RAPID THREE-DIMENSIONAL SURFACE DIGITIZER

Abstract: Method and apparatus for automatic mensuration of three-dimensional surfaces. The surface is sensed via non-contact optical triangulation and the optically produced image measured by electronic means. The electronic measurements are encoded as binary data and passed to a digital computer for storage, analysis and other functions. The invention is significant because of the speed and relative simplicity of the method and a provision for alleviating shadowing from large surface irregularities and its accommodation of a wide deviance range of light reflections. The apparatus has no moving parts, excepting a mechanism to provide for movement of the apparatus relative to the surface so as to progressively view the entire surface. This invention projects planes of light which intersect the surface at known angles, producing illuminated lines. Said lines are viewed by a video type camera placed at a known angle away from the planes so that displacements in the line may be sensed. Electronic circuits detect and measure points on the line and transfer the data to a digital computer.

Description: Rapid Three-Dimensional Surface Digitizer Field of the Invention This invention relates to non-contact surface mensuration methods and apparatus which provide data to digital computers and more particu¬ larly apparatus capable of high sampling rates so that movement of the surface, such as occurs uith live subjects, can be accommodated. The Prior Art Surface mensuration apparatus has been limited to use on inanimate subjects because of methods that are inherently slou. Examples are U.S., Pat No. 4,089,608 and 4,373,804 uhich use mechanical scanning or mechani¬ cal error correction. To make a high resolution measurement of the sur¬ face of a living subject, such as the human head, measurement rates greater than 10,000 samples per second are required. fit these high rates samples can be collected before movement of the subject causes excessive distortion in the image. ft second problem uith optical non-contact apparatus is shadαuiπg of the light beam or obstruction of the sensor's vieu by features of the subject surface. U.S. Pat No. 4,089,608 is an example uhich has thi3 limitation. ft third problem is the determination of the exact location of a light spot. The high sampling rate requirement necessitates a πoπ- mechanical scanning sensor such as a television camera and so the loca¬ tion determination becomes a pulse timing measurement on the camera's video signal. Earlier timing measurements relied on either simple ampli¬ tude thresholds or differentiation of the signal amplitude to determine the slope reversal occurring at maximum. Amplitude threshold methods result in large errors as the signal amplitude varies. This is espe- cially prevalent uhen measuring human surfaces. The light diffuses through the flesh, yielding a louer level beam uidth many times the incident beam uidth. In order to respond to the lou levels of light reflected from hair or unfavorable surface slopes, the threshold must be set close to the detector noise level. Under these conditions the thres¬ hold uill be reached much too soon, along the broad base of the light response curve. These errors are illustrated in Figure 5. ft lou level reflection is shoun at 27, the threshold level is set at amplitude 28 and measures the point to be at time 29, uhile the optimum point is at the response curve maximum. ft larger response is shoun at 30 and the threshold indicates the point to be at time 31. If the light reflects from human flesh additional broadening at the base uill occur as shoun at 32 and the meas¬ ured time uill be at 33, an error 34. Non-linear amplifiers, though helpful, are not sufficient and tend to suppress the sharpness of the response curve. Automatic gain control systems can be used but need more than one signal response for each point, requiring more time than is available for live subjects. Curve slope change sensing can be done at high speed but is basi¬ cally a high pass filter so the signal to noise ratio deteriorates, lim¬ iting the dynamic range. High levels of incident light might be used to avoid the ueak reflection region but result in involuntary movements of the human subject uhen near the eyes, and do not reduce the total dynamic range requirement. Summary of the Invention It is an object of the present invention to minimize the time required to collect the surface samples. It is another object of the present " invention to provide a geometry and signal processing method uhich ameliorates the shadouing problems of complex surfaces. It is another object of the present invention to accommodate the uide dynamic range of reflected light from live subjects uithout loss of resolution or time. Other objects and advantages of the present invention uill become apparent as the description proceeds. The invention provides one or more thin planes of light uhich illuminate the subject providing contour lines on the surface of the sub- ject. The contour lines are vieued from one or more points an acute angle auay from the light planes by a scanning optical sensor, for exam¬ ple a video camera. Part of the apparatus provides for moving the sub¬ ject relative to the light projection and sensing assembly so that the entire surface of interest may be sampled. The coordinates of points on the subject surface can be derived from the image of the contour curve on the sensor and the position of subject surface reference points relative to the light projection and sensing assembly. The scanning sensor detects points along the contour curve and digi¬ tal data are generated uhich represents the position of these points on the surface of the sensor. The data along uith indexing data are stored sequentially in a computer's memory. To minimize the volume of data uhich is stored during the exposure of the subject, only one coordinate is stored. The other coordinates of the point may be derived from the relative positions of the data in the memory. ft method is provided for accurate measurement of the contour image position on the sensor by»use of an amplitude insensitive trigger. Each light plane can measure the subject surface from a different angle and therefore ameliorate the shadouing problem encountered if only a single light plane is used. The light beam need not intersect the axis of a rotating subject, nor does the relative motion of the sub ect and light plane need to be rotary. The best configuration depends upon the topography of the sur¬ face to be measured. Brief Description of the Drauiπgs Figure 1 illustrates the geometry of the apparatus for the first embodiment. Figure 2 represents a typical image at the plane of the video camera sensor 7 of Figure 1. Figure 3 is a block diagram of the electronic signal processing and synchronization circuits. Figure 4 is a schematic representation of an amplitude insensitive trigger circuit. Figure 5 represents typical input uavβforms associated uith the trigger circuits of prior art and the present invention. Figure 6 illustrates the geometry of the apparatus for the second embodiment. Figure 7 is a detailed vieu of the sensor of Figure 6. Description of the Preferred Embodiments Figure 1 shous one practical embodiment of the invention. Samples uill be collected in cylindrical coordinate form. ft rotating table 1 upon uhich the subject surface is placed has an axis of rotation 2. Intersecting this axis are tuo thin planes of light 3 and 4 perpendicular to the drauiπg plane. The angles betueeπ light sources and camera 7 are small to allou measurement into πarrou depressions of the subject sur¬ face. Light sources 5 and 6 can be incandescent or laser, ft video cam¬ era sensor 7 vieus both contour lines generated by the intersections of the light planes and the subject. The center of the camera field of vieu is aligned on the table axis 2. Figure 2 is a representation of the image ield of the camera. . The horizontal scan point begins at the upper left boundary 8 and proceeds through the center of the field 9 to the right boundary 10 retraces and continues dounuard. Images of the tuo subject ' contours are at 11 and 12. The block diagram of Figure 3 illustrates the electronic circuitry for controlling the video camera sensor 7 and processing the camera out¬ put. All operations are timed by a clock 13 through synchronization cii— cuit 16. During each horizontal sueep an up-douπ counter 14 counts douπ from maximum at the beginning of the sueep to zero at the center or table axis image position and back up to maximum at the horizontal sueep termi¬ nation. A coincidence betueen the counter value and a video response from a contour line yields a counter value proportional to the radius of that surface point on the subject. The most straight oruard scanning system avoids the commonly used interlaced video scanning giving a single video field per frame. Uith this single field the point coordinate along the axis of subject rotation is proportional to the vertical deflection or vertical sueep count. The remaining coordinate is directly related to the rotational "position of the subject and can be derived from the sequential frame position. It is not necessary to synchronize the table rotation as long as its velocity is constant and the precise start and end of one revolution is knouπ. The video signal outputs of the video camera sensor have a uide range of amplitude and are frequently asymmetrical due to variations in the reflectivity and slope of the subject surface. If a high accuracy measurement of radius is to be achieved special treatment of the camera video is required. The effect of amplitude variation on the time measurement can be greatly reduced by a technique that compares the video to a slightly delayed but otheruise high fidelity copy of itself. Figure 4 illustrates the amplitude insensitive trigger circuit. The camera video 20 is divided into tuo equal parts, one of uhich is delayed by delay line 21 and then applied to one input of a high speed voltage comparator 23. The sum of the uπdelayed video and an adjustable bias signal 22 is applied to the other input. Figure 5 illustrates the rela¬ tionship betueeπ the tuo signals. The comparator uill provide an output uheπever the amplitude of the delayed signal 24 exceeds that of the unde- layed signal 25. The undelayed video amplitude of Z5 uill exceed the delayed video amplitude of 24 until the tuo curves cross measuring the spot to be at 35. For responses that are symmetrical about their tops, the error 36 is a constant at one-half the delay of 26 and so is easily subtracted out in later processing by the computer. The bias adjustment adjusts the tuo signals relative to one another so that sensitivity can be set above system noise. Returning to Figure 3, the output of the amplitude insensitive trigger 15 enables the latch 17 causing the counter 14 value to be stored. To maximize data handling speed the radius data are packed into the uord size of the computer 19. This uord of data is then passed to a direct memory access circuit 18 uhich stores it in the computer random access memory. The process continues uhile the subject rotates one full turn. Frame synchronization data can be stored in the computer memory by reserving the highest value radius reading for that purpose, and storing it immediately preceding or follouiπg the data for each frame. Another embodiment has several advantages over the first: The tuo images are combined optically before the introduction of system noise, doubling the sensitivity. The images can also be more precisely added optically than electronically. There is no ambiguity for points lying behind the rotation axis if only one light plane is used. ε Both vieus of a surface point are digitized at the same instant so the effect of movement of a live subject is minimized. Refer to Figure 6. The subject 37 uhose surface is to be measured is placed on a rotating table 38. A light source 39 generates a thin plane of light 47 perpendicular to the plane of the Figure. The contour line generated by the intersection of the light plane and the subject surface is vieued from a small angle auay from the light plane by mirrors 40 and 42. The path of light 41 from point 50 on the contour line is reflected from mirror 40, passes through the beamsplitter 44 to the sen ¬ sor 46. A second light path 43 from the same point 50 is reflected by mirror 42 and the surface of beamsplitter 44 to the sensor 46. The com¬ bined images follou light path 45. For the embodiment of Figure 6 to function properly, three condi¬ tions must be satisfied: The vieuiπg angles between the mirrors and the light plane must be equal. Within the sensors vieu the tuo light path lengths betueen any one of the set of points in the light plane to the sensors must be equal. There must be an additional reflection, therefore image reversal, in one path. These conditions are readily met by mounting the reflecting surfaces and camera imaging surface perpendicular to the plane of Figure 6 and adjusting the lengths of the paths. Light ray tracing uill shou that all points in the light plane uill coincide at the beamsplitter 44 and sensor 46 if the above conditions are met. Tt follous that the illuminated contours uill coincide for all sur¬ faces although one path may be shadoued by an unfavorable slope uith respect to -one mirror's vieu. Refer to Figure 7. Because the tuo light path lengths vary in a regular manner as a function of the radius coordinate of the surface, the depth of field error of sensor lens 49, can be compensated for by tilting the focal plane 48 of the sensor. The associated electronic system is the same for both embodiments. More than the tuo vieuing directions can be combined by iteration of either preferred embodiment. The details of the present invention are given by uay of example and are not intended to limit the spirit or scope of the invention as set forth in the claims.

Claims: What is claimed is: 1. An apparatus for performing high speed non-contacting mensuration of 3-dimensional surfaces comprising: means for illuminating the surface uith tuo or more planes of light intersecting the surface and producing contour lines; means for moving the 3-dimensional surface relative to the planes of light; means for scanning the surface and sensing points on the con¬ tour lines reflected from the surface; means for determining the posi'tioπ of the points over a uide dynamic range of reflected light intensity and waveform distor¬ tion; means for storing the point position data in a computer by direct memory access; means to provide synchronization for all parts of the apparatus so that all three coordinates of the subject surface can be derived from the point position data and the sequence of the point position data in memory. 1 . An apparatus as defined in Claim I wherein the subject is rotated relative to the sensing system and uhereiπ the planes of light intersect the axis of relative rotation. 3. An apparatus as defined in Claim 1 uherein the sensing means comprises a video camera sensor uith the field of vieu divided so as to observe multiple contour lines simultaneously. 4. An apparatus as defined in Claim 1 uhereiπ the means for determining the position of the points comprises the sensor output divided into tuo parts, the first being delayed before connection to one input of an amplitude comparator, the second connected to the remaining input of the comparator resulting in an amplitude and distortion insensi¬ tive timing detector. 5. An apparatus as defined in Claim 1 uherein the means for storing data comprises the measurement of the radius coordinate uhich is then stored in the order measured allouing the remaining coordinates to be derived from the order of storage in the memory. 6. An apparatus for performing high-speed non-contacting mensuration of 3-dimenβional surfaces comprising: means for providing a plane of light intersecting the surface and producing a contour line; means for moving the surface relative to the plane of light; means for viewing the contour line from both sides of the light plane; means for combining the vieus of the contour line so that there is only one optical contour line image; means for determining the position of the points over a uide dynamic range of reflected light intensity and uaveform distoi— tion; means for storing the point position data in a computer by direct memory access; means to provide synchronization for all parts of the apparatus so that all three coordinates of the subject surface can be derived from the point position data and the sequence of the point position data in memory. 7. An apparatus as defined in Claim 6 uhereiπ the sensing means comprises a sensor, the focal plane of uhich is tilted to compensate for depth of field error. 8. An apparatus as defined in Claim 6 wherein the moving means comprises .rotation of the light plane and subject relative to one , another, the light plane intersecting the axis of rotation. 9. An apparatus as defined in Claim 6 uherein the means for vieuing the contour line comprises mirrors on each side of the light plane positioned to reflect the contour line images to the combining means. 10_ An apparatus as defined in Claim 6 uherein the vieu combining means comprises a beamsplitter positioned to superimpose the contour line images. _» 11. An apparatus as defined in Claim S uhereiπ the means for providing a plane of light is a laser uith a cylindrical lens to spread the beam.

Inventors: ADDLEMAN DAVID ANDREW, ADDLEMAN LLOYD ANDREW, ADDLEMAN, DAVID, ANDREW, ADDLEMAN, LLOYD, ANDREW
Applicants: ADDLEMAN DAVID ANDREW, ADDLEMAN LLOYD ANDREW, ADDLEMAN, DAVID, ANDREW, ADDLEMAN, LLOYD, ANDREW
Application year: 1986
Main classification: G01B11
IPC codes: G01B11, G05B19