Study of laser reflection of infrared cameras with germanium optics

摘要

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 reflected back to the direction of the laser source. An IR detector positioned close to the laser source can then be used to detect the reflected beam. This effect may reveal the location of the cameras and thus increases the risk of covert operations. In the present work, the characteristics of the retroreflection were studied. It was found that the power intensity of a reflected beam is high when the incident beam enters through the center of the lens while the power density is low through the outer part of the lens due to beam divergence. 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 KC1 with refractive indices of 2.49 and 1.54, respectively) with different thickness (2mm, 4mm and 6mm) were placed in the optical system. The result showed 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 was also studied.