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The surface emissivity of the object being imaged is the biggest unknown affecting infrared thermal measurements. At present there is no model to describe the precise relation between the skin temperature and the body temperature. The practical way of correction is just to setanoffset, say 0.5 ◦ C. Further work has to be done to explore this issue for the thermal camera in SARS application. The emissivity should be determined for each specific measurement situation. Other factors that can introduce uncertainties are ambient temperature, atmospheric transmittances, path radiance, as well as the intrinsic non-linearities of the instrument’s electronics. Remote infrared calibration sources placed in the detection plane can help to suppress the uncertainty from environment. Typical radiometric system has a radiometric accuracy of 2%, which corresponds to a temperature accuracy ±2 ◦ C for a detection range of −20 ∼ 500 ◦ C. Since human skin temperature varies in a small range, narrowing down the detection range (i.e. 0 ∼ 40 ◦ C) greatly reduces the instrument errors. Now, many infrared cameras manufacturers are able to achieve ±0.2 ◦ Cwithadetection rangeof0− 40 ◦ C with the above improvements. References [1] R.James Seffrein, “Thermal Imaging for Detecting Potential SARS Infection”. Photonics Research Centre Seminars in 2003 Date Speaker Topic [2] Maxtech International Inc., http://www.maxtechintl.com/infrar.htm [3] http://www.howstuffworks.com/nightvision.htm [4] Joel Yang from Network Research Centre (NTRC) is currently working on a project in collaboration with Photonitech Pte Ltd., Singapore. [5] A. Rogashki, “Infrared Photon Detectors”, Bellingham, Washington: SPIE Optical Engineering Press, c1995. [6] P. W. Kruse, “Principles of Uncooled Infrared Focal Plane Arrays” in Semiconductors and Semimetals, vol. 47: Uncooled Infrared Imaging Arrays and Systems ed by P.W. Kruse and D.D. Skatrud, pp. 17–42, San Diego: Academic Press, c1997. [7] John Wallace and Kathy Kincade, “Infrared cameras find hot spots in nuclear reactor”, Laser Focus World, July 2003. (http://lfw.pennnet.com/Articles/Article_Display.cfm? Section=ARTCL&ARTICLE_ID=182133&VERSION _NUM=1) 24 January A/P Chin Mee Koy Photonic Integrated Circuits 7March Ast/P Yu Siu Fung Modeling of Optoelectronics Devices - Past, Present and Future 24 March Prof. Paul French Imperial College, U.K. Time-resolved Fluorescence Microscopy 24 March Prof. Kenny Weir Imperial College, U.K. Fibre Optic Sensors 4April Ast/P Ricky Ang Lay Kee Intense Beam Interaction with Surrounding Structures 25 April Ast/P John Ngo Quoc Nam Electrically tunable dispersion compensator with fixed center wavelength using fiber Bragg grating 5 June Ast/P Pita Kantisara Electrochromic Materials and Devices 13 August Dr. Chye Yew Hee Ferromagnetic/Semiconductor Hybrid Structures for Spin Ma- IMRE, Singapore nipulation 18 August Dr. Zhu Furong Low temperature transparent conducting oxide for flexible or- IMRE, Singapore ganic light emitting devices 20 August Ast/P Rajesh Menon Zone-Plate-Array Lithography (ZPAL):A novel approach to maskless nanolithography 8SeptemberDr Han Mingyong IMRE, Singapore TBD 24 hotonics'a
FEATURE ARTICLE Optical Tweezers Lee Woei Ming, Zhang Dianwen, Tao Shaohua and Yuan Xiaocong In nature, light from the Sun provides plants with the means of making food, and hence energy that sustains the cycle of life on Earth. In science, light from laser beams is providing biologists with a new means of studying the intricate details and functions within single living cells. It has become a tool to discover life itself. Laser beams used to trap and manipulate microscopic particles are called optical tweezers. The first optical tweezer was demonstrated by Dr Arthur Ashkin [1] and co-workers in the 1980s at Bell Labs. They demonstrated that light can move matter because photons carry linear momentum (a photon of wavelength λ has a momentum p = hc/λ, where h is the Planck’s constant, and c is the speed of light). In their experiment, a highly focused Gaussian beam was used to create an optical gradient force on a particle with a higher refractive index than the surrounding medium. As light refracts when it passes through the micro-objects, the refraction of light will cause a change in the photon linear momentum, and due to the conservation of momentum, a force will result which acts on the particle in the opposite direction. This force is directed towards the area of the highest intensity; hence the particle is trapped in the center of the beam. Figure 1: 1st type of optical tweezers Another type of optical tweezer is based on the orbital angular momentum property of a Laguerre-Gaussian (LG) beam, TEM∗ 0l , also known as an optical vortex [2]. In this case, the particles are trapped in the region of zero intensity at the center of the LG beam [see Fig. 2]. The optical traps created by the helical LG beams of light (due to a phase singularity) can also exert optical torques on the trapped objects. The special intensity profiles of these beams are created by engineering the phase of the laser beam’s wavefront. Figure 2: 2nd type of optical tweezers Figure 3: Irregular shaped optical tweezers In NTU, we have begun a research project on optical tweezers only in the past year. Our initial focus is to build a strong foundation for the fundamental understanding of optical tweezers. At the moment, we are able to generate irregular shaped beams to be used as a new form OPTICAL TWEEZERS September 2003 25
Page 1 and 2: hotonics'a A Publication of the Pho
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Page 7 and 8: Biophotonics Research Ast/P Ng Beng
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Page 15 and 16: Polymer Photonics Polymeric materia
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