5 September 2003 Study of laser reflection of infrared cameras with germanium optics
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Infrared cameras are widely used in today's battlefield for surveillance purpose. Because of retroreflection, an incident laser beam entering the camera optics results in a beam reflecting back to the direction of the laser source. An IR detector positioned close to the laser source can then detect the reflected beam. This effect can reveal the location of the cameras and thus increases the risk of covert operations. In the present work, the characteristics of the retroreflection is studied. It is found that the reflection intensity is high when the incident beam enters through the middle part of the lenses while it is low and the beam is diverged when entering through the outer part of the lenses. The reflection is symmetric when the incident beam is normal to the lenses while asymmetric when it is incident with an angle to the lenses. In order to study the potential effects on retroreflection of modified camera optics, IR low index slides (ZnSe and KCl with refractive indices of 2.49 and 1.54, respectively) with different thicknesses (2mm, 4mm and 6mm) are placed in the optical system. The result shows that the focal point of the lenses is changed by the addition of the slide but the optical paths of the reflection remain unchanged. The relationship between the different slides and beam intensity is also studied.
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Patrio Chiu, Ishiang Shih, S. Shi, and Philips Laou "Study of laser reflection of infrared cameras with germanium optics", Proc. SPIE 5075, Targets and Backgrounds IX: Characterization and Representation, (5 September 2003); doi: 10.1117/12.497585; https://doi.org/10.1117/12.497585

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