Patent Publication Number: US-9885608-B2

Title: Passive infrared detector

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation application claiming priority to U.S. patent application Ser. No. 11/974,425, filed Oct. 12, 2007, having the same title, and the same inventor, which application is allowed; which application claims the benefit of priority to U.S. Provisional Application No. 60/851,659, filed Oct. 13, 2006, having the same title, and the same inventor, and which application is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to passive thermal infrared devices, and more particularly to passive infrared detectors with configurations leading to narrow fields of view. 
     BACKGROUND OF THE DISCLOSURE 
     Infrared detectors have become ubiquitous in the security industry, due to the several advantages these detectors offer over competing systems. In particular, because the energy being detected is emitted heat rather than reflected light, such detectors are suitable for both daytime and nighttime operations. Also, because infrared wavelengths are much longer than visible wavelengths of light, infrared detectors can detect through dust, smoke, clouds, haze, and light rain. Infrared detectors are also highly sensitive, and are generally capable of detecting temperature variations of a fraction of a degree centigrade. In addition, because they operate outside of the visible region of the spectrum, infrared detectors are completely passive and non-intrusive. 
     Passive infrared (PIR) detectors are a type of infrared detector commonly used in security systems. PIR detectors commonly consist of a housing containing optics to focus thermal infrared energy, a pyroelectric detector onto which the optics focus, and circuitry that can amplify and process the electrical signal from the pyroelectric detector. A simple configuration is a single cylindrical lens in front of a dual-element pyroelectric detector. This configuration leads to a broad, double vertical barrier, with the width of the barrier dependent on the width and spacing of the elements in the detector and on the focal length of the lens. 
     While this approach can address security against, for instance, attempts to enter a window or to cross a barrier in front of an artwork, it does not address nuisance alarms associated with such security requirements. For example, in some installations of motion detectors, the detector is required to sense motion across a window, storefront or other opening which may be situated adjacent to a busy sidewalk or street. In such installations, if the angular range covered by the detector (that is, the angle of coverage measured perpendicularly to a major plane of coverage) is overly broad, the detector may sense unintended targets, such as innocent passersby or normal street traffic. Even if the coverage area of the detector is tilted toward the window in order to reduce the portion of the coverage area overlapping the public areas, reflections from the window glass of thermal energy from passersby or traffic can be detected, and nuisance alarms may result. 
     In order to minimize such nuisance alarms, detectors in such installations can be configured with a very long focal length. Since the angular range covered by the detector decreases with increasing focal length (the tangent of the angle of coverage is given by dividing the dimensions of the pyroelectric detector by the focal length of the lens), this approach causes the image of the detector elements to be very narrow, and hence establishes a very narrow sheet of protection in front of the opening to be protected. However, the use of a long focal length also means that the image of an intruder close to the lens will not be focused on the pyroelectric detector. Hence, the narrow angle of coverage achieved by this approach (and the associated reduction in nuisance alarms) comes at the expense of poor detection performance (and in particular, the existence of blind spots) in regions close to the lens of the detector. 
     Other approaches aimed at solving this problem may utilize multiple pyroelectric detectors and sophisticated electronic circuitry to achieve a narrow detection zone. However, these approaches have the drawback of increased cost and complexity of the detector. 
     There is thus a need in the art for an infrared detector that overcomes the aforementioned infirmities. In particular, there is a need in the art for an infrared detector which has a reasonable focal length and minimal degradation in detection performance close to the lens, and which avoids some of the cost and complexity inherent in some approaches to achieving a narrow detection angle detector. These and other needs may be addressed by the devices and methodologies described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of the area protected by a passive, infrared (PIR) sensor; 
         FIG. 2  is an illustration of an embodiment of an infrared detector in accordance with the teachings herein; 
         FIG. 3  is an illustration of a pyroelectric detector suitable for use in the infrared detectors disclosed herein; 
         FIG. 4  is an illustration of a cap for a pyroelectric detector of the type disclosed herein; 
         FIG. 5  is a cross-sectional view of the cap of  FIG. 4  taken along LINE  5 - 5 ; 
         FIG. 6  is an illustration of a pyroelectric detector in accordance with the teachings herein; 
         FIG. 7  is a top view illustration of the coverage area attainable with a pyroelectric detector of the type disclosed herein; 
         FIG. 8  is a front view illustration of the coverage area attainable with a pyroelectric detector of the type disclosed herein; 
         FIG. 9  is a ray tracing of a pyroelectric detector equipped with a lens; 
         FIG. 10  is a cross-sectional side view of a sense element for a pyroelectric detector giving rise to the ray tracing shown in  FIG. 9 ; 
         FIG. 11  is a ray tracing of a pyroelectric detector equipped with a lens array; 
         FIG. 12  is a cross-sectional side view of a sense element for a pyroelectric detector giving rise to the ray tracing shown in  FIG. 11 ; 
         FIG. 13  is a ray tracing of a pyroelectric detector equipped with a strip that is opaque to infrared electromagnetic radiation; 
         FIG. 14  is a cross-sectional side view of a sense element for a pyroelectric detector giving rise to the ray tracing shown in  FIG. 13 ; 
         FIG. 15  is a cross-sectional illustration of a lens array of the type disclosed herein which is equipped with a septum; 
         FIG. 16  is a view taken along LINE  16 - 16  of  FIG. 15 ; 
         FIG. 17  is a schematic illustration of a quad element pyroelectric detector; 
         FIG. 18  is a schematic illustration of a quad element pyroelectric detector with a masking element; 
         FIG. 19  is an illustration of a pyroelectric detector with a septum disposed downward from the lens; and 
         FIG. 20  is an illustration of a pyroelectric detector with a septum disposed downward from the lens. 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, the term “aperture stop” refers to an element (in an infrared detector of the type described herein) which is placed in the optical path of a detector to modify the ray cone angle at an image point. This element may be, for example, a diaphragm or septum, or the edge of a lens or mirror. 
     In one aspect, an infrared detector is provided which comprises a pyroelectric detector having first and second sensing elements, an aperture stop, and a Fresnel lens array. 
     In another aspect, an infrared detector is provided which comprises a window; first and second sensing elements disposed on a first side of a window; an aperture stop disposed on a second side of a window; and a Fresnel lens array; wherein the Fresnel lens array has a finite focal length. 
     In still another aspect, a method is provided for detecting an intrusion. The method comprises (a) disposing an infrared detector adjacent to an opening such that the infrared detector is positioned to detect the movement of an infrared emitting body across the opening, wherein the infrared detector includes (a) a pyroelectric detector equipped with a window and having first and second sensing elements disposed on a first side of said window, (b) a diffusely reflective septum disposed on a second side of said window, and (c) a Fresnel lens array disposed on said second side of said window; (b) monitoring the opening with the infrared detector for the movement of an infrared emitting body across the opening; and (c) generating an alarm signal when the movement of an infrared emitting body across the opening is detected. 
     It has now been found that the aforementioned needs in the art may be met through the provision of an infrared motion detector which is equipped with a pyroelectric detector having an aperture stop disposed at, or near, a front window thereof. Such a detector may be configured as a compact, low-cost passive infrared detector unit for detecting intrusion into openings, such as store fronts, windows and doors, where there is a possibility of nuisance alarms due, for example, to the proximity of innocent foot traffic to the openings. 
     In a preferred embodiment, the detector employs a dual-element pyroelectric detector coupled with an aperture stop and a simple Fresnel lens array with a reasonable focal length. Such a configuration affords a narrow sheet of protection across an opening being protected, thereby minimizing the incidence of such nuisance alarms. Since the detector uses a lens array of reasonable focal length, degradation of detector performance near the lens is minimized. Moreover, the use of an aperture stop permits the detector to achieve a narrow sheet of protection with a single dual-element pyroelectric detector. While it might be possible to achieve a similar result through the use of multiple dual-element pyroelectric detectors and complicated circuitry, the infrared detectors described herein provide a comparatively less expensive and less complex solution to the aforementioned problems. 
     Referring now to  FIG. 1 , a typical mode of deployment  201  of an infrared detector  203  of the type disclosed herein is depicted. As seen therein, the infrared detector  203  is mounted near one extreme of, and slightly ahead of, an opening or area  205  to be protected. In operation, the infrared detector  203  establishes a narrow sheet of protection for the detection of intrusion attempts into the protected area  205 , while minimizing nuisance alarms. 
     The details of the infrared detector  203  depicted in  FIG. 1  may be further appreciated with reference to the particular, non-limiting embodiment thereof which is illustrated in  FIGS. 2-6  (for ease of illustration, certain elements of the infrared detector  203 , such as the control circuitry, have been omitted). As seen therein, the infrared detector  301  comprises a (preferably low-cost) dual-element pyroelectric detector  303  which is positioned in opposing relation to a lens array  305 . Suitable pyroelectric detectors  303  which may be used in such an embodiment include, for example, the LHi 878 pyroelectric infrared detector, which is available commercially from Perkin-Elmer Corporation, Fremont, Calif. 
     The pyroelectric detector  303  comprises a chassis or can  315  (see  FIG. 2 ) having a plurality of lead wires  313  extending from one end thereof. The lead wires  313  provide power to the pyroelectric detector  303  and allow it to interface with control circuitry or devices. The opposing, operational end of the pyroelectric detector  303  is equipped with an infrared window  309  which, in this particular embodiment, has an aperture stop  311  mounted thereon. 
     The aperture stop  311  is preferably approximately centered over the blank space between the sensitive elements  345  of the detector as indicated in  FIG. 6 . If the aperture stop  311  is small, the effect will also be small (that is, the coverage area will be wider than desired). If the aperture stop is large, there is a danger that tolerance buildups will cause one of the pyroelectric detector&#39;s sensitive elements to be completely blocked, preventing the effect. In a preferred embodiment, the aperture stop  311  is a strip of material which is opaque to the 8 to 14 μm region of the spectrum and which is positioned perpendicular to the long direction of the sensitive elements  345  (see  FIG. 4 ). 
     The aperture stop in this embodiment may have any suitable width (w) (see  FIG. 6 ), but preferably has a width that is within the range of more than 1 but less than 3 mm, and more preferably, within the range of about 2.25 to about 2.75 mm. Most preferably, the width of the aperture stop  311  is about 2.5 mm. The aperture stop  311  may also be of any suitable thickness. 
     If the aperture stop has any appreciable thickness, the sides of the aperture stop  311  are preferably roughened as shown in  FIGS. 5-6  to minimize the quantity of infrared electromagnetic radiation reflected from the sides of the aperture stop  311  that reaches the pyroelectric detector  303  (in this respect, it is to be noted that such reflections are undesirable in that they can effectively widen the angle of protection). Such roughening may be achieved by molding grooves or protrusions into the sides of the aperture stop  311 , by coating the sides of the aperture stop  311  with a composition comprising a suitable particulate matter of appropriate particle size disposed in a polymeric matrix, or by chemically roughening this surface through treatment with a solvent or etchant. In some embodiments, the sides of the aperture stop  311  may be treated with a composition that absorbs or diffusely scatters infrared radiation. 
     The aperture stop  311  may comprise various metals or metal alloys, including, for example, aluminum or stainless steel. The aperture stop  311  may also comprise various plastics which are suitably opaque to infrared wavelengths, including, for example, various acrylics, polycarbonates, or ABS, in which case the aperture stop  311  may be formed by injection molding or through other suitable techniques as are known to the art. The aperture stop  311  may also be made of other non-metal, non-plastic materials that are opaque in the thermal infrared, such as glass. The aperture stop  311  may also be equipped on one surface with an adhesive with which it may be applied to one surface of the pyroelectric detector&#39;s window  309  or can  315 . 
     The construction of the infrared detector  301  with an injection molded aperture stop held to the pyroelectric detector  303  by one form of integral alignment may be understood with reference to  FIGS. 3-6 . As seen therein, the infrared detector  301  has a pyroelectric detector  303  that is fitted with a cylindrical sheath  307  which slidingly engages, and which is complimentary in shape to, the chassis  315  or “can” of the pyroelectric detector  303 . The sheath has two through holes  321  that admit thermal infrared radiation. The sheath  307  is equipped with a notch  323  that keys to a tab  319  provided on the chassis  315 . This arrangement ensures that the aperture stop  311  will be properly oriented over the window  309  of the pyroelectric detector  303 . The chassis  315  is generally a TO-5 metal can, while the sheath  307  may also comprise metal or a suitable plastic. 
     Referring now to  FIG. 6 , it will be appreciated that it may be advantageous in some embodiments of the infrared detectors disclosed herein to make the aperture stop  311  of sufficient height so that it effectively blocks all or most infrared electromagnetic radiation which is intended for one of the sensitive elements  345  from reaching the other element. In a common pyroelectric detector design, the sensitive elements  345  are recessed behind a window  309  of silicon or another infrared transmitting material and are disposed within the interior of the chassis  315 . Typically, the sensitive elements  345  are recessed at an optical distance d of approximately 0.8 mm, have a width of 1 mm, and are spaced 1 mm apart. 
     As shown in  FIG. 6 , the aperture stop  311  of the pyroelectric detector/aperture stop combination has a width w and a height h, and the focal length of the lenslets  341 ,  343  in the lens array  331  is f. To prevent all rays from one lenslet reaching the sensitive element associated with the other lenslet, the following inequality applies: 
                       3   -   w         2   ⁢     (     h   +   d     )       )       &lt;     3   f             (     INEQUALITY   ⁢           ⁢   1     )               
It follows from INEQUALITY 1 that the height h of the strip should be in accordance with the following inequality:
 
                   h   &gt;       ~     f   2       -   d   -     wf   6               (     INEQUALITY   ⁢           ⁢   2     )               
However, the elimination of all rays is not strictly necessary, so that the height of the aperture stop can be less than INEQUALITY 2 requires.
 
     Referring again to  FIG. 6 , the lens array  331  consists of two cylindrical lenslets  341  and  343  which are of approximately the same focal length, and which are disposed with their cylindrical axes parallel to the longitudinal axis of the sensitive elements  345  of the pyroelectric detector. It will be appreciated that, in this orientation, the cylindrical axes of the lenslets  341  and  343  will be parallel to the plane of protection. If a pyroelectric detector  303  is used which does not have a longitudinal axis, then the cylindrical axes of the lenslets  341  and  343  should still be parallel to the plane of protection. The cylindrical axis of each lenslet  341  and  343  is positioned directly ahead of the outside edge of the sensitive area  345  of the pyroelectric detector  303 , one on each side. 
     Several variations of the embodiment depicted in  FIGS. 2-6  are possible. For example, in some embodiments, the angular coverage in the plane of protection provided by the infrared detector may be extended, if desired, through the addition of annular lens segments arranged with their optical centers off of the physical borders of the lenslet. The resulting device can be made to mimic a cylindrical lens whose optical properties extend beyond the boundaries of the physical lens. 
       FIGS. 13-14  depict the resulting protection pattern afforded by an infrared detector  301  of the type depicted in  FIGS. 2-6 . The example shown in  FIGS. 13-14  represents the protection pattern generated with an infrared detector which utilizes cylindrical lenses having a focal length of 6 mm, and which produces a pattern 4.8° wide with a transition from a negative to a positive signal at the approximate center of the pattern. It will be appreciated that practical devices might use a longer focal length. Hence, a focal length of 20 mm would be needed to give a total angle of approximately 1.4°, corresponding to a barrier which is 20 cm wide at a distance of 8 meters. While the optimum focal length can vary depending on the particular application and area to be protected, in a typical situation, the focal length will be less than 50 mm, and preferably, will be within the range of about 10 to about 35 mm. More preferably, the focal length will be within the range of about 15 to about 32 mm. 
       FIG. 9  shows the protection pattern  501  afforded by a conventional single cylindrical lens infrared detector  502  (shown in  FIG. 10 ) which is devoid of an aperture stop. The lens  502  has the same focal length as each of the two cylindrical lenses  505  ahead of a conventional dual-element  503  pyroelectric detector in the previous example. Each element  503  of the detector is approximately 1 mm×2 mm. For a focal length of 6 mm, the pattern consists of two “fingers”  509 ,  511  of opposite polarity, each approximately 9.5 degrees wide, and separated by a “dead zone”  513  approximately 9.5 degrees wide. 
       FIG. 11  shows the protection pattern  601  afforded by a conventional dual-element pyroelectric detector  602  (shown in  FIG. 12 ). The detector  602  is devoid of an aperture stop, and is equipped with a lens array  605  having two cylindrical lenslets  607 ,  609 . The resulting protection pattern  601  has first and second optical centers C 1  and C 2 , and consists of first  611  and second  613  groups of “fingers” of opposite polarity, each finger being approximately 9.5 degrees wide, and each group  611  and  613  of fingers being associated with one of the elements  603  of the detector  602 . 
     As seen from the protection pattern  601 , the use of such a lens array  605  has eliminated the “dead zone”  513  present in the protection pattern of  FIG. 9 , but for a trivial non-diverging “dead zone” 3 mm wide. Consequently, the pattern is now 19° wide, with a transition from a negative to a positive signal at the approximate center of the pattern. Taking into account the pattern due to imaging the opposite sensitive element  603 , the total protection pattern  601  is 57° wide. 
       FIG. 13  illustrates the protection pattern  701  afforded by the lens array ahead of a dual-element pyroelectric detector  702  equipped with an aperture stop  709  in accordance with the teachings herein (in this particular embodiment, the aperture stop  709  is a thin, infrared-opaque strip). The detector  702  is equipped with a lens array  705  having two cylindrical lenslets  707 ,  709 . The resulting protection pattern  701  consists of two “fingers”  711 ,  713  of opposite polarity. For the 6 mm focal length example, the pattern  701  is now only 4.8° wide, plus the trivial 3 mm constant-width “dead zone”  715 . For longer focal lengths, there is a possibility that each of the sensitive elements  703  may have some energy imaged onto it from the opposite lenslet; hence the thick aperture stop discussed earlier. 
       FIGS. 15-16  depict a particular, non-limiting embodiment of a lens array  801  useful in the devices and methodologies described herein. The lens array  801  depicted therein comprises first  803  and second  805  lenslets and is constructed such that the area between the two cylindrical lens centers  807  and  809  is rendered opaque or scattering to infrared wavelengths. This may be achieved, for example, by running a septum  811  of infrared opaque material from the lens array  801  toward the pyroelectric detector, or by incorporating a diffusing section  813  between the two lens centers  807  and  809  consisting of, for example, short focal length cylindrical lenses with their cylindrical axis running perpendicular to the cylindrical axis of first  803  and second  805  cylindrical lenslets. This step may be necessary if the aperture stop is thin and the focal length is long. The septum  811  has the additional advantage of stiffening the lens array  801 . 
     The lens array  801  can consist of ordinary cylindrical (or acylindrical) lenses, provided that their material is sufficiently transparent to infrared radiation to allow the required thickness at the optical centers. The lens array  801  can also consist of two rows of ordinary spherical (or aspherical) lenses with their optical centers spaced approximately the length of the long dimension of the pyroelectric detector elements (with the same thickness caution as above), or may consist of cylindrical (or acylindrical) Fresnel lenses or rows of spherical (or aspherical) Fresnel lenses. The rows of spherical or aspherical lenses constitute synthetic cylindrical (or acylindrical) lenses. In some embodiments, the angular coverage can be extended beyond the endpoints of the cylindrical or acylindrical lenses or synthetic lenses through the use of off-axis spherical or aspherical lenses with their optical centers in line with the other optical centers and beyond the extent of the lens array. 
     In light of the foregoing, it will be appreciated that the protection pattern may be narrowed through the use of the devices and methodologies disclosed herein in one or more of the following three ways: 
     (1) The central “dead zone” may be narrowed arbitrarily close to zero through the use of two cylindrical lenses with parallel axes located ahead of the extremes of the sensitive area; 
     (2) The remainder of the pattern may be narrowed by obscuring a portion of each of the two sensitive elements of the pyroelectric detector of the example; and 
     (3) Other artifacts contributing to a wider pattern (the image of one sensitive element of the pyroelectric detector through the cylindrical lens whose cylindrical axis is ahead of the other sensitive element) may be eliminated by the use of a thick aperture stop on top of the pyroelectric detector as shown, for example, in  FIG. 6 , and/or by adding a septum between the two cylindrical lenses. 
     In the most preferred embodiment, the lens array is an array of two cylindrical or acylindrical plano convex Fresnel lenses. The material of the lens array is preferably high-density polyethylene, but could also be any material transparent to thermal infrared radiation. From an optical design standpoint, it is preferable that the grooved side of the lens array faces away from the pyroelectric detector, although other considerations may dictate that the grooved side should face the pyroelectric detector so that a smooth (piano) surface faces the protected area. The lens array is preferably curved at approximately its focal length (as measured when curved) about the pyroelectric detector, and the pyroelectric detector itself is preferably a common dual-element type detector packaged in a TO-5 metal can. 
     The aperture stop is preferably in the form of a strip which is opaque to infrared radiation, and is preferably incorporated into an injection molded cap with a cylindrical portion that fits over the TO-5 package and that has a notch for alignment with the tab on the TO-5 package. The strip is preferably 2.5 mm wide, has a height approximately equal to or in excess of f/2−d−wf/6 mm high (see INEQUALITY 2), and has sides that are roughened or angled to avoid reflecting unwanted rays onto the sensitive elements of the pyroelectric detector. 
     It will be appreciated that the pyroelectric detectors disclosed herein may have various other features and elements as are known to the art. These include, for example, suitable electronics to operate the device and to generate an alarm signal when intrusion is detected, a suitable power source, and a suitable housing to protect or enclose the detector. 
     It will further be appreciated that, while the use of dual element pyroelectric detectors is preferred in some embodiments of the devices and methodologies described herein, pyroelectric detectors having more than two elements may also be utilized. A particular, non-limiting embodiment of such a device is depicted in  FIGS. 17-18 . 
     With reference to  FIG. 17 , a quad-element pyroelectric detector  901  is depicted which is equipped with a housing  903  having four sensing elements  905  disposed therein. In the particular embodiment depicted, the sensing elements  905  are 1 mm×1 mm in size and are spaced 1 mm apart from each other, though it will be appreciated that the dimensions of these sensing elements  905  may vary from one device or end use to another. 
     The device  911  of  FIG. 18  is similar to the device of  FIG. 17  and also has a housing  913  with four sensing elements  915  disposed therein, but has been further modified in accordance with the teachings herein through the provision of a septum  917  or other masking element. This device functions in an analogous manner to the device  303  of  FIG. 2 . 
     It will also be appreciated that various dispositions of the septum or septa are possible in the devices and methodologies described herein. Two such dispositions are illustrated in  FIGS. 19 and 20 . 
     With reference to  FIG. 19 , the device  951  depicted therein comprises a lens  953  having a septum  955  which is disposed downward from the lens  953  and toward the detector  957 . Preferably, the septum  955  in this type of embodiment forms an integral part of the lens  953  and hence, these two elements may be molded as a single construct. An example of this type of embodiment is depicted in  FIG. 15 . 
     With reference to  FIG. 20 , the device  961  depicted therein comprises a lens  963  having a septum  965  which is disposed upward from the detector  967 . The septum  965  in this type of embodiment may form part of a “hat” of the type which may be used for a thin aperture stop in some embodiments made in accordance with the teachings herein. 
     The above description of the present invention is illustrative, and is not intended to be limiting. It will thus be appreciated that various additions, substitutions and modifications may be made to the above described embodiments without departing from the scope of the present invention. Accordingly, the scope of the present invention should be construed in reference to the appended claims.