Abstract:
A remote sensing method for detecting and analyzing gases, vapors and flame plumes using an imaging spectrometer. The spectrometric instrument uses Image Multispectral Sensing (IMSS) technology, enhanced by advanced imaging processing techniques and micro-miniature circuitry. These enhancements provide a portable instrument with the capability to remotely detect and image gases, including gas leaks. The technology also provides an analysis of the gas including chemical species and concentrations. The instrument can also remotely detect, image and analyze flames and plumes in the same manner, providing an analysis of the chemical species and concentrations in the flame. Advanced image processing techniques are used to provide gas and plume images and analysis to the operator. These processing algorithms are implemented in micro-miniature circuits such as digital signal processors (DSP&#39;s) and field programmable gate arrays (FPGA&#39;s) to provide a field portable instrument.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is an imaging spectrometer useful for measuring the spectral composition of gases emanating from a gas leak at a remote location, and, more particularly, provides a method and apparatus for locating gas leaks. 
     2. Prior Art 
     Gas producers and distributors lose millions of dollars annually due to gas leakage from a distribution line, processing or containment facility. A portable instrument with the capability to remotely detect and image gases, including gas leaks, is needed. Such a device and method for finding and identifying gas leaks should preferably further provide an analysis of the gas including chemical species and concentrations. 
     Spectrophotometers (color meters) are widely used in the area of target recognition. The principle underlying the use of spectrophotometers in target recognition is that different targets reflect, emit or absorb light differently. Alternatively, different targets represent independent light sources, the light emanating from each target having an observable spectra which is an identifiable inherent characteristic of that target which may be used for target identification. 
     Image multispectral sensing (IMSS) records the spectrum of individual luminous objects (targets) within an image or scene. IMSS is capable of simultaneously recording the spectrum of light emerging from many different discrete light sources contained within a single field of view. U.S. Pat. No. 5,479,258 to Hinnrichs et al., the contents of which is incorporated herein by reference thereto, discloses an image multispectral sensing device, which provides good spectral resolution for images comprising luminous point objects which have good contrast ratios with respect to the background. The ability of IMSS spectrometers to distinguish between an object and a background may be extended to the detection of a non-homogeneous distribution of gases in a volume of gas. There is a need for an IMSS apparatus which is adapted to be portable and operable for detecting gas leaks at remote locations. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a spectrometric apparatus for measuring the spectral composition of infrared, visible or ultraviolet light emanating from a volume of gas located at a distance from the apparatus. 
     It is yet another object of this invention to provide a imaging spectrometer apparatus which can determine the spectral composition of light emanating from different portions of a large volume of gas comprising a remote field of view. 
     It is another object of this invention to provide a imaging spectrometer apparatus useful for comparing the spectral composition of light (color signature) emanating from a remote luminous volume of gas within a field of view with the color signature of previously identified luminous gases or mixtures of gases. 
     It is a particular object of the invention to provide a IMSS apparatus operable for performing the above objects and which can detect the presence of a gas leak from a gas containment device, or components is a processing or distribution system. 
     These and other aspects of the invention will soon become apparent in view of the drawings and description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing red and blue spectral components of dispersed polychromatic light emerging from a diffractive lens in sharp focus at different focal planes along the optic axis. 
     FIGS.  2 ( a ) and  2 ( b ) show the intensity distribution of dispersed spectral components of light from a single point source illuminating a photosensitive surface and show the relative intensity of the in-focus spectral component having a wavelength λ 1  with respect to the superimposed intensity of out-of-focus spectral components of the dispersed light at the focal plane of λ 1  and generally illustrating the relationship between spectral resolution in of the in-focus image and the out-of-focus background. 
     FIG. 3 is a schematic diagram of an IMSS spectrophotometer showing the optical relationship of the addressable transmissive spatial mask ( 35 ) with respect to the input optics for controlling transmission of the image projected thereon to the diffractive lens ( 31 ) for dispersion and to the photodetector ( 32 ) for spectral detection. 
     FIG. 4 is a schematic diagram of a gas detection instrument in accordance with the present invention, adapted for detecting gas leaks. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides an apparatus and method for identifying the presence of a gas leak at a location remote from the apparatus. The apparatus is a IMSS spectrometer incorporating an addressable spacial mask which permits the spectral composition of a single pixel to be determined even when the image comprises more than one independent source of light. A prior art IMSS system is shown in FIG.  3 . The apparatus  30  comprises a diffractive lens  31  having an optical axis, a photodetector  32  having a planar photosensitive surface orthogonal to the optical axis and intersecting the optical axis at a distance f from the diffractive lens, a stepping motor  33 , which may include an optional shaft position encoder, adapted to change D a known amount in response to a control signal, a programmable signal processor  34  which includes programmable computer means adapted to provide a control signal to the stepping motor  33  for changing D and provide control signals to the gates and identify and synchronously record the output signal of the pixel in optical communication with the gate. The signal processor is adapted to receive and organize the output signals from each pixel within the photodetector into a sequence of frames, each frame in the sequence containing the signal output of each pixel at a focal plane f corresponding to an input of the position encoder  33  identifying a focal plane, process the frames to spectrally filter the spectral data and present the processed spectral data to a spectral correlator  35  for comparison, identification storage, or for future reference purposes. Light  36  emanating from a remote target  37  within a field of view is collimated by input optics  38  which directs the light  36  comprising an image of the field of view to impinge upon a diffractive lens  31 . The light  36  is focused by the diffractive lens  31  onto the photodetector  32 . The distance f between the photodetector array  32  and the diffractive lens  31  is controlled by mounting one of the components  31  or  32  on translating means  33  such as a stepper motor, a piezoelectric translation device or other such translating device capable of varying D to traverse the range of focal lengths encompassing the spectral components of the light  36 . The distance D, which is the instantaneous distance between the diffractive lens  31  and the photodetector  32  along the optic axis, is determined by a stepping motor  33 , which is controlled by the signal processor  34 . The signal processor records the signal output of each pixel (not shown in FIG. 3) in the photodetector  32 . Thus, for every value of D, only luminous objects comprising the image having a spectral component is in sharp focus at the plane of the photodetector  32  will be recorded within the frame corresponding to the focal plane for the spectral component. If the image includes multiple light sources or targets, the intensity of the spectral component of light emanating from targets  1  through n in the image are recorded within a frame for each value of D as shown. The record of the signal output of each pixel within the photodetector corresponding to a particular focal distance (wavelength λ) comprises a single frame. In FIG. 3, only a single target  37  is shown. The relative intensity and wavelength of the spectral components λ 1 -λ 9  of the light  36  emanating from the target  37 , may be compared with spectral data stored within the memory of a spectral correlator  35 . The correlator  35  provides a means for storing and comparing spectral data. 
     As stated earlier, the signal output of each pixel within the photodetector array  12  is electronically scanned into a signal processor  34 . The signal processor compares the signal output of each pixel with the signal output of pixels immediately adjacent thereto. If the signal output of the adjacent pixels is uniformly lower than the output of the pixel being processed, the uniform output signal may be attributed to noise and subtracted from the measured signal output of the pixel to provide a measure of the intensity of the in-focus spectral component. The process is repeated for each pixel having a signal output greater than a threshold value and the electronically filtered spectral intensity data is stored in a frame. The procedure is repeated at incremental focal planes until frames encompassing the spectral range of interest are recorded. For example, with reference to FIG. 1, when the photodetector  12  is at the position of f r , only the red spectral component will be in focus at pixel  16  and remain in the frame corresponding to f r  after signal processing at the position f b  only the blue spectral component will pass through the signal processing filter and appear as the only signal output in the frame corresponding to f b . 
     The change in distance between the diffractive lens and the photodetector required for IMSS must be sufficient to span the range of focal lengths for the spectral components of interest in the target-derived light. For example, a piezoelectric driven movable element may be used to mount the diffractive lens. Focal length scans of 160 millimeters per second are practical with this approach. A stepper motor or a pneumatic/hydraulic translational device can also be employed for changing the distance f between the diffractive element and the photodetector when either of the components are moved thereby thereon. 
     The defocused spectral components of light comprising the spectra of nearby independent sources of light within the image will also impose noise upon a pixel&#39;s output signal. If the out-of-focus spectral component from an adjacent target is very intense, it can dramatically and asymmetrically effect the signal output of the pixel receiving the in-focus spectral component of the less luminous light source. The apparatus employs an addressable spatial mask, shown in phantom at  35  in FIG. 3, disposed between the input optics  38  and the diffractive lens  11  to shadow one of a plurality of adjacent light sources in an image to reduce interspectral noise at the photodetector. The mask has an image receiving surface which consists of a planar array of discrete, addressable, independently controllable optical gates disposed to receive an image projected thereon by the input optics. The function of the mask is to select only a pixel-sized portion of the image incident thereon and sequentially direct the gated pixel-sized portion of the image to be dispersed by the diffractive lens. The diffractive lens focuses the spectral components to illuminate the photodetector. Light from other portions of the image are blocked out while the signal output of the pixel receiving the gated light is synchronously recorded. Each of the plurality of switchable gates forming the image-receiving surface of the mask are sequentially “switched” in response to a control signal from the signal processor and will direct only the portion of the image incident thereon to the diffractive lens in synchronization with the electronic readout of the pixel upon which the gated light is focused. In this manner each spectral component comprising the image is sequentially mapped onto the photodetector surface a “gate-full” at a time in synchronization with the sequential output sampling of the gate-sized pixel in the photodetector upon which the gated light is focused. Out-of-focus light from other spatially separated portions of the image is blocked out. 
     With reference now to FIG. 4, a preferred embodiment  40  of the present invention is illustrated in diagrammatic view. A diffractive optical element  41  is used to focus an image of the target chemical under analysis on a photosensitive surface  42  of a detector  43  (such as a focal plane array (FPA)). An image of the scene under view is formed on the detector  43 . As discussed above, the IMSS diffractive optical element  41  focuses different wavelengths of light at different distances or focal lengths. The distance between the diffractive optical element  41  and the detector  43  is changed to form a series of very narrowband spectral images. The images are stored in the image processing electronics  44 . The image processing electronics  44  uses these spectral images to detect and image the target chemical by techniques such as, for example, by comparing the image at the absorption band of the target chemical to images outside the absorption band, using motion detection algorithms, and applying techniques such as principle components analysis. A raw image and a processed image of the target chemical can be provided to the operator via operator display  45 . 
     The instrument  40  can also identify an unknown chemical. The spectrum of an unknown target chemical obtained by the instrument is compared with a spectral database of chemical spectra stored in the image processing electronics to identify the chemical species of the target chemical. Concentrations of the target chemical are obtained by determining the absorption (or emission in the case of a flame or plume) of the target chemical at its absorption wavelength (or emission wavelength) compared to images outside the absorption or emission region. 
     In view of the foregoing, and in light of the objectives of the invention, it will be apparent to those skilled in the art that the subject matter of this invention is capable of variation in its detail, and we do not therefore desire to be limited to the specific embodiments of the apparatus selected for purposes of explanation of the invention. For example, it is possible to employ a diffractive lens for the chromatic dispersion of light wherein the particular wavelength of light in focus at a particular point in space depends on the electromechanical forces applied to the lens. A diffractive optical element can be made using polymer or spatial light modulators which change the characteristics of the lens such that the chromatic focal length of the lens can be adjusted by application of an appropriate electrical signal to the lens. In such a case there is no need to move the diffractive lens relative to either the spatial mask or the photodetector array to perform Image Multispectral Sensing. The foregoing has been merely a description of one embodiment of the imaging spectrophotometer that may be used to detect gas leaks. The scope of the invention can be determined by reference to the claims appended hereto.