Infrared filter disc for radiation detectors.

p0001The invention relates to an infrared filter disc according to the preamble of claim 1. Such infrared filter discs are generally known. They serve as a radiation-permeable window for closing the radiation entrance apertures of housings of radiation detectors to protect located in the interior of the radiation detectors, pyroelectric radiation receiver from air turbulence.

p0002Radiation detectors of the above type are used, inter alia, for producing electrical signals in response to the incident radiation in the infrared intrusion detection systems and in flame detectors. For example, in the US-A-3,703,718 an infrared intrusion detection system is described in which a filter is arranged between a sensor element and a mirror for focusing infrared radiation on the sensor element to electromagnetic radiation in the range of 4.5 20 microns (body radiation of living organisms) is permeable. As an example, for use in flame detectors, US-A-4,280,058 may be mentioned in which a flame detector is described, the different infrared filter discs used which electromagnetic radiation in the range from 4.1 to 4.8 microns (the absorption band of carbon dioxide) or in the region transmit 6 to 6.7 microns (typical radiation of stray radiation).

p0003For other fields of technology, it is known for example from DE-B1-24'43'164 (Hoechst), for the protection of documents or means of payment to use thermoplastic plastic foils containing undissolved particles which radiated, invisible, electromagnetic radiation in visible light to convert. An indication of the use of such fluorescent materials for use in infrared filter discs for radiation detectors which are to have a particular permeability range in Infraot area, can the said Auslegeschrift not to enter.

p0004In WO-A-88'04'025 (GRAVISSE) filters are described, which contain materials which fluoresce in the infrared range and, in effect, prevent the passage of electromagnetic radiation in particular wavelength ranges. According to the teaching of this patent, it is not necessary that the filter in certain spectral electromagnetic radiation are permeable. The fluorescent substances described above are structurally complex and because of their radiation absorption in the range of 6 to 12 microns unsuitable as a window for passive infrared intrusion detectors-.

p0005In the above-mentioned US-A-3,703,718 an infrared intrusion detection system is described, can be used on the sensor element both mirrors and lenses in which for focusing the infrared radiation. Particularly with the use of mirrors, the interior of the housing in which the sensor is arranged largely to the atmosphere open. To protect the sensor to fluctuations of the ambient air, etc., it has proved to be advantageous to close the inlet opening of the infrared radiation in the interior of the housing through a permeable for infrared radiation window. For this purpose are particularly suitable window of polyethylene, which can easily be processed by extrusion into films or injection molded into plates. Polyethylene has the advantage that it is substantially transparent in the desired wavelength range of infrared radiation. In the case of the use of lenses as a focusing means polyethylene has the further advantage that the lenses in the form of Fresnel lenses in the manufacture of the films or sheets can be embossed.

p0006It is known (AHPfund; Journal of the Optical Society of America, Vol 24, 143 [1934].) To use powder films as an infrared transmission filter, for example those with powders of quartz, zinc oxide or magnesium oxide. Experiments were (RLHenry; Journal of the Optical Society of America, Vol 38, No. 9, 775 [1948].) Made with different materials (quartz, and zinc sulfide powder), the powder layers were examined for plates of glass or rock salt. It has been found that the factors which determine the filter transmission, the particle concentration, the particle size distribution, and the refractive indices of particle material and matrix.

p0007An window for closure of radiation inlet openings of infrared intrusion detectors include the following requirements:<ul><li>a) They are a detector, ie the area have adapted coloration, </li><li>b) they are short-wavelength light having a wavelength Keep &lt;4.0 microns by absorbing, reflecting or scattering from the sensor and</li><li>c) they should let through radiation with a wavelength in the range of 4 to 15 microns unhindered.</li></ul>

p0008In practice, as a window for infrared intrusion detectors usually used polyethylene films. The light scattering takes place in two ways, once by a (colored or colorless) pigment in polyethylene as a matrix or by micro-crystallites in the polyethylene. The micro-crystallites sprinkle with suitable dimensioning shortwave radiation (visible light), the windows appear therefore know; longwave radiation (Infraotstrahlung in the body radiation of living organisms) let through almost unhindered.

p0009Various passive infrared intrusion detectors commercially in which the light scattering in the entrance windows is performed by embedded into the polyethylene matrix microcrystallites. Examples of infrared intrusion detectors with windows that are tinted with white pigments, are for example the products marketed by Cerberus AG PIR IR210 and IR212 whose entry window appears with titanium dioxide, which is colorless in itself, but knows by scattering colored ,

p0010To increase the security against false alarms from passive infrared intrusion detectors, there has been more and more required in the course of time, to reduce the influence of disturbance light. In particular, the passive infrared intrusion detectors are to have a low sensitivity to radiation below 3 to 4 microns wavelength for flame detectors and 6 microns for passive infrared intrusion detectors. The filter directly used on the sensors, for example, with thin multiple layers of dielectric material coated germanium or silicon plates are very expensive and can not be configured so that they comply with the requirements set with a reasonable financial cost. For example, the most commonly used filters on sensors for PIR detector on the wavelength range between 1.6 microns and 2.5 microns still residual transmission peaks of a few tenths per mille. Since the sensors used, inter alia, are pyroelectric, so thermal sensors whose sensitivity is independent of the wavelength, the light of a strong headlamp in sufficient residual starch can pass through the filter, to trigger an alarm.

p0011However, is not yet sufficiently the coloring of polyethylene films with white pigments, such as by commercially available titanium dioxide, scored impermeability of the window for stray light in the range of 1.6 microns to 2.5 microns, to the requirements for protection against false alarms of passive infrared to satisfy intrusion detectors. For this, the particles of titanium dioxide available on the one hand too small, whereby their scattering effect occurs when too small wavelengths, on the other hand, titanium dioxide may be used because of its broad absorption band in the range of> 11 microns with a pronounced maximum at 15 microns only in small concentrations. For example, a one percent concentration in polyethylene already a weakening of the intrusion signal to approximately 75% (see. The curve S the attached figure). The weakening of the interference in the range of 0.16 microns to 2.5 microns is at this concentration is too low to effectively suppress the flare.

p0012If the light scattering generated in the entry windows by embedding micro-crystallites in the polyethylene matrix, it can be spread by suitable dimensioning of the micro-crystallites short-wave radiation in the desired manner (the window appear white), with long-wave radiation is allowed to pass almost unhindered. The method appears elegant, but unfortunately it is procedurally not easy to adjust the particle size and concentration of the micro-crystallites requirements. In practice, windows of this type usually consist of two layers, a thin layer of polyethylene microcrystallines for light scattering and a thick layer of pure polyethylene in order to achieve the necessary stability. Furthermore, thick polyethylene film of the required Kristallitkonzentration not be injection molded, since the high temperatures required during injection molding to bring the micro-crystallites disappear. It is therefore not possible to produce by this method in a technically simple manner and inexpensive suitable infrared filter discs.

p0013From this prior art, the object of the invention seeks to avoid the disadvantages of the infrared filter disks previously used for radiation detectors and particularly to provide such infrared filter discs for radiation detectors, the improved impermeability to stray light in the range between 1.6 microns and 2.5 microns have without the permeability for infrared radiation in the desired range of 4 .mu.m to 15 .mu.m is reduced in any manner, with the range from 4 microns to 6 microns for flame detectors and the range of 6 microns to 15 microns for infrared penetration detection systems applies.

p0014This object is achieved with an infrared filter disc of the type mentioned by the characterizing features of claim 1. Preferred embodiments of the invention and embodiments are defined in the dependent claims.

p0015According to a preferred embodiment, the infrared filter disc made of polyethylene and contains zinc sulfide particles in an amount of 2 to 50, preferably from 5 to 30 weight percent, in particular in an amount of about 13 weight percent. The particle size of the zinc sulfide particles is preferably &lt;5 .mu.m, in particular &lt;2 microns.

p0016According to a further preferred embodiment, the infrared filter disc is designed as a thin, plane-parallel plate. A particularly preferred embodiment of the infrared filter disc according to the invention is that it has a surface roughness of about 1 micron.

p0017A preferred embodiment of the Infrared filter disc according to the invention is that it is constructed as an optical element, comprising at least one Fresnel lens.

p0018Which consists of polyethylene, inventive infrared filter disc is preferably used as cover window for passive infrared intrusion detectors. For use in flame detectors is preferably composed of a fluorocarbon polymer, in particular of polytetrafluoroethylene.

p0019An infrared filter disc according to the present invention is prepared according to the following embodiment:

<u>example</u>

p0020An infrared filter disc according to the invention is obtained as follows: zinc sulfide, for example that obtainable from Fluka, is screened in a suitable manner, so that a particle size in the range of 1 .mu.m to 5 .mu.m is obtained. 85 parts by weight of this zinc sulfide and 15 parts by weight of polyethylene granules, eg Hostalen GB 6450 of Messrs. Hoechst AG, are mixed and granulated in a compounding. 15 parts by weight of the thus obtained granulated premix are blended with 85 parts by weight of polyethylene granules. It will be in the normal extrusion process polyethylene window in a thickness of 0.4 mm, containing 12.7 weight percent zinc sulfide produced (film E2).

p0021A second infrared filter disc according to the invention is prepared by the same methods described above, only the granulated pre-mixture of zinc sulfide and polyethylene granules are used instead of 15 parts by weight used 1.18 parts by weight, so that the Polyetylenfolie 1 weight percent zinc sulphide (film E1).

p0022For comparison, according to the same known methods an infrared filter disc according to the prior art is produced which has a content of titanium dioxide of 1 weight percent comprises (sheet S).

p0023The spectral transmittance of the three films is measured in the range of 0.5 .mu.m to 15 .mu.m. In the accompanying figure, the transmissions of the pigments are plotted in percent against wavelength. The absorption bands of the polyethylene are not included because they overlap all three curves evenly.

p0024Curve S represents the transmission curve of the infrared filter disc of the prior art, that is, a 0.4 mm thick Poyethylenfolie with 1 percent by weight titanium dioxide, represent; Curve E1 shows the transmission curve of an infrared filter disc according to the invention, that is, a 0.4 mm thick Poyethylenfolie with 1 weight percent zinc sulfide, and curve E2 shows the transmission of an infrared filter disc according to the invention with a content of 12.7 percent by weight of zinc sulfide.

p0025It can be seen that the transmittance of the film with 1 percent zinc sulfide (curve E1) in the region of the disturbance light is between 1 micron and 2.5 microns is less than that of the film with 1 percent titanium dioxide (curve S). The average particle diameter of the zinc sulfide particles is greater than in commercially available titanium dioxide, and the transmission curve is therefore shifted to greater wavelengths. From about 11 microns, the transmission is almost 100 percent. It is therefore readily possible to reduce the transmission in the area of ​​the disturbance light, to increase the concentration of zinc sulfide particles without the permeability to infrared radiation in the desired range of 4 microns to 15 microns is reduced in an unacceptable manner.

p0026Housed in a typical flame simulation test, eg in the form prescribed by Association of Insurers in the Federal Republic of Germany test, measured residual signal weakens by the scattering effect of 12.7 percent zinc sulfide in a 0.4 mm thick polyethylene film as compared to a pigmentation with titanium dioxide to 5 to 20 per cent, depending on the optical system of the passive infrared detector in which it is measured.

p0027Modifications of the infrared filter disc described above are common in the context of the invention as claimed possible, and the skilled person.