Pyrodetector

Claim 1

A pyrodetector with a sharply limited directional range of detection sensitivity for the detection of a body entering into said directional range from a distance greater than the maximum sensing distance from the detector, said body having a temperature which deviates from that of its environment, comprising: a concave mirror means for focusing thermal radiation emanating from the body; a pyroelectric first sensor element positioned at the focus of the predetermined directional range of the mirror means and formed as a flat body comprising an electrically non-conductive material with permanent orientation polarization and which is provided with a pair of electrodes; cover means transmissive to the incident thermal radiation from the body and ambient thermal radiation and being positioned for protecting the first sensor element against damage and air drafts; at least one further sensor element comprised of the same material as the first sensor element and having substantially a same thickness so as to provide substantially identical heating time constants for the first and further elements; the further sensor element having a pair of electrodes and being located outside a focal plane of the minor means containing the focus and perpendicular to a focus axis but adjacent the first sensor element at a locus such that both the further sensor element and the first sensor element are subjected to an equal extent to the ambient thermal radiation emanating from said cover means; the first and further sensor elements comprising central portions of a single flat body having opposite major surfaces, and a mounting means holding the single body at edges thereof; the single flat body being aligned such that its major surfaces are parallel to and the body lies on the focus axis passing through the focus and extending in the direction of the detection range so that the first sensor and further sensor lie adjacent one another along said focus axis; the concave mirror means having a parabolic shape and the focus lying within an inner half of the mirror; the first and further sensor elements formed by the body absorbing only radiation reflected by the concave mirror means at the opposite major surfaces; the electrode pairs of all sensor elements being connected to an electrical evaluation means; the pairs of electrodes each being terminated with an impedance such that a time constant K of a decay time of the sensor element R.sub.X ·C.sub.X =K is valid for all pairs of electrodes, wherein C X is a capacitance of a capacitor formed from the respective pair of electrodes, R X is a respective terminating impedance, and K<10 seconds; and an electronic evaluation means for forming a difference between an output signal of the first and of the further sensor elements.