of this paper is to design an IR zoom collimator for the focal collimator method of measuring focal length shown in lower case of Figure 2. This technique is basic and applicable to a wide variety of systems. The IR focal collimator consists of an objective with a calibrated reticle (i.e. four bar target) at its focal point. The focal length of the objective and the size of the reticle must be accurately known. The test IR camera is set up and the size of the image formed by the lens is accurately measured with the measuring instrument. From Figure 2 it is apparent that the focal length (F x ) of the IR camera under test is given by: And the width of four bar target is: F = A. (1) A F x 0 F = A F x A 0 and the focal length of IR collimator objective shows: A F0 = Fx (2) A where A is the measured size of the IR CCD pixel size, A is the size of the reticle, and 0 F is the focal length of the IR collimator objective. It is apparent from Equation 1 that any inaccuracies in the values of A', A, or F 0 are reflected directly in the resultant value of the focal length. In setting up a 145

4 focal collimator it is necessary to determine the collimator constant (F 0 / A) to as high a degree of accuracy as possible to get F x. 2. IR ZOOM LENS DESIGN 2.1 System Requirements An IR zoom lens (Mikš, Novák & Novák, 2008) has a varying focal length ranged by test target spaces. This project was to establish a standard processes to design an IR zooming collimator in the waveband from 3μm to 5μm. For the first order lens design, the zooming focal lengths and image quality are the basic parameters to be determined and we can find them by the following: (1) Zooming focal lengths of IR collimator The spacing of the testing targets (A) and CCD pixel (A'), and the focal length of the test IR camera (F x ) are shown in table 1. Applying Equation (2), we get F 0 shown in the first column of Table 1. Table 1. The parameters of focal collimator method F 0 (focal length of collimator) A (the spacing of four bar target) A' (IR CCD pixel spacing) F x (the focal length of the test IR camera) 49.2 mm 25μm 49.2 mm mm 36.73μm 25μm 49.2 mm mm 65μm 25μm 49.2 mm (2) Image quality of IR collimator The Modulation Transfer Function (MTF) is the most frequently used method to evaluate the image quality of lenses. According to the Nyquist limit, the minimum frequency to be resolved is the replica of twice the spacing of the smallest bar target. The cut off frequency of MTF is found by (Chrazanowski, Lee & Wrona, 2005; Chraznowski, 2007): 1 1 Cut off frequency= = = 20( lp / mm) (mm/lp) 0.05( mm / lp) (3) Image height of IR collimator Referring to Figure 2, the image height of IR CCD corresponds to the bar target. As the largest size of the IR CCD is 13.4(mm) 10.08(mm), then the image height is half of the diagonal of IR CCD 146

5 2 2 Image height= / 2 = / 2 = / 2 = ( mm). 2.2 Zoom Lens Design of IR Collimator The idea of zoom lens design of an IR collimator comes from US patent US Refer to US patent US , for a wavelength range of 3 to 5 μ m depicted in Figure 3 scaled to an F number of 1.2 and EFL s of 50 mm to 200 mm. As shown in Figure 3, the IR zoom lens comprises five lens groups, namely, in order from the object side to the image side, a positive power first lens group comprising one or two lens elements, a negative power second lens group comprising one or two elements, a third lens group comprising a single component of a meniscus lens element with a concave object side surface, a fourth lens group comprising a single component of convex lens element, and a positive power fifth lens group comprising at least four lens elements including a positive meniscus lens with a convex object side surface arranged nearest an image plane. Figure 3. US IR zoom lens system. In Figure 3, the second and third lens groups are axially movable in predetermined relation relative to each other and relative to the first, fourth and fifth lens groups which are stationary to focus and vary the focal length of the IR zoom lens shown in Figure Novel IR zoom collimator design According to the first order scheme in 2.1 and the zooming idea from 2.2, we have the novel design to meet the requirement of an IR camera test, shown as follows: (1) To scale down the fifth lenses group shown in Figure 3. The sixth lens of the fifth group is eliminated because of its smallest power. Then the third lens of the fifth group is removed because the ray deviation is the smallest. 147

6 Figure 4. The zooming position of US , upper: wide-angle end middle: intermediate, lower: telephoto end. Figure 5. The novel scheme of IR zoom collimator. 148

7 Figure 6. The first order novel design of IR zoom collimator. (2) The fifth lenses are two filters to be removed. (3) After the above processes, the IR zoom lens collimator is modified as shown in Figure 5. The fifth lens group being changed from five lenses to three lenses. (4) Comparing the residual aberrations after the further elimination of the fifth lenses group, we can see the removing of the second lens gives less changes of aberrations. (5) Finally, we have the first order novel design of IR zoom collimator shown in Figure OPTIMIZATION OF IR ZOOM COLLIMATOR To set the parameters of lens design, we have the following guidelines for lens designers. 3.1 Thickness Setting (1) Lens central thickness (CT) In general, lens thickness is chosen to be larger than lens diameter (CA, clear aperture) 6%. (2) Edge thickness (ET) of lens A. CA 5(mm), ET 0.5(mm) B. 5<CA 50(mm), ET 1~2(mm) C. CA 50(mm), ET 2(mm) 149

8 (3) Operators of thickness used to optimize We use the operators: MNCG, MXCG, MNEG, MXEG, MNCA, MXCA, and TOTR to optimize the thickness of central glass, edge glass, air space, and total length of the IR zoom collimator. 3.2 Operators of Aberrations Used to Optimize We use LONA to optimize longitudinal aberration at target wavelength (4μm), AXCL to optimize axial chromatic aberration at the rest wavelengths (3 μm and 5 μm) and LACL to optimize lateral chromatic aberration at the rest wavelengths (3 μm and 5 μm). 3.3 Operator of Effective Focal Length Used to Optimize As usual, EFFL is used to optimize the effective focal length in different configuration (wide, mid, and telephoto). 4. IR ZOOM COLLIMATOR DESCRIPTION The final lens system description is shown in Table 2. Table 2. Lens data of IR zoom collimator 150

9 The aspheric parameters of surface 13 are a4= e-007, a6=3.7822e-12. And the multi-c onfiguration data are shown in Table 3. Then we get the novel IR zoom collimator shown in Figure 7. Table 3. Multi-configuration of IR zoom collimator Figure 7. IR zoom collimator. 5. IMAGE ASSESSMENT OF IR ZOOM COLLIMATOR The aberration behaviors and MTF are shown in Figure 8 and Figure

10 Figure 8. Longitudinal chromatic aberration of IR zoom collimator. Figure 9. MTF of IR zoom collimator. 152

11 6. DISCUSSION AND FURTHER RESEARCH From Figure 8, we can see the spheric chromatic aberration (Walker, 1995) is corrected well. Figure 9 shows the image contrast is better than 60% in the field area that fit to the specification required. Further research goes to minimize the size of the IR collimator and the diffractive optical element can be one of the options. REFERENCES Cabib, D., Buchwald, R. A., Nirkin, S., Lavi, M., Neria, O., Ben Yaakov, C., Tzafrir, E., Blau, M., & Dolev, J. (2006). Advanced, Manpower and Time Saving Testing Concept for Development, Production and Maintenance of Electro-Optical Systems. Proc. of SPIE, 6207, 62070R R-8. Chen, D. C., & Lin, C. S. (2006). Recent introduction to electro-optical engineering. Taiwan: CHWA [in Chinese]. Zhao, Y., Mao, M., Horowwitz, R., Majumdar, H., Varesi, J., Norton, P., & Kitching, J. (2002). Optomechnical Uncooled Infrared Imaging Syatem: Design, Microfabrication, and Performance. Journal of Microelectromechanical System, 11(2), Watanabe & Fumio (1998). US Patent 6,091,551, January, 26, Mikš, A., Novák, I., & Novák, P. (2008). Method of Zoom Lens Design. Applied Optics, 47(32), Chrazanowski, K., Lee, H. C., & Wrona, W. (2005). A Condition on Spatial Collimator for Testing of Thermal Imaging Systems. Optical Engineering, 39(5), Chraznowski, K. (2007). Evaluation of Infrared collimators for testing thermal imaging systems. Opto-Electronics Review, 15(2), Walker, B. H. (1995). Lens Design for the Near IR Correction of Primary Chromatic Aberration. Applied Optics, 34(34), Der-Cheng Chen received a B. S. degree in physics from Soochow University in 1980, an M. S. degree in astronomy and physics from Central University in 1983, and a Ph. D. degree in Optics and Photonics from Central University in Dr. Chen joined Feng Chia University in Taiwan in August 2004 and is an Associate Professor in the Department of Electrical Engineering. 153

12 Rung-Sheng Chen received a B. S. degree in survey engineering from Chung Cheng Institute of technology in 1982, an M. S. degree in applied optics from Reading University (U.K.) in 1989, and a Ph. D. degree in photonics from Imperial College London (U.K.) in Dr. Chen joined Asia University in Taiwan in August 2006 and is an Assistant Professor in the Department of optoelectronics and Communication Engineering. Yu-Cheng Su received a B. S. degree from the department of electronic engineering at I-Shou University in 2005 and an M. S. degree from the Department of Electrical Engineering at Feng Chia University in