CA1278842C - Thermal wave imaging apparatus - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A thermal wave imaging apparatus generates a real time image of the surface and subsurface of an opaque solid object.
AC electrical signals indicative of the configuration of the surface and subsurface of the object which are generated during a thermal wave scan of the object are stored in an image memory under the control of a central processor. A refresh counter generates sequential, incremental signals used to control the X
and Y deflection of a display monitor. Such signals also address the image memory and generate output data controlling the intensity of the display point at each generated X and Y
deflection point. A modulation and intensity regulation circuit generates an optical beam having a constant amplitude inspite of any fluctuations in the output of a laser.

Description

` Our Reference: WSU-105-A 1~7~4~ PATENT

THERMAL WAVE IMAGING APPARATUS
BACK(iROUND OF THl~ INVENTION
This invention relate~, in general, to method~ and devices for non-destructive testing of opaque articles to detect surface and sub-surface crack~, flaws, voids, etc.
Various methods have been proposed to detect surface and subsurface cracks, flaws, voids, etc. in opaque solids. One common method utilizes photo-acoustic techniques in which periodic, localized heating of a ~ample within a gas-filled cell is caused by focu~ed inten~ity modulated ligh-tJ electro-magnetic radiation or a particle beam. The heat generates æound within the gas medium which are detected by a transducer, such as a microphone mounted within the gas cell. The transducer or microphone generates electrical signals which are analyzed to locate surface and ~ubsurface defects.
In actual use, an argon-ion laser whose output is modulated is focused onto the surface of interest through an optical window spaced from the surface of the sample by a ~mall volume of air vf ga~. The transducer mounted within~the cell detects the amplitude and phH~e of pressure variations with the cell caused by the temperature profile at the ~ur-~ace of the sample. Howevar, while this imaging technique is effective at detecting certain crack orientations, it cannot detect strictly vertical, closed cracks. While in practice many cracks are not quite vertical or not quite closed or both, any cracks which are strictly vertical and closed would be missed when employing this technique.
Mirage effect -thermal wave imaging has proven effective at detecting strictly vertical closed cracks within opsque solids. This technique~utilizes a laser source -to probe the air just above the surface of an opaque solid which is heated by a Z
secon~ modulated laser. ~n ac electrical signal i8 produced by using a phototransistor to monitor the de~lection of -the probe beam in a plane parallel or perpendicular to the sample sur-~ace.
Indexing o~ the sample underneath the heating laser beam or indexing the heating laser beam over -the surface of the sample results in a series of data signals which are usef~ll in detec-ting subsurface and surface cracks, flaws, voids and other defects.
Other imaging techniques currently being used or investigated include gas cell, photothermal displacement, infrared detection and piezoelectric detection.
The signals by themselves cannot yield any useEul in~orma-tion as to the existence of surface or subsurface cracks without additional analysis. Heretofore, on-line, real time analysis techniques have been minimal for data generated by the various thermal wave imaging techniques. This lack o~ useful date anRlysis techniques has hampered the use of thermal wave imaging techniques for detecting surface and subsurface cracks in opaque solid objects.

Thus, it will be desirable to provide an analysis technique which overcomes the deficiencies in analyzing and displaying information generated during a thermal wave scan of opaque solids. It would also be desir~ble to provide an analysis -technique for use with a thermal wave scan of opaque solids which generates a vi~ual image of the surface and immediate subsurface of the solid illustrating any cracks, flaws, which may exi~t within the solid. ~inally, it would be desirable to provide an analysis technique for use with thermal wave imaging of opaque solid~ which generates a visual image of the exis-tence of any crack~ or flaws within the sample luring real time when the sampla is being probed by the thermal wave scan.

%

There is di~closed herein a unique thermsl wave imaging apparatus which includes unique data acqui~ition features for genera-ting an on-line, real-time image which is useful in detecting the presence of any surface or subsurface cracks, flaws or voids in an opaque solid object.
The -thermal wave imaging apparatus o~ the present inven-tion can be used with any imaging process including ga~
cell, photothermal displacement, mirage effect, infrared detec-tion, piezoelectric detection and photoacoustic detection.

By way of example only, the present invention will be described in use with a mirRge effect imaging apparatus.
As i8 conventional, in the mirage effect technique, a heating laser generate~ an output which is intensity modulated to provide a periodic optical signal used to periodically heat a point on the surface of an object. The optical beam from a probe laser pa~ses parallel to the surface of the object through the heated zone. This probe beam i~ deflected from a normal path due to density variations in the air above the surface of the sample caused by uneven heating due to the presence of surface or subsurface cracks, flaws, voids, etc., in the object. The amount of deflection of the probe beam is detected to provide an indica-tion of the existence of any surface or subsurface defects in the object.
According to the present i~vention, the deflection data are converted to digital signals and stored in an image memory under the control of a central processor. The stored defection data are u~ed to control the intensity of poin-ts or pixels on a display monitor.
Separate means are provided for generating ~equential, incremental signal~ used to control the X snd Y axis defection of -the moni-tor. Such signals are also used to address the image memory and to output the\refrom -the stored data at each address location corresponding to each generated X and Y deflection datum to control the intensity of the displayed point or pixel on the monitor. In this manner, as the ob;ect is probed point by point across its surface, a real time, on-linP image is generated on the monitor which provides a visible indication of the presence of any surface of subsurface cracks, flaws, voids, etc. in the ob~ect.

The thermal wave imaging apparatus o~ the present invention also includes a unique feedback circuit which controls an acousto-optic modulator such that the lntensity or amplltude of -the heating optical beam directed onto the surface of the ob~ect remains constant despite any fluctuations or varlations in the output of the heating laser itself.

Thus according -to the present invention in a thermal wave imaging apparatus for detecting surface and subsurface cracks, flaws and voids in opaque solid ob~ects in which A.C.
electrical signals indicative of the configuration of the surface and subsurface of an opaque, solid ob;ect are generated by a thermal wave scan of the ob;ect in which a first laser generates a heating energy ~eam directed through the ob;ect to generate a surface temperature gradient, a second optical probe beam is directed through the ob~ect and deflected by the surface temperature gradient, and means for generating the A.C.
electrical signals indicative of the amount of deflection of the second optical probe beam, the improvement comprising: means for converting A.C. electrical signals to digital signals; memory means for storing the digital signals; central processor means for controlling the transfer of the digital signals to the memory means; means for displaying an image of the surface and subsurface of the ob~ect; and means for generating control signals for controlling the X and Y axis point deflection of the displaying means and for addressing the memory means to output thereErom the digital signals to control the intensity of each displayed point on the displaying means; modulation means for modulating the laser output beam to a pulsed beam which strikes and hats a localizPd point on the object: means for controlling the amplitude of the output beam from the modulation means, the means for controlling the amplitude of the optical output beam of the modulation means comprises- means for detecting the amplitude of the op-tical beam output from the modulation means; means for '~ generating an RF slgnal for controlling the modulation means; and means, responsive to the detecting means, for attenuating the RF
signal so as to control the modulation means such that the output optical beam from the modulation means remains at a constant amplitude despite fluctuations in the output optical beam from the laser.

The thermal wave imaging apparatus of the present invention overcomes may of the deficiencies of previously devised 1~ techniques for analyzing data generated by the various thermal wave imaging techniques. The data acquisition apparatus of the present invention uniquely enables a real time, on-line image to be generated to provide a visible indication of the presence of any surface or subsurface cracks, flaws, voids, etc. in the 2~ ob~ect.
The various features, advantages and other uses o the present invention will become more apparent by referring to the following detailed description and drawing in which:

Figure 1 is a pictorial representation of the temperature profile generated by a mirage effect thermal wave imaging technique:

Figure 2 is a pictorial representation showing a mlrage 3~ effect thermal wave imaging apparatus;

Figure 3 is a block diagram of the laser beam modulator and intensity regulation circuit shown in general Figure 2;
3~
- 4a -8~:
Figure 4 i~ a block diagram showing the data acquisition and imaging sy~tem of the present invention;
Figure 5 is a detniled ~chematic and block diagra~ of one data acquisition channel; and igure 6 is a schematic and block diagram of the image memory subsystem shown in general in Figure 4.
Throughout the following description and drawing, an identical reference number is utilized to refer to the same component shown in multiple figures of the drawing.

The thermal wave imaging apparatu~ of the present invention i3 configured -to control the acquisition of data during a thermal wave scan of an opaque solid object and to ; display an image of surface and subsurface of the object 3howing any cracks, flaws, void or other defects in the surface and subsurface of the object. The apparatus of the pre~ent invention may be used with any imaging technique including, but not limited to, gas cell monitoring, photothermal displacement, mirage effect detection, infrared detection, photoacoustic and piezoelectric monitoring. In each o-f these imaging techniques, ac electrical s:ignals are generated by a detector, such as a microphone or pho-totransi~tor, which can be analyzed to provide information about the structure of the object.
By way of example only, the apparatu~ of the present invention will be de~cribed in conjunction with apparatus for u~ing the mirage effect thermal wave imaging technique. It will be understood, however, that the present invention may be employed with any thermal wave imaging apparatus.
Before describing in detail a preferred construction of the thermal wave imaging apparatus of the present invention, a brieE description of the theory behind the mirage effect ; technique for thermal wave imaging will be initially described to provide a ba~ic understanding of -the principles employed in ~he thermal wave imaging apparatu~ of the prssent invention.
~ ccording to the mirage effect method of thermal wave imaging, as shown in Figure 1, an optical or laser heating beam 1 is u-tilized -to provide periodic, localized heating of a point 2 on an opaque solid object 3. Such teaching of the object crea-tes a temperature profile 4 above the surface of the object in which the density of the air just above and aro~lnd the laser focal spot varies with tempera-ture variations on the surface which in turn are influenced by variations in the ~djacent subsurface of the object. Thus, the presence of cracks, flaws, voids, on the surface or immediately below the surface of the object will cause density variations in the air above the ~urface of the object.
A second probe beam passing through this temperature profile parallel to the surface of the object will be deflected by such density changes in the air immediately above the surface of the object 3. Detection of these deflections 6, 7 and 8 of the second probe beam 5 can be utilized to provide an indication of the presence of a surface or sub3urface crack, etc.
Re~erring now to Figure 2, there is illustrated a thermal wave imaging apparatus which is constructed to make use of the mirage effec-t to detect surface and subsurface cracks, etc., in an opaque solid object. The apparatus 10 include~ 8 first heat source 12, such a8 a laser. Any type of laser 12, such as an argon-ion laser, may be employed in the apparatu~
10. Furthermore, the la~er 12 may be provided with any power output and in any wave length. Preferably, however, vi~ible wave length are employed for ease in aligning and adju3ting the apparatus 10.
As the laser 12 provides a continuous ou-tput beam 14, the beam must be perioclically interrupted or modulated to ~Y~7~
provide the desired periodic, localized heating of the objec-t.
Thus, the output beam 14 from the laser 12 i~ pa~sed through Qn acousto-op-tical chopper or modulator 16, whose output is a modulated optical bea~ 18. The beam 18 i3 directed onto an object 20 -through a len~ 21 to cause the de~ired periodic localized heating of a point on the surface of an opaque, solid object 20.
A second probe laser 22 i9 oriented such that its output beam 24 focused by lens 25 passe~ through the heated areQ or temperature profile generated by the fir~t laser 12 nnd substantially parallel to the surface of the sample 20. As noted above, deflections of the output beam 24 caused by density variations in the air im~ediately above the surface of the object 20 can be detected by means of a photodetector 26, such as photodiode array. As is conventional, the photodlode array 26 includes -two pairs of perpendicularly oriented photodiode~.
The diodes in each pair are electrically connected such that an output signal will be generated on line 28 which will indicate by means of i-ts magnitude the point on the photodetector 26 on which the probe beam 24 impirges.
According to one feature of the subject invention, the thermal wave imaging apparatu3 10 is provided wi-th a feedback circuit to provide a constant amplitude for the modulated laser beam 18 despite nny fluctuations or variation~ which may occur in th0 output beam 14 of the heating laser 12. In effecting this feedback, Q portion of the optical beam 18 from the modu-lator 16 is split by means of a conventional beam splitter 30, such as a partially reflec-tive mirror, which deflects a portion 32 of the optical beam 18 to the feedback circuit 34.

In gener~1, -the beam modulator and intensity regulation circuit 34 controls the modulator 16 in such a way -that the amplitude of the output ~eam 18 remains constan-t in~pite of any fluctuations or varia-tions in the intensity of the outpu-t optical beam 14 generated by the laser 12.
~ s shown in greater detail in Figure 3, the split portion 32 of the optical beam strikes a photodiode 40 which generates an electrical output signal proportional to the intensity or amplitude of the beam 32. The output from the photodiode 4~ passes -through a low-pass filter circuit 42 to a gnin control circuit 44. Another input to the gain control circuit ~4 is a signal from an offset control circuit 46 to provide reference levels for the feedback signal.
At the same time, a square wave signal 50 at the desired modulation frequency is fed to a conventional RF switch 54 which modulates the output signal from a RF ~ignal genera-tor 56. The amount of RF signal which i3 passed to the modulator 16 is determined by means of an RF attenuator circuit 58 which receives the modulated RF signal from the RF switch 54 and the outpu-t of the gain control 44, with the magnitude o~ the output of the ~ain control 44 being proportional to the difference be-tween the output signal from the low-pass filter 4Z and the dc level of the signal from the offset control ~6. The output of the attenuator 58 is amplified and fed to the acousto-optic modulator 16.
The acousto-optic modulator 16 is constructed in a conventional manner to diffract a portion of the input optical laser beam 14 into the output beam 18 only when an RF signal is present. In the ab~ence of an RF signsl as determined by the RF
switch 5~ described above, the output optical beam 18 is interrupted. In the presence of an RF signal, the intensity of the output beam 18 is controlled -to a fixed level set by -the dc offset control 46. In this manner, the intensity of the output optical beam 18 from the acousto-optic modula-tor 16 remains z cons-tant despite any fluctuations or variatlons in the output optical beam 1~ from the laser 12.
Referrirlg now to Figure 4, there i8 illustra-ted a block diagram of a data acquisition and contrGl system 80 which controls the acquisition of d~ta -from the photo- detector 26 and the generation of a visible image which displays the presence of any surface cracks, etc., on the object 20. The data acquisi-tion system 80 is controlled by a central processor unit 82 which can be any conventional microprocessor such HS a microprocessor sold by Motorola, Model No. 6800. The central processor 82 functions to control the tran~fer of data between the various subsystems of the data acquisition system 80 by genera-ting appropriate timed, control signals. In controlling such data transfer, the central processor 82 executes a stored control program shown in Appendix A.
A~ illustratad in Figure 4, the output 2~ from the photodetector 26 together wit the reference signal 50 is input to a lock-in amplifier 84 which generates two outputs, one indicating the magnitude of the output from the photodetector 26 and the other the phase of the output ~ignal from the photodetec-tor 26 relative to that of the reference signal 50.
Alternately, the two outputs can indicate the in-phase and quadrature components of the output from the photodiode 26. The outputs from the amplifier 8~ are input to two separate data acquisition channels 83 and 86. labeled channel one and channel two, respectively, corresponding to -the pha~e and magnitude or the in-phase and quadrature outputs from the photodetector 26.
Only one of the data acqui~ition channels 83 and 85, such as the firs-t channel 83 corresponding to the magnitude signal from the photodetector 26, will be described in greater detail hereafter since both data acquisition channels 83 and 85 are identically construc~ted. As shown in Figure 5, the output ~7~ 2 from the amplifier 84 is input to an ampli*ier 86 whose output is input to an A~D converter 88.
The A/D converter 88 i~ a~signed to three addres~
locations of the central processor 82, one for starting analog conver3ion, one for reading the busy state of the A/D con~erter 88 and a third for reading the converted data. In operation, the central processor 82 initiates a data conversion by sending a signal on control line 92 to the A/D converter 88, wait~ for a busy state completion signal on con-trol line 94 -from the A/D

converter 88 and then generate~ a read data signal on control line 90. The output from the A/D converter 88 is the input on data bus lines 96 to the central processor 82.
The output data signals from the central processor 82 on lines 96 are input to two D/A converters 98 aDd 100. Each of the D/A conv0rters 98 and 100 is assigned a single address and behaves as a random access memory for the central processor 82.
A number written in the D/A converter 98 is converted into an analog voltage which is summed by the amplifier 86 with the analog input signal from the lock-in amplifier 84. This provides a programmable offset. Control line 104 is the chip select line and line 106 is the read/write control line for this operation. In a similar fashion, the D/.A converter 100 provides a programmable voltage which is input to the A/D converter 88 as a reference voltage for conversion, thereby providing a programmable gain control. Control lines 102 and 106 are the control lin0s for this operation.
Referring again to Figure 4, the output data from the first data acquisition channel 83 is transferred under the contro]. of signals generated by the central proce~sor 82 to an image memory ~ubsystem 110. The image memory subsystem 110 serves as an image memory for a digi-tized microscopic pic-ture of the objec-t 20. The output of the memory contained with the image memory subsystem 110 is continuously displayed on a monitor 112 which, in a pref~rred embodiment, is a high resolution, -fla-t ~creen display with a linear intensity response so as to generate a high quality picture suitable for photography. Furthermore, the data conten-t of the image memory subsystem 110 is displayed in gray scale on the monitor 112 with 256 di~ferent intensity levels Eor each pixel and a total of 65,536 pixels.
In general, the basic cycle of the central processor 82 is divided into two hslves~ with the central processor 82 addressing memory only during the first half of each cycle.
Then, during the second half of each cycle, the address line~ of the memory are multiplexed to the output of a 16-bit refresh counter which is constantly incremented by the CPU clock.
Referring now to Figure 6, there is shown a detailed block diagram of the image memory subsystem 110. The image memory subsystem 110 includes a random nccess memory 120 which, in a preferred embodiment, includes 64~ of 8 bit memory locations. During each half cycle, the central processor 82 will generate sequential addresses on addre3s bus 122 which are input to an address multiplexer 12~. The address multiplexer 124 controls the selection of addresses to be used -to address -the memory 120. During the memory read or write half cycle of the central processor 82, the address multiplexer 124 will select addresses -from address bus 122 so as to direct the data on data bus 96 to the appropriate locations within the memory As shown in Figure 6, the data bus 96 is also input to a data line multiplexer 128 which controls the flow of da-ta either between the central processor 82 and the memory 120 or between the memory 120 and the D/A converter 144.

8~2 In this manner, data corresponding to the magnitude or phase o-f the deflection of the probe beam for each sequentially sampled spot on a surface of -the object 20 i~ stored in sequential memory locations within the memory 120. The magnitude or phase of the deflection of the probe beam corresponds to the intensity of the displayed point image on the monitor 112. In a preferred embodiment, the two data acquisition channels 83 and 86 operate in parallel so that both magn.itude and phase images appear on the monitor 112 simultaneou~ly. Alternately, only one of the channels 83 and 85 may be activa-ted to di~play only a ~ingle image.
The image memory subsystem 110 also includes a control multiplexer 130 which controls the read-write mode of the memory lZ0 as well as the selection of addres~e~ by the address multiplexer 124 and the data multiplexer 128.
A 16-bit refresh counter 140 ~enerotes a new 16-bit address upon each ENABLE signal from the central processor 82.
The address multiplexer 124 will select the output from the 16-bit counter 140 during each non-read or write h~lf cycle of the central processor 82. The output of the counter 140 is input to two D/A converters 142 which convert two 8-bit signals from the counter 140 to two signals u~ed to control the X and Y
deflection of the monitor llZ. This controls the position of the next point to be displayed on the monitor 112. At the same time, the ~ddres~ generated by the counter 140 is input through the address multiplexer 124 to the memory 120. Data stored at the ffpecified address location i~ output through -the data lirle multiplexer 128 to D/A converter 144. The output signal from the D/A converter 14~ is smplified and fed to the 30 monitor 112 to control the intensity of the point being displayed on th~ monitor 112.

~7~3~342 ~ 190 shown in Figure 6 are mode control signals Mo~ M
nnd M2 which are input to gates 146, 148 and 160. The mode control signals M~, Ml and Mz are generated by the csntral processor 82 and are used to control the size of the display image on the monitor 112. For small sample sizes, only one-eight, one-quarter or one-half of the display screen need be used. Thus, depending upon -the binary code input on line~ Mo~
Ml and M~ to the 3elected mode control gates 146, 14~ and 150, onLy one-eighth, one-quarter, one-hal~ or a full screen will be displayed. When a partial ~creen on the monitor 112 is displayed, the monitor 112 will be refreshed at a faster rate which reduces the flickering of the image.
In summary, there has been disclosed a unique thermal wave imaging apparatus which generates an on-line, real time image of surface and subsurface crac~s, flaw , etc. OD an opaque ~olid which is probed by means of a thermal wa~e imaging technique. A laser beam modulation and regulation control circuit has also been disclosed which generates a modulated optical beam of a const~nt amplitude or intensity inspite of any fluctua-tions or variation3 in the output beam of a laser.

s

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a thermal wave imaging apparatus for detecting surface and subsurface cracks, flaws and voids in opaque solid objects in which A.C. electrical signals indicative of the configuration of the surface and subsurface of an opaque, solid object are generated by a thermal wave scan of the object in which a first laser generates a heating energy beam directed through the object to generate a surface temperature gradient, a second optical probe beam is directed through the object and deflected by the surface temperature gradient, and means for generating the A.C. electrical signals indicative of the amount of deflection of the second optical probe beam, the improvement comprising: means for converting A.C.
electrical signals to digital signals; memory means for storing the digital signals; central processor means for controlling the transfer of the digital signals to the memory means; means for displaying an image of the surface and subsurface of the object; and means for generating control signals for controlling the X and Y axis point deflection of the displaying means and for addressing the memory means to output therefrom the digital signals to control the intensity of each displayed point on the displaying means; modulation means for modulating the laser output beam to a pulsed beam which strikes and hats a localized point on the object; means for controlling the amplitude of the output beam from the modulation means, the means for controlling the amplitude of the optical output beam of the modulation means comprises:
means for detecting the amplitude of the optical beam output from the modulation means; means for generating an RF signal for controlling the modulation means; and means, responsive to the detecting means, for attenuating the RF signal so as to control the modulation means such that the output optical beam from the modulation means remains at a constant amplitude despite fluctuations in the output optical beam from the laser.