Source: http://www.google.com/patents/US5517575?dq=7143430
Timestamp: 2017-07-21 20:52:44
Document Index: 763876382

Matched Legal Cases: ['art 470', 'art 470', 'art 470', 'art 470', 'art 470', 'art 470', 'art 470', 'art 470']

Patent US5517575 - Methods of correcting optically generated errors in an electro-optical ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsMethods of correcting optically generated errors in images formed of test object surface profiles include the use of a bias image to correct errors resulting from problems associated with components of the gauging system. The bias image is also useful for correcting errors resulting from the nature of...http://www.google.com/patents/US5517575?utm_source=gb-gplus-sharePatent US5517575 - Methods of correcting optically generated errors in an electro-optical gauging systemAdvanced Patent SearchTry the new Google Patents, with machine-classified Google Scholar results, and Japanese and South Korean patents.Publication numberUS5517575 APublication typeGrantApplication numberUS 08/136,624Publication dateMay 14, 1996Filing dateOct 13, 1993Priority dateOct 4, 1991Fee statusLapsedPublication number08136624, 136624, US 5517575 A, US 5517575A, US-A-5517575, US5517575 A, US5517575AInventorsTheodore B. LadewskiOriginal AssigneeLadewski; Theodore B.Export CitationBiBTeX, EndNote, RefManPatent Citations (15), Referenced by (30), Classifications (15), Legal Events (7) External Links: USPTO, USPTO Assignment, EspacenetMethods of correcting optically generated errors in an electro-optical gauging system
US 5517575 AAbstract
Methods of correcting optically generated errors in images formed of test object surface profiles include the use of a bias image to correct errors resulting from problems associated with components of the gauging system. The bias image is also useful for correcting errors resulting from the nature of the test object surface. Further methods also include the use of a mask to selectively vary the amount of radiation that is used across the test object surface when gauging the surface profile. The mask effectively uniformly irradiates the test object surface to correct errors caused by irregular or extreme surface contours or coloration in the test object surface.
1. A method of correcting optically generated errors in an image of a test object surface profile, the image being formed by irradating the test object, attenuating radiation reflected from the test object and determining the intensity of the radiation across the surface to thereby form the image, comprising the steps of:(A) filtering the radiation used to irradiate the test object before such radiation irradiates the test object; and (B) forming a bias image of the test object surface profile, by performing the substeps of:forming a first image of a preselected calibration surface at a first wavelength; forming a second image of the calibration surface at a second wavelength; digitizing the first and second images, respectfully; and forming the bias image by subtracting the first digitized image from the second digitized image, whereby the bias image is used to correct optically generated errors caused by undesirable imperfections in a reference surface used to determine the test object surface profile as a function of the spacing between the test object surface and the reference surface. 2. A method of correcting optically generated errors in an image of a test object surface profile, the image being formed by irradiating the test object, attenuating radiation reflected from the test object and determining the intensity of the radiation across the surface to thereby form the image, comprising the steps of:(A) filtering the radiation used to irradiate the test object before such radiation irradiates the test object; and (B) forming a bias image of the test object surface profile, by performing the substeps of:forming a first image at a first wavelength of a calibration object having a preselected surface profile and essentially the same reflectance across the surface as the reflectance across the test object surface; forming a second image of the calibration object at a second wavelength; digitizing the first and second images, respectfully; and forming the bias image by subtracting the first digitized image from the second digitized image, whereby the bias image is used to correct optically generated errors caused by nonuniform reflectance across the test object surface. Description
The methods associated with the presently preferred embodiments of this invention are for correcting optically generated errors in the image of a test object surface profile that is formed by an optical gauging system designed in accordance with the teachings of this invention. The presently preferred methods of correcting optically generated errors in such a system include filtering the radiation that is used to irradiate a test object before such radiation is incident upon the test object. A bias image is formed of the test object surface profile and such a bias image is used to correct the optically generated errors present in the image that is otherwise generated of the test object surface profile.
In a second approach as illustrated for example in FIG. 2, the effects of surface reflectivity are removed by making measurements at two separate average wavelengths λ1 and λ2. An optical filter 52 is used to select the wavelength recorded by the camera. The filter is coupled to a conventional translation device 54 controlled by computer 42 for selectively translating filter 52 into and out of the path of the reflected radiation that effectively provides the test object surface image incident on camera 34. A first image of the test object surface is obtained with filter 52 removed from the image path, as illustrated in FIG. 2. This first image is taken at an averaged spectral wavelength λ1 to which the dye has a spectrally averaged absorption coefficient α1. A second image is obtained with filter 52 intersecting the image path. This second image is taken at an averaged spectral wavelength λ2 to which the dye has a spectrally averaged absorption coefficient α2. For the image obtained with wavelength λ1, the measured intensity of the returned or reflected light Iml at a given pixel location is described by the equation:
Im1 =Ii1 exp(-2&#945;1 d)R1 where I1l is the effective incident intensity at that pixel location, d is the thickness of the dye layer at that pixel location, and R1 is the reflectivity of the surface at that pixel location. Similarly, the measured intensity of the returned or reflected light Im2 for wavelength λ2 at that same pixel location is given by the equation:
Im2 =Ii2 exp(-2&#945;2 d)R2 where Ii2 is the effective incident intensity at that pixel location, d is the thickness of the dye layer at that pixel location, and R2 is the reflectivity of the surface at that pixel location. Assuming that the surface reflectivity is independent of wavelength, which is a reasonably good approximation for most metals, R1 equals R2 in the two above equations for each pixel location. The ratio of the measured intensities at the two wavelengths is thus given by the following equation:
D1 =d/cos(&#952;c1)+d/cos(&#952;s1)
Im1= Ii1 R1 exp(-D1 /&#954;)
where Im1 is the intensity of electromagnetic radiation received by image sensor 34, I1l is the intensity of illuminating ray 240, R1 is the reflectivity of the test object surface 12 for angles θc1 and θs1, and κ is the extinction coefficient of the attenuating medium. The above two equations can be solved for:
d={-1n(Im1 /(Ii1 R1))/&#954;}{1/cos(&#952;cl) +1/cos(&#952;s1)}
Im =Ii1 R1 exp(-D1 /&#954;)+Ii2 R2 exp(-D2 /&#954;)
&#961;=1-(sqrt(1+4tan2 (&#952;'))-1)2 /(2tan2 (&#952;'))
D=-1n(Im1 /(Ii1 R1))/&#961;.
&#961;=1-(sqrt(1+4tan2 (&#952;'))-1)2 /(2tan2 (&#952;')).
In the embodiment of FIG. 19, a gas attenuating medium is especially preferred. Gas is preferred, in part, over the liquid dyes discussed above, because a liquid attenuating medium would tend to change the temperature of the test object. For purposes of simplification, whether the attenuating medium consists of a liquid, a gas, or both the attenuating medium will simply be called a dye. Infrared-absorbing dyes that are preferred for use in accordance with this invention include CO2, water vapor and nitrous oxide. CO2 absorbs strongly at 2.7 micrometers, 4.3 micrometers and 14.5 micrometers. Water vapor absorbs strongly at 1.4 micrometers, 1.9 micrometers, 2.7 micrometers and 6.5 micrometers. FIG. 20A illustrates the high resolution spectrum for CO2 (courtesy of Gall Anderson, Phillips Laboratory). FIG. 20A includes wave number axis 434, axis 436 and wavelength axis 437. The high resolution spectrum plot for CO2 is shown by the curve indicated at 438. The spectrum is plotted relative to the scales just described.
Implementing the embodiment of FIG. 19 preferably includes two basic tasks for obtaining surface profile information relating to any point along test object surface 424. These two basic steps include determining the intensity of the infrared radiation emitted in the direction of image sensor 426 from a particular point on test object surface 424. Secondly, the intensity of the radiation received by image sensor 426 from that same point is determined. Determining the radiation intensity received by the camera is accomplished using a high quality infrared camera. In one embodiment, low light intensity or radiation intensity requires exposures for at least several seconds. In such an embodiment, image sensor 426 necessarily includes variable integration capability. Infrared cameras or image sensors having a variable integration capability are commercially available from several sources. Such sources include Amber Engineering, Inc. and Electrophysics Corporation. The Amber Engineering model AE-4256 has a 256×256 pixel sensor with a 12 bit direct digital output. The Amber Engineering model is especially preferred because it provides a depth resolution as great as one part in 4000, and it has a sensor sensitivity up to a wavelength of 5.5 micrometers. Electrophysics series 5400 cameras have sensitivities up to a wavelength of 20 micrometers and provide analog video RS-170 output. Image data from such cameras can be acquired to a microcomputer using a commercially available frame grabber such as from DIPIX Technologies, Inc.
I&#955;1 (x,y)=Io &#955;1 (x,y)&#964;&#955;1,p (D(x,y))                  (4)
where Iλ1 (x,y) is the image intensity measured at point (x,y) of the sensor;
τλ1,92 (z) is defined as the transmittance of radiation at wavelength λ1 through a distance z of the attenuating medium 414 having density ρ. The transmittance of light is the fraction of radiation transmitted with the attenuating medium 414 present.
I&#955;2 (x,y)=Io &#955;2 (x,y)(&#964;&#955;2,p (D(x,y))                 (5).
(2) a known emittance condition wherein (Io λ2 (x,y)/Io λl (x,y)); and
1-&#948;1 &#8806;&#964;&#955;1,p (z)&lt;1   (6)
&#948;1 =max(1-&#964;&#955;1,p (z)), 0&#8806;z&#8806;zmax                               (7)
I&#955;1 (x,y)/(1-&#948;1)&#8807;Io &#955;1 (x,y)&#8807;I&#955;1 (x,y)                        (8)
Io &#955;1 (x,y)=I&#955;1 (x,y)/((1-&#948;1 /2)±&#948;1 /2)                                  (9)
The known emittance condition; i.e., the second preferred condition mentioned above is met if the relative emitted radiation at wavelengths λ1 and λ2 are known. This condition is described by the equation: ##EQU1## or in different, more concise, notation:
Io &#955;2 (x,y)={(1±&#948;2)R(&#955;1,&#955;2,T)}{Io &#955;1 (x,y)}                                     (11)
where W is the emitted radiation in watts/(cm2 micrometers);
R(&#955;1,&#955;2,T)=(W(&#955;1,T,&#949;,&#966;).DELTA.&#955;1)/(W(&#955;2,T,&#949;,&#966;) &#916;&#955;2)                                    (13)
R(&#955;1,&#955;2,T)={&#949;(&#955;1)C(&#966;)F(.lambda.1,T)&#916;&#955;1}/{&#949;(&#955;2)C(&#966;)F(&#955;2,T)&#916;&#955;2 }.                          (14)
R(&#955;1,&#955;2,T)=(&#949;(&#955;1)/&#949;(.lambda.2))(F(&#955;1,T)&#916;&#955;1 /F(&#955;2,T)&#916;&#955;2).               (15)
F(&#955;,T)=2&#960;hc2 /(&#955;5 (ehc/&#955;kT -1)) (16)
R(&#955;1,&#955;2,T)={&#949;(&#955;1)/(&#955;.sub.1 5((ehc /.sup.&#955;1 kT -1) &#916;&#955;1)}/{&#949;(&#955;2)/(&#955;2 5(ehc /.sup.&#955;2 kT -1)&#916;&#955;2)}(17)
R'(&#955;1,&#955;2 T)=(&#955;2 5(ehc/&#955; 2kT -1)/(&#955;1 5(ehc/&#955; 1kT -1)).
&#948;2 '=1-{R'(&#955;,&#955;+&#916;&#955;o,T)/R'(&#955;,&#955;+.DELTA.&#955;o,T+&#916;T)}                           (19)
The first method, mathematically inverting τλ2,ρ(z), requires an analytic expression for τλ2,ρ(z). For example, if τλ,ρ(z) obeyed Beer's law (a condition that would be true for a liquid dye) then τλ2,ρ(z) would be exponential and its inverse would be logarithmic. Such a logarithmic approach is used in the first embodiment described earlier in this application relating to diffuse surface test objects as illustrated in FIGS. 1-5. Infrared absorption by gases or attenuating media 414 that include gases follows Beer's law only over narrow wavelength bands. An analytic expression for τλ2,ρ(z) for a gas over a broad infrared band typically is difficult or impossible to find. Therefore, the logarithmic method is not preferred with the embodiment of FIG. 19.
The third method, tabulating τλ2,ρ-1 (r) empirical measurements, preferably includes performing a dye calibration procedure illustrated diagrammatically in FIG. 23A, the description of which follows.
FIG. 23A shows attenuating medium 414 and calibration part 470. Calibration part 470 has a known surface geometry 472 and a known orientation relative to reference surface 422 such that distances zi indicated by the lines at 474 are known. Filters 476 and 478 are provided for taking image measurements at wavelengths λ1 and λ2, respectively. Filters 476 and 478 are translated into and out of the field of vision of image sensor 426 according to the direction arrow shown in FIG. 23A. The rays of infrared radiation shown at 428 corresponding to each zi 474 are received and measured by image sensor 426. These intensity values are then measured at the second wavelength λ2. FIG. 23B illustrates a calibration plot 480 showing data that would correspond to the ratio of intensities taken at the wavelength λ2 (i.e., with filter 478 in place) divided by the intensities of the image taken at λ1 shown along axis 482 versus the distances zi shown on axis 484. That is, distances zi are tabulated with the ratio R.sub. i =I.sub.λ2 (zi)/Iλ1 (zi) of measured intensities I.
The calibration procedure illustrated diagrammatically in FIG. 23A is preferably carried out by first preparing calibration part 470. Part 470 may be constructed of any material but must have a known contour on one surface, the calibration surface 472. For example, calibration part 470 may be a solid bar of known surface contour or a sheet of thin material stretched on a frame. The accuracy of the calibration obviously depends, in part, on the accuracy to which the contour of calibration surface 472 is known. For convenience, the calibration surface 472 is preferably flat, deviating from a plane by dimensions which are less than the desired accuracy of the optical gauging system used in accordance with the preferred embodiment of this invention. Calibration part 470 is put into an optical gauging unit that includes attenuating medium 414 at the proper concentration used for gauging test objects. Calibration part 470 is oriented such that the calibration surface faces image sensor 426 and is slanted relative to image sensor 426 such that the distances zi 474 span the range of distances reasonably anticipated to be gauged on test objects 416. Calibration part 470 is preferably brought to a uniform temperature, T. An image is acquired by image sensor 426 at λ1 to obtain the intensity profile I.sub.λ1 (zi). Next, an image at λ2 is obtained by image sensor 426 to obtain the intensity profile I.sub.λ2 (zi). The ratio Ri of measured intensities is then calculated and τ.sub.λ2ρ-1 (ri) is then tabulated from a plot of zi versus Ri as illustrated in FIG. 23B. Therefore, calibration is accomplished.
I&#955;2 (x,y)=Io &#955;2 (x,y)(&#964;&#955;2,.sub.&#961; (D(x,y))                  (5)
I&#955;2 (x,y)={((1±&#948;2)R(&#955;1,&#955;2,T))(Io &#955;1 (x,y)}T&#955;2,&#961;(D(x,y))        (20)
I&#955;2 (x,y)={((1±&#948;2)R(&#955;1,&#955;2,T)) (I&#955;1 (x,y)/((1-&#948;1 /2)±&#948;1 /2))}T&#955;2,&#961;(D(x,y))                        (21)
&#964;&#955;2,&#961;(D(x,y))=I&#955;2 (x,y)/{((1±&#948;2)R(&#955;1,&#955;2,T))((I&#955;1 (x,y)/((1-&#948;1 /2)±&#948;1 /2))   (22)
&#964;&#955;2,&#961;(D(x,y))={I&#955;2 (x,y)/I&#955;1 (x,y)}{(1-&#948;1 /2)±&#948;1 /2)(1±&#948;2)R(&#955;1,&#955;2,T)}(23)
E(&#955;1,&#955;2,T)=((1-&#948;1 /2)±&#948;1 /2)(1±&#948;2)R(&#955;1, &#955;2,T) (24)
D(x,y)=&#964;&#955;2,&#961;-1 (E(&#955;1,&#955;2,T)(I&#955;2 (x,y)/I&#955;1 (x,y))                                                    (25).
Assume λ1 equals 4.8 micrometers, λ2 equals 5.8 micrometers, T equals 298 degrees K (25 degrees C.), and ΔT equals 0.1 degrees C. Assume an attenuating medium 414 being water vapor at the concentration equivalent to 0.1 mm precipital water at Zmax, the maximum depth to gauge being 0.1 mm. From FIGS. 20B and 20C, τλ1 appears to be 0.999 such that equation 7 yields δ1 0,001. Also from FIGS. 20B and 20C, the transmittance is 0.50 such that equation 19 yields δ2 at 0.00058. Using equation 18, R(λ1,λ2, T) is 2.20, then from equation 24: ##EQU2##
Io &#955;2 (x,y)=Io &#955;1 (x,y)+RT (Io &#955;1 (x,y)-Io &#955;3 (x,y))        (26)
RT =(Io &#955;2 (x,y)-Io &#955;1 (x,y))/(Io &#955;1 (x,y) -Io &#955;3 (x,y)) (27)
Ep =hc/&#955;                                       (28).
Assuming a 10 micrometer×10 micrometer pixel, an f8 lens, a black body emissivity of 0.1 (typical for a machined metal), a sensor quantum efficiency of 0.5, and a filter bandwidth of 10 nanometers, an image of a 300° K black body would then generate, at one pixel, 2×106 electrons per second at a wavelength of 4 micrometers and 24×106 electrons per second at 7 micrometers. At 1000° K and 4 micrometers, 4000×106 electrons per second per pixel are generated. Since a dynamic range of 1000 requires 10002 =106 electrons, adequate data can be obtained between a fraction of a second and a few seconds exposure time, depending on temperature, emissivity, optical design and sensor characteristics.
The embodiments illustrated in FIGS. 19-24 provide optical gauging systems having advantages over the inventive embodiments shown in relation to FIGS. 1-18 above. Such advantages include the capability to gauge a surface of a test object regardless of the nature of that surface. Test objects having a diffuse or specular surface or any combination of the two can be measured without modifying the apparatus, system or software of the embodiment of FIG. 19. Further, because the preferred embodiment Just discussed does not require an external source of radiation, it includes the capability of measuring surfaces that are oriented relative to the image sensor up to an angle of orientation approximately 90°. This latter feature overcomes the difficulties discussed in relation to the embodiment of FIG. 8 above which is designed especially for specular surfaced object gauging.
The embodiment of this invention illustrated in FIG. 25 has several advantages associated with it. The use of a range camera 502 eliminates the requirement that the test object be placed within a test chamber containing a controlled attenuating medium. Therefore, image sensor 502 can be used in any environment to gauge the surface of very large test objects. Similarly, the embodiment illustrated in FIG. 25 could be used outdoors. The embodiment of FIG. 25 is adaptable to gauging objects having diffuse, specular or a mixture of surfaces. Therefore, a variety of test objects can be gauged or measured without modifying the apparatus or software associated with the embodiment of FIG. 25. The embodiment of FIG. 25 is capable of measuring various surfaces, in part, because no external source of radiation is required and therefore no correction for varying reflectivity is involved. Similarly, because no external source of radiation is required the embodiment of FIG. 25 can measure surfaces oriented at angles of approximately 90° relative to image sensor 502. Lastly, the embodiment of FIG. 25 includes a passive instrument and therefore can determine range without detection. Determining the range to a test object and gauging its surface without detection may be advantageous in military operations, for example.
Fifth, some test object surfaces introduce what is called a dynamic range problem. A test object surface having a large depression, for example, is difficult for optical gauging system 100 to accurately and correctly gauge. The optical gauging system, as described above in relation to FIGS. 1-18 is limited to a depth resolution of (dmax -dmin)/g, where dmax is the maximum depth of a depression on the test object surface and dmin is the minimum depth of such a depression and where g is the number of gray levels of the image sensor (e.g., 256 for an 8-bit camera). Higher resolution requirements demand higher image sensor performance. Such requirements may exceed the capabilities of commercially available image sensors or cameras and, at times, may require the use of undesirably expensive image sensors.
Computer unit 602 utilizes computer software and conventional computer hardware to generate an image which represents the distance between the test object surface 612 and reference surface 618 by taking information from image sensor 634 and converting that into a digitized image of test surface 612. The calculation of the distance between test surface 612 and reference surface 618 across the image of test object 614 begins by taking the logarithm of the intensity data received from the image sensor by the computer unit 602. The information or intensity data received as taken through filter 626 is stored in image 640. Similarly, the intensity data received from image sensor 634 as received through filter 628 is stored in image 642. Image 640 is then subtracted from image 642 modulo g pixel-by-pixel from image 642 yielding image 644. "g" in "modulo g" represents the number of gray levels, for example 256 for an eight-bit data. Offset value g0 is then subtracted from image 644 at 646. Typically, g0, which establishes the reference point, is the gray level in image 644 of either the highest or lowest point on test object surface 612. In this instance, again, subtraction is performed modulo-g. The result is a digitized image which, when multiplied by a constant determined by calibration (as generally described above) describes the distance between the test object surface 612 and reference surface 618.
In FIG. 35 computer unit 602 includes the means for correcting optically generated errors due to oblique distortion. Image 640 is generated by gauging test object 614 with filter 626 in place and image 642 is similarly generated by gauging test object 614 with filter 628 properly in place in accordance with the general description above. Bias image 660 is created in one of the manners described above. Image 642 is subtracted from image 640 and image 660 is subtracted from that difference. The arithmetic difference gives the resulting image 690. Offset value g0 is subtracted at 646. The logarithm of the image 690 minus the g0 offset is then calculated in order to produce image 692.
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