Patent Publication Number: US-7218222-B2

Title: MEMS based space safety infrared sensor apparatus and method for detecting a gas or vapor

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention relates generally to the field of intrusion detection systems for animate, inanimate or gaseous substances relying on infrared signal detection and, more specifically, to a space safety infrared signal intrusion detection system which incorporates a micro-electro-mechanical system (MEMS) mirror array. 
   2. Description of Related Art 
   Passive infrared (IR) sensors detect intruders moving within the field of view (FOV) by measuring the temperature gradient caused by an intruder. The sensor&#39;s FOV is fixed and is determined by the optical properties of the lens or mirror system. The FOV is subdivided into static active and inactive zones; the motion of an intruder from an active to an inactive zone is detected as an alarm. The IR energy from each active zone is focused on the IR detector and the IR detector cannot determine which active zone is collecting the energy. The problem with this arrangement is that other sources of IR energy within a zone or zones can be detected as alarm signals as well. Examples include a space heater cycled on and off or a sunlit shade moving from a breeze within the detector&#39;s zones. Other sources of noise include a pet such as a small dog. Also, the inactive zones offer a path that an intruder can traverse without detection. Others have tried to solve these problems as follows: One product has an algorithm to detect repetitive motion within a zone and desensitize the detector to ignore this signal. This also desensitizes the sensor to intruders as well. Another approach uses a CCD camera to monitor the protected space and employs video processing algorithms to detect motion. The problem with this approach is that the protected space needs to be illuminated to detect the motion. Another approach uses a second lens system to minimize the inactive zones but this approach still suffers from the other shortcomings. 
   SUMMARY OF THE INVENTION 
   To address the above and other issues, the present invention is directed to a space safety apparatus monitoring a volume of space encompassing a field of view, the space safety apparatus for detecting an intrusion within the volume of space, the apparatus comprising a micro-electro-mechanical system (MEMS) having mirror elements in a mirror array for reflecting infra-red (IR) energy beam collected from the FOV; and an IR energy detector for detecting the IR energy reflected by the MEMS array and converting the IR energy to an output signal. The present invention is also directed to a method for moving the IR zone within the FOV of an intrusion protected space or volume by means of a multi-axis MEMS mirror array. This motion of the IR zone effectively scans the IR signature of the protected space or volume. The intrusion can be an effect caused by the presence within the volume of space of an animate or inanimate object, for example a robotic vehicle, or a gas or vapor. 
   In a particular aspect of the invention, a first embodiment of the present invention is directed to a space safety apparatus for detecting an intrusion in a volume of space comprising: a focusing element for focusing an infra-red (IR) energy beam collected from the volume of space; a filter element for filtering the infra-red (IR) energy beam collected from the volume of space; a micro-electro-mechanical system (MEMS) having mirror elements in a mirror array for reflecting the IR energy; an IR energy detector for detecting the IR energy reflected by said MEMS array and converting the IR energy to an output signal; an amplifier for amplifying the output signal; an analog to digital converter for converting the output signal from analog to digital; a processor for processing the output signal, a memory storage for storing the output signal; a controller for adjusting an angle of at least one element of said MEMS mirror array; and an alarm for annunciating detection of an intrusion resulting from a change in amplitude of the output signal corresponding to a change in amplitude of the IR energy beam. The output signal can be one of electrical, magnetic, optical, acoustical, pneumatic and hydraulic pressure. The controller can adjust an angle by varying a control signal to said at least one element of said MEMS mirror array. The control signal can be one of electrical, magnetic, optical, acoustical, pneumatic and hydraulic pressure. The controller can derive a reference signal by switching said MEMS mirror array between the FOV and an IR reference. Varying an electrical control signal to said MEMS mirror array can cause motion of at least one mirror element of said MEMS mirror array, the motion being by at least one of thermal expansion and electrostatic force. The controller can actuate the MEMS mirror array to traverse the FOV of said IR detection apparatus by traversing the FOV in a non-chopping mode, either in incremental, overlapping steps or in discrete, finite steps. 
   The controller can actuate the MEMS mirror array to traverse the FOV of said IR detection apparatus by traversing the FOV in a chopping mode, either in incremental, overlapping steps or in discrete, finite steps. The space safety apparatus can further comprise an IR source providing a reference value for detecting at least one of tampering with and degradation of said space safety apparatus. The MEMS mirror array can be comprised of mirror elements each capable of rotation to simulate a finite element representation of a curved mirror or the mirror elements can be configured to simulate a finite element representation of a flat mirror. 
   A detector assembly of the first embodiment can comprise: said filter element; said MEMS mirror array disposed on a ceramic substrate; and said IR energy detector disposed to detect the IR energy reflected by said MEMS array. The detector assembly can further comprise: a detector assembly housing enclosing at least said filter element; said MEMS mirror array disposed on a ceramic substrate; said IR energy detector disposed to detect the IR energy reflected by said MEMS array; and a detector assembly housing base for coupling to said detector assembly housing. The detector assembly housing base can further comprise at least four pins for coupling to a printed circuit board, at least one of said pins receives power, one of said pins is a ground one of said pins sends a signal, and one of said pins provides MEMS mirror array control signal. The detector assembly can be coupled to a printed circuit board. The printed circuit board can comprise: said amplifier; said analog to digital converter; said processor; said memory storage; said controller for adjusting an angle of at least one mirror element of said MEMS mirror array; and said alarm for annunciating detection of an intrusion. The printed circuit board and said detector assembly can be disposed within an enclosure housing and disposed on an enclosure base for coupling to said enclosure housing such that said MEMS mirror array within said detector assembly can receive the IR energy through a window within said enclosure housing. The window can be comprised of a focusing element for focusing the IR energy. The detector assembly can be disposed on said printed circuit board such that said MEMS mirror array within said detector assembly is parallel to said printed circuit board and said printed circuit board is disposed at an angle of about 10° with respect to said enclosure base. The enclosure housing can further comprise an IR source disposed in proximity to said window such that said MEMS mirror array can receive and reflect IR energy from said IR source onto said IR detector elements, said IR source providing a reference value for detecting at least one of tampering with and degradation of said space safety apparatus. 
   In another aspect of the invention, a second embodiment of the present invention is directed to a space safety apparatus for detecting an intrusion in a volume of space comprising: a plurality of focusing elements for focusing infra-red (IR) energy collected from within the volume of space; a filter element for filtering the IR energy collected from within the volume of space; a micro-electro-mechanical system (MEMS) mirror array for reflecting the IR energy; an IR energy detector for detecting the IR energy reflected by said MEMS array and converting the IR energy to an output signal; an amplifier for amplifying the output signal; an analog to digital converter for converting the output signal from analog to digital; a processor for processing the output signal, a memory storage for storing the output signal; a controller for adjusting said MEMS array by switching from one to another of said plurality of focusing elements; and an alarm for annunciating detection of an intrusion resulting from a change in amplitude of the output signal corresponding to a change in amplitude of the IR energy beam. The output signal can be one of electrical, magnetic, optical, acoustical, pneumatic and hydraulic pressure. The controller can derive a reference signal by switching said MEMS mirror array between the FOV and an IR reference. The plurality of focusing elements can comprise at least one of (a) a lens element, and (b) a mirror focusing element. The controller can adjust the MEMS array by switching from one to another of said plurality of focusing elements by traversing the FOV either in incremental, overlapping steps or in discrete, finite steps. 
   The controller can actuate the MEMS mirror array to traverse the FOV of said IR detection apparatus by traversing the FOV in a chopping mode, either in incremental, overlapping steps or in discrete, finite steps. The space safety apparatus can further comprise an IR source providing a reference value for detecting at least one of tampering with and degradation of said space safety apparatus. The MEMS mirror array can be comprised of mirror elements each capable of rotation to simulate a finite element representation of a curved mirror or the mirror elements can be configured to simulate a finite element representation of a flat mirror. 
   A detector assembly of the second embodiment can comprise: said filter element; said plurality of focusing elements; said MEMS mirror array disposed on a ceramic substrate; and said IR energy beam detector disposed to detect the passive IR beam reflected by said MEMS array. The detector assembly can further comprise: a detector assembly housing enclosing at least said plurality of focusing elements; said filter element; said MEMS mirror array disposed on a ceramic substrate; and said IR energy detector disposed to detect the IR energy reflected by said MEMS array; and a detector assembly housing base for coupling to said detector assembly housing. The detector assembly housing base further comprises at least four pins for coupling to a printed circuit board, at least one of said pins receives power, one of said pins is a ground, one of said pins sends a signal, and one of said pins provides MEMS control signal. The detector assembly can be coupled to a printed circuit board. The printed circuit board can comprise: said amplifier; said analog to digital converter; said processor; said memory storage; said controller for adjusting of said MEMS mirror array; and said alarm for annunciating detection of an intrusion. The printed circuit board and said detector assembly can be disposed within an enclosure housing and disposed on an enclosure base for coupling to said enclosure housing such that said MEMS mirror array within said detector assembly can receive the IR energy beam through a window within said enclosure housing. The window can be comprised of a focusing element for focusing the IR energy. The detector assembly can be disposed on said printed circuit board such that said MEMS mirror array within said detector assembly is parallel to said printed circuit board and said printed circuit board is disposed at an angle of about 10° with respect to said enclosure base. The enclosure housing can further comprise an IR source disposed in proximity to said window such that said MEMS mirror array can receive and reflect IR energy from said IR source onto said IR detector elements, said IR source providing a reference value for detecting at least one of tampering with and degradation of said space safety apparatus. 
   In yet another aspect of the invention, a third embodiment of the present invention is directed to a space safety apparatus where the space safety apparatus is for detecting an intrusion within a volume of space encompassing a FOV, wherein the intrusion is a gas or vapor in the volume of space encompassing the FOV, wherein the FOV comprises: an infra-red (IR) energy reference source emitting an IR energy beam; an air path from the volume of space providing a potential gas or vapor sample to be detected and through which the IR energy beam passes; a collimating lens between the IR energy source and the air path for collimating the IR energy beam emitted by said IR energy reference source; a focusing element for focusing the collimated IR energy beam from the air path; the space safety apparatus further comprising a narrow band bandpass filter element for filtering the collimated IR energy beam, the IR energy beam passing through said air path prior to passing through said narrow band filter element; a micro-electro-mechanical system (MEMS) mirror array for reflecting the narrow band IR energy beam from said narrow band bandpass filter; an IR energy detector for detecting a change in the narrow band IR energy beam reflected by said MEMS array and converting the narrow band IR energy beam to an output signal; an amplifier for amplifying the output signal from the narrow band detector; an analog to digital converter for converting the output signal from the narrow band detector from analog to digital; a processor for processing the output signal from the narrow band detector; a memory storage for storing the output signal from the narrow band detector; a wide band bandpass filter element for filtering the collimated IR energy beam, the IR energy beam passing through said air path prior to passing through said wide band filter element; a micro-electro-mechanical system (MEMS) mirror array for reflecting the wide band IR energy beam from said wide band bandpass filter; an IR energy detector for detecting the wide band IR energy beam reflected by said MEMS array and converting the wide band IR energy beam to an output signal, said IR energy detector for detecting the wide band IR energy beam; an amplifier for amplifying the output signal from the wide band detector; an analog to digital converter for converting the output signal from the wide band detector from analog to digital; a processor for processing the output signal from the wide band detector; a memory storage for storing the output signal from the wide band detector; an IR reference enabling a reference signal to be derived by switching said MEMS mirror array between the IR Source and said IR reference; a controller for adjusting an angle of at least one element of said MEMS mirror array; and an alarm for annunciating detection of a gas or vapor in response to a change in output signal corresponding to a change in the ratio of the IR energy beams received from said narrow band detector. The output signal can be one of electrical, magnetic, optical, acoustical, pneumatic and hydraulic pressure. The controller can adjust an angle by varying a control signal to said at least one mirror element of said MEMS mirror array. Varying a control signal to said MEMS mirror array causes motion of at least one mirror element of said MEMS mirror array, varying an electrical control signal causing motion by at least one of thermal expansion and electrostatic force. The controller can actuate said MEMS mirror array to traverse the FOV of said IR detection apparatus by traversing the FOV in a chopping mode, the traversing of the FOV in a chopping mode can be achieved by traversing the FOV in incremental, overlapping steps or in discrete, finite steps. The space safety apparatus for detecting a gas or vapor can further comprise an IR source providing a reference value for detecting at least one of tampering with and degradation of said space safety apparatus. The MEMS mirror array can be comprised of mirror elements each capable of rotation to simulate a finite element representation of a curved mirror or configured to simulate a finite element representation of a flat mirror. 
   A detector assembly of the third embodiment can comprise: at least one of said narrow band filter element and said wide band filter element; at least one of said narrow band and said wide band MEMS mirror array disposed on a ceramic substrate; and said IR energy beam detector disposed to detect the IR beam reflected by said MEMS array. The detector assembly can further comprise: a detector assembly housing enclosing at least one of said narrow band and said wide band IR filter element; at least one of said narrow band and said wide band said MEMS mirror array disposed on a ceramic substrate and disposed to detect the IR beam reflected by said MEMS array and a detector assembly housing base for coupling to said detector assembly housing. The detector assembly can comprise both said narrow and said wide band IR energy beam detectors, and a partition can separate the narrow band IR energy beam detector from the wide band IR energy beam detector; or the detector assembly can comprise both said narrow band and said wide band MEMS mirror arrays, and a partition can separate the narrow band MEMS mirror array from the wide band MEMS mirror array; or the detector assembly can comprise both said narrow band and wide band filter elements, and a partition can separate the narrow band filter element from the wide band filter element. The detector assembly housing base can further comprise at least five pins for coupling to a printed circuit board, one of said pins receiving power, one of said pins being a ground, one of said pins sends a signal from said narrow band IR detector, one of said pins sends a signal from said wide band IR detector, and one of said pins provides MEMS control signal. The detector assembly can be coupled to a printed circuit board, the printed circuit board can comprise: at least one of said amplifiers; at least one of said analog to digital converters; said processor; said memory storage; said controller; and said alarm for annunciating detection of the gas or vapor in response to the ratio of the output signals from the narrow band and wide band detectors. The printed circuit board and said detector assembly can be disposed within an enclosure housing and disposed on an enclosure base for coupling to said enclosure housing such that said at least one MEMS mirror array within said detector assembly can receive the IR energy beam through a window within said enclosure housing. The detector assembly can be disposed on said printed circuit board such that said MEMS mirror array within said detector assembly is parallel to said printed circuit board and said printed circuit board is disposed at an angle of about 30° to 45° with respect to said enclosure base. The window can be comprised of a focusing element for focusing the IR energy beam. The enclosure housing can further comprise an IR source disposed in proximity to said window such that said MEMS mirror array can receive and reflect IR energy from said IR source onto said IR detector elements. The IR source can provide a reference value for detecting at least one of tampering with and degradation of said gas or vapor detection apparatus. The output signal filtered by the narrow band filter can comprise a plurality of peak values. The ratio of narrow band to wide band detector when less than one indicates the presence of a gas or vapor within the air path. When the ratio is close to unit, it indicates a change in the output power of the IR source or a change in ambient lighting. 
   In yet another aspect of the invention, a fourth embodiment of the present invention is directed to a space safety apparatus for detecting an intrusion within a volume of space encompassing a FOV, wherein the intrusion is a gas or vapor in the volume of space encompassing the FOV, wherein the FOV comprises: an infra-red (IR) energy reference source emitting an IR energy beam; an air path from the volume of space providing a potential gas or vapor sample to be detected and through which the IR energy beam passes; a collimating lens between the IR energy source and the air path for collimating the IR energy beam emitted by said IR energy reference source; and a plurality of focusing elements for focusing the collimated IR energy beam from the air path, the space safety apparatus further comprising a narrow band bandpass filter element for filtering the collimated IR energy beam, the IR energy beam passing through said air path prior to passing through said narrow band filter element; a micro-electro-mechanical system (MEMS) mirror array for reflecting the narrow band IR energy beam from said narrow band bandpass filter; an IR energy detector for detecting a decrease in the narrow band IR energy beam reflected by said MEMS array and converting the narrow band IR energy beam to an output signal; an amplifier for amplifying the output signal from the narrow band detector; an analog to digital converter for converting the output signal from the narrow band detector from analog to digital; a processor for processing the output signal from the narrow band detector; a memory storage for storing the output signal from the narrow band detector; a wide band bandpass filter element for filtering the collimated IR energy beam, the IR energy beam passing through said air path prior to passing through said wide band filter element; a micro-electro-mechanical system (MEMS) mirror array for reflecting the wide band IR energy beam from said wide band bandpass filter; an IR energy detector for detecting the wide band IR energy beam reflected by said MEMS array and converting the wide band IR energy beam to an output signal, said IR energy detector for detecting the wide band IR energy beam; an amplifier for amplifying the output signal from the wide band detector; an analog to digital converter for converting the output signal from the wide band detector from analog to digital; a processor for processing the output signal from the wide band detector; a memory storage for storing the output signal from the wide band detector; an IR reference enabling a reference signal to be derived by switching said MEMS mirror array between the IR source and said IR reference; a controller for adjusting said MEMS array by switching between focusing elements in a chopping mode alternating between said IR source and said IR reference; and an alarm for annunciating detection of a gas or vapor in response to a change in output signal corresponding to a change in the IR energy beam received from said narrow band detector. The output signal can be one of electrical, magnetic, optical, acoustical, pneumatic and hydraulic pressure. The focusing element can be at least one of (a) a lens element and (b) a mirror focusing element. The controller can actuate said MEMS mirror array to switch between focusing elements in a chopping mode between focusing elements in incremental, overlapping steps or in discrete, finite steps. The space safety apparatus for detecting a gas or vapor can further comprise an IR source providing a reference value for detecting at least one of tampering with and degradation of said space safety apparatus. The MEMS mirror array can be comprised of mirror elements each capable of rotation to simulate a finite element representation of a curved mirror or configured to simulate a finite element representation of a flat mirror. 
   A detector assembly of the fourth embodiment can comprise: at least one of said narrow band and said wide band filter elements; at least one of said narrow band and said wide band MEMS mirror array disposed on a ceramic substrate; and said IR energy beam detector disposed to detect the IR beam reflected by said MEMS array. The detector assembly can further comprise: a detector assembly housing enclosing at least one of said narrow band filter element and said wide band filter element; at least one of said narrow band and wide band MEMS mirror arrays disposed on a ceramic substrate; and at least one of said narrow band and wide band IR energy beam detectors disposed to detect the IR energy reflected by said MEMS array; and a detector assembly housing base for coupling to said detector assembly housing. The detector assembly can comprise both said narrow and said wide band IR energy beam detectors, and a partition can separate the narrow band IR energy beam detector from the wide band IR energy beam detector; or the detector assembly can comprise both said narrow band and said wide band MEMS mirror arrays, and a partition can separate the narrow band MEMS mirror array from the wide band MEMS mirror array; or the detector assembly can comprise both said narrow band and wide band filter elements, and a partition can separate the narrow band filter element from the wide band filter element. The detector assembly housing base can further comprise at least five pins for coupling to a printed circuit board, one of said pins receiving power, one of said pins being a ground, one of said pins sends a signal from said narrow band detector, and one of said pins sends a signal from said wide band detector. The detector assembly can be coupled to a printed circuit board, the printed circuit board can comprise: at least one of said amplifiers; at least one of said analog to digital converters; said processor; said memory storage; said controller; and said alarm for annunciating detection of an intrusion in response to the output signal. The printed circuit board and said detector assembly can be disposed within an enclosure housing and disposed on an enclosure base for coupling to said enclosure housing such that said at least one MEMS mirror array within said detector assembly can receive the IR energy beam through a window within said enclosure housing. The detector assembly can be disposed on said printed circuit board such that said MEMS mirror array within said detector assembly is parallel to said printed circuit board and said printed circuit board is disposed at an angle of about 30° to 45° with respect to said enclosure base. The window can be comprised of a focusing element for focusing the IR energy beam. The enclosure housing can further comprise an IR source disposed in proximity to said window such that said MEMS mirror array can receive and reflect IR energy from said IR source onto said IR detector elements. The IR source can provide a reference value for detecting at least one of tampering with and degradation of said gas or vapor detection apparatus. 
   In both the third and fourth embodiments, the processor calculates the ratio of the instantaneous peak values of the output signal of the narrow band detector to the instantaneous peak values of the output signal of the wide band detector during a given time period. The processor can also calculate the ratio of the average of the instantaneous peak values of the output signal of the narrow band IR detector to the average of the instantaneous peak values of the output signal of the wide band IR detector during a given time period. The processor can also average the ratios of the instantaneous peak values of the output signal of the narrow band IR detector to the instantaneous peak values of the wide band IR detector during a given time period. In all cases, occurrence of ratios having a value significantly less than one (1) during the given time period indicates concentration of a gas or vapor within the air path and occurrence of ratios having a value close to one (1) during the given time period indicates a shift in at least one of IR output and ambient light to enable self-calibration of the narrow band and wide band IR detectors by the processor. The magnitude of the ratios calculated is proportional to the concentration of gas or vapor present. The magnitude of the ratio of the signal drop indicates the percentage of gas present. 
   In a method of detecting an intrusion in a volume of space encompassing a field of view (FOV), the method comprises the steps of: a) positioning a micro-electro-mechanical system (MEMS) mirror array of rows and columns of mirror elements to reflect an infra-red (IR) energy beam with respect to active elements of an IR detector corresponding to the FOV; and b) collecting the IR energy from an i th  portion of the FOV at a pre-determined scan rate. The step (b) of collecting the IR energy from an i th  portion of the FOV at a pre-determined scan rate can comprise the steps of: (b′1) focusing the IR energy beam; (b′2) filtering the IR energy beam; (b′3) reflecting the IR energy beam by the MEMS mirror array onto a detector; (b,4) detecting the IR energy beam by means of the detector; (b,5) converting the IR energy beam to an output signal; (b′6) amplifying the output signal; (b′7) converting the output signal from analog to digital; and (b′8) processing the output signal by means of a processor prior to annunciating detection. The output signal can be one of electrical, magnetic, optical, acoustical, pneumatic and hydraulic pressure. The method can further comprise the step of: (b′9) controlling the MEMS mirror array to measure all mirror array elements corresponding to the entire FOV by scanning. The method of detecting an intrusion can further comprise the steps of: (c) determining whether all mirror array elements have been measured; d1) if no, repeating step (b); d2) if yes, storing the scan of the mirror array elements; e) processing the results of the scan; f) determining if an intrusion has been detected based on the results of the scan by detecting a change in the IR energy beam level; g1) if yes, annunciating an alarm; g2) if maybe, returning to step (b) of collecting IR energy from an i th  portion of a field of view (FOV) by re-scanning a limited volume of the space where an intrusion appears to be detected, and g3) if no, returning to step (b). The method the step (b) of collecting the IR energy from an i th  portion of the FOV can further include the steps of at least one of: b1′) actuating the MEMS mirror to traverse the FOV; and b1″) directing a signal controller to adjust the MEMS mirror to switch from one to another focusing element. At least one of the step (b1′) of actuating the MEMS mirror to traverse the FOV, and (b1″) directing a signal controller to adjust the MEMS mirror to switch from one to another focusing element can include the steps of at least one of: (b2) traversing the FOV in a non-chopping mode, and (b3) traversing the FOV in a chopping mode. The step (b2) of traversing the FOV in a non-chopping mode can include the steps of at least one of: (b2′) traversing the FOV in incremental, overlapping steps; and (b2″) traversing the FOV in discrete, finite steps. The step (b3) of traversing the FOV in a chopping mode can include the steps of at least one of: (b3′) traversing the FOV in incremental, overlapping steps; and (b3″) traversing the FOV in discrete, finite steps. The step (b) of collecting the IR energy from an i th  portion of the FOV can include the step of: (b4) adjusting an angle of at least one mirror element of said MEMS mirror array, wherein the step (b4) of adjusting an angle includes the step of: (b5) varying a control signal to said at least one element of said MEMS mirror array. The control signal can be one of electrical, magnetic, optical, acoustic, pneumatic and hydraulic pressure. The step (b5) of varying a control signal to said at least one element of said MEMS mirror array can cause motion of said at least one mirror element of said MEMS mirror array, said step (b5) of varying of a control signal can cause motion by at least one of thermal expansion and electrostatic force. The focusing element can comprise at least one of (a) a lens element; and (b) a mirror focusing element. The step of (g2) of re-scanning a limited volume of the space where an intrusion appears to be detected can include the steps of at least one of: (g2′) re-scanning at the pre-determined scan rate; and (g2″) re-scanning at a different scan rate. The step (b2) of traversing the FOV in a non-chopping mode can produce an output signal with a peak value, such that a shift in the peak value indicates movement of a heat source within the FOV. The step (b3) of traversing the FOV in a chopping mode can produce an output signal with a plurality of peak values, such that a shift in amplitude of at least one of the plurality of peak values indicates movement of a heat source within the FOV. 
   In a method of detecting an intrusion within a volume of space encompassing a FOV, wherein the intrusion is a gas or vapor in the volume of space encompassing the FOV, the method comprises the steps of: (a) positioning a micro-electro-mechanical system (MEMS) mirror array to reflect a collimated infra-red (IR) energy beam with respect to active elements of an IR detector, a portion of the collimated beam filtered by a narrow IR band bandpass filter, a portion of the collimated beam filtered by a wide IR band bandpass filter, an IR energy source disposed at a distal end of the air path with respect to the MEMS mirror array; (b) measuring, at a predetermined scan rate, the IR energy of the IR heat source at the distal end of the air path through the narrow IR band bandpass filter and a narrow IR band detector; (c) measuring, at the pre-determined scan rate, the temperature of a point at a known reference temperature in the MEMS mirror array through the narrow IR band bandpass filter and a narrow IR band detector; (d) measuring, at the pre-determined scan rate, the IR energy of said IR heat source at the distal end of the air path through the wide IR band bandpass filter and the wide IR band detector; (e) measuring, at the pre-determined scan rate, the temperature of a point at a known reference temperature in the MEMS mirror array through the wide IR band bandpass filter and the wide IR band detector; (f) measuring the IR energy beam received by the detector with the wideband filter. The step (c) of measuring, at the pre-determined scan rate, the temperature of a point at a known reference temperature in the MEMS mirror array through the narrow IR band bandpass filter and a narrow IR band detector and (d) of measuring, at the pre-determined scan rate, the energy of an IR heat source in the air path through the wide IR band bandpass filter and the wide IR band detector can each comprise the steps of: (b1) focusing the IR energy beam; (b2) filtering the IR energy beam; (b3) reflecting the IR energy beam by the MEMS mirror array onto a detector; (b4) detecting the IR energy beam by means of the detector; (b5) converting the IR energy beam to an output signal; (b6) amplifying the output signal; (b7) converting the output signal from analog to digital; and (b8) processing the output signal by means of a processor prior to annunciating detection. The output signal can be one of electrical, magnetic, optical, acoustical, pneumatic or hydraulic pressure. The method can further comprise the step of: (b9) controlling the MEMS mirror array to measure all mirror array elements by scanning. The method can further comprise the steps of: (g) determining whether all mirror array elements have been measured; (h1) if no, repeating steps (b) through (f); (h2) if yes, storing the scan of the field of view; (i) processing the results of the scan; (j) determining if a gas or vapor has been detected based on the results of the scan by detecting a change in the ratio of the IR energy beam received by the detector with the narrowband filter to the IR energy beam received by the detector with the wideband filter during a given time period; (k1) if yes, annunciating an alarm; (k2) if maybe, returning to steps (b) through (f) of measuring the temperatures by re-scanning the air path where the gas or vapor appears to be detected, and (k3) if no, returning to steps (b) through (f). The step (j) can be performed by the step (j″) of calculating the ratio of the instantaneous peak values of the output signal of the narrow band detector to the instantaneous peak values of the output signal of the wide band detector during a given time period. The step (j) can be performed by the step (j″) of calculating the ratio of the average of the instantaneous peak values of the output signal of the narrow band IR detector to the average of the instantaneous peak values of the output signal of the wide band IR detector during a given time period. The step (j) can be performed by the step (j′″) of averaging the ratios of the instantaneous peak values of the output signal of the narrow band IR detector to the instantaneous peak values of the wide band IR detector during a given time period. In all cases, occurrence of ratios having a value significantly less than one_(1) during the given time period indicates concentration of a gas or vapor within the air path and occurrence of ratios having a value close to one (1) during the given time period indicates a shift in at least one of IR output and ambient light to enable self-calibration of the narrow band and wide band IR detectors. The magnitude of the ratios calculated is proportional to the concentration of gas or vapor present. The steps (b) through (f) of measuring the IR energies and temperatures can include the steps of at least one of: (b1′) directing a signal controller to adjust an angle of at least one mirror of said MEMS mirror array; and (b1″) directing a signal controller to adjust the MEMS mirror to switch from one to another focusing element in a chopping mode following measurement of the energy of the IR source and the temperature of IR reference. The step (b1′) of directing a signal controller to adjust the angle of at least one mirror element can be performed by toggling the angle position. The step (b3) of adjusting an angle can include the step of: (b4) varying a control signal to said at least one element of said MEMS mirror array. The step (b2) of varying a control signal to said at least one element of said MEMS mirror array causes motion of said at least one mirror element of said MEMS mirror array, the control signal can be one of electrical, magnetic, optical, acoustical, pneumatic and hydraulic pressure, said step (b2) of varying of an electrical control signal causing motion by at least one of thermal expansion and electrostatic force. The focusing element can comprise at least one of (a) a lens element; and (b) a mirror focusing element. The step of (k2) of re-scanning the air path where a gas or vapor appears to be detected includes the steps of at least one of: (k2′) re-scanning at the pre-determined scan rate; and (k2″) re-scanning at a different scan rate. 
   In an alternate configuration, the present invention is directed to the space safety apparatus of the first and second embodiments wherein said detector assembly further comprises a viewing port and said mirror elements of said MEMS mirror array are disposed within the detector assembly. The mirror elements are start and end position mirror elements that are configured in rows and columns. All rows and columns of said start and end position mirror elements can be oriented in start and end positions such that all of said mirror elements view inside said detector assembly housing. Alternatively, at least a portion of said rows and columns of said start and end position mirror elements can be oriented in start and end positions such that at least a portion of said mirror elements view outside said detector assembly housing. 
   The method of detecting an intrusion in a volume of space can further include said mirror elements that are start and end position mirror elements disposed in a detector assembly housing having an IR filter window for viewing outside said detector assembly housing, said method comprising the step of: orienting in start and end positions all rows and columns of said mirror elements to view inside said detector assembly housing. Alternatively, the method of detecting an intrusion in a volume of space can comprise the step of: orienting in start and end positions at least a portion of said rows and columns of said mirror elements to view outside said detector assembly housing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, benefits and advantages of the present invention will become apparent by reference to the following text and figures, with like reference numbers referring to like structures across the views, wherein: 
       FIG. 1A  illustrates a plan view of a infra-red sensor of the prior art as viewed from the bottom. 
       FIG. 1B  illustrates a section view along section line  1 B— 1 B of the prior art infra-red sensor of  FIG. 1A . 
       FIG. 1C  illustrates a section view along section line  1 C— 1 C of the prior art infra-red sensor of  FIG. 1A . 
       FIG. 2  is an isometric view of the prior art infra-red sensor of  FIG. 1A–1C . 
       FIG. 3  is an elevation cross-sectional view of the infra-red sensor of the prior art of  FIGS. 1A–1C  as mounted within an enclosure. 
       FIG. 4A  is a plan view of the IR beam exposure pattern of the prior art sensor of  FIGS. 1A–1C  and of the present invention. 
       FIG. 4B  is an elevation view of the IR beam exposure pattern of the prior art sensor of  FIGS. 1A–1C . 
       FIG. 5  is a block diagram of a prior art IR sensor detection system. 
       FIG. 6A  illustrates a plan view of the MEMS-based infra-red sensor of the present invention. 
       FIG. 6B  illustrates a section view along section line  6 B— 6 B of the MEMS-based infra-red sensor of  FIG. 6A . 
       FIG. 6C  illustrates a section view along section line  6 C— 6 C of the MEMS-based infra-red sensor of  FIG. 6A . 
       FIG. 7  is an isometric view of the prior art infra-red sensor of  FIG. 6A–6C . 
       FIG. 8  is an elevation cross-sectional view of the MEMS-based infra-red sensor of  FIGS. 6A–6C  as mounted within an enclosure. 
       FIG. 9  illustrates one type of MEMS mirror sensor element of the present invention. 
       FIG. 10A  illustrates a plan view two-dimensional representation of a finite element equivalent of a curved mirror of the present invention. 
       FIG. 10B  illustrates a side view two-dimensional representation of a finite element equivalent of a curved mirror of the present invention. 
       FIG. 10C  is a plan view of a finite element simulation of a flat mirror. 
       FIG. 10D  is a side view of the finite element simulation of the flat mirror of  FIG. 10C . 
       FIG. 11A  is a block diagram of one aspect of the MEMS-based IR sensor detection system of the present invention. 
       FIG. 11A-1  illustrates incremental scanning of the IR zones within the FOV. 
       FIG. 11A-2  illustrates discrete, finite scanning of the IR zones within the FOV. 
       FIG. 11B  is a block diagram of a second aspect of the MEMS-based IR sensor detection system of the present invention. 
       FIG. 12  is a side elevation view of an area coverage pattern of the MEMS-based IR sensor detection system of the present invention. 
       FIG. 13  is a method diagram of the steps of operating the MEMS-based IR sensor detection system of the present invention. 
       FIG. 13A  is a method diagram of a first alternative method of operating the MEMS-based IR sensor detection system of the present invention. 
       FIG. 13B  is a method diagram of a second alternative method of operating the MEMS-based IR sensor detection system of the present invention. 
       FIG. 14  illustrates a third embodiment of the present invention wherein a MEMS-based IR detector assembly is arranged to detect gases or vapors. 
       FIG. 15A  illustrates an elevation cross-sectional view of the infra-red sensor detector assembly of  FIG. 14  designed for the detection of gases or vapors. 
       FIG. 15B  illustrates an elevation cross-sectional view of a variation of the infra-red sensor detector assembly of  FIG. 15A  designed for the detection of gases or vapors. 
       FIG. 16  illustrates a plan view of a MEMS based IR detector assembly which houses both narrow band and wide band detectors for gas detection. 
       FIG. 16A  is a section view along section line  16 A— 16 A of  FIG. 16 . 
       FIG. 16B  is a section view along section line  16 B— 16 B of  FIG. 16 . 
       FIG. 16C  is a section view along section line  16 C— 16 C of  FIG. 16 . 
       FIG. 16D  is a perspective view of the detector assembly of  FIG. 16 . 
       FIG. 16E  is a cutaway view of the detector assembly of  FIG. 16D . 
     FIG.  16 A 1  illustrates a non-chopping scan across an FOV for the method of  FIGS. 13 and 13A . 
     FIG.  16 A 2  illustrates a chopping scan across an FOV for the method of  FIGS. 13 and 13A . 
     FIG.  16 B 1  illustrates a non-chopping scan for switching on/off of a lens element for the method of  FIGS. 13 and 13B . 
     FIG.  16 B 2  illustrates a chopping scan for switching on/off of a lens element for the method of  FIGS. 13 and 13B . 
     FIG.  16 B 2 ′ illustrates a detail of FIG.  16 B 2 . 
       FIG. 17  illustrates a chopping scan for the gas detection method of  FIGS. 13 and 13B . 
       FIG. 18-1  illustrates an electrical signal output for the non-chopping scan of FIGS.  16 A 1  and  16 B 1 . 
       FIG. 18-2  illustrates an electrical signal output for the chopping scan of FIGS.  16 A 2  and  16 B 2 . 
       FIG. 19  illustrates an electrical signal output for the gas detection chopping scan of  FIG. 17 . 
       FIG. 20  illustrates IR absorption peaks for the gas detection chopping scan of  FIG. 19 . 
       FIG. 20A  illustrates signal levels detected by both narrow band and wide band detectors and the ratio of the narrow band to wide band signal levels in the presence of a gas. 
       FIG. 20B  illustrates signal levels detected by both narrow band and wide band detectors due to change in IR source output or ambient IR noise levels. 
       FIG. 21A  illustrates a plan view of a MEMS mirror array incorporating MEMS mirror elements of  FIG. 9 . 
       FIG. 21B-1  illustrates a MEMS mirror element in an unactuated position. 
       FIG. 21B-2  illustrates a MEMS mirror element in an actuated position. 
       FIG. 21C  illustrates an IR ray trace for a MEMS device which has only a start and an end position. 
       FIG. 22  illustrates actual electrical output from a mockup simulation of the non-chopping (sweeping) mode of the passive IR sensor of FIG.  16 A 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is directed to a space safety apparatus monitoring a volume of space encompassing a field of view. The present invention uses a multi-axis MEMS array to redirect the IR energy of a beam within the FOV of the protected space. This effectively scans the IR signature of the room. The scanned IR signature is stored in memory, and compared to successive scans for changes in IR signature. Processing algorithms determine if changes in the scanned IR signature are consistent with the signature of the motion of an intrusion. When the proper change in signature is detected an alarm is annunciated. This solves the problems described above because the sensor can determine where within the FOV the IR energy is changing and the sensor therefore can monitor the movement of the IR energy within the FOV. Sources of false alarms can be filtered by monitoring the magnitude and width of the output signal to determine the size and shape of the source of variation. Areas which cause false alarms can be omitted from the scans or given less importance in the processing algorithm. 
   In addition, the protected space does not need to be illuminated for this system to work. The intrusion can be an effect caused by the presence within the volume of space of an animate or inanimate object, or example a robotic vehicle, and including a liquid or a gas or a vapor. Therefore, this system can also be used to detect any gas or vapor including, but not limited to harmful, toxic, explosive or flammable vapors or gases such as: carbon monoxide (CO), volatile organic compounds (VOCs), hydrogen (H 2 ), hydrocarbon gases such as methane (CH 4 ) and propane (C 2 H 6 ) or other beneficial or non-toxic gases such as oxygen (O 2 ) or carbon dioxide (CO 2 ). The detection is achieved by adding narrowband IR bandpass filters centered around the frequency of the IR absorption of the specific gas to the optical path and comparing the IR absorption of the FOV with a reference signal. The reference signal is derived by using the MEMS array as an IR chopper that switches between the FOV and an IR reference. A second detector with a wide band filter, such as the detector used for motion detection, can be used to self calibrate the system. 
     FIG. 1A  illustrates a plan view of an infra-red sensor detector assembly  100  of the prior art showing the bottom of detector housing base  102 , which comprises typically three connection pins  1 ,  2  and  3 . Optical beam  106  is directed towards the sensor  100 . Those skilled in the art recognize that while  FIG. 1A  illustrates a single element, a dual or quad element detector could also be used. 
     FIG. 1B  is a section view along section line  1 B— 1 B of the prior art infra-red sensor detector assembly  100  of  FIG. 1A . The detector housing base  102  is formed with the connection pins  1 ,  2  and  3 , which are typically three in number. The optical beam or beams  106  penetrate the infrared filter window  108  within the detector housing cover  110 . The optical beams  106  are sensed by detector element or elements  112  which are mounted on spacers  116  for thermal isolation and then electrically coupled to ceramic substrate  114 . The IR energy collected from the optical beam or beams  106  is converted by the detector element or elements  112  to an electrical signal which is conditioned by a field effect transistor (FET) and other components on the ceramic substrate  114 . 
     FIG. 1C  illustrates a section view along section line  1 C— 1 C of the prior art infra-red sensor detector assembly  100  of  FIG. 1A . Essentially the same components are shown as described for  FIG. 1B  but in the orientation more closely corresponding to the installed configuration for operation. 
     FIG. 2  is an isometric view of the prior art infra-red sensor detector assembly  100  of  FIGS. 1A–1C . A cut-away view shows the optical beam or beams  106  being received by the detector elements  112 . Again, essentially the same components are shown as described for  FIG. 1B . 
     FIG. 3  is an elevation cross-sectional view of the infra-red sensor detector assembly  100  of the prior art of  FIGS. 1A–1C  as mounted within an enclosure  200 . The enclosure  200  is comprised of an enclosure base  202  to which an enclosure cover  204  mates to form the enclosure  200 . The enclosure cover  204  includes focal element or elements  206 . The detector assembly  100  is mounted on a printed circuit board (PCB) assembly  220  and positioned within the enclosure  200  at an angle so as to receive the optical signal beam or beams  106  through focal element or elements  206 .  FIG. 3  shows a lens or lenslet optical system. Those skilled in the art recognize that a mirrored optical system could be used as an alternate approach. 
     FIG. 4A  is a plan view of an IR beam exposure pattern of the prior art sensor detector assembly  100  of  FIGS. 1A–1C .  FIG. 4B  is an elevation or side view of the IR beam exposure pattern of the prior art sensor detector assembly  100  of  FIGS. 1A–1C . The enclosure  200  with detector assembly  100  is mounted on a wall  410  and above the floor or ground  420  of a room or an outdoor area to detect an intrusion in a volume of space formed by the wall  410  and the floor or ground  420 . The IR energy from each active zone is focused on the IR detector elements  112  as a static pattern in which the first tier of beams B 1  above the ground  420  through beams B 2 , B 3 , B 4 , B 5  and ending with beam B 6  is received simultaneously. A second tier of beams simultaneously lands on the ground to receive beams B 7 , B 8  and B 9  to detect an intrusion  430 . In this example, there are a total of 9 beams in two tiers: 6 long beams and 3 short beams. The summation of the beams B 1  to B 9  forms the field of view (FOV)  440 . The FOV  440  is bounded by the outer borders of the beams, in this example, beams B 1  to B 9 . 
     FIG. 5  is a block diagram of the prior art IR sensor detector assembly  100  arranged in a detection system. The filter element  108  is included within the detector assembly  100  of  FIGS. 1A–1C  and  2 . The focusing element  206  is included within the enclosure  200  of  FIG. 3 . The IR optical beams  106  from the FOV penetrate the focusing elements  206  and the filter element  108  where the beams are received by IR detector element(s)  112  which convert the IR beam energy to an electrical signal. The electrical signal is then conditioned and amplified by amplifier  502  and converted from analog to digital by A/D converter with sufficient resolution  504 . The signal is then forwarded to processor  506  where the signal can be stored in memory  508  and a threshold detection algorithm applied. If appropriate, a signal to activate an alarm  510  can be generated. The passive infrared (IR) sensor detector assembly  100 , as noted previously, detects intruders moving within its field of view (FOV)  440  by measuring the temperature gradient caused by an intruder. The sensor&#39;s FOV  440  is fixed and is determined by the optical properties of the lens system. The FOV  440  is subdivided into static active and inactive zones; the motion of an intruder from an active to an inactive zone is detected as an alarm. The IR energy from each active zone is focused on the IR detector and the IR detector cannot determine which active zone is collecting the energy. At least one problem with this arrangement is that other sources of heat within a zone or zones can be detected as alarm signals as well. 
     FIG. 6A  illustrates a plan view of the MEMS-based infra-red sensor detector assembly  600  of the present invention showing the bottom of detector housing base  102 , which comprises typically the three connection pins  1 ,  2  and  3 . Optical beam  106  is directed towards the sensor assembly  600 . 
     FIG. 6B  is a section view along section line  6 B— 6 B of the infra-red sensor detector assembly  600  of  FIG. 6A .  FIG. 6C  illustrates a section view along section line  6 C— 6 C of the infra-red sensor detector assembly  600  of  FIG. 6A . The detector housing base  102  is formed with the connection pins  1 ,  2 ,  3  and  4 , which are typically four in number, one for power, one for ground, one for detector output signal, and one for MEMS control signal. The output signals and control signals are disclosed herein as being electrical but the signals can be generally electromagnetic, i.e., electrical or magnetic or optical, or can be of other types such as, for example but not limited to, acoustical, pneumatic and hydraulic pressure. Electrical power is typically supplied from the general security system within which the MEMS-based infra-red sensor detector assembly  600  is typically included. The optical beam or beams  106  penetrate the infrared filter window  108  within the detector housing cover  110 . The optical beam  106  is now first reflected by MEMS mirror array  604  prior to being sensed by the detector element or elements  112  which are mounted on, and electrically coupled to, ceramic substrate  614 . The detection signal information provided by the optical beam or beams  106  is transmitted by the detector element or elements  112  for processing by the ceramic substrate  614 . The IR energy collected by the optical beam or beams  106  is converted by the detector element of elements  112  to an electrical signal which is conditioned and amplified by a FET and other components on the ceramic substrate  614 . Due to the small size of the MEMS mirror array  604 , a separate power supply typically is not required. The output signal is disclosed as being electrical but the signal can be generally electromagnetic, i.e., electrical or magnetic or optical, or of other mechanical types such as acoustical or fluidic pressure such as pneumatic or hydraulic. The pneumatic is not limited to air but includes any suitable gas such as nitrogen. The hydraulic is not limited to water but includes oils or other liquids. 
     FIG. 6C  shows essentially the same components as described for  FIG. 6B  but in the orientation more closely corresponding to the installed configuration for operation. In addition, the optical beam or beams  106  is shown reflected from the MEMS mirror array  604  as optical beam  606  that is directed towards the detector elements  112 . The detector element or elements  112  are mounted on, and electrically coupled to, ceramic substrate  614  by means of supports  616 . 
     FIG. 7  is an isometric view of the infra-red sensor detector assembly  600  of  FIGS. 6A–6C . A cut-away view shows the optical beam or beams  106  first reflected by MEMS mirror array  604  prior to being sensed by the detector element or elements  112  as reflected optical beam  606 . Again, essentially the same components are shown as described for  FIG. 6B . 
     FIG. 8  is an elevation cross-sectional view of the infra-red sensor detector assembly  600  of  FIGS. 6A–6C  as mounted within an enclosure  700 . The enclosure  700  is comprised of an enclosure base  702  to which an enclosure cover  704  mates to form the enclosure  700 . The enclosure cover  704  includes a viewing port  705  for positioning focal element or elements  706 . The detector assembly  600  is mounted on printed circuit board (PCB) or circuit assembly  720  and is now positioned within the enclosure  700  at an angle so that the MEMS mirror array  604  first receives the optical signal beam or beams  106  through focal element or elements  706 . The optical signal beam or beams are then reflected as beam  606  to the detector elements  112 . 
   To achieve lens supervision, i.e., to detect any unauthorized tampering with, or degradation of, the enclosure  700  or the detector assembly  600 , an IR source  802  can be located at a suitable location outside the enclosure cover  704  to provide a known reference signal when the enclosure  700 , including the focal elements  706 , and the detector assembly  600  are in their normal configuration.  FIG. 8  shows a lens or lenslet optical system. A mirrored optical system could be used but is not illustrated as those skilled in the art recognize that the alternate approach can be used. 
     FIG. 9  shows one type of an element  900  of a MEMS mirror segment of MEMS mirror array  604 . The mirror element  900  is comprised typically of active mirror area  902 . The active mirror area  902  is coupled to support structure  908  by means of rotating springs  904  to provide one axis of rotation and to another set of rotating springs  906  to provide a second axis of rotation. In such a configuration, the mirror element  900  can be considered to have an unactuated or start position, as shown, and an actuated or end position (not shown) in which the active mirror area  902  can be rotated to the angular limits permitted by the rotating springs  904  and  906 . Such a start and stop MEMS mirror array provides a less expensive means for fabricating a MEMS mirror array. In a more sophisticated version, the active mirror element  902  can be rotated to any intermediate position of angular rotation permitted by the rotating springs  904  and  906 . 
   For the IR detector elements  112 , an active element is the area on the surface of the detector material which has been blackened to allow IR absorption. On the MEMS mirror array element  900 , the active area  902  is the area which is selectively plated to be an IR reflective surface. The remaining area around the mirror array element which is for support structure  908 , rotating springs  904  or  906  or other mechanism to allow movement and the control mechanism is the inactive area or non IR reflective surfaces. 
     FIG. 10A  shows a 2-dimensional representation of a finite element equivalent of a curved mirror using the MEMS mirror array  604 . In this example, the mirror elements are shown as a center element  1000  and four adjacent elements  1000 A,  1000 B,  1000 C and  1000 D. The approximate imaging plane occurs transversely to the optical axis of the center element  1000 . The multi-axis MEMS mirror array  604 , comprised of the center element  1000  and the four adjacent mirror elements  1000 A,  1000 B,  1000 C and  1000 D, is placed in close proximity to the active elements of the IR detector  112 . The actuated elements are those mirror elements which rotate or otherwise change position, i.e.,  1000 A,  1000 B,  1000 C and  1000 D are actuated elements while mirror element  1000  is stationary and therefore unactuated. The mirror array  604  collects IR energy  606 F from an IR source in the far field in the FOV of the mirror elements  1000 ,  1004  and  1004 A– 1004 D. The IR energy  606 F from the IR source in the far field is filtered by IR filter  108 . In  FIG. 10A , the IR energy  606 F from the IR source in the far field originates from a position such that the IR energy  606 F is reflected by the center element  1000  and four adjacent elements  1000 A,  1000 B,  1000 C and  1000 D as IR energy beam  606 R onto a spot  620 C of active elements in the IR detector  112  having a spot size Sc. The center element  1000  and four adjacent elements  1000 A,  1000 B,  1000 C and  1000 D are positioned so that the focal point  622  of the combination of the elements occurs beyond the IR detector  112  but before the IR filter  108  (shown before the IR filter but may occur on the other side). The focal point  622  is determined by the intersection of the lines  624  perpendicular (normal) to the center element  1000  and four adjacent elements  1000 A,  1000 B,  1000 C and  1000 D. The focal length L of the curved mirror elements is in the order of 30 mm or less while the IR source  606  is effectively at an infinite distance, i.e., in the far field at a distance typically of 2.4 meters (8 feet) or more away. The angles φ A , φ B , φ C , and φ D , between the optical axis of the MEMS mirror array elements  604  and the optical axis of the IR sensing detector elements  112  defines the placement of the zone within the FOV. By varying the angles α A , α B , α C , and α D , of each of the flat elements  1000 A,  1000 B,  1000 C and  1000 D in the mirror array  604 , a finite element equivalent of a curved mirror can be created to represent variation in the width of the FOV of the center element  1000  and four adjacent actuated elements  1000 A,  1000 B,  1000 C and  1000 D, effectively representing changing of the zone size. The center element  1000  and four adjacent actuated elements  1000 A,  1000 B,  1000 C and  1000 D are in actuality part of a 2 dimensional N×M array. 
     FIG. 10B  illustrates a side view of the simulated curved mirror comprised of the mirror element  1000  and four adjacent elements  1000 A,  1000 B,  1000 C and  1000 D. As an example, the four rotating elements  1000 A to  1000 D rotate at different angles β A  to β D  to focus the optical beam  606 F reflected from the center element  1000  and rotating elements  1000 A to  1000 D as optical beam  606 R onto the spot  620 C of the active elements of detector  112 . In some types of MEMS mirrors commercially available, it is possible for all of the elements to rotate in unison. The elements can rotate each at the same angle β or at different angles to reflect the optical beam  606 R onto the detector elements  112 . In other types of commercially available MEMS mirrors, for example, in only one-quarter of a 2×2 array do all of the elements move in unison. In  FIGS. 10A and 10B , the 2-dimensional N×M array is represented, as an example, by a 5×1 array. Those skilled in the art recognize that any different array dimensions can be applied, depending on the intended application. 
     FIG. 10C  is a plan view of a finite element simulation of a flat mirror. Again, center element  1000  and adjacent elements  1000 A to  1000 D are positioned to reflect IR energy  606 F from an IR source in the far field. However, in this configuration, the IR energy  606 F is reflected by the flat configuration of the center element  1000  and adjacent elements  1000 A to  1000 D towards the detector  112  with a distributed spot  620 F having a spot size SF.  FIG. 10D  is a side view of the finite element simulation of the flat mirror of  FIG. 10C . In the flat mirror configuration of both  FIGS. 10C and 10D , as expected, there is no focal point. 
     FIG. 11A  is a block diagram of the MEMS-based IR sensor detector assembly  600  arranged in a detection system. The filter element  108  is included within the detector assembly  600  of  FIGS. 6A–6C  and  7 . The focusing element  706  is included within the enclosure  700  of  FIG. 8 . The MEMS mirror array  604  first receives the optical signal beam or beams  106  through focal element or elements  706 . The optical signal beam or beams are then reflected as optical beams  606  to the detector elements  112  which convert the IR energy to an electrical signal. Since the electrical signal is only in the range of 50 microamps, it is necessary to have a high resolution A/D converter or an amplifier  1102 . The signal is converted from analog to digital by A/D converter  1104 . The signal is then forwarded to processor  1106  where the signal can be stored in memory  1108  and an algorithm applied. From the memory  1108 , following processing by the processor  1106 , a signal can be generated for different modes of operation by controller  1110  to adjust the orientation of the MEMS mirror array elements  900  by controller  1110 . If appropriate, a signal to activate alarm  1112  can be generated. The amplifier  1102 , the A/D converter  1104 , the processor  1106 , the memory  1108  and controller  1110  typically are part of the printed circuit board or circuit assembly  720 . 
   Specifically, microprocessor  1106  sends a signal to the controller  1110  to change the voltage V to the elements of the MEMS mirror array  604 . Changing this voltage V generates electrical resistance heating which, for example by thermal expansion or electrostatic force, moves the mirror array elements  900 . Controller  1110  can perform several different modes of operation. In a non-chopping mode wherein IR reference source  1114  is ignored and the IR energy detection is confined solely to the FOV  440 , there are two sub-modes possible. In the first sub-mode, voltage variation changes the orientation of the mirror elements  900  in incremental overlapping steps within the FOV  440 . The elements  900  receive the optical beam  106  by traversing the FOV  440  incrementally in steps in a continuous scan. The angles α A , α B , α C , and α D  change and correspondingly the angles φ A , φ B , φ C , and φ D , between the mirror array  604  and the IR detector element  112  also change. The first sub-mode is illustrated schematically in  FIG. 11A-1  where the IR zones  1150  within the FOV  440  are scanned in incremental steps in a continuous manner. In the first sub-mode, the electrical output signals are produced as the convolution of the target with the FOV  440  which is then shaped by the IR filter  108 . 
   In a second sub-mode in the non-chopping first mode of operation, as illustrated in  FIG. 11A-2 , the elements  900  receive the optical beam  106 , i.e. as IR energy, by traversing the IR zones  1150  within the FOV  440  in discrete, non-continuous steps. Again, the electrical output signals are produced as the convolution of the target with the FOV which is then shaped by the IR filter. 
   Other means for moving the mirror array elements include translation such as by application of an electrostatic force to move the elements either in a linear or non-linear manner. 
   Referring again to  FIG. 11A , in a second mode of operation, a reference signal Sr is derived by using the MEMS array  604  as an IR chopper that switches between the FOV  440  and an IR reference  1114 . As such, the microprocessor or process controller  1106  and controller  1110  can move the IR zone within the FOV and use the reference to compute the target temperature. 
   By providing a signal to the controller  1110 , the microprocessor  1106  steps the voltage V to the mirror array  604 , records the IR energy in the zone, then steps the voltage V to move the zone an incremental amount within the FOV  440 . The electrical signal produced by the IR detector  112  is now an AC signal with a DC bias. 
   As before, the first sub-mode illustrated schematically in  FIG. 11A-1  can be applied to the chopping mode of operation such that the IR zones  1150  are scanned in incremental steps in a continuous manner. In the first sub-mode, the electrical output signals are now produced as AC signals with a DC bias. 
   Similarly, the second sub-mode can be applied to the chopping mode of operation, as illustrated in  FIG. 11B  such that the elements  900  receive the optical beam  106 , i.e., as IR energy by traversing the IR zones  1150  within the FOV  440  in discrete, non-continuous steps. Again, the electrical output signals are produced as AC signals with a DC bias. 
   For both the first and second modes of operation of the embodiment of  FIG. 11A , the process is repeated, left to right, up and down, until the entire FOV has been scanned. Once the entire FOV has been scanned the process is repeated and the new IR scan is compared with the previous scan. The IR scans are analyzed for changes in magnitude and position. Changes consistent with motion of an intruder annunciate an alarm signal. 
   Also, as discussed previously, lens supervision can be achieved to detect tampering or degradation of the detector assembly  600  by verification of a reference signal from IR source  802  to the processor  1106 . 
     FIG. 11B  is a block diagram of a second embodiment of the MEMS-based IR sensor detection system of the present invention. This second aspect is essentially identical to the first embodiment described for  FIG. 11A , except that controller  1110  is replaced by controller  1116 . Those skilled in the art recognize that the functions of controllers  1110  and  1116  can be combined to be performed by a single controller. 
   In a first mode of operation, which is a non-chopping mode of operation, in place of angle adjustment by voltage variation by way of controller  1110 , the MEMS mirror array directs the IR energy beam to one of a plurality of focusing elements  706  representing a zone of interest within the FOV  440 . The processor  1106  then signals the controller  1116  to adjust the MEMS mirror array  604  to switch to another of the plurality of focusing elements  706  in discrete, finite steps. The focusing elements  706  can comprise a lens element, e.g., a lenslet, or a mirror focusing element. 
   As before, either a non-chopping sub-mode of operation can be implemented wherein the IR zones  1150  within the FOV  440  are scanned in incremental steps in a continuous manner, as illustrated in  FIG. 11A-1 . In this sub-mode, again the electrical output signals are the convolution of the target with the FOV  440  which is then shaped by the IR filter  108 . 
   In a second sub-mode in the non-chopping mode of operation, as illustrated in  FIG. 11A-2 , the elements  900  receive the optical beam  106 , i.e., as IR energy, by traversing the IR zones  1150  within the FOV  440  in discrete, non-continuous steps. Again, the electrical output signals are the convolution of the target with the FOV  440  which is then shaped by the IR filter  108 . 
   In a second mode of operation, which is a chopping mode of operation, in place of angle adjustment by voltage variation by way of controller  1110 , the processor  1106  signals the controller  1116  to adjust the MEMS mirror array  604  to switch between one of the plurality of focusing elements  706  to another of the focusing elements  706 . The focusing elements  706  can comprise a lens element, e.g., a lenslets or a mirror focusing element. The controller  1116  adjusts the MEMS mirror array  604  to switch between one of the plurality of focusing elements  706  to another focusing element in discrete, finite steps. 
   In that the second mode of operation is a chopping mode of operation, again a reference signal S R  is derived by using the MEMS array  604  as an IR chopper that switches between the FOV  440  and the IR reference  1114 . As such, the microprocessor or process controller  1106  and controller  1116  can step the IR zone within the FOV  440 . 
   In both the first and second modes of operation of the embodiment of either  FIG. 11A  or  FIG. 11B , the process is repeated, left to right, up and down, until the entire FOV has been scanned. Once the entire FOV has been scanned the process is repeated and the new IR scan is compared with the previous scan(s) to determine the presence of an intruder. The IR scans are analyzed for changes in magnitude and position. Changes consistent with motion of an intruder annunciate an alarm signal. The ability to switch modes of operation such as to do non-chopping or chopping, or to change the scan rates can further improve false alarm immunity. 
   Also, as discussed previously, lens supervision can be achieved to detect tampering or degradation of the detector assembly  600  by verification of a reference signal from an IR source outside of the enclosure  802  by the processor  1106 . 
     FIG. 12  is an elevation or side view of the IR beam exposure pattern of the detector assembly  600  of  FIGS. 6A–6C . The detector assembly  600  is shown mounted within enclosure  700  on circuit assembly  720 . The enclosure  700  is mounted on the wall  410  so as to receive IR energy of optical beam or beams  106  emanating from the volume of space bordered by the floor  420 . The enclosure  700  with detector assembly  600  is mounted on a wall  410  and above the floor or ground  420  of a room or an outdoor area to detect an intruder in a volume of space formed by the wall  410  and the floor or ground  420 . The IR energy from the beam  106 , sweeps the FOV 440  such that the entire FOV  440  is covered. 
   The enclosure  700  with detector assembly  600  is mounted on the wall  410  and above the floor or ground  420  of the room or outdoor area of  FIG. 4B  to detect an intruder  430  in the volume of space formed by the wall  410  and the floor or ground  420 . The scan signals  106  are separated by an angle γ to detect an intruder  430 . The angle theta (θ) between the vertical and the circuit assembly is generally about 30° to 45°. 
     FIG. 13  is a method diagram of the steps of operating the MEMS based passive IR sensor detector assembly  600  of the present invention. In particular, step S 1300  directs positioning the MEMS mirror  604  with respect to the active elements of the IR detector assembly  600 . Step S 1302  directs collecting the IR energy from the i th  portion of the field of view (FOV) at a pre-determined scan rate. Step S 1302  is achieved by performing either step S 1302 A or S 1302 B illustrated in  FIGS. 13A and 13B , respectively. Step  1302 A directs activating the MEMS mirror  604  of the first embodiment of the present invention to traverse the FOV  440  of the IR detector assembly  600 . Step  1302 A is performed either by performing step S 1302 A 1  or step S 1302 A 2 . Those skilled in the art recognize that step S 1302  of collecting the IR energy inherently includes the steps of focusing the IR energy beam, filtering the IR energy beam, reflecting the IR energy beam by the MEMS mirror array onto a detector, detecting the IR energy beam by means of the detector, converting the IR energy beam to an electrical signal, amplifying the electrical signal, converting the electrical signal from analog to digital, processing the electrical signal by means of a processor prior to annunciating detection, and storing the results in a memory. The method can further include the steps of controlling the MEMS mirror array to scan or traverse the field of view  440 . All of the foregoing method steps are analogous to the apparatus functions disclosed in  FIGS. 11A and 11B . 
   Step S 1302 A 1  directs traversing the FOV  440  in a non-chopping mode by either performing step  1302 A 1 ′ which directs traversing the IR zones  1150  of the FOV  440  in incremental, overlapping steps or by performing step S 1302 A 1 ″ which directs traversing the IR zones  1150  of the FOV  440  in discrete, finite steps. 
   Alternatively, step S 1302 A 2  directs traversing the FOV  440  in a chopping mode by either performing step  1302 A 2 ′ which directs traversing the IR zones  1150  of the FOV  440  in incremental, overlapping steps or by performing step S 1302 A 2 ″ which directs traversing the IR zones  1150  of the FOV  440  in discrete, finite steps. 
   In  FIG. 13B , the alternative step S 1302 B of the second embodiment of the present invention directs the signal controller  1116  to adjust the MEMS mirror  604  to switch to another focusing element  706  to traverse the FOV  440  of the IR detector assembly  600 . Step S 1302 B is performed either by performing step S 1302 B 1  or step S 1302 B 2 . 
   Step S 1302 B 1  directs switching to another focusing element  706  during traversal of the FOV  440  in a non-chopping mode by either performing step  1302 B 1 ′ which directs traversing the IR zones  1150  of the FOV  440  in incremental, overlapping steps or by performing step S 1302 B 1 ″ which directs traversing the IR zones  1150  of the FOV  440  in discrete, finite steps. 
   Alternatively, step S 1302 B 2  directs switching to another focusing element  706  during traversal of the FOV  440  in a chopping mode by either performing step  1302 B 2 ′ which directs traversing the IR zones  1150  of the FOV  440  in incremental, overlapping steps or by performing step S 1302 B 2 ″ which directs traversing the IR zones  1150  of the FOV  440  in discrete, finite steps. The chopping mode alternates directing the beam  106  between a portion of the FOV  440  and the reference  1114 . 
   Once step S 1302  has been completed by performing step S 1302 A or step S 1302 B either separately or in combination, step S 1304  directs determining whether all IR zones  1150  within the FOV  440  have been measured. If No, the process returns to step S 1302 . If Yes, step S 1306  directs storing the scan. Step S 1308  directs processing the results and determining whether an intruder  430  has been detected. If No, the process returns to step S 1302 . If Yes, step S 1310  directs annunciating an alarm. If Maybe, step S 1310 ′ directs re-scanning a limited area of the room where the intruder is suspected and determining whether an intruder has actually been detected. If Yes, step S 1310  of annunciating an alarm is performed. The re-scanning process of step S 1310 ′ can be implemented either by step S 1310 ′A of re-scanning at the pre-determined rate or by step S 1310 ′B of re-scanning at a different scan rate to minimize the chances of initiating a false alarm. 
   Although, as noted previously, it is generally intended to screen out as intruders small animate objects such as pets and children, the system and method can also be used to detect such “intruders” in locations where their safety is jeopardized. In indoor locations, such locations include a furnace room or a kitchen area surrounding a stove or other such appliance. The system and method can also be applied in outdoor locations such as swimming pools. In addition, the system and method can be used to detect children, pets and animals in blind spots around mobile vehicles such as the rear end or front end of sports utility vehicles (SUVs), mini-vans, trucks, buses (especially school buses), or construction equipment. 
   As noted, this system can also be used to detect any gas or vapor with IR absorption characteristics including, but not limited to harmful, toxic, explosive or flammable vapors or gases such as: carbon monoxide (CO), volatile organic compounds (VOCs), hydrogen (H 2 ), methane (CH 4 ), propane (C 2 H 6 ), or other beneficial or non-toxic gases such as oxygen (O 2 ) or carbon dioxide (CO 2 ). It can also be used to detect flames. 
     FIG. 14  illustrates a third embodiment of the present invention wherein the IR detector assembly  1400  is arranged to detect gases or vapors. The third embodiment is identical to that shown in  FIG. 11A  except that an ambient air path or IR absorption path  1410  which provides a potential gas or vapor sample is positioned between a collimating lens  1414  and a focusing element or lens  1416  IR, i.e., thermal, energy reference source  1412  is positioned behind the collimating lens  1414 . The collimating lens  1414  collimates the IR energy beam  1420  emitted by the IR reference source  1412  prior to passing through the air path  1410 . The IR detector assembly  1400  is identical to detector assembly  600  previously shown in  FIGS. 6A–6C ,  7  and  8 , except that a narrow IR band pass filter element  1408 N is provided in addition to the wide IR band pass filter element  108 W. The narrow IR band pass filter element  1408 N and the wide IR band pass filter element  108 W each are positioned in parallel. 
   During operation, IR energy from the IR energy source  1412  is directed either to the narrow IR band pass filter element  1408 N or the wide IR band pass filter element  108 W, or to both. Upon emerging from the narrow IR band pass filter element  1408 N, the IR energy beam from the IR energy source  1412  is directed sequentially to MEMS mirror array  604 N, IR detector element(s)  112 N, amplifier  1102 N and A/D converter  1104 N, and finally to processor  1106 /memory  1108 . Similarly, upon emerging from the wide IR band pass filter element  1408 W, the IR energy beam from the IR energy source  1412  is directed sequentially to MEMS mirror array  604 W, IR detector element(s)  112 W, amplifier  1102 W and A/D converter  1104 W and finally to processor  1106 /memory  1108 . 
   As before, the signal emerging from the A/D converter  1104  is processed by the processor  1106 /memory  1108  by an algorithm and a feed back signal is provided through the controller  1110  to adjust either or both of the MEMS mirrors  604 N and  604 W. In one manner of operation, the controller  1110  adjusts an angle of at least one mirror element  900  of either or both of the MEMS mirror arrays  604 N and  604 W. In an alternate manner of operation, the controller toggles the angle position of either or both of the MEMS mirror arrays  604 N and  604 W. Changing the voltage to the mirror elements  900  causes motion by at least one of thermal expansion and electrostatic force. In still another manner of operation, the controller  1110  can activate either or both of the MEMS mirror arrays  604 N and  604 W to switch in a chopping mode between the IR source  1412 , which is focused by the focusing lens  1416 , and the IR reference  1114 . The IR reference  1114  bypasses the collimating lens  1414 , the air absorption path  1410 , and the focusing lens  1416 , and supplies reference signal S R  directly to the wide IR band filter  108 W and to the narrow IR band filter  1408 N. With the narrow band pass filter element  1408 N added to the optical path detected by the detector assembly  1400 , detection is achieved in an IR chopping mode by comparing the IR absorption characteristics of any gases or vapors present within the FOV  440  with the reference signal S R . The reference signal S R  is derived by using the MEMS mirror array  1400  as an IR chopper that switches between the FOV  440  and IR reference  1114 . As noted, the IR source  1412  emits a wideband signal. The narrowband IR filter  1408 N limits the spectrum to the portion of interest for a given gas. 
     FIG. 15A  illustrates an elevation cross-sectional view of the infra-red sensor detector assembly  1400  as mounted within an enclosure  1500  designed for the detection of gases or vapors. The enclosure  1500  is in the form preferably of a “C” shape to enable the ambient air path  1410  to be disposed between the IR (thermal) energy source  1412  and the narrow band pass filter element  1408 . Since the housing  1500  is in the form preferably of a “C” shape, there are two opposite legs  1502  and  1504 . Leg  1502  has a surface  1502   a  which faces surface  1504   a  of leg  1504 . The IR source  1412  is mounted within an interior region of one of the legs,  1502  as shown. The MEMS-based IR detector  1400  is shown mounted within an interior region of the opposite leg  1504 . The collimating lens  1414  is positioned to penetrate the surface  1502   a  while the focusing lens  1416  is positioned to penetrate the surface  1504   a . The IR source  1412  therefore emits the uncollimated beam  1420  which is collimated by lens  1414  and emerges from the lens  1414  as collimated beam  1422 . The ambient air absorption path  1410  comprises any potential gas or vapor sample to be detected. The collimated beam  1422  passes through the ambient air absorption path  1410  and passes through any potential gas or vapor sample. The collimated beam  1422  then penetrates the focusing lens  1416  and proceeds through the narrow band pass filter  1408 N of the MEMS based IR detector assembly  1400  where it is reflected by a MEMS mirror array and reflected onto a detector element in the same manner as described previously with respect to detector assembly  600 . The detector assembly  1400  is mounted on printed circuit board  1520  which comprises the logic circuitry and memory storage of data regarding the IR spectrum absorption characteristics of the gases or vapors of interest to be detected by the detector assembly  1400 . The system is self calibrated by using a second detector with a wide band IR filter such as that used in the first embodiment to compensate for variations such as intensity of the IR source and ambient lighting. The FOV in this case is limited to the IR zone defined by the collimated beam  1422 . 
   The IR heat source  1412  is at the distal end  1524 A of the optical path  1524  and of the air (absorption) path  1410  and the temperature of the IR heat source  1412  is measured in the vicinity of the IR heat source  1412  and the sidewall  1502 . The optical path  1524  includes the collimating lens  1414 , the air (absorption) path  1410 , the focusing lens  1416 , the filter windows  108 W and  1408 N, the MEMS mirror arrays  604 N and  604 W, and the detector element(s)  112 N and  112 W. The temperature of a point at a known temperature is measured at the distal end  1524 B of the optical path  1524 . The proximal end of the optical path  1524  includes the side wall  1504  of the detector housing  1500 , the MEMS mirror arrays  604 N and  604 W, and the detector element(s)  112 N and  112 W. 
   When a decrease is detected in the ratio S NB /S WB  of the narrow band S NB  to wide band S WB  detector signals, which indicates the presence of the gas or vapor of interest, a series of measurements can be taken to minimize the occurrence of false alarms. Once a positive identification is made of a gas or vapor of interest, the alarm  1112  is annunciated in the same manner as with respect to detection of an intruder. 
   In reality, traversing the FOV for gas detection is a generalization. Only one point on the FOV need be looked at. Traversing the FOV requires either one large IR source or multiple IR sources. In such a case, the housing  1500  would be separated into two parts: one for the IR source(s)  1412  and one for the detector  1412 . 
     FIG. 15B  is cross-sectional elevation view of a variation of the third embodiment of the present invention wherein the wideband elements, i.e., wide IR band filter  108 W; MEMS mirror  604 W; IR detector  112 W; amplifier  1102 W; and A/D converter  1104 W are each enclosed in or associated with a discrete wide band MEMS based IR detector  1400 W while the narrow band elements, i.e., narrow IR band filter  108 N; MEMS mirror  604 N; IR detector  112 N; amplifier  1102 N are each enclosed in or associated with a discrete narrow band MEMS based IR detector  1400 N. The two detectors  1400 W and  1400 N are separated by a distance d which is minimized to reduce the area which the collimated IR beam needs to be focused onto. An alternative approach is to make one detector housing which contains both the narrow IR band and wide IR band elements and separate them with a partition in the middle so as to minimize reflections and/or cross talk between the two detectors  1400 W and  1400 N. 
     FIG. 16  illustrates a plan view of a MEMS based IR detector assembly  1600  which houses both narrow band and wide band detectors for gas detection.  FIG. 16A  is a section view along section line  16 A– 16 A.  FIG. 16B  is a section view along section line  16 B— 16 B.  FIG. 16C  is a section view along section line  16 C— 16 C. The detector assembly  1600  includes five pins  1 ,  2 ,  3 ,  4  and  5 : one for power, one for ground, one for signal out from the narrow band detector portion  1602 N, one for signal out from the wide band detector portion  1602 W, and one for MEMS control signal.  FIG. 16D  is a perspective view of the detector assembly  1600 .  FIG. 16E  is a cutaway view of the detector assembly  1600 . A partition  1604  can be used to combine the narrow IR band detector portion  1602 N and wide IR band detector portion  1602 W into one detector housing  1610 . The partition  1604  is included inside the detector assembly  1600  to separate the wideband elements, i.e., wide IR band filter  108 W; MEMS mirror  604 W; IR detector  112 W; amplifier  1102 W; and A/D converter  1104 W, from the narrow band elements, i.e., narrow IR band filter  1408 N; MEMS mirror  604 N; IR detector  112 N; amplifier  1102 N; and A/D converter  1104 N. The purpose of the partition is to reduce reflections and/or cross talk between the two sections of the detector  1400 . Typically, the detector  1400  is sealed with the wide band filter  108 W; the narrow band filter  1408 N is placed either on top of or below the wide band filter  108 W, as shown in  FIG. 16E . Those skilled in the art recognize that the diameter or perimeter of the housing  1610  is generally larger than the embodiment shown in  FIGS. 6A–6C  and  7  and is a function of the optics and physical size of the narrow band and wide band portions of the detector. 
   FIG.  16 A 1  illustrates an example of a scan output for detecting an intruder by traversing the FOV  440  in a non-chopping mode corresponding to step S 1302 A 1  of  FIG. 13A . The x-axis represents the time in seconds. The y-axis represents the angle α 1 , of the mirror, φ of  FIG. 4A . It should be noted that the plan view shown in  FIG. 4A  is the same for the present invention as it is for the prior art. The pyroelectric detector  600  is a rate of change or second order detector. Therefore, a signal is generated when a change in temperature is detected. A room at a constant temperature produces no electrical signal. A room divided into three temperatures “0”, “+1”, “−1” produces a positive signal on the transition between 0 and +1 and a negative signal of twice the magnitude between +1 and −1, assuming the crystal is positively polarized. The width of the signal generated is a function of the sensitivity of the detector, the scan rate or the system and the shape of the beam. FIG.  16 A 1  shows pointing angle α 1  of the system and the duration of time spent t in seconds. The actual electrical signal is a function of the type of scan performed and the IR characteristics of the room. This discussion applies as well to the following FIGS.  16 A 2 ,  16 B 1  and  16 B 2 . It should be noted that FIGS.  16 A 1 ,  16 A 2 ,  16 B 1  and  16 B 2  are planar representations of the horizontal movement. There are several, typically three (3), vertical tiers. 
   FIG.  16 A 2  illustrates an example of a scan output for detecting an intruder by traversing the FOV in a chopping mode corresponding to step S 1302 A 2  of  FIG. 13A . The x-axis represents the time in seconds. The y-axis represents the angle α 1 , within the FOV φ of  FIG. 4A . 
   FIG.  16 B 1  illustrates an example of a scan output for detecting an intruder by switching on/off of a lens element in a non-chopping mode corresponding to step S 1302 B 1  of  FIG. 13B . The x-axis represents the time in seconds. The left side y-axis represents the angle α 1 , within the FOV φ of  FIG. 4A . Specifically, the right side y-axis represents a plurality of focal elements A 1  . . . A N  where in this example, N=7, each of which corresponds to an angular range in an angle α 1  of the FOV. The plurality of focal elements corresponds to the focal element(s)  706  of  FIG. 8 . 
   FIG.  16 B 2  illustrates an example of a scan output for detecting an intrusion by switching from one to another lens element in a chopping mode corresponding to step S 1302 B 2  of  FIG. 13B . The x-axis represents the time in seconds. The left side y-axis represents the angle α 1 , within the FOV φ of  FIG. 4A  and specifically, as before, the right side y-axis represents a plurality of focal elements A 1  . . . A N . FIG.  16 B 2 ′ is an enlarged detail of the chopping mode corresponding to FIG.  16 B 2  of switching from one to another lens element. Following a dwell time T D , there is a transition time T T . 
     FIG. 17  illustrates an example of a scan output for detecting a gas or vapor by scanning in a chopping mode the air absorption path as the FOV, as corresponds to use of the gas detector apparatus of  FIGS. 14 and 15  in a chopping mode according to step S 1302 A 2  of FIG.  13 A 2 . The x-axis represents the time in seconds. The y-axis represents the FOV of the system. The air or absorption path  1410  points at the IR heat source  1412  and a location outside of view of the heat source, IR reference  1114 , thereby providing radiant contrast by alternating between the IR source  1412  and room temperature. 
   In other words, the gas detection scheme measures IR energy at two points: the IR energy of the IR heat source  1412  which is on the other side of the air (absorption) path  1410  and the IR energy of a point at a known temperature, i.e., IR reference  1114  in the side wall of the MEMS-based IR detector  1400 .  FIG. 17  represents the IR energy level of the FOV, whereas, the output of the detector  1400  is illustrated in  FIG. 19 , which is discussed later. Specifically, the FOV in this case is effected by switching between the IR source  1412  and the IR reference  1112 . Since the output of the narrow band detector decreases by approximately 30–40% when the gas of interest is present and the output of the wide band detector decreases by 1–4% when the gas of interest is present, the ratio of the narrow band signal to the wideband signal is far less sensitive to noise (ambient light, IR source variations) fluctuations, and represents more definitive evidence of the presence of a gas or vapor of interest within the air path  1410 . 
   The method of operation of the gas or vapor detection system of the third embodiment of the present invention is analogous to implementing steps S 1302 A or S 1302 B in the chopping modes of steps S 1302 A 2  or S 1302 B 2 , respectively. Those skilled in the art recognize that the steps S 1302 A or S 1302 B of collecting the IR energy inherently include the steps of focusing the IR energy beam, filtering the IR energy beam, reflecting the IR energy beam by the MEMS mirror array onto a detector, detecting the IR energy beam by means of the detector, converting the IR energy beam to an electrical signal, amplifying the electrical signal, converting the electrical signal from analog to digital, and processing the electrical signal by means of a processor prior to annunciating detection. In addition, the method can include the step of controlling the MEMS mirror array. All of the foregoing method steps are analogous to the apparatus functions disclosed in  FIG. 14 . 
   However, the gas detection method does not include a step of scanning of the FOV. Rather, the method includes the steps of measuring the IR energy of the IR heat source  1412  which is on the other side of the air (absorption) path  1410  and measuring the IR energy of a point at a known temperature, i.e., IR reference  1114  in the side wall of the MEMS-based IR detector  1400 . Each IR energy is measured through both the narrow IR band filter  1408  and narrow IR band detector  112 N and through the wide IR band filter  108  and wide IR band detector  112 W. The step of detecting of gas occurs by measuring a decrease in the IR energy beam received by the detector with the narrowband filter  112 N. In addition, the step of calibrating the detector system occurs by measuring the IR energy beam received by the detector with the wideband filter  112 W. 
     FIG. 18-1  illustrates an example of detector output versus the FOV as a comparison of two scans  1  and  2  in the non-chopping mode for the intrusion detection system corresponding to either FIGS.  16 A 1  or  16 B 1 . A comparison between Scan  1  and Scan  2  shows that the peak P of the electrical signal has shifted from left to right within the FOV. The shift of the signal peak P indicates the movement of a heat source within the room, potentially providing grounds for annunciation of an alarm signal. 
     FIG. 18-2  illustrates an example of an electrical signal output in millivolts, mv, versus the FOV as a comparison of two scans  1  and  2  in the chopping mode for the intrusion detection system corresponding to either FIGS.  16 A 2  or  16 B 2 . A difference in amplitude ±ΔA in the electrical signal gives rise to peak signals +P 1  to +P 8  and −P 1  to −P 8 . In a comparison between Scans  1  and  2 , detection of the same amplitude ±ΔA of the peaks ±P 1  to ±P 2  indicates the presence of a heat source in the room but not movement of the heat source. A change in the amplitude of the peaks ±P 3  to ±P 6  indicates movement of the heat source within the room, potentially providing grounds for annunciation of an alarm signal. The shape of the curves is approximate. The actual output is dependent upon the chopping rate and the responsivity of the detector. Responsivity is a measure of the time constant of the detector. 
     FIG. 19  illustrates an example of detector output versus the FOV, i.e., the air path as a comparison of two scans  1  and  2  in the chopping mode for the gas detection system corresponding to  FIG. 17 . Scan  1  includes both a narrowband detector scan output  1 N and a wideband detector scan output  1 W. Similarly, scan  2  includes both a narrowband detector scan output  2 N and a wideband detector scan output  2 W. The narrowband scans  1 N and  2 N each include peaks ±P 10  to ±P 30 , while wideband scans  1 W and  2 W each include peaks ±P 100  to ±P 300 . 
   A large difference in amplitude ±ΔA N  in the peaks ±P 10  to ±P 30  of the electrical signal of the narrowband scans  1 N and  2 N indicates the presence of a gas or vapor of interest. The magnitude of the difference in amplitude indicates the percentage of gas or vapor that is present. Correspondingly, only a small difference in amplitude ±ΔA W  occurs in the peaks ±P 100  to ±P 300  of the electrical signal of the wideband scans  1 W and  2 W due to the presence of the gas or vapor of interest. 
     FIG. 20  illustrates a typical IR absorption spectrum for a gas of interest for gas detection of  FIG. 19 . Within the narrowband filter limits L N  an absorption peak P N1  is observed as a result of a gas or vapor of interest being detected. Correspondingly, within the wideband filter limits L W  one or more absorption peaks P N1  and P N2  are observed. The figure illustrates two peaks. The absorption peaks P N1  and P N2  are offset within the spectrum. The bandwidth of the wideband filter is selected such that the presence of the gas does not significantly affect the signal produced by the IR source. 
     FIG. 20A  is a graph representing the output signals generated by the narrow band and wide band IR detectors  112 N and  112 W, respectively, in the presence of a gas. The x-axis represents the percentage concentration of gas present, designated as CG. The y-axis represents the percentage of the output signal S WB  generated by the wideband detector  112 W. The output signal S NB  generated by the narrow band detector  112 N is shown as 50% of the signal S WB  generated by the wide band detector  112 W. In reality, S NB  will be closer to 100 times smaller due to the absorption of a significant amount of IR energy by the narrow band filter window  1408 N. This reduction in IR energy absorption is compensated for with additional gain in the amplification circuit of the narrow band signal S NB . In this example, for a 20% concentration of the gas, CG in the air path, the output signal S NB  from the narrow band decreases by approximately 40%, while the output signal S WB  from the wide band decreases by approximately 4%. The actual value of interest is the ratio of the narrow band to the wide band output signals, S NB /S WB . This ratio, S NB /S WB , is directly proportional to the concentration of the gas or vapor present in the air path. 
   The ratio of the narrow band to the wide band output signals S NB /S WB  can be calculated by the processor  1106  in several ways. Typically, this ratio S NB /S WB  is calculated by comparing the average of the instantaneous narrow band peak values to the average of the instantaneous wide band peak values over a given time period. Alternatively, this ratio S NB /S WB  can be calculated by averaging the ratios S NB /S WB  based on the instantaneous narrow band peak values to the instantaneous wide band peak values over a given time period. The ratio S NB /S WB  can also be calculated based on unaveraged instantaneous peak values. The different methods of calculating the ratios are considered depending upon the responsivity required for the particular application. Greater responsivity to the presence of a gas might be desired for application in a home environment as opposed to an industrial environment, for example. 
   The normalized signal ratio S NB /S WB  is presented so that all of the data can appear on one chart. The signal ratio S NB /S WB  typically is characterized by one or more thresholds. A signal ratio S NB /S WB  significantly less than 1 represents the presence of a gas or vapor. An alert threshold, T ALERT , indicates a possible problem and an alarm threshold, T ALARM , indicates an emergency. For example, the LEL (lower explosion level) of methane gas is approximately 4% i.e. the percentage of gas necessary to cause an explosion, T LEL . Lower percentages will only cause a flame. Therefore, an alert threshold T ALERT  of 20% of the LEL or C G =0.8% gas and an alarm threshold T ALARM  of 50% of the LEL or C G =2% gas would be reasonable. The actual value of the thresholds is dependent upon the requirements of the application. Likewise, for carbon monoxide, danger levels are determined by PPM (parts per million) of gas. Again the limits are dependent upon application, where permissible levels in a commercial environment would be higher than those levels permissible in residential or educational environments. 
     FIG. 20B  is a graph representing the output signals S NB  and S WB  generated by the narrow band and wide band IR detectors  112 N and  112 W, respectively, in the presence of a gas, as affected by changes in output from IR energy source  1412 . The basis for calculating the ratio S NB /S WB  is the same as discussed previously with respect to  FIG. 20A . The processor  1106  calculates the ratio S NB /S WB  for self calibration with respect to changes in the IR energy which reach the top surface of the filter windows  108 W and  1408 N that are caused by changes in intensity of the IR source  1412  or ambient lighting. For example, a 10% decrease in the output power of the IR source  1412  would translate to 10% lower signals S NB  and S WB  from the narrow band and wide band detectors  112 N and  112 W, respectively. However, the ratio of the signal S NB /S WB  from the two detectors does not change significantly. Therefore, the processor  1106  can monitor and self calibrate the narrow band and wide band detector  112 N and  112 W respectively by observing how close the ratio S NB /S WB  is to 1. 
     FIG. 21A  illustrates a plan view of MEMS mirror array  2100 , which is comprised of individual mirror elements  2110   j  that are arranged in rows  2112   k  and columns  2214   1 . In the example shown, a 3×3 arrangement of elements  2110   j  forms a 3×3 group  2116   m  within a 5×5 array of groups  2118   n . Therefore, the MEMS mirror array  2100  is a 15×15 array. 
   As discussed previously with respect to  FIG. 9 , MEMS mirrors generally operate in two different modes. In the first mode, the MEMS mirror array  2100  operates with mirror elements  2110   j  operating between start and end positions in a manner of operation similar to that of a mechanical relay. That is, in the first mode, the start and end position are fixed, and the MEMS mirror array  2100  is either in an unactuated or actuated mode. Once a control signal is applied, the MEMS mirror active elements move rapidly to the end or final position. In the second mode, the MEMS mirror  2100  operates with a smooth transition across the angular adjustment, or a series of angular steps can be effected. Also, the range of motion is limited to a specified angle, say +20 to 0 degrees. This range of motion provides a 40 degree field of view, FOV  440 . 
     FIG. 21B-1  illustrates how an IR ray  2120  hitting the active area of an unactuated individual MEMS mirror element  2110   j  is reflected as a ray  2122  at an angle η. In the example shown, η i  in the unactuated or initial position=40°. 
     FIG. 21B-2  illustrates how the IR ray  2120  hitting the active area of now actuated individual MEMS mirror element  2110   j  is reflected as ray  2122  at angle η where, in the example shown, η f  in the actuated or final position=80°. That is, there occurs twice the angular movement (η f −η i ) of the element  2110   j  from its unactuated position in  FIG. 21B-1 , to its actuated position in  FIG. 21B-2 , i.e., 20 degrees of angular movement of the MEM&#39;s mirror element provides a reflected beam movement of 40 degrees from the initial angle η i =40° to the final angle η f =80°. 
     FIG. 21C  illustrates an IR ray trace for a MEMS mirror device which is comprised of elements each capable of operating between only a start and an end position. By manipulating only portions of the start and end mirror array  2100  at any given time, discrete steps can be created with a MEMS mirror device which comprises mirror elements  2102   a  through  2102   f  that are only capable of being actuated between a start and end position. The actual orientation of the detector assembly  600  is the same as shown in  FIG. 12 . In the example (a), rows of active elements  2102   a  and  2102   b  are at the +20° position and the four rows of elements  2102   c  through  2102   f  at the 0° position with none of the elements viewing through the port  705  outside of the detector housing cover  110 . In (e), the three rows of mirror elements  2102   a  through  2102   c  are moved to the +20° position with a beam pointed at 14° from the normal to the detector housing cover  110  and coming from element  2102   c , thereby viewing outside the detector housing cover  110  through IR filter window  108 . The opening in detector assembly  600  for the IR filter window  108  is in reality an optical field stop. In (f), the third row  2102   c  is returned to the 0° position and the fourth row  2102   d  is moved to the +20° position with a beam pointing at 19° from the normal to the detector housing cover  110  and coming from element row  2102   d . For clarity, the IR filter  108  is not shown in the detector housing cover  110 . 
     FIG. 22  illustrates actual electrical output from a mockup simulation of the non-chopping (or simulated sweeping) scan of the passive IR sensor of the present invention illustrated in FIG.  16 A 1 . 
   As can be appreciated from the previous discussions, there are four modes of operation for motion detection:
     (1) a large number of steps or a continuous movement of the MEMS mirror array simulating a sweep or a non-chopping scan;   (2) a finite number of discrete steps in which each lens element is evaluated one at a time;   (3) a chopping scan with a large number of steps or a continuous movement of the MEMS mirror array;   (4) a chopping scan with a finite number of discrete steps.   

   Therefore,  FIG. 22  represents scan output from the first mode of operation. 
   The invention has been described herein with reference to particular exemplary embodiments. Certain alterations and modifications may be apparent to those skilled in the art, without departing from the scope of the invention. The exemplary