Abstract:
Two light beams from respective light-emitting devices (e.g., lasers or lamps) cross each other and strike a surface (e.g., of a fluid) at respective oblique orientations relative to the surface (e.g., oblique but nearly vertical orientations that are equal and opposite to each other). A camera captures the surface scattering of the beams in a photographic “double-beam” image containing two respective photographic forms corresponding to the two respective surface scattering locations. The measured distance between the two photographic forms is trigonometrically indicative of the height and slope of the surface in the vicinity of the two surface scattering locations. Some inventive embodiments effect “single-beam” images that are trigonometrically indicative of height only. Plural (e.g., numerous) individual or paired light-emitting devices can be arranged so that a camera snaps an instantaneous photograph containing corresponding forms that are mathematically informative of a surface&#39;s configuration at plural (e.g., numerous) locations.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. provisional application No. 60/500,235, filed 5 Sep. 2003, hereby incorporated herein by reference, entitled “Global Laser Rangefinder Profilometry,” joint inventors Paisan Atsavapranee and Jerry W. Shan. 

   STATEMENT OF GOVERNMENT INTEREST 
   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 
   The present invention relates to interfaces between solids and fluids or between two kinds of fluids, more particularly to methods and apparatuses for performing measurements relating to surface waves described by such interfaces. 
   Seakeeping characteristics of a surface ship are of the utmost concern to the designer of the marine vessel, whether it be a naval combatant, a commercial cargo ship or a pleasure cruise liner. A common paradigm in the design process involves the usage of computational fluid dynamics (CFD) techniques to compute the influence of surface waves on the motion of the ship in different sea states. The surface wave input to the ship motion calculation can be from an assumed wave spectrum, or from a solution of a CFD computation, or from a measured wave based on a physical experiment. Experimental measurement of surface waves is thus important either as a direct input to the computation of the ship motion or as a verification of the accuracy of CFD computation of the surface wave field. 
   Many measurement techniques, such as sonic probes and finger probes, have been employed to measure surface wave elevation at a few discrete locations. However, to date no technique exists that would allow the instantaneous measurement of the surface wave field at a large array of points around a ship model. In order to use discrete point measurements as input to the ship motion calculation, certain assumptions need to be made regarding the spatial uniformity of the wave field. The validity of these assumptions depends highly on the complexity of the wave field and can therefore put the results into question. Simultaneous field measurements of the surface waves taken at a large array of points, and covering a large physical area, would be more suitable—both for the purpose of providing a direct input to the ship motion calculation and for the purpose of validating a CFD computation of a wave field. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the present invention to provide a surface wave measurement methodology that permits instantaneous measurement of a relatively large number, as compared with conventional wave measurement methodologies, of discrete points describing a surface wave field. 
   It is a further object of the present invention to provide a surface wave measurement methodology that permits instantaneous measurement of a relatively large area, as compared with conventional wave measurement methodologies, of a surface wave field. 
   The present invention&#39;s “Global Laser Rangefinder Profilometry” (acronymously referred to by the inventors as “GLRP”) represents a novel optical methodology and technique for rendering instantaneous field measurement of surface waves. The inventive GLRP surface measurement system enables optical mapping of a distinct liquid/air interface or a solid/air interface. The inventive GLRP surface profiling system, when effected in conjunction with proper treatment of the measured surface, allows for instantaneous multi-point measurement of the shape of the surface of interest. Although the inventive GLRP surface measurement system can be used in a wide range of applications, it was first developed by the inventors for the purpose of measuring a surface wave field around a model-scale naval combatant in different sea states. 
   The principle of the present invention&#39;s GLRP is somewhat analogous to that of a conventional laser rangefinder using triangulation. A conventional laser rangefinder projects a beam of visible laser light to create a spot on a target surface. Scattered light from the surface (e.g., light that has been deflected, or light that has been absorbed and re-emitted) is viewed at an angle by a line-scan detector, and the target&#39;s distance is computed from the image pixel data. The inventive GLRP is similar insofar as a beam of visible laser light is projected upon a surface, and light that is scattered from the surface (e.g., light that has been deflected, or light that has been absorbed and re-emitted) is viewed using a device; however, the inventive GLRP avails itself of different geometric principles and uniquely implements a photographic device such as a digital camera for detection purposes. The term “scattering,” as used herein in relation to light, broadly refers to the diffusion or redirection, in any manner, of light energy that encounters particles. Scattering of light can occur, for instance, at or near the interface between two mediums (e.g., either at the surface of, or inside, the medium containing particles associated with the scattering). Scattering of light can be associated with any of various physical processes, including (i) reflection (deflection) of light, and/or (ii) absorption and reemission of light (such as exemplified by flurorescence). 
   In accordance with typical inventive embodiments, a method for determining at least one configurative characteristic of a surface comprises emitting at least one light beam, generating a photographic image, measuring at least one separation, and calculating at least one distance. Each light beam is disposed at an oblique angle with respect to a geometric normal to the geometric plane generally defined by the surface. Each light beam is scattered by the surface at a corresponding surface location. The photographic image contains at least one photographic form. Each photographic form is associated with the scattering by the surface of a corresponding light beam. Each separation is between a photographic form and a corresponding photographic reference location in the photographic image. Each distance is between a surface location and a corresponding geometric reference location in a direction normal to the geometric plane generally defined by the surface. The calculating of each distance includes trigonometrically relating an oblique angle and a separation. According to frequent inventive practice, the trigonometrically relating includes equating the tangent of the oblique angle to the quotient represented by the division of the separation by the distance. 
   According to some inventive embodiments, the method comprises performing sequentially, at least twice, the combination of steps including the generating, the measuring and the calculating; for instance, if the surface is the surface of a fluid wave, the method further comprises causing the fluid wave to be in motion, and the sequential performance yields at least two different values of the distance. The present invention may be particularly beneficial in its capability of performing the combination of steps concurrently for each of plural light beams corresponding to plural locations on the surface; such inventive embodiments can be informative as to the shape of a surface or a portion thereof. 
   Typical inventive apparatus is for evaluating the configuration of a surface. The inventive apparatus comprises: (a) a pair of laser devices for projecting two laser beams crossing each other; (b) a camera for photographing the scattering of said two laser beams by said surface; and, (c) computer means for determining the slope of said surface at a slope location. Each laser device projects a laser beam upon the surface, which generally describes a geometric plane. The camera creates an image including two separate image spots that respectively manifest two separate scatter locations. Each scatter location is a location on the surface at which a laser beam is scattered by the surface. The slope location is a location on the surface that is between the two scatter locations. The determination of the slope includes consideration of: (a) the respective orientations of said two laser beams relative to the geometric plane generally described by the surface; and, (b) the separation distance of each image spot in the image with respect to a corresponding photographic reference location. 
   The inventive GLRP typically projects at least one laser beam upon a target surface at an acute (usually, small) angle with respect to the geometric normal to the geometric plane generally described by the target surface configuration. A portion of the beam hits and is scattered from a “scatter point” of the target surface so as to result in a photographic image upon the image sensing component of a camera, which is situated so that the image sensing component defines a geometric plane which is parallel to the geometric plane generally described by the target surface configuration. A photographic image (typically in the form of a rather nebulous spot) is created in or on the image sensing component of the camera, the photographic image lying in a geometric line that passes through the scatter point and is normal to the geometric plane generally described by the target surface configuration. The originally transmitted beam is disposed at a selected angle with respect to the scatter beam creating the photographic image. 
   A computer is typically employed to determine the relative displacement of the photographic image, such displacement being commensurate with the distance traveled by the projected laser beam; that is, longer beam paths are associated with longer image displacements (and, conversely, shorter beam paths are associated with shorter image displacements). Application of geometric principles to known and measured values (vis., the original beam&#39;s angle from normal, and the photographic image&#39;s distance from reference point) yields a value indicative of the elevation of a point on the target surface. According to inventive “single-beam” embodiments, a computer is used to calculate the distance of the photographic image from a reference point (e.g., a marker or projected image). The computer determines the elevation at the scatter point of the target surface as part of a single geometric triangle. 
   According to inventive “double-beam” embodiments, two beams are concurrently generated at equal and opposite angles with respect to the normal so as to criss-cross (intersect) each other, thereby forming two corresponding geometric triangles. Each beam is scattered so as to leave a photographic image, the two resultant photographic images being separated from each other. A computer is used to calculate the distance between the two photographic images. The elevation at a measurement point (located between the respective scatter points) on the target surface is determined using the computer, based on the supposition that the two geometric triangles together form a single isosceles triangle. Furthermore, according to many inventive embodiments, the slope at a measurement point (located between the respective scatter points) on the target surface is determined using the computer, based on the geometric properties of the two adjacent geometric triangles each corresponding to a scatter point. 
   The inventive GLRP is comparable to conventional laser rangefinding insofar as availing of sound geometric principles; however, the inventive GLRP achieves effective results using different apparatus and different geometric principles. According to typical inventive practice a basic abstract trigonometric concept is availed of, namely that in a right triangle the tangent of an interior angle equals the ratio of the opposite (non-hypotenuse) side over the adjacent (non-hypotenuse) side. Advantageously, because of the unique features of the present invention, the inventive GLRP methodology is capable of performing cost-effective measurements at many locations on a surface simultaneously, thereby evaluating the “topography” of the surface. This simultaneous multi-measurement capability is especially propitious when the surface is dynamic (rather than static), such as a surface of a wave or other moving fluid. In such applications, at a certain point in time the present invention can render a photographic “snapshot” encompassing multiple locations of a fluid surface that is constantly changing. The snapshots can be inventively rendered on plural occasions to demonstrate how the surface configuration is changing. In accordance with many embodiments of the present invention, plural (e.g., multiple) laser beams are utilized for tagging plural (e.g., a large number) of spots, an area-scan camera is utilized as a detector, and the distance from the beam origin to the target surface at an array of locations is measured. For inventive embodiments that use plural (e.g., two or three) laser beams for each measurement spot, the surface slope in plural directions can be obtained as well. 
   Existing techniques and methodologies such as those implementing sonic probes and finger probes are capable of profiling a liquid/air interface or a solid/air interface at, at most, only a few discrete locations. The present invention&#39;s GLRP is a field measurement technique capable of performing instantaneous measurements at a large array of points, covering a large physical area. The inventive field measurement technique meets requirements for a wide range of applications, such as involving direct input to a ship motion calculation or involving CFD validation of a computed wave field. 
   Other objects, advantages and features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein like numbers indicate the same or similar components, and wherein: 
       FIG. 1  is a schematic representation of an embodiment of a multi-laser, two-beam GLRP system in accordance with the present invention. The digital camera includes an image sensor that is diagrammatically shown both in edge view and plan view, the former view illustrating the reception by the camera&#39;s image sensor of light scattered by the surface being measured, the latter view illustrating the resultant rendering of raw imagery upon the camera&#39;s image sensor. 
       FIG. 2  is a diagram illustrating typical inventive practice in terms of important geometric relations associated with transmission, surface scattering and camera imaging of a laser beam. In particular,  FIG. 2  reveals how the distance corresponding to a camera image (e.g., relative to another location on the imaging plane) is commensurate with the distance corresponding to the laser beam 
       FIG. 3  is a diagram illustrating the present invention&#39;s two-beam GLRP concept, as typically embodied, for measurement of surface elevation at a single point on a water wave surface. 
       FIG. 4  is a diagram illustrating the present invention&#39;s two-beam GLRP concept, as typically embodied, for measurement of surface slope at a single point on a water wave surface. 
       FIG. 5  is a side pictorial view of a wave that was measured through experimental practice of a two-beam GLRP system in accordance with the present invention, similar to that depicted in  FIG. 3  and  FIG. 4 . As shown in  FIG. 5 , the wave is traveling from right to left. 
       FIG. 6  is a magnified view of a raw photographic image taken of two laser spots in inventive experimentation associated with the wave shown in  FIG. 5 . 
       FIG. 7  is a magnified view of a processed image of the two laser spots shown in  FIG. 6 , wherein the two laser spots are at the peak of the wave shown in  FIG. 5 . The two closed peripheral delineations indicate the two areas where the present invention&#39;s software has identified (e.g., via light borderline) two distinct “blobs” corresponding to the two laser spots shown in  FIG. 6 . 
       FIG. 8  is a magnified view of a processed image of the two laser spots that are at the trough of the wave shown in  FIG. 5 . Again, the two closed peripheral delineations indicate the two areas where the present invention&#39;s software has identified (e.g., via light borderline) two distinct blobs corresponding to the two laser spots shown in  FIG. 6 . Note that the distance between the two blobs in  FIG. 8  is larger than that in  FIG. 7 . 
       FIG. 9  is a graph showing measured wave height versus time spots in inventive experimentation associated with the wave shown in  FIG. 5 . 
       FIG. 10  is a schematic representation, largely in perspective, of inventive laser apparatus effecting three-beam GLRP, which is suitable for measuring the surface slope of the wave in each of two different directions. 
       FIG. 11  is a schematic representation of an embodiment of a multi-laser, one-beam GLRP system in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIG. 1 , inventive GLRP system  200  comprises three pairs of double-beam laser devices  20 , an area-scan camera  30 , and a computer system  40 . Each laser device  20  emits a laser beam  21 . Each beam  21 , in turn, strikes and is scattered by surface  100  in a scatter path  22  that reaches digital camera  30  so as to form an image spot  32  on the image sensor  31  of digital camera  30 . The raw image spots  32  are processed by computer  40 . Not shown but appreciable as present are electrical power means and structural support means for the inventive apparatus including lasers  20 , camera  30  and computer  40 . 
   Laser  20  can be either a diode laser (also known as a laser diode or injection laser) or a conventional laser (such as helium-neon, ruby, and gas types). A laser diode is a semiconductor device that, when current passes therethrough, produces coherent radiation (wherein the waves propagate at the same frequency and phase) in the visible or infrared spectrum. As compared with conventional lasers, laser diodes usually are smaller and lighter, have lower power requirements, and are lower in intensity. 
   According to typical inventive practice, GLRP simultaneously effectuates measurements at numerous locations on a surface such as water wave surface  100 . For illustrative purposes, only three pairs of lasers  20  are shown in  FIG. 1 . A total of six laser beams  21  is transmitted, each by its respective laser  20 . The six laser beams  21  indicate (“tag”) three measurement points (locations) p that are situated on surface  100 . Each surface point p has associated therewith two roughly rounded image spots  32 , slightly separated from each other, that are generated via scattering paths  22  onto camera film  31  by the corresponding pair of lasers  20 . The ordinarily skilled artisan who reads this disclosure will appreciate that the present invention can be practiced using practically any plural number of lasers  20 , and that use of numerous lasers  20  may be propitious for many applications. 
   Still with reference to  FIG. 1  and also with reference to  FIG. 2  through  FIG. 4 , each surface point p has associated therewith a pair of lasers  20 , a pair of laser beams  21 , a pair of scatter paths  22 , a pair of scatter points s, a geometric medial line (median) m, a pair of equal and adjacent geometric angles θ, and a pair of image spots  32 .  FIG. 1 ,  FIG. 3  and  FIG. 4  each depict how the respective laser beams  21  of two paired lasers  20  correspond to a surface point p. Each median m is situated equidistantly intermediate two corresponding laser beams  22 , and is situated either equidistantly or non-equidistantly intermediate two corresponding scatter paths  22 . Each median m is perpendicular to the geometric plane s that is generally defined by surface  100 , and is coplanar with its corresponding two beams  21  and two scatter paths  22 . Median m intersects the corresponding surface point p and bisects the angle defined by the corresponding pair of beams  21 . Two equal, adjacent angles e are formed wherein each angle θ is formed by median m and a beam  21 .  FIG. 1  shows a preferred inventive approach wherein each intersecting pair of beams  21  is associated with the same oblique angle, viz., angle θ; nevertheless, inventive practice permits variation in the value of this oblique angle between two or more different pairs of beams  21 . 
   Every laser  20  in inventive GLRP system  200  is situated at the same height with respect to surface plane w; hence, every laser emission point e (e.g., the tip of the laser  20 ) is situated at the same height h E-W  with respect to surface plane w, which is the geometric plane generally defined by surface  100 . Further, every laser emission point e is situated at the same distance d E-F  with respect to image sensor plane f (the geometric plane defined by image sensor  31 ) and at the same distance d E-I  with respect to the corresponding beam criss-cross intersection point i. Area-scan camera  30  (more specifically, image sensor  31 ) is situated at a constant camera height h F-W  with respect to surface plane w. The laser emission points e of all beams  21  are collinear and/or coplanar, are parallel to surface plane w, and are parallel to the geometric plane f defined by image sensor  31  of area-scan camera  30 . Each pair of laser beams  21  is propagated so as to cross at the same angle θ with respect to median m, which is perpendicular to surface plane w. Each beam  21  is scattered by surface  100  at a scatter point s. Each measurement point p is generally interposed between the corresponding pair of scatter points s, e.g., s 1  and s 2 . 
   The laser emission points e are at a constant same height h E-W , and each pair of laser beams is characterized by the same beam-crossing configuration describing the same pair of angles θ. An array of image spots  32  is formed on camera image sensor  31  so as to be coherently indicative of both the elevation (height) and the slope of surface  100  at any given point p. Area-scan camera  30  represents a kind of detector that is capable of taking a “snapshot” photograph, in two dimensions, of the scattered laser beams. It is noted that inventive practice does not require constancy or uniformity of angles θ or laser heights or laser emission point heights h E-W  (such heights being measured relative to the surface plane w), such as illustrated in  FIG. 1 ; some inventive embodiments provide for variation in angles θ and/or laser heights and/or laser emission point heights h E-W . 
   Accordingly, the actual distance from a beam origin e to the corresponding point p on the target surface  100  equals h+d E-I , where h is the distance from intersection point i to the target surface point p, and d E-I  is the distance from intersection point i to beam origin e. Similarly, the actual distance from image sensor plane f to a point p on the target surface  100  equals h+d E-I +d E-F , where d E-F  is the distance from beam origin e to image sensor plane f. These laser-to-surface (or camera-to-surface) distances can be computed at each of an array of locations, such as through the use of a computer  40  having a computer program product that is capable of performing data processing of measurement data. As elaborated upon hereinbelow with reference to  FIG. 4 , many inventive embodiments use plural beams for each measurement spot, thereby obtaining one or more surface slopes in addition to or instead of the surface elevation. 
   Typical inventive embodiments implement a digital camera  30  (having image sensor  31 ), the photographic information of which is directly input to a computer  40  for processing. Nevertheless, the present invention can be practiced implementing an “old-fashioned” film camera (having film  31 ), whereby the photographic information is digitized (as by a digitizer such as analog-to-digital converter  67 ) and is then input to a computer  40  for processing. 
   As shown in  FIG. 1 ,  FIG. 3  and  FIG. 4 , two laser beams  21  are respectively emitted from a pair of commercial, off-the-shelf laser diodes  20 , and are projected onto the water surface  100  at an incident angle of nearly (but appreciably less than) ninety degrees (90°). Beams  21  each form the same positive or negative angle θ with respect to median m, which are parallel to each other. Each beam  21  is strongly scattered by “floaters”  80  (e.g., micron-sized buoyant hollow glass spheres or dye mixed into the surface  100  layer) on the water surface  100 ; floaters  80  are shown in a magnified view in  FIG. 1 . The image of the scattered beam  22  is recorded via an area-scan camera  30 . 
     FIG. 2  illustrates the variation of horizontal distance d in accordance with vertical distance h. Image spot  32   a  is formed by vertical scatter path  22   a ; image spot  32   b  is formed by vertical scatter path  22   b ; image spot  32   c  is formed by scatter path  22   c . The distance d is actually taken with respect to centroid  330  of image spot  32 , centroid  330  having been determined using computer  40 . Distance d is the distance between an image spot  32  and median m, which equals the distance between the corresponding scattering point s and median m. Distance d c  is greater than distance d b , which is greater than distance d a ; height-wise distance h c  is greater than height-wise distance h b , which is greater than height-wise distance h a . It is seen that horizontal distance d increases with increasing vertical distance h. 
   Because each projected beam  21  makes a slight angle θ to the surface normal (such as indicated by median m), the illuminated spots are horizontally displaced from a neutral position (such as median m) as the water-surface  100  elevation changes. By judiciously choosing the beam  21  angle θ for an expected peak-to-peak wave height, each beam  21  can be contained within a certain distance d (e.g., less than half grid spacing) from its neutral position, viz., medial line m. In this way, an array of surface elevation measurements and/or surface slope measurements can be made without confusing the neighboring beams  21 . 
   The height value h is understood to be a value relative to a reference height (e.g., h+d E-I , or h+d E-I +d E-F ), and is thus indicative of the height of the surface  100  of the wave. Computer  40  processes the separation information regarding image spot  32 , received from camera  30 , computer  40  thereby finding a value for d, the horizontal distance between median m and a scattering point s. Computer  40  also contains in its database the value of tan θ, since a value of θ is pre-selected for the inventive apparatus configuration. Computer  40  uses the following trigonometric relationship to determine height h:
 
 h=d /tan θ
 
where θ is the angle formed by laser beam  21  relative to median m, and d is the horizontal distance between an image spot  32  and median m (or, equally, the horizontal distance between the corresponding scattering point s and median m).
 
   As shown in  FIG. 3 ,  2 θ is the crossing angle of the paired beams  21 . The triangle formed by vertices i, s 1  and s 2  is isosceles, or approximately so; that is, the distance between point i and point s 1  equals the distance between point i and point s 2 , median m bisects this isosceles triangle, and points s 1  and s 2  are horizontally even with each other. For some inventive embodiments (or for some inventive measurements), an assumption is thus made that horizontal distance d 1  equals horizontal distance d 2 ; therefore, the horizontal distance T between image spots  32  (or, equally, between scatter points s) equals the sum d 1 +d 2 , which equals 2d, which equals T. In other words, since angle θ is small (as is frequent in inventive practice), it is assumed that vertical distances h, h 1  and h 2  are equal to each other, and that 2d is the horizontal distance between the points s 1  and s 2  on water surface  100 . It is noted generally that, according to inventive principles, angle θ can be any value greater than zero degrees and less than ninety degrees. 
   Angle  2 θ also represents the crossing angle of the paired beams  21  shown in  FIG. 4 . However, as distinguished from  FIG. 3 , a significant slope exists in surface  100  at surface point p in  FIG. 4 . That is, the horizontal distance T between image spots  32  (or, equally, between scatter points s) still equals the sum d 1 +d 2 , but d, does not equal d 2 , and hence T does not equal 2d. As shown in  FIG. 4 , two right triangles are formed that are adjacent to each other, viz.: (i) the triangle having vertices i, r, and s 1 ; and, (ii) the triangle having vertices i, r 2  and s 2 . In the first triangle, tan θ=d 1 /h 1 . In the second triangle, tan θ=d 2 /h 2 . Hence, in accordance with an inventive two-beam GLRP system such as depicted in  FIG. 4 , the slope of the water surface  100  in one direction is trigonometrically determined as follows: 
                   h   1     =         d   1     /   tan     ⁢           ⁢   θ                   h   2     =         d   2     /   tan     ⁢           ⁢   θ                 slope   =         Δ   ⁢           ⁢   h         d   1     +     d   2         =           d   2     -     d   1           d   2     +     d   1         ⁢     1     tan   ⁢           ⁢   θ                       
where d 1  and d 2  are the respective horizontal displacements of the paired image spots  32  (or, equally, of the paired scatter points s) from medial line m. Medial line m is the reference location for image spots  32  (or, equally, for the paired scatter points s) in such calculations.
 
   A single-measurement point, two-beam inventive GLRP prototype, similar to the inventive GLRP systems illustrated in  FIG. 1 ,  FIG. 2  and  FIG. 4 , was built and tested in the Miniature Water Basin at the Naval Surface Warfare Center, Carderock Division (NSWCCD), located in West Bethesda, Md. The inventive prototypical apparatus included two laser diode modules  20  (Radio Shack) and a Roper Scientific ES4.0 digital camera  30  (2k×2k pixel resolution). It is pointed out that these commercial off-the-shelf laser diodes  20  were purchased from Radio Shack at retail for $10 per unit; the inexpensiveness in this regard suggests the economic feasibility of rendering measurements at a large number of surface points p. A wave train was generated at the far end of the basin, and measurement was taken roughly in the middle of the basin. The wave train traveled from right to left, as shown in the snapshot of  FIG. 5 . A large contact angle θ was chosen, primarily for visual impact. 
     FIG. 6  through  FIG. 8  portray examples of raw image spots  32  ( FIG. 6 ) and processed image spots  320  ( FIG. 7  and  FIG. 8 ),  FIG. 7  representing the processed version of  FIG. 6 .  FIG. 6  and  FIG. 7  show corresponding pictures of the scattered beams (raw and processed, respectively) at the peak of the wave. To process the camera  30  raw photographic image shown in  FIG. 6 , a blob analysis routine was used that was resident in the memory of computer  40 . Blobs  32   1  and  32   2  (shown in  FIG. 6 ) were processed into spots  320   1  and  320   2  (shown in  FIG. 7 ), respectively. The two distinct “blobs”  32 , and  322  shown in  FIG. 6  were first identified by the blob analysis program, as delineated by peripheries  340   1  and  340   2  of the processed image spots  320   1  and  320   2  shown in  FIG. 7 . As also illustrated in  FIG. 7 , the centroid c for each blob  32  was then determined, wherein processed spot  320   1  was characterized by centroid  330   1 , and processed spot  320   2  was characterized by centroid  330   2 . In accordance with inventive principles described hereinabove with reference to  FIG. 3 , the horizontal distance between the centroids  330   1  and  330   2  was T=2d, and this horizontal distance was used to calculate wave height h.  FIG. 8  shows the processed image at the trough of the wave. As demonstrated by  FIG. 7  versus  FIG. 8 , the two spots are farther apart at the trough of the wave than at the peak of the wave. 
   As demonstrated by the comparative resolutions of the processed image spots  320  shown in  FIG. 7  and  FIG. 8 , the lefthand laser beam  21  (resulting in processed image spot  320   1  in  FIG. 7 , and in processed image spot  320   3  in  FIG. 8 ), is of a higher quality than is the righthand laser beam  21  (resulting in processed image spot  320   2  in  FIG. 7 , and in processed image spot  320   4  in  FIG. 8 ). It is evident in  FIG. 7  and  FIG. 8  that using beams of lower quality (e.g., the righthand beams shown in  FIG. 7  and  FIG. 8 ) will not adversely affect the measurements, as for most inventive applications the centroids  330  will remain sufficiently coincident regardless of beam quality. This suggests robustness of the present invention&#39;s methodology in that the inventive practitioner who integrates the inventive system is not required to cull for “perfect” laser diodes  20 . 
     FIG. 9  shows the measured surface  100  wave heights, as a function of time. The measurements are taken at a single measurement point p of wave surface  100 , wherein measurement point p was associated with the same pair of intersecting beams  21 . On a generally continual basis, the location of measurement point p changed in accordance with the changing configuration of wave surface  100 . As shown in  FIG. 9  about a hundred measurements are taken sequentially in a time span of about seven seconds. The water wave is moving from right to left as portrayed in the snapshot of  FIG. 5 . Note that  FIG. 5  shows the typical profile of a wave on the verge of breaking, with a higher slope in the front of the wave. The measurements shown in  FIG. 9  correspondingly demonstrate a higher slope in the front of the waves than in the back of the waves. 
     FIG. 1 ,  FIG. 3  and  FIG. 4  illustrate inventive practice involving two beams per surface measurement point p.  FIG. 3  is illustrative of inventive determination of surface elevation.  FIG. 4  is illustrative of inventive determination of surface slope, but in only one direction (which can be obtained instead of or in addition to the surface elevation). With reference to  FIG. 10 , if surface slopes in two directions are required, a three-beam inventive GLRP system may conceptually utilize up to three laser beams that form a diverging triangular pattern. As shown in  FIG. 10 , a first horizontal distance, T x , is the distance between the computer-processed centroid  330 ′ (of blob  32 ′) and the computer-processed centroild  330 ″ (of blob  32 ″). A second horizontal distance, T y , is the distance between the computer-processed centroid  330 ′ (of blob  32 ′) and the computer-processed centroild  330 ′″ (of blob  32 ′″). Distance T x  and distance, T y  define directions that are perpendicular to each other. 
   Now referring to  FIG. 11 , some inventive embodiments describe a single-beam mode rather than a plural-beam mode. If only surface elevations are desired, the inventive measurement can be made using only one beam. This simplifies the inventive GLRP concept when surface slopes are not required. For trigonometric purposes in inventive single-beam embodiments, a distance d s  is measured between a single blob  32  and a reference location such as reference point  37 , both blob  32  and point  37  existing in or on image sensor  31 . The reference point  37  can be demarcated in any of various ways, such as by a digital marker or by a digital image projected by a laser beam emitted (e.g., by another laser device) in an upward vertical direction from a location atop laser device  20 . The height h s  is determined, based on distance d s  and angle θ, according to the trigonometric equation h s =d s /tan θ. 
   An inventive one-hundred-point, single-beam GLRP prototype was built and tested in the Miniature Water Basin at the Naval Surface Warfare Center, Carderock Division (NSWCCD), located in West Bethesda, Md., using commercial off-the-shelf laser-diode modules  20  and a high-resolution area-scan camera  30 . The single-beam prototypical setup was similar to that shown in  FIG. 11 . Wave trains of various heights and propagation speeds were generated using a wave maker. Measurements of the wave heights along a line near the side wall of the basin were taken via the inventive GLRP system and were found to favorably compare with flow visualization. 
   The present invention is not to be limited by the embodiments described or illustrated herein, which are given by way of example and not of limitation. Other embodiments of the present invention will be apparent to those skilled in the art from a consideration of this disclosure or from practice of the present invention disclosed herein. Various omissions, modifications and changes to the principles disclosed herein may be made by one skilled in the art without departing from the true scope and spirit of the present invention, which is indicated by the following claims.