Abstract:
An apparatus and method permits measuring of near-direct forward scattering functions in water to enable acceptable underwater imaging for detection, classification, and identification of objects, such as mines. A source of light mounted on a housing member receiving ambient water emits a beam of light along an axis to a scattering detector assembly mounted on the base member. The detector assembly has a central active region disposed in the axis to receive portions of the light beam emitted along the axis and a plurality of concentric active regions are located radially outwardly from the central active region and the axis to receive scattered portions of the light beam. The central and concentric active regions provide signals representative of the magnitudes of the axial and scattered portions of the light beam for determination of the scattering function of the ambient water.

Description:
STATEMENT OF GOVERNMENT INTEREST  
       [0001] The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to an apparatus for measuring light scattering. More particularly, this invention measures near direct-forward scattering function of light in water for underwater imaging.  
           [0003]    Currently there is renewed interest in developing electro-optic sensors for underwater imaging used in the detection, classification, and identification of a number of submerged objects, such as mines. The performance of these sensors is strongly influenced by the characteristics of the water in which they are operated. For acceptable imaging one of the most influential environmental parameters which affects the performance is scattering. In particular, the parameter, or function of near-direct forward scattering that is attributed to the water medium and dissolved and particulate matter in the water medium dominates the ability to resolve fine details of images.  
           [0004]    Historically, this scattering function has been difficult to measure, and no available sensors are known to accurately perform this measurement. In fact, direct measurement of the scattering phase function has not been attempted often. The report, “ Volume Scattering Functions for Selected Ocean Waters” by Scripps Institution of Oceanography for Naval Air Development Center, October  1972 , National Technical Information Service AD -753 474, is the only known published account of direct measurements available. The device of the Scripps report used a movable detector that could be positioned over essentially the full arc from the direction the beam was emitted to the direction the beam was directly reflected (0-180 degrees relative to the direction of the emitted beam). The arc the movable detector traveled was large, making it acceptable for effective real-time scientific measurements; however, the device was not suitable for general use at work sites in the ocean.  
           [0005]    Indirect measurements of other phenomena in the water have been made on occasion in the form of Modulation Transfer Functions (MTFs) and Point Spread Functions (PSFs). But these measurements are not directly relatable to the underlying scattering function, as they provide a measurement of integrated effects from which the scattering function cannot be extracted.  
           [0006]    Thus, in accordance with this inventive concept, a need has been recognized in the state of the art for a detector apparatus deployable in the ocean to measure near-direct forward scattering of ambient water.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides an apparatus for and method of measuring near-direct forward scattering in water. A source of light is mounted on a housing member to emit a beam of light along an axis of emission and a scattering detector assembly is mounted on the housing member a predetermined distance from the light source. The detector assembly has a central active region disposed in the axis to receive portions of the light beam emitted along the axis, and a plurality of concentric active regions located radially outwardly from the central active region and the axis to receive scattered portions of the light beam. The central active region and the concentric active regions provide signals representative of magnitudes of the axial portions and scattered portions of the light beal.  
           [0008]    An object of the invention is to provide an apparatus for and method of measuring near-direct forward scattering in water.  
           [0009]    Another object of the invention is to provide an apparatus for and method of measuring the near-direct forward scattering function in water in the harsh marine environment.  
           [0010]    Another object of the invention is to provide an apparatus for and method of measuring near-direct forward scattering in water that is uncomplicated and reliably used in the harsh marine environment  
           [0011]    Another object of the invention is to provide an apparatus for and method of reliably measuring near-direct forward scattering in the harsh marine environment to permit detection, identification, and classification of submerged objects, such as mines.  
           [0012]    Another object of the invention is to provide an apparatus for and method of measuring scattered light in water that can compensate for the progressively, rapidly decreasing magnitude of light scattered outside of the direction of the beam.  
           [0013]    Another object is to provide an apparatus for and method of directly comparing received signals to obtain a calibrated return based on known performance of the active areas present to simultaneously obtain the scattering curve in the entire near-forward scattered region.  
           [0014]    Another object of the invention is to provide an apparatus for and method of measuring the near-direct forward scattering function in water using a common supply voltage for all active regions of the detector to ensure commonality in the process of optical reception.  
           [0015]    Another object of the invention is to provide an apparatus for and method of measuring the near-direct forward scattering function in water having differently shaped and located active regions for detection of optical scattering of light to achieve different measurement goals.  
           [0016]    Another object of the invention is to provide an apparatus for and method of measuring the near-direct forward scattering function in water using photo-detecting active mediums like photo-conductive components, photo-diodes.  
           [0017]    Another object of the invention is to provide an apparatus for and method of measuring the near-direct forward scattering function in water using one or more avalanche photo diodes to improve strength of signals.  
           [0018]    These and other objects of the invention will become more readily apparent from the ensuing specification when taken in conjunction with the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 Is a schematic side view of the apparatus of the invention for measuring the near-direct forward scattering function in water in the harsh marine environment.  
         [0020]    [0020]FIG. 2 is a schematic front view of the scatter detector assembly taken generally along lines  2 - 2  in FIG. 1. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    Referring to FIG. I, detector  10  of this invention reliably measures the scattering, or scattering function of light. Detector  10  is particularly adapted to measure phenomena known as the near-direct forward scattering function in mediums, such as water  40 . Knowing what this scattering function is enables more responsive detection, identification, and classification of objects, such as a mine  50  submerged in the harsh marine environment by a sensor module  60  connected to a processing system  70 . The more responsive operation is assured since the performance of sensor module  60  is strongly influenced by the characteristics of the ambient water in which it is operating. Detector  10  of this invention can be compactly packaged and may be mounted adjacent sensor module  60  on a submersible platform  80  or other underwater mounting surface to provide real-time data representative of scattering functions for processing system  70  associated with sensor module  60 . Optionally, detector  10  and processing system  70  could be removed from the other constituents and combined to work as a separate Instrument in the ocean to make environmental measurements for a variety of different tasks.  
         [0022]    Detector  10  has a rigid can-shaped, or cylindrical shell-shaped housing member  11  provided with a plurality of elongate elliptical openings  12  that extend nearly its length between disc-shaped end caps  13   a  and  13   b . Openings  12  may be shaped differently than elliptical so long as they permit flow of water. Housing member  11  is made from non-corrosive metal or some rigid plastic material to provide structural integrity for the components of detector  10 , and elongate elliptical openings  12  in housing member  11  permit substantially unrestricted access to and communication with its interior  14  by ambient water  40 . In other words, elliptical openings  12  in housing  11  let water  40  flow freely into and fill interior  14  when detector is immersed in ambient water  40  to quickly permit measurements of scattering.  
         [0023]    End cap  13   a  has a laser  15  secured to it and oriented to emit a beam of light  16  along an axis of emission  17  through interior  14 . Light beam  16 , having a diameter of about 3 mm for example, is emitted along, or on axis  17  at the proper intensity for a desired measurement of scattering in ambient water  40 . A source of power  18  mounted on end cap  13   a  is connected via leads  18   a  to laser  15  to maintain the intensity of light beam  16  constant or selectively variable if desired throughout a measurement procedure. Power source  18  could be inside submersible  80  if desired. Light beam  16  is emitted to travel along axis  17  in the direction toward scattering detector assembly  20 .  
         [0024]    Scattering detector assembly  20  is secured to end cap member  13   b  at a predetermined distance along axis  17  from laser  15 . The exact distance, or separation between laser  15  and scattering detector assembly  20  can range from a few centimeters to meters, or whatever separation is needed to provide accurate measurements in different mediums. Scattering detector assembly  20  of detector  10  receives illumination from laser  15  to measure the near-forward scattering function at a number of angles in the near-forward direction that diverge up to about five degrees from the direction of travel of emitted light beam  16  along axis  17 . This degree of angular diversion around light beam  16  is the region of primary interest in determining ability to image fine details underwater.  
         [0025]    Referring also to FIG. 2, scattering detector assembly  20  has a disc-shaped central active region  22  and a series of ring-shaped concentric active regions  24 ,  26 , and  28  surrounding central active region  22 . Central active region  22  is axially aligned to receive light beam  16  to measure the intensity of portions  16   a  of light beam  16  that are not scattered and directly impinge on region  22  along axis  17 . Ring-shaped concentric active regions  24 ,  26 , and  28  are concentrically disposed about axis  17  and the direction of emitted light beam  16  to measure the light intensity of portions  16   b  of near-forward scattered light outside of axis  17  and the direction of emitted light beam  16 . Only three concentric active regions  24 ,  26 , and  28  made up of segments  24   a ,  26   a , and  28   a , respectively are depicted. It is understood that scattering detector assembly  20  of detector  10  can have more or less concentric active regions as desired.  
         [0026]    Central active region  22  and arc-shaped segments  24   a ,  26   a , and  28   a  of ring-shaped concentric active regions  24 ,  26 , and  26  are all connected to a common power supply  30  via leads  30   a  to provide the same supply voltage to all of the active regions uniformly. This feature permits the photo-gain from central active region  22  and from each of the segments  24   a ,  26   a , and  28   a  of active regions  22 ,  24 ,  26 , and  28  to be comparable to each other (i. e. similar in magnitude) or another standard created in interconnected processing system  70 . The operating voltage coupled to active regions of scattering detector assembly  20  from power supply  30  is set so that central active region  22  produces a full-scale output signal without saturation when it is illuminated by light beam  16  in air, i. e. interior  14  is filled with air.  
         [0027]    Central active region  22  and segments  24   a ,  26   a , and  28   a  of concentric active regions  24 ,  26 , and  28  may be any suitable photo-detecting active medium, such as photo-conductive components, (photo-diodes). An annular, shallow light baffle  21  can be placed around disc-shaped central detector region  22  to eliminate, or at least reduce surface scatter on the surface of central active region  22  from reaching ring-shaped concentric active regions  24 ,  26 , and  28 . The height annular baffle  21   a  extends above central active region  22  is relatively small, about 1 mm, to avoid interference with the scattering measurement by concentric active regions  24 ,  26 , and  28 .  
         [0028]    A first annular non-active region  23  of scattering detector assembly  20  is disposed radially outwardly from and adjacent to central active region  22 . First annular non-active region  23  surrounds central active region  22  to preclude, or prevent stray light impinging outside the active area of central active region  22  from affecting the reading of impinging light intensity on central active region  22 . A second annular non-active region  25  of scattering detector assembly  20  is disposed radially outwardly from and adjacent to concentric active region  24 . Second annular non-active region  25  surrounds concentric active region  24  to preclude, or prevent stray light impinging outside the active area of concentric active region  24  from affecting the reading of impinging light intensity on concentric active region  24 . A third annular non-active region  27  of scattering detector assembly  20  is disposed radially outwardly from and adjacent to concentric active region  26 . Third annular non-active region  27  surrounds concentric active region  26  to preclude stray light impinging outside the active area of concentric active region  26  from affecting the reading of impinging light intensity on concentric active region  26 . A fourth annular non-active region  29  of scattering detector assembly  20  is disposed radially outwardly from and adjacent to concentric active region  28 . Fourth annular non-active region  29  surrounds concentric active region  28  to preclude stray light impinging outside the active area of concentric active region  28  from affecting the reading of impinging light intensity on concentric active region  28 . All the nonactive regions are made from materials that do not produce signals in response to impinging light and/or can be or have coatings that absorb light, for example.  
         [0029]    Ring-shaped concentric active regions  24 ,  26 , and  26  provide active areas substantially larger than central active region  22 . The segments of concentric active regions  24 ,  26 , and  28  can be arranged in any of several fashions in addition to the arrangement of different arc-shaped segments  24   a ,  26   a ,  28   a  of ring-shaped concentric active regions  24 ,  26 , and  28  shown in FIG. 2. However, irrespective of the arrangement, the aggregate of active regions of arc-shaped segments  24   a ,  26   a , and  28   a  of each of concentric active regions  24 ,  26 , and  28  are made progressively larger in area as their distances from axis  17  of light beam  16  and central active region  22  are made greater, or increased. This progressive increase in areas of concentric active regions  24 ,  26 , and  28  is to accommodate the scattering that diminishes, or falls off rapidly as the distances, or separations increase radially outwardly from axis  17  of light beam  16  in most waters of interest. In other words, the progressive increase of active areas compensates for this fall-off by capturing additional scattered light to enhance signals for effectively use in processing system  70 .  
         [0030]    Since active regions  22 ,  24 ,  26 , and  26  are connected to common power supply  30  that provides the same supply voltage to all these regions, the photo-gain from each region is directly comparable. That is, the signal generated by each square mm of active area of active regions  22 ,  24 ,  26 , and  28 , will be the same for the same unit of optical energy that impinges on it. Consequently, impinging light in concentric active region  24  that is the same magnitude as the impinging light that is received in center active region  22  will equate to a specific signal equal to 1 times (area of center active region  22 /area of concentric active region  24 ). If the area ratio between central active region  22  and concentric active region  24  is 1:10, then the comparison signal generated by concentric active region  24  will be 0.1 times the signal generated by central active region  22 . While it would obviously be desirable to have the output signal from each of the detection segments similar in magnitude, since the scattering function will vary depending on individual environmental considerations, this can only be approximated in any hardware realization of detector  10 . Since the detection process is reasonably linear over at least two orders of magnitude of input signal, the compensation is adequate in most cases.  
         [0031]    Detector  10  of this invention provides the ability to measure scattered light in water and compensates for the rapidly decreasing magnitude of the scattered light as concentric active regions are located from the axis and direction of light beam  16 . Detector  10  provides the ability to directly compare received signals to obtain a calibrated return (through knowledge of the active areas present), to simultaneously obtain the scattering curve in the entire near-forward scattered region, and uses common supply voltage  30  to ensure commonality in the optical reception process.  
         [0032]    Detector  10  can be implemented in several ways without departing from the scope of this invention. Laser  15  and components of scattering detector assembly  20  of detector  10  can be selected from a number of commercially available units that have been appropriately modified to operate successfully in ambient water  40  while undersea tasks are being completed. The size and location of the constituents of scattering detector assembly  20  can be varied, and can be segmented in various ways. Different arrangements of active regions of scattering detector assembly  20  of detector  10  can be used to achieve specific, or different measurement goals. For example, a hemi-circular arrangement of active regions that are rotated in the plane perpendicular to the beam might be used to examine the cylindrical symmetry tacitly assumed in most theoretical models of the ocean optical process. Similarly, in some waters, the photo-detection process may yield signals that are too low for practical use, in which case detector  10  can use avalanche photodiodes in active regions  22 ,  24 ,  26 , and  28 , or other configurations of active regions to improve signal strengths.  
         [0033]    Having the teachings of this invention in mind, modifications and alternate embodiments of detector  10  may be adapted. Its uncomplicated, compact design lends itself to numerous modifications to permit its use in the hostile marine environment and on land. For examples, detector  10  can be made larger or smaller in different shapes and fabricated from a wide variety of materials to assure resistance to corrosion, sufficient strength, and long term reliable operation under different operational requirements. Rigid housing member  11  could have different shapes, such as being an elongate rigid member having laser  15  mounted at one end and scattering detector assembly  20  at the other end, and ambient water  40  in-between. Furthermore, concentric active regions  24 ,  26 , and  28  could have different arrangements of differently shaped segments  24   a ,  26   a , and  28   a  or more or less concentric active regions could be provided. Clamp-like structure or other connective means could be mounted on housing member  14  to allow quick and secure connections to other structural members.  
         [0034]    The disclosed components and their arrangements as disclosed herein, all contribute to the novel features of this invention. Detector  10  is a compact, cost-effective, unattended means for measuring scattering and scattering functions on land or underwater. Therefore, detector  10 , as disclosed herein is not to be construed as limiting, but rather, is intended to be demonstrative of this inventive concept. It should be readily understood that many modifications and variations of the present invention are possible within the purview of the claimed invention. It is to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.