- Email: [email protected]
An automatic holographic monitoring system for metal surfaces corrosion
277
An automatic holographic monitoring system for metal surfaces corrosion P. Carelli, D. Paoletti and G. Schirripa Spagnolo Dipartimento di Energetica, Universitd degli Studi di l'Aquila, Localitd Monteluco di Roio, 67040 Roio Poggio (l'Aquila), Italy
Abstract. A contouring technique based on multiple-source sandwich holography is described for evaluation of the corrosion rate of a metal or alloy. An image processing system for automatic fringe analysis is used as a real-time inspection technique. Some experimental results obtained on models are reported.
Keywords. Holographic metrology, corrosion monitoring, digital image processing.
1. Introduction
A very interesting optical technique which has considerable potential in industrial applications is that of contouring. Conventional shape measurements use mechanical probes and give either point-by-point or line-scan information about shape. Optical methods have the advantages of being noncontacting, and providing information over the full field of view; overall features of the surface can immediately be assessed. Thus, in recent years, there has been considerable effort directed towards the development of these techniques, based on moir6 methods, fringe projection or holographic methods. The applicability depends on their merits in terms of sensitivity, depth of field, size of test specimen, general simplicity. We shall direct our attention to a holographic method, based on a multiple-source system utilizing a sandwich technique, with the purpose of obtaining contouring fringes which can be easily interpreted and used for a quantitative automatic evaluation of gradual corrosion of a metal or alloy in quasi real time.
Elsevier Industrial Metrology 1 (1991) 277-284 0921-5956/91/$3.50 © 199i. Elsevier Science Publishers B.V.
278
P. CareIli et al. / Automatic holographic monitoring system
Corrosion monitoring is an important tool for plants maintenance, and it could be useful for the evaluation of the corrosion resistance of different materials in different environments.
2. The principle of the method In conventional holographic interferometry the observed fringe patterns are determined by the object displacement and deformation, and by the illumination and observation configurations. The obtained information, in some cases, m a y not be in the most convenient form for further data processing. To overcome this problem and to create new possibilities, holographic fringe patterns can be changed by modifying the optical set up [1-4]. As a result of these modifications contours are generated on or near holographic images. C o n t o u r generation means the formation of a fringe pattern giving the topographical map of a three-dimensional object or of its virtual image, like the level lines on a map. Each fringe is the locus of points of the surface that are at a constant distance from a fixed plane. It is possible to choose the spacing of the fringes and consequently it is possible to adjust the sensitivity as a function of the Pasquale Carelli is a Professor of Physics in the Faculty of Engineering of the University of L'Aquila and leader in the design of superconductor devices by lithographic techniques. His research interests include optical metrology by automatic pattern recognition.
Domenica Paoletti has been an Associate Professor of Physics in the Faculty of Engineering of the University of L'Aquila since 1977 and leader of the optical group in the laser laboratory. Her research interests include optical metrology, holography, speckle, and environmental monitoring.
Giuseppe SchirripaSpagnolo has been a physics researcher in the Faculty of Engineering of the University of L'Aquila since 1985. His research interests include coherent optics and the design of scientific instruments.
P. Carelli et al. / Automatic holographic monitoring system
279
size of the object. There are many ways to produce interference contouring fringes. If the source that illuminates the object is changed in position or frequency during the exposure, the hologram contains more object information than in the singlesource or frequency case; in the beginning contouring methods have been based on the use of a couple of wavelengths differing by A2 ~ 2 but the system utilizing multiple illuminating sources seems the easiest and most obvious way to improvement [5]. A result of these multiple source positions or multiple frequencies is the construction of contours of a complex three-dimensional object. Starting from these works we propose a contouring method based on translation of the holographic plate and an associated shift of the illumination beam. We have adopted the experimental layout shown in Fig. 1. A laser beam is split into two by a variable beam splitter. One beam is a reference beam which illuminates the holographic plate, the other beam is reflected by a rotating mirror and illuminates the object. Scattered light from the object interferes with the reference beam at the holographic plate to create an interference pattern. N-exposures can be effectuated, one for each source position. An exposure is taken of the surface to be contoured with the illuminating source in an initial position; then the developed holographic plate is illuminated with the same reference beam used for recording. The wave front scattered from the object is reconstructed, the observation can be performed either in real time or by using a holographic sandwich technique. If the observation is made in real time, after repositioning of the holographic plate, the analysed body is made to interfere with its initial image, the beam object is rotated, so as to displace the virtual image of the source and a system of fringes is observed. These fringes contain contour information; they will be the same as if interference fringes had been projected into the object as in projected fringe contouring, the spacing between them is the contour interval. In particular if we assume to have plane surfaces separated by a step, illuminated by plane waves, a fringe originally at an arbitrary position xl, Zl moves to the position x2, z2 after the introduction of a phase variation; this implies a displacement in the x-direction
MIRROR
BEAM
EXPANDER
SANDWICH H O L O G R A M
/A
BEAM SPMTTER
OBJECT
~DD
TV CAMERA
PRINTER MIRROR LASER
!
ROTATING MIRROR
Fig. 1. Experimental set-up for sandwich contouring hologram.
280
P. Carelli et al. / Automatic holographic monitoring system
ot the fringe pattern equal to: Ax = Az tan ~,
(1)
where Az is the depth of the step or the change of position with respect to a reference plane normal to the direction of observation, Ax is a fraction (k) of the pitch (P0) of the fringe pattern and e is the inclination of the object beam with respect to the normal to the plate [-4,5]. Control of the localization of the fringes can be effectuated by displacing the hologram plate, but the quality of the fringe pattern is very poor; the method can be optimized by using sandwich hologram interferometry [6]. The set up is the same as used in the production of standard double-exposure holograms, with a mirror mounted on a calibrated rotation stage. In particular one holographic plate is exposed in a plate holder, with its emulsion layer toward the object, after the point source of light illuminating the object is displaced laterally by rotation of the mirror. Sequential exposures are made for different positions of the illuminating beam on holographic plates placed in the plate holder behind a glass plate; the holographic plates are processed and repositioned in the plate holder. The first exposed plate is placed in front of the back plate during reconstruction. The resulting pairs may be considered to be the sum of two separate holograms giving rise to two images that occupy the same space but differ in phase, so that the object appears covered by fringes that behave just as any contouring fringes. The fringes appear fixed to the object surface; by shearing the two plates in relation to each other the control of the localization of the fringes may be optimized and the fringe pattern observed is useful for measurement of the depth of erosion, long-term wear and for corrosive surface changes. In the past the quantitative analysis required precise manual measurements, which were inherently slow and complex. With the incorporation of recently developed electronics and computers into the holographic system, it is now possible to obtain rapid acquisition and analysis of a large number of interferograms. In this paper we propose a relatively simple system that is capable of rapidly acquiring the interferometric data, processing and analyzing it virtually in real time.
3. Fringe analysis system The processing sequence of the fringe patterns is as follows. A high-resolution charge coupled device (CCD) camera (480 x 512) supported by an image digitized board (OCULUS-200), with software in C language is used to read the photographic contoured image or directly the holographic image. Setting conditions and line data (amplitude of pixel) taken with a telecamera are written in an ASCII file. There is a one-to-one correspondence between a byte on a line in the ASCII file and a pixel of the CCD. All mathematical analysis of data is performed with a simple BASIC program running on the same PC (Olivetti M24 with mathematical coprocessor). The BASIC routine reads all the stored data and converts them back
P. Carelli et al. / Automatic holographic monitoring system
281
into an integer array X(I, J) with two indices, the first one corresponding to the line, the second one to the column. Then, for each line, an autocorrelation algorithm is used to find out the fringe period expressed in units of pixels. The autocorrelation is defined as: 0.5N
Ak=
~
X(I, J)X(I, J 4- K).
(2)
J=O.1N
The maximum of the autocorrelation (a function of k), is obtained only if at least five autocorrelations corresponding to the shift of 1 pixel are on a parabola and their difference is greater than a fixed threshold. The period T (i.e. kmax) is calculated with a precision better than one pixel, fitting the three data Akm,~-1, Akmox, Akm,x+1 with a parabola. The period of each line is averaged and those that differ from the others by more than 10% are disregarded. In the default condition this analysis requires about 10 s. The following step is to find the maximum of correlation between all the lines (U) before and after (W) the step to be measured, using the same procedure as for the autocorrelation. O,5N+O.56T
G ( c , w) =
x(ty, J)X(W, J + K).
(3)
J=O.SN-O.56T
The k maximum is averaged, with the same proceduce used to calculate the period. At this point the step in the same unit as the pitch is calculated with eqn. (1). Use of a faster computer allows almost real-time analysis.
4. Experimental results Preliminary experiments have been made on an aluminium specimen with calibrated steps (20, 104, 305, 505 gm) simulating several thickness changes. Figures 2 and 3 show fringe patterns obtained from sandwich holograms effectuated on an aluminium model with a step of 305 gm, with two different rotations of the object beam. Figure 4 shows a tracked fringe pattern, directly recorded from a sandwich hologram with the CCD camera. In Table 1 are reported the quantitative measurements effectuated with a compuier system on steps with different depths. Argon laser light of 514.5 nm wavelength was used, with Agfa plates 10E56. The mechanical measurement of the calibrated steps was made with an automated stylus surface profile measure with precision of better than 1 ~tm.
5. Conclusion This holographic contouring method is very promising for nondestructive evaluation of wear or thickness reduction in corrosive environments. With this system unwanted patterns that can obscure the true contours or degrade the quality of the holographic map may be eliminated a posteriori during reconstruction of the
282
P. Carelli et al. / Automatic holographic monitoring system
Fig. 2. Fringe pattern corresponding to the measurement of a 305 gm step on a model.
Fig. 3. Fringe pattern corresponding to the same step with a different rotation of the object beam.
P. Carelli et al. / Automatic holographic monitoring system
.........11.22~~.~ c ~ ~ J
, , , ~ ...... ~:
~,,,:i ~ 2
~ Fi2_!K.f
283
"i~.;~
. _
i
i,---17
. . . . . . . . . . . . . . . . . . . . . .
Fig. 4. An example of a tracked fringe pattern captured in real time by a C C D camera.
Table 1 Observed values of d e p t h of different steps Method stylus
optical
305 305 505 104 20
287 _+ 23 327 _+ 16 504 _+ 25 89 _+ 21 19_+3
I~m lam Ixm gm ~tm
c~
p
46 ° 65 ° 46 ° 46 ° 86 °
1596 4361 1300 810 2402
gm gm gm gm gm
284
P. Carelli et al. / Automatic holographic monitoring system
sandwich hologram and the useful information relating to corrosion can be processed automatically. Monitoring of the gradual corrosion can be performed with a series of sandwich plates and with a varying sensitivity, able to reach 5 Ixm or even less. A computer system can process the fringe pattern data and compute steps also of bad quality because the use of correlation function for computation guarantees that also fringe systems of bad periodicity give pretty good results. The digitized fringe pattern is limited in resolution by the size of the pixels; consequently there is a limit to the accuracy of measurement if the scale of the fringes and changes between the fringe patterns approach the pixel size. In conclusion we think that the proposed set-up can be a practical industrial tool for controlling complex surfaces of objects of several species in corrosive environments.
References [1] K.A. Haines and B.P. Hildebrand, Multiple-wavelength and multiple-source holography applied to contour generation, J. Opt. Soe. Am. 57 (1967) 155-159. [2] J.S. Zelenka and J.R. Varner, A new method for generating depth contours holographically, Appl. Opt. 7 (1968) 2107-2110. [3] C.A. Sciammarella, Holographic moir6, an optical tool for the determination of displacements, trains, contours and slopes of surfaces, Opt. Eng. 21 (1982) 447-457. [-4] P. De Mattia and V. Fossati Bellani, Holographic contouring by displacing the object and the illumination beam, Opt. Comun. 26 (1978) 17-21. [5] N. Abramson, The Making and Evaluation of Holograms, Academic Press, New York, 1981. [6] K. Creath, Holographic contour and deformation measurement using a 1.4 million element detector array, Appl. Opt. 28 (1989) 2170-2176.
An automatic holographic monitoring system for metal surfaces corrosion P. Carelli, D. Paoletti and G. Schirripa Spagnolo Dipartimento di Energetica, Universitd degli Studi di l'Aquila, Localitd Monteluco di Roio, 67040 Roio Poggio (l'Aquila), Italy
Abstract. A contouring technique based on multiple-source sandwich holography is described for evaluation of the corrosion rate of a metal or alloy. An image processing system for automatic fringe analysis is used as a real-time inspection technique. Some experimental results obtained on models are reported.
Keywords. Holographic metrology, corrosion monitoring, digital image processing.
1. Introduction
A very interesting optical technique which has considerable potential in industrial applications is that of contouring. Conventional shape measurements use mechanical probes and give either point-by-point or line-scan information about shape. Optical methods have the advantages of being noncontacting, and providing information over the full field of view; overall features of the surface can immediately be assessed. Thus, in recent years, there has been considerable effort directed towards the development of these techniques, based on moir6 methods, fringe projection or holographic methods. The applicability depends on their merits in terms of sensitivity, depth of field, size of test specimen, general simplicity. We shall direct our attention to a holographic method, based on a multiple-source system utilizing a sandwich technique, with the purpose of obtaining contouring fringes which can be easily interpreted and used for a quantitative automatic evaluation of gradual corrosion of a metal or alloy in quasi real time.
Elsevier Industrial Metrology 1 (1991) 277-284 0921-5956/91/$3.50 © 199i. Elsevier Science Publishers B.V.
278
P. CareIli et al. / Automatic holographic monitoring system
Corrosion monitoring is an important tool for plants maintenance, and it could be useful for the evaluation of the corrosion resistance of different materials in different environments.
2. The principle of the method In conventional holographic interferometry the observed fringe patterns are determined by the object displacement and deformation, and by the illumination and observation configurations. The obtained information, in some cases, m a y not be in the most convenient form for further data processing. To overcome this problem and to create new possibilities, holographic fringe patterns can be changed by modifying the optical set up [1-4]. As a result of these modifications contours are generated on or near holographic images. C o n t o u r generation means the formation of a fringe pattern giving the topographical map of a three-dimensional object or of its virtual image, like the level lines on a map. Each fringe is the locus of points of the surface that are at a constant distance from a fixed plane. It is possible to choose the spacing of the fringes and consequently it is possible to adjust the sensitivity as a function of the Pasquale Carelli is a Professor of Physics in the Faculty of Engineering of the University of L'Aquila and leader in the design of superconductor devices by lithographic techniques. His research interests include optical metrology by automatic pattern recognition.
Domenica Paoletti has been an Associate Professor of Physics in the Faculty of Engineering of the University of L'Aquila since 1977 and leader of the optical group in the laser laboratory. Her research interests include optical metrology, holography, speckle, and environmental monitoring.
Giuseppe SchirripaSpagnolo has been a physics researcher in the Faculty of Engineering of the University of L'Aquila since 1985. His research interests include coherent optics and the design of scientific instruments.
P. Carelli et al. / Automatic holographic monitoring system
279
size of the object. There are many ways to produce interference contouring fringes. If the source that illuminates the object is changed in position or frequency during the exposure, the hologram contains more object information than in the singlesource or frequency case; in the beginning contouring methods have been based on the use of a couple of wavelengths differing by A2 ~ 2 but the system utilizing multiple illuminating sources seems the easiest and most obvious way to improvement [5]. A result of these multiple source positions or multiple frequencies is the construction of contours of a complex three-dimensional object. Starting from these works we propose a contouring method based on translation of the holographic plate and an associated shift of the illumination beam. We have adopted the experimental layout shown in Fig. 1. A laser beam is split into two by a variable beam splitter. One beam is a reference beam which illuminates the holographic plate, the other beam is reflected by a rotating mirror and illuminates the object. Scattered light from the object interferes with the reference beam at the holographic plate to create an interference pattern. N-exposures can be effectuated, one for each source position. An exposure is taken of the surface to be contoured with the illuminating source in an initial position; then the developed holographic plate is illuminated with the same reference beam used for recording. The wave front scattered from the object is reconstructed, the observation can be performed either in real time or by using a holographic sandwich technique. If the observation is made in real time, after repositioning of the holographic plate, the analysed body is made to interfere with its initial image, the beam object is rotated, so as to displace the virtual image of the source and a system of fringes is observed. These fringes contain contour information; they will be the same as if interference fringes had been projected into the object as in projected fringe contouring, the spacing between them is the contour interval. In particular if we assume to have plane surfaces separated by a step, illuminated by plane waves, a fringe originally at an arbitrary position xl, Zl moves to the position x2, z2 after the introduction of a phase variation; this implies a displacement in the x-direction
MIRROR
BEAM
EXPANDER
SANDWICH H O L O G R A M
/A
BEAM SPMTTER
OBJECT
~DD
TV CAMERA
PRINTER MIRROR LASER
!
ROTATING MIRROR
Fig. 1. Experimental set-up for sandwich contouring hologram.
280
P. Carelli et al. / Automatic holographic monitoring system
ot the fringe pattern equal to: Ax = Az tan ~,
(1)
where Az is the depth of the step or the change of position with respect to a reference plane normal to the direction of observation, Ax is a fraction (k) of the pitch (P0) of the fringe pattern and e is the inclination of the object beam with respect to the normal to the plate [-4,5]. Control of the localization of the fringes can be effectuated by displacing the hologram plate, but the quality of the fringe pattern is very poor; the method can be optimized by using sandwich hologram interferometry [6]. The set up is the same as used in the production of standard double-exposure holograms, with a mirror mounted on a calibrated rotation stage. In particular one holographic plate is exposed in a plate holder, with its emulsion layer toward the object, after the point source of light illuminating the object is displaced laterally by rotation of the mirror. Sequential exposures are made for different positions of the illuminating beam on holographic plates placed in the plate holder behind a glass plate; the holographic plates are processed and repositioned in the plate holder. The first exposed plate is placed in front of the back plate during reconstruction. The resulting pairs may be considered to be the sum of two separate holograms giving rise to two images that occupy the same space but differ in phase, so that the object appears covered by fringes that behave just as any contouring fringes. The fringes appear fixed to the object surface; by shearing the two plates in relation to each other the control of the localization of the fringes may be optimized and the fringe pattern observed is useful for measurement of the depth of erosion, long-term wear and for corrosive surface changes. In the past the quantitative analysis required precise manual measurements, which were inherently slow and complex. With the incorporation of recently developed electronics and computers into the holographic system, it is now possible to obtain rapid acquisition and analysis of a large number of interferograms. In this paper we propose a relatively simple system that is capable of rapidly acquiring the interferometric data, processing and analyzing it virtually in real time.
3. Fringe analysis system The processing sequence of the fringe patterns is as follows. A high-resolution charge coupled device (CCD) camera (480 x 512) supported by an image digitized board (OCULUS-200), with software in C language is used to read the photographic contoured image or directly the holographic image. Setting conditions and line data (amplitude of pixel) taken with a telecamera are written in an ASCII file. There is a one-to-one correspondence between a byte on a line in the ASCII file and a pixel of the CCD. All mathematical analysis of data is performed with a simple BASIC program running on the same PC (Olivetti M24 with mathematical coprocessor). The BASIC routine reads all the stored data and converts them back
P. Carelli et al. / Automatic holographic monitoring system
281
into an integer array X(I, J) with two indices, the first one corresponding to the line, the second one to the column. Then, for each line, an autocorrelation algorithm is used to find out the fringe period expressed in units of pixels. The autocorrelation is defined as: 0.5N
Ak=
~
X(I, J)X(I, J 4- K).
(2)
J=O.1N
The maximum of the autocorrelation (a function of k), is obtained only if at least five autocorrelations corresponding to the shift of 1 pixel are on a parabola and their difference is greater than a fixed threshold. The period T (i.e. kmax) is calculated with a precision better than one pixel, fitting the three data Akm,~-1, Akmox, Akm,x+1 with a parabola. The period of each line is averaged and those that differ from the others by more than 10% are disregarded. In the default condition this analysis requires about 10 s. The following step is to find the maximum of correlation between all the lines (U) before and after (W) the step to be measured, using the same procedure as for the autocorrelation. O,5N+O.56T
G ( c , w) =
x(ty, J)X(W, J + K).
(3)
J=O.SN-O.56T
The k maximum is averaged, with the same proceduce used to calculate the period. At this point the step in the same unit as the pitch is calculated with eqn. (1). Use of a faster computer allows almost real-time analysis.
4. Experimental results Preliminary experiments have been made on an aluminium specimen with calibrated steps (20, 104, 305, 505 gm) simulating several thickness changes. Figures 2 and 3 show fringe patterns obtained from sandwich holograms effectuated on an aluminium model with a step of 305 gm, with two different rotations of the object beam. Figure 4 shows a tracked fringe pattern, directly recorded from a sandwich hologram with the CCD camera. In Table 1 are reported the quantitative measurements effectuated with a compuier system on steps with different depths. Argon laser light of 514.5 nm wavelength was used, with Agfa plates 10E56. The mechanical measurement of the calibrated steps was made with an automated stylus surface profile measure with precision of better than 1 ~tm.
5. Conclusion This holographic contouring method is very promising for nondestructive evaluation of wear or thickness reduction in corrosive environments. With this system unwanted patterns that can obscure the true contours or degrade the quality of the holographic map may be eliminated a posteriori during reconstruction of the
282
P. Carelli et al. / Automatic holographic monitoring system
Fig. 2. Fringe pattern corresponding to the measurement of a 305 gm step on a model.
Fig. 3. Fringe pattern corresponding to the same step with a different rotation of the object beam.
P. Carelli et al. / Automatic holographic monitoring system
.........11.22~~.~ c ~ ~ J
, , , ~ ...... ~:
~,,,:i ~ 2
~ Fi2_!K.f
283
"i~.;~
. _
i
i,---17
. . . . . . . . . . . . . . . . . . . . . .
Fig. 4. An example of a tracked fringe pattern captured in real time by a C C D camera.
Table 1 Observed values of d e p t h of different steps Method stylus
optical
305 305 505 104 20
287 _+ 23 327 _+ 16 504 _+ 25 89 _+ 21 19_+3
I~m lam Ixm gm ~tm
c~
p
46 ° 65 ° 46 ° 46 ° 86 °
1596 4361 1300 810 2402
gm gm gm gm gm
284
P. Carelli et al. / Automatic holographic monitoring system
sandwich hologram and the useful information relating to corrosion can be processed automatically. Monitoring of the gradual corrosion can be performed with a series of sandwich plates and with a varying sensitivity, able to reach 5 Ixm or even less. A computer system can process the fringe pattern data and compute steps also of bad quality because the use of correlation function for computation guarantees that also fringe systems of bad periodicity give pretty good results. The digitized fringe pattern is limited in resolution by the size of the pixels; consequently there is a limit to the accuracy of measurement if the scale of the fringes and changes between the fringe patterns approach the pixel size. In conclusion we think that the proposed set-up can be a practical industrial tool for controlling complex surfaces of objects of several species in corrosive environments.
References [1] K.A. Haines and B.P. Hildebrand, Multiple-wavelength and multiple-source holography applied to contour generation, J. Opt. Soe. Am. 57 (1967) 155-159. [2] J.S. Zelenka and J.R. Varner, A new method for generating depth contours holographically, Appl. Opt. 7 (1968) 2107-2110. [3] C.A. Sciammarella, Holographic moir6, an optical tool for the determination of displacements, trains, contours and slopes of surfaces, Opt. Eng. 21 (1982) 447-457. [-4] P. De Mattia and V. Fossati Bellani, Holographic contouring by displacing the object and the illumination beam, Opt. Comun. 26 (1978) 17-21. [5] N. Abramson, The Making and Evaluation of Holograms, Academic Press, New York, 1981. [6] K. Creath, Holographic contour and deformation measurement using a 1.4 million element detector array, Appl. Opt. 28 (1989) 2170-2176.