To detect moving objects with a pyrosensor the heat radiation of the object has to be imaged into the plane in which the pyroelectric elements are located. Usually this is done with the help of fresnel lenses. These lenses are made of high-density polyethylene (HD-PE). Due to their special construction these lenses are very thin. This is necessary because the heat radiation of the human body is strongly attenuated while passing the lens material. The shorter the optical path through the lens, the less the damping of the radiation. A Fresnel lens is usually built of several lens sections, which are located on a curved surface. The sections are oriented in different directions in space. By proper design of the different section orientations, the sensitivity angle of the movement detector can be adjusted.