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United States Patent m
Hall et al.
[ii] 4,425,502  Jan. 10,1984
 PYROELECTRIC DETECTOR
 Inventors: Raymond F. Hall, Crawley; Andrew A. Turnbull, Reigate, both of England
 Assignee: U.S. Philips Corporation, New York, N.Y.
 Appl. No.: 269,307
 Filed: Jun. 2,1981
 Foreign Application Priority Data
Jun. 2, 1980 [GB] United Kingdom 8017945
 Int. C1.3 G01J 1/00
 U.S. CI 250/338; 250/349
 Field of Search 250/338, 339, 340, 342,
250/352, 353; 374/208, 163
 References Cited
U.S. PATENT DOCUMENTS
3,539,803 11/1970 Beerman 250/338
3,581,092 5/1971 Pearsall et al 250/349
3,813,550 5/1974 Abrams et al 250/338
3,839,640 10/1974 Rossin 250/353
U.S. Patent Jan. 10, 1984 Sheet 1 of 2 4,425,502
The invention relates to a pyroelectric detector and to a method of making such a detector. 5
More specifically, one aspect of the invention relates to a pyroelectric detector comprising an element of pyroelectric material for generating electrical charges at two opposed faces of the element when the temperature of the element changes, flexible film means resil- 1° iently supporting the element, the element being substantially less flexible than the flexible film means, and a respective electrical connection to each of said faces for detecting the electrical charges, wherein the electrical connection to one face comprises an electrically con- 15 ductive layer extending along and supported by the flexible film means.
Pyroelectric detectors are used for detecting infrared radiation, for example in the wavelength range of 8-14 u.m. A pyroelectric detector generally comprises an 20 element of pyroelectric material with two electrodes respectively on opposite faces of the element. When the temperature of the element changes, for example as a result of the incidence on the element of radiation from
a scene being viewed, electrical charges are generated at the electrodes. If the element is arranged as a capacitor in a suitable amplifying circuit, the resultant current or the voltage developed across a suitable resistor can be detected. Since the pyroelectric charge is produced 3Q only while the temperature of the element is changing, , it is necessary for the temperature to be varied continuously to obtain a continuous electrical signal. This may be done by chopping the incident radiation at a uniform frequency, the element being exposed to radiation at a ^ reference temperature while the radiation from the scene being viewed is cut off.
Pyroelectric materials are also piezoelectric, and hence a pyroelectric detector will produce an electrical output if it is subjected to vibration which subjects the 40 pyroelectric element to varying stress, for example if the detector is mounted on a vehicle. This phenomenon, termed microphony, constitutes undesired background noise which interferes with the detection of radiation.
The pyroelectric element in commercially available 45 detectors is generally rigidly mounted, but it has been found that such a manner of mounting can result in an undesirably large degree of microphony. It is also known (see, for example, Applied Optics, Vol. 7, No. 8, September 1968, pp. 1687-1695) to mount a pyroelec- 50 trie element on a thin plastics film with an electrically conductive layer on the film providing an electrical connection to an electrode adjacent the film. Such a manner of mounting may reduce microphony, and has the additional advantage of providing a path of only 55 low thermal conductance from the element, thereby increasing the responsivity of the detector at low chopping frequencies. However, it has been found that the detector may still be susceptible to significant microphony. Furthermore, a lead providing an electrical 60 connection to the electrode remote from the film may constitute a path of fairly high thermal conductance.
According to a first aspect of the invention, a pyroelectric detector as set forth in the second paragraph of this specification is characterised in that the electrical 65 connection to the other face comprises a further electrically conductive layer extending along and supported by the flexible film means.
The element can thus be supported and electrical signals derived therefrom via the electrical connections in such a manner that the element is subject to substantially less stress and has substantially smaller thermal losses than in known detectors and furthermore is relatively isolated from vibration. The combination of the flexible film means, which support the element, and the electrical connections to both faces into what may effectively be a single mechanical integer may substantially remove a potential cause of strain that exists in known detectors where the means for supporting the element and the electrical connection to at least one of the faces are mechanically separate. In the latter case, the electrical connection may impose a substantial additional mechanical constraint on the system which comprises the element and its supporting means. The electrical connection differs from the supporting means both in position and in mechanical characteristics, and is likely to co-operate with the supporting means in exerting a stress on the element, especially when the detector is subjected to shock or vibration. In a detector embodying the invention, the flexible film means and the electrical connections can accommodate their shapes more readily to the mechanical constraints imposed by the element of pyroelectric material, which may be relatively stiff but fragile, than can known combinations of supporting means for the element and electrical connections at least one of which is independent of the element supporting means, particularly if such an independent connection is sufficiently stiff to be at least partly self-supporting (as is generally the case with a wire).
In addition, an electrically conductive layer may readily be provided with a cross-sectional area substantially less than that of a connecting wire which typically may have a diameter of 25 /xm with a small element and hence the total thermal conductance between the element and its surroundings may be reduced.
The electrically conductive layers may extend from the element along, and be supported by, a common flexible film. This may simplify construction in that only a single film may be required. Said other face may be remote from the common flexible film and the electrical connection thereto may comprise another electrically conductive layer extending over another face of the element between said two opposed faces.
Preferably, the electrically conductive layers extend from the element along, and are supported by, respective flexible films. This may be a simpler and more reliable way of mounting the element is mounted between the films. The mounting of the element may then by symmetrical and the element may be protected by the films. The films may be secured to one another around the element by an adhesive. This can help to provide additional protection for the element and may be used to hold the films against said faces of the element without having to provide a bond directly between the element and a film.
An electrical connection to a face of the element may comprise a distinct electrode provided on said face by, for example, vacuum deposition. The electrically conductive layer may be on a surface of a flexible film remote from a face of the element, with the electrical connection to that face provided by capacitative coupling through the film between the layer and such an electrode on the face or the face itself.
The flexible film means may consist of a plastics material, such as a polyimide which has good mechanical