ACOUSTO-OPTIC TUNABLE FILTER-BASED SURFACE SCANNING SYSTEM AND PROCESS

Abstract: A scanning system (10) for inspecting a surface (16) including a light source (30) which generates a beam of light (32) that is reflected, scattered or causes fluorescence at the surface to be inspected. An optical interface (14) receives the beam of light and directs it along a predetermined path extending to and from the surface. An acousto-optic tunable filter (34) tuned to pass light having a wavelength corresponding to a known optical property of a predetermined material is positioned within the path of light. A detector (42) is positioned to receive light emanating from the surface and is configured to monitor the intensity of light at each predetermined wavelength being monitored and generate a corrsponding signal. The system is preferably attached to a scan board (90) thereby enabling the system to be used in scanning a surface. The system also includes a signal processor (22) which processes the signal generated by the detector. The resulting data is displayed by an output device (26).

Description: ACOUSTO-OPTIC TUNABLE FILTER-BASED SURFACE SCANNING SYSTEM AND PROCESS BACKGROUND 1. The Field of the Invention The present invention is related to a system and process for inspecting surfaces. More particularly, the present inven¬ tion is related to a system for obtaining near real time, non¬ destructive detection and evaluation of various materials on surfaces by directing light at the surface and analyzing the in- tensity and polarity of the light emanating from the surface at a wavelength corresponding to a known optical property of a pre¬ determined material. 2. Technical Background A typical manufacturing process utilized in many applica- tions is the bonding of two materials. The criticality of the strength of the bond will vary depending on the particular ap¬ plication for which the bonded material is to be used. For example, in the manufacture of solid rocket motors, bond strength is particularly critical. The bonds in a solid rocket motor can be subjected to for¬ ces of high magnitude due to acceleration, ignition pressuriza- tion and thermal loads. A weak bond or area of debonding can be the source of stress risers which can result in further weaken¬ ing of the bond, eventually leading to failure of the bond, and can distort the geometry of the bonded material thereby adverse¬ ly affecting the firing characteristics of the motor. In the manufacture of a solid rocket motor, a variety of materials must be successfully bonded to one another. For exam¬ ple, some of the bonds found in a typical solid rocket motor are the bond between the case and the insulator, between the insula¬ tor and the liner, between the liner and the propellant and between the nozzle phenolic and the metal nozzle housing. A weak bond or debond in any of these bonds could result in catas¬ trophic failure of the rocket motor. When two materials are bonded together, contaminants on the surface of either of the materials can weaken the bond and, in some instances, cause areas of debonding. Organic materials such as greases, hydraulic fluids and mold release agents are the primary source of contamination of bonding surfaces in solid rocket motors. Other contaminants include particulates such as sand or dust. Oil vapors are often present in environments where hydraulic systems and electric motors are present. These vapors can condense on surfaces to be bonded. Even small levels of these contaminants, not visible to the human eye, can degrade bond strength. The extent to which a surface can be cleaned prior to bond- ing and the method to be utilized in cleaning the surface vary according to the nature of the surface. For example, the rocket case of the space shuttle is a grit-blasted steel surface. It is typically cleaned by a vapor degrease process. According to one such process, the case is suspended within a pit in the bot- torn of which boiling methylchloroform is located. The methyl- chloroform evaporates and condenses on the rocket case. As it runs off the rocket case, it dissolves any grease in its path. While this process works well in cleaning small amounts of grease from the rocket case, if there are areas of localized buildup of grease, not all of the grease may be removed by the cleaning process. Using a solvent such as methylchloroform to clean a bonding surface may not be viable if the bonding surface is a phenolic material. In a solid rocket motor the nozzle is typically made of a phenolic material. The nozzle is made by wrapping uncured tape onto a mandrel, permitting the tape to cure and then ma¬ chining the part into the desired shape. Phenolic materials will absorb virtually any type of clean¬ ing solvent with which they come into contact. These solvents can alter the surface chemistry and/or carry dissolved contami¬ nants into the phenolic. In applications such as those discus¬ sed herein, the surface properties of the phenolics must remain unchanged. Presently, the preferred method of cleaning a phenolic material is to place it on the mill and machine a new surface, thereby removing the contaminated surface. However, this can only be done if the tolerances of the part permit a portion of the surface to be removed. Otherwise, a contaminated part may have to be replaced. Because even small levels of contaminants, not visible to the human eye, can degrade bond strength, bonding surfaces must be inspected prior to bonding to ensure that there is no con¬ tamination, or that if there is contamination, it is within acceptable limits. A crude method of conducting a surface inspection is to place some solvent on a wipe and stroke the surface with the wipe thereby transferring surface contaminants to the wipe. The wipe may then be analyzed using standard spectroscopy methods to verify the existence of contaminants on the wipe and determine their identity. A principal obstacle to the successful use of this method is that it can only be used as a check method. It cannot be used as an inspection method on the entire bonding surface. And, while the method may provide information about the exis¬ tence of a contaminant and its identity, it cannot be used to determine the thickness of the contamination. It is a qualita¬ tive method and therefore does not provide a quantitative meas¬ urement of the contamination. Additionally, this method cannot be used with phenolic materials because the surface chemistry of the phenolics would be altered by passing a wipe permeated with solvent over it. A more versatile surface inspection method is to conduct a visual inspection with the aid of an ultraviolet light. Some contaminants, particularly grease such as that used for rust protection, fluoresce under ultraviolet light. Thus, by visual¬ ly inspecting the surface under ultraviolet light, any contami¬ nants which fluoresce under the light can readily be detected. A disadvantage of this method is that the method cannot be reliably used to detect low levels of contamination as it is limited by what can be seen with the human eye. Additionally, this method, being manual in nature, does not provide machine- readable data. Consequently, the person performing the visual inspection must attempt to record the location and size of the contaminated area. As with many manual methods, the possibility of human error renders this method inadequate for many applica¬ tions. Automated inspection methods include an optically stimulat¬ ed electron emission ("OSEE") method. This method is based on the photoelectric effect. By shining ultraviolet light on the surface to be inspected, electrons are emitted from the surface. By placing an electrode near the surface and raising the elec¬ trode to a predetermined voltage, an electric field is generat- ed, drawing an electron current from the surface whose strength can be monitored. If there is contamination on the surface, the current is impeded. A disadvantage with the OSEE method is that it is subject to many variables which are not relevant to the determination of contamination. Such variables may include air currents surrounding the device being tested, relative humidity and moisture on the surface. Also, the OSEE method only works effectively on metals. It is ineffective as a tool to inspect phenolic or rubber surfaces. Thus, it would be an advancement in the art to provide a system for the inspection of bonding surfaces which would detect the presence of thin films, including low-level contamination or surface coatings, which may not be detectible with prior-art visual inspection methods. Indeed, it would also be an advancement in the art if such a surface inspection system could work effectively to detect contamination on a variety of surfaces and with different levels of roughness, including metal, phenolic and rubber surfaces. It would be yet a further advancement in the art to provide such a system that could work efficiently and effectively in inspecting large surface areas. Such a system for inspecting surfaces is disclosed and claimed herein. BRIEF SUMMARY AND OBJECTS OF THE INVENTION The present invention is directed to a novel system for inspecting surfaces to detect and characterize thin films, in¬ cluding contaminants. The system includes a light source ca¬ pable of generating a beam of light and an optical interface for receiving the beam of light from the light source. The optical interface directs the beam of light along.a predetermined path extending to and from the surface. An acousto-optic tunable filter is positioned within the path of light and is tuned to pass light having a wavelength corresponding to a known optical property of the material for which inspection is sought. Such optical properties may include traditional physical properties, such as absorption characteristics, as well as other, more gen¬ eral properties, such as spectral signatures which are indica¬ tive of a particular material. A detector is positioned to receive light emanating from the surface. The detector is capable of monitoring the inten¬ sity of light at at least one predetermined wavelength and gen¬ erates a signal corresponding to the intensity of each wave¬ length being monitored. The signal generated by the detector is fed into a signal processor which processes the signal and gen¬ erates data concerning the characteristics of the surface. The system also includes means for moving the system relative to the surface such that the surface may be scanned with the beam of light. In one embodiment, the system may be used to detect and measure thin films, such as contamination or coatings, for which absorption properties are known. A presently preferred system includes a light source optimized for near to mid infra¬ red wavelengths. The incident beam of ligh_ is passed through a spectrometer having an acousto-optic tunable filter. The spec¬ trometer is preset to monitor the absorbance of at least the absorption band of one predetermined material and at least one reference band outside the absorption band. An optical interface is provided to receive the incident beam of light from the spectrometer and focus it onto a discrete location on the surface to be inspected. The optical interface is also configured to gather a portion of the beam scattered off the surface and direct it into a detector. The detector gener¬ ates a signal corresponding to the intensity of the detected light and transmits that signal to a computer for processing. The data processed by the computer is preferably translated into a graphical image by an output device, either in the form of a color (including a gray scale) image/display or a surface map of the contamination. For rough metal surfaces, including machined or grit-blast¬ ed metal surfaces, the optical interface is preferably adjusted to gather a portion of the back-scatter component of the scat¬ tered beam. For smooth surfaces or rough non-metallic surfaces, it is presently preferred to adjust the optical interface to gather a portion of the specular component of the scattered beam. The angle of incidence for smooth surfaces and rough non- metallic surfaces is chosen to be at or near the Brewster angle. The incident beam is polarized when it is passed through the acousto-optic tunable filter. The filter separates the beam into two orthogonal components of linearly polarized light which exit the filter at different angles. In a preferred embodiment, the optical interface includes a partition positioned to block one of the components of polarized light from being directed onto the surface. It is currently preferred that the incident beam be vertically polarized, i.e., that component of the inci¬ dent beam which is polarized parallel to the incident plane of light. When utilizing a polarized incident beam, the gathered portion of the scattered beam is preferably passed through an analyzing polarizer. The orientation of the analyzing polarizer with respect to the incident polarized beam may be adjusted to maximize the ability to detect absorbance. When inspecting rough metal surfaces, it is preferred to orient the analyzing polarizer to pass the 90 degree depolarized portion of the beam. In a preferred embodiment, a scanning apparatus is employed to rapidly change the point on the surface at which the beam of light is directed, thereby permitting the inspection of various locations on the surface or of large surface areas. By synchro- nizing the signal processing and the scanning of the surface, data concerning materials on the surface is generated. In one embodiment of the invention, successful scanning for contamina¬ tion has been accomplished by directing the beam of light at discrete locations on the surface which are spaced about 0.10 inches apart and changing the point on the surface at which the beam of light is directed about every 0.01 seconds. To obtain data concerning the thickness of a material on the surface as well as the existence of the material, an em¬ bodiment of the invention measuring absorbance of the incident beam of light is utilized in combination with calibration plates. Such calibration plates may include one plate with no contamination and one plate with a known amount of contamina¬ tion. By scanning calibration plates prior to inspecting a surface, the linear relationship between absorbance and thick- ness of contamination may be determined. Because the thickness of the contamination is proportional to the absorption band size, once the linear relationship between absorbance and thick¬ ness is defined, the thickness of the contamination may readily be determined. In another embodiment of the invention, the infrared light source is replaced with an ultraviolet light source capable of generating an incident beam of light including wavelengths in the ultraviolet range, i.e. generally from about 150 nm to about 400 nm. The incident beam is preferably polarized with a polarizer before being directed onto the surface. Also, it is preferred to modulate the incident beam with a chopper wheel so that the effects of ambient light may be eliminated. The polarized incident beam of ultraviolet light is direct¬ ed onto the surface by the optical interface. Upon striking the surface, the ultraviolet light including light in the fluores¬ cence inducing wavelength of the surface causes excitation of valence electrons inducing them to temporarily jump to a higher energy state. The fluorescence inducing wavelength is that wavelength of light which causes the material for which inspec¬ tion is sought to fluoresce. Upon dropping to an intermediate energy state, photons in the visible spectrum corresponding to the fluorescent wavelength of the material are emitted from the surface. Because the wavelength of the emitted fluorescent light generated by this phenomenon is characteristic of the material producing it, the existence of a particular material on the surface can be ascertained by monitoring for light at a fluorescent wavelength of that material. In this embodiment which utilizes an ultraviolet incident beam of light, the optical interface is also configured to gather at least a portion of the light emitted from the surface. The acousto-optic tunable filter is positioned to receive the gathered portion of the fluorescent beam and is tuned to pass light corresponding to the fluorescent wavelength of the mate¬ rial for which inspection is sought. Because of the positioning of the acousto-optic tunable filter, it acts as an analyzing polarizer. Thus, the acousto- optic tunable filter polarizes the gathered fluorescent beam and separates it into two orthogonal components of linearly polar¬ ized light which exit the filter at two different angles. Detec¬ tors are positioned to receive each component of polarized light transmitted by the acousto-optic tunable filter and generate a signal corresponding to the intensity of the detected light. In accordance with the teachings of the present invention, the light source, optical interface and acousto-optic tunable ilter may be mounted on a scan board and included as part of the end effector of a robotic arm or other apparatus to accom- plish scanning of the surface to be inspected. So configured, the system of the present invention may be utilized to provide near real-time data concerning the characteristics of a surface. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic of the components comprising one embodiment of the surface scanning system of the present inven¬ tion. Figure 2 is a schematic illustrating the components com¬ prising the spectrometer and the optical interface of the system of Figure 1 and illustrating a plan view of the path of the beam of light through the system. Figure 3 is a perspective view of the paraboloid mirror and inspection surface of Figure 1, illustrating how a portion of the back-scatter component of the scattered beam is gathered by the mirror. Figure 4 is a plan view of one embodiment of the present invention illustrating how a portion of the specular component of the scattered beam is gathered. Figure 5 is a schematic illustrating an alternative embodi¬ ment of the present invention. Figure 6 is a graph charting the amount of absorbance measured on a rough metal surface as a function of angle of orientation of the analyzing polarizer. Figure 7 is a schematic illustrating an additional alterna¬ tive embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference is now made to the figures wherein like parts are referred to by like numerals throughout. With particular refer¬ ence to Figure 1, one embodiment of a system for inspecting a surface for contamination in accordance with the present inven¬ tion is generally designated at 10. The system of the present invention may be used to inspect for a variety of materials for which certain optical properties are known or can be ascer¬ tained. Indeed, because of the use of the acousto-optic tunable filter in the system of the present invention, near real-time analysis may be conducted for a variety of materials having an optical property characterized by a signature wavelength. By way of illustration, such optical properties may include absorp- tion characteristics or fluorescence inducing characteristics. Other optical properties may also be utilized within the scope of the present invention. The present invention is particularly useful when the material for which inspection is sought is known or suspected to be found on the surface. For example, in the production of solid rocket motors wherein data concerning contamination on bonding surfaces is needed, inspection may be conducted for specific contaminants such as silicone mold release agents. In a manufacturing facility, the existence of hydraulic systems or electric motors frequently leads to the presence of oil vapors in the ambient air which condense on bonding surfaces. By utilizing the present invention, whether these vapors have con¬ densed on bonding surfaces can be ascertained. Indeed, the present invention has been used successfully to inspect for oil and grease, such as HD2 grease commonly used for rust protec¬ tion. In one embodiment, the system 10 of the present invention includes a spectrometer having an acousto-optic tunable filter 12, sometimes referred to herein as an "AOTF spectrometer." It has been found that an AOTF spectrometer is capable of providing an optimal combination of fast processing time and spectral res¬ olution. In a presently preferred embodiment of the invention, spectrometer 12 is a solid state spectrometer based on the acousto-optic tunable filter, such as is marketed by Infrared Fiber Systems, Inc. of Silver Spring, Maryland. In communication with the spectrometer 12 is an optical interface 14. As explained below in greater detail, the optical interface directs a beam of light from the spectrometer 12 to a surface 16 being inspected. It also collects a portion of the scattered beam and directs it into the spectrometer for analy¬ sis. In one embodiment of the present invention, the surface or substrate 16 being inspected is supported by a scan table 18. The scan table is controlled by a scan controller 20. Scan table 18 and scan controller 20 may be any of those controllers and tables which are commercially available, such as the 4000 Series controller and the HM-1212 table, both of which are sold by Design Components, Inc. of Franklin, Massachusetts. In accordance with the embodiment of the present invention illustrated in Figure 1, the spectrometer 12 and optical in¬ terface 14 are held in a stationary position while the surface 16 being scanned is moved by the scan table 18. While such an embodiment is presently preferred for a laboratory scale model of the invention wherein small surfaces are being scanned, it is not the preferred embodiment if the surface to be inspected is a large surface, such as the bonding surfaces in a large solid- rocket motor. Thus, it will be appreciated by one of skill in the art that the spectrometer 12 and optical interface 14 may be util¬ ized in combination with a robotics system to accomplish surface inspection of large surfaces. In such an embodiment, the sur¬ face to be scanned is held in a stationary position while the spectrometer and optical interface are moved relative to the surface to obtain data from various discrete locations on the surface. A signal processor such as a computer 22 is provided to control the motion of the scan controller 20 and process the signal produced by the spectrometer 12. Use of computer 22 permits the synchronization of the motion of the scan controller 20 with the processing of data acquired from the spectrometer 12, thereby providing information concerning the location of any contamination detected on the surface 16 during scanning. Com- puter 22 may be any type of computer commonly known among those skilled in the art for use in this type of application. An IBM- AT compatible computer has been found to work satisfactorily. An analog-to-digital converter 24 is provided between the AOTF spectrometer 12 and the computer 22 for converting the analog signal generated by the spectrometer into a digital sig¬ nal which can be processed by the computer 22. It will be ap¬ preciated by one of skill in the art that analog-to-digital con¬ verter 24 may be integral with either the spectrometer 12 or the computer 22, as many AOTF spectrometers currently available on the market are equipped with such a converter. Alternatively, the converter 24 may be a separate component of the system 10. An output device 26 is provided in communication with the computer 22 for providing a display of the data generated during the examination of surface 16. The output device 26 may include any device known among those skilled in the art for displaying data, including a video monitor or plotter. It may provide the data in either human-readable or machine-readable form. In one embodiment of the present invention, an EGA color graphics system has been found to provide satisfactory output. The display of data may be accomplished in either graphical or numerical form. In a presently preferred embodiment of the invention, the data is displayed formatted in a manner to illus¬ trate a surface map or a color scale image of the contamination. For graphical output, a color monitor may be used to display contour corresponding to various preassigned colors. Alterna¬ tively, a similarly formatted output may be illustrated in shades of gray. As illustrated in Figure 2, the AOTF spectrometer 12 in- eludes a light source 30 which generates a beam of light 32. In this embodiment, light source 30 is preferably a quartz, halogen lamp such as that made by Gilway Technical Lamp of Woburn, Mass¬ achusetts. Such a light source 30 is optimized for near to mid infrared wavelengths. In most commercially available AOTF spec- trometers, light source 30 will be housed within the spectromet¬ er. The spectrometer 12 is configured such that the beam of light 32 passes through the AOTF crystal 34 within the spectro¬ meter. The crystal 34 acts to filter out all wavelengths of light from the beam 32 except those to be monitored by the sys¬ tem 10 during the surface inspection. Before the beam 32 exits the AOTF spectrometer 12, the beam is transformed into a collimated beam. Upon its exit from the 5 spectrometer 12, the collimated beam of light 32, including only those wavelengths of light to be monitored during the surface inspection, comes into contact with a first paraboloid mirror 36. First mirror 36 focuses the beam onto the discrete location on the surface 16 to be inspected. In this embodiment of the 10 invention, first mirror 36 acts both to focus the incident beam on the surface and to gather a portion of the scattered compo¬ nent of the beam. If the surface 16 to be inspected is a rough surface, such as is the case with most metal surfaces, first paraboloid mirror 15 36 is preferably positioned with respect to the surface such that it will gather a portion of the back-scatter component of the scattered beam, as is illustrated in Figures 2 and 3. As used herein, a surface is considered to be "rough" if its RMS (root mean square) roughness is on the order of a wavelength or " 20 greater than the wavelength of the light being employed by the method used to evaluate the surface. If the surface being evaluated is one-dimensionally rough, as may be the case with a metal surface that has been machined, first paraboloid mirror 36 is preferably positioned with respect 25 to the surface such that the incident beam is perpendicular to the parallel lines which comprise the roughness. One of the principal advantages of the present invention is that even if the surface is randomly rough, such as a grit-blasted metal sur¬ face, by positioning the paraboloid mirror 36 to collect a por- 30 tion of the diffuse reflectance of the incident beam, meaningful data may be obtained from which contamination may be detected. Particularly where the surface roughness is fairly uniform, the effect roughness may be removed from of the data when the signal is processed. 35 Importantly, in accordance with the teachings of the pres¬ ent invention, surface roughness actually enhances the ability of the system of the present invention to detect and quantita¬ tively measure surface contamination. Generally, the sensiti¬ vity of the present invention in detecting and measuring conta- mination is proportional to the intensity of the electric field created by the incident beam at the surface. Hence, as surface roughness increases, there is greater tendency for multiple scattering of light to occur at the surface which results in increased intensity in the electric field at the surface. Because of this ability to successfully inspect rough sur¬ faces, the present invention may be used to inspect surfaces of phenolic materials —materials which have proved particularly difficult to inspect by other methods. Carbon phenolics, for example, which have a surface which is generally treated as ran¬ domly rough even when machined, can be efficiently and effec¬ tively inspected by practicing the teachings of the present in¬ vention. For a rough metallic surface, such as that illustrated in Figures 2 and 3, it is presently preferred to direct the beam at the surface at an incident angle in the range of from about 30 degrees to about 40 degrees. The present invention may also be used on smooth surfaces, defined as surfaces having a RMS roughness less than the wave- length of light being used by the inspection method. For smooth surfaces, or rough surfaces of non-metallic materials, the first paraboloid mirror 36 is preferably positioned with respect to the surface 16 such that the mirror 36 will gather a portion of the specular component of the scattered beam, as illustrated in Figure 4. The angle of incidence a of the beam is at or near the Brewster angle. It is at the Brewster angle that the elec¬ tric field intensity near the surface is the strongest for the normal component of the electric field. For a typical polymer, the Brewster angle would be approximately 45 to 50 degrees at infrared wavelengths. The gathered portion of the scattered beam, whether it be taken from the back-scatter component (mirror 36 of Figures 2 and 3) or the specular component of the beam (mirror 48 of Fig¬ ure 4) , is converted back into a collimated beam and directed into a second paraboloid mirror (mirror 38 of Figure 2 or mirror 50 of Figure 4) . The second paraboloid mirror focuses the beam onto the detector 42 via a directing mirror 40. The detector signal is digitized by the analog-to-digital converter 24 and received by the computer 22 for analysis. The use of directing mirror 40 is optional. In a presently preferred embodiment of the invention in which a cryogenically cooled detector 42 is utilized, a directing mirror is employed because the beam must be directed horizontally into the detector to avoid spilling the liquid nitrogen used to cool the detector. It will be appreciated by one of skill in the art, however, that a variety of configurations may be employed in connection with the optical interface 14 to accomplish the purpose of the opti¬ cal interface —directing and focusing the beam onto the surface and gathering a portion of the scattered component of the beam and directing it back into the spectrometer. In operating this embodiment of the invention, the AOTF spectrometer 12 is initially set to monitor the absorbance band of a predetermined material. It is presently preferred that the band selected be that corresponding to the peak absorbance of the material sought to be located by the inspection. For exam¬ ple, if the material is a hydrocarbon, the absorption band is centered from between about three microns to about four microns, with 3.4 microns being preferable. In a presently preferred embodiment of the invention, the AOTF spectrometer 12 is set to inspect for a single material. However, if it is desired to simultaneously inspect for a variety of materials, the AOTF spectrometer could be set to monitor the peak absorbance of each. Simultaneously monitoring two or more materials may be even more practical as spectrometer technology improves to the point that AOTF spectrometers having a wider band capability become available on the market. The AOTF spectrometer should also be set to monitor at least one reference band outside of the absorption band of any of the materials being monitored. It is presently preferred that two reference bands be monitored, one on each side of the absorption band of the material being monitored. Monitoring a reference band provides a basis for evaluating the absorption band of the material to determine whether variations in the measured absorbance of the absorption band are due to the pre¬ sence of the material or due to external factors such as fluc¬ tuations or variations in surface roughness. For example, if the surface is being inspected for the presence of a hydrocarbon having an absorption band of 3.4 microns, preferred reference bands are 3.24 microns and 3.6 microns. If it is desired to inspect a surface for silicone release agents, an absorption band of about eight microns may be monitored. When inspecting for silicone release agents it is presently preferred to monitor an absorption band of 7.95 microns and monitor reference bands of 7.7 microns and 8.3 microns. Once the AOTF spectrometer 12 has been preset, the system is preferably calibrated prior to use. Because the relationship between the thickness of the material on the surface and the amount of absorbance is approximately linear, the zero point and slope of that linear relationship must be determined by calibra¬ tion in order to calculate the thickness of the material from the absorption data. Calibration is performed by obtaining a calibration plate made of the same material and having the same roughness as the substrate to be inspected. In a preferred embodiment, five pre¬ determined thicknesses of contamination are applied to approx¬ imately five different locations on the plate, thereby providing a sufficient number of data points that the relationship between absorption and thickness can readily be determined. The calib¬ ration plate should be representative of both the material type and the roughness level of the surface to be inspected. The system 10 should be calibrated each time the substrate to be inspected is changed. Also, each time the mirrors are adjusted or the angle of incidence of the beam is altered, the system should be calibrated to regenerate the calibration curve. With the system calibrated, it is ready to be used to inspect surface 16. In use, as illustrated in Figures 1 through 4, the beam of light 32 is focused onto a discrete location on the surface 16 by the optical interface 14. The optical inter¬ face 14 then gathers up a portion of the scattered beam and directs the beam into the detector 42 of the AOTF spectrometer 12. As discussed previously, if the surface being inspected is rough and metallic, it is preferred that a portion of the back- scatter component of the scattered beam be analyzed; if the sur¬ face is smooth, or if it is rough and non-metallic, a portion of the specular component of the scattered beam is preferred. The detector 42 of the AOTF spectrometer 12 analyzes the absorbance of the bands being monitored by generating a signal corresponding to the intensity of light at the absorption band. This analog signal is converted to a digital signal by the analog-to-digital converter 24. The digital signal is then pro¬ cessed by the computer 22. Having been previously calibrated, the computer compares the absorbance of the absorption band with that of the reference band and generates data indicating whether the for which inspection is sought is present and provides in¬ formation concerning its thickness and location on the surface. An alternative embodiment of the present invention is illustrated in Figure 5. As with the previously discussed em¬ bodiment, light source 30 is optimized for near to mid infrared wavelengths. In this embodiment, the optical interface includes a lens 60 configured to receive the beam of light from the light source 30 and direct it into the acousto-optic tunable filter 34. Another lens 62 receives the light exiting from the filter 34. The acousto-optic tunable filter 34 is tuned to pass light corresponding to the absorption band of the material for which inspection is sought and at least one reference band outside the absorption band, as discussed above. The filter 34 is inherent¬ ly configured to linearly polarize the incident beam to produce two orthogonal components of polarized light, a vertical compo¬ nent 64 and a horizontal component 66, exiting the filter 34 at different angles. The "vertical" component 64 is termed verti- cal because the polarization is vertically oriented with respect to the plane containing the incident beam, i.e., the plane nor¬ mal to the paper in Figure 5. In this embodiment, the two com¬ ponents of light exiting the filter are separated by an angle of about 12 degrees. It has been found that the ability of the system to measure absorbance is enhanced if the vertical component 64 of the inci¬ dent beam is utilized. Thus, a partition 68 is included in the optical interface, positioned to block the horizontal component 66 from being directed onto the surface 16. The optical interface further includes a lens 70 through which the incident beam is collimated and directed to an inci¬ dent mirror 72 where it is focused on the surface 16. A collec¬ ting mirror 74 is included in the optical interface for gath¬ ering a portion of the scattered beam 76. As described above, the roughness of the surface will generally dictate how the col¬ lecting mirror 74 is positioned to gather a particular portion of the scattered light. The polarization of the incident beam is modified upon iή- teraction with the surface 16. Thus, by passing the gathered portion of the scattered beam 76 through a polarizing analyzer, the amount the incident beam has been depolarized by the surface can be analyzed. Thus, an analyzing polarizer 78 is positioned to receive the gathered portion of the scattered beam 76. Anal- yzing polarizer 78 may include virtually any polarizers, such as those which are commercially available. A detector 80 is positioned to receive the gathered portion of the scattered beam 76 as it exits the analyzing polarizer 78. As with the detector in the previously discussed embodiment, de- tector 80 generates a signal corresponding to the intensity of light it detects. As will be appreciated by one of skill in the art, the processing of the data and the hardware necessary for such processing is substantially the same as that outlined in connection with the previously described embodiment. It has been found in some applications that by varying the angular orientation of the analyzing polarizer 78, the ability of the system to measure absorbance data varies. In particular, when scanning rough metal surfaces, by orienting the analyzing polarizer 78 to pass the 90 degree depolarized portion of the beam, the ability of the system to detect absorbance appears to be maximized. The graph of Figure 6 charts the amount of absor¬ bance measured on a rough metal surface as a function of angle of orientation of the analyzing polarizer. As illustrated in Figure 6, absorbance is maximized at an analyzing polarizer angle of approximately 90 degrees. Accordingly, when utilizing this embodiment of the present invention to inspect rough metal surfaces, the analyzing polar¬ izer 78 is preferably positioned to pass the 90 degree depolar¬ ized portion of the beam 76. This is generally achieved by rotating the analyzing polarizer 90 degrees with respect to the incident polarization (in this embodiment, provided by the acousto-optic tunable filter 34) . This is illustrated in Figure 5 with the analyzing polarizer 78 positioned to pass the horiz¬ ontal component of the gathered portion of the scattered beam. An additional alternative embodiment of the present inven¬ tion is illustrated in Figure 7. This embodiment of the present invention is illustrated with the light source, optical inter¬ face and acousto-optic tunable filter mounted on a scan board 90. When attached to such a scan board, the present invention may easily be included as part of the end effector of a robotic arm or other apparatus to accomplish scanning of the surface to be inspected. When positioned on a scan board, a source optics train 92 and a receiving optics train 94 are generally defined. The source optics train 92 generates the incident beam, prepares it for application to the surface and directs it to the surface. The receiving optics train 94 is configured to gather a portion of the light emanating from the surface, process the gathered light and generate a signal corresponding to detected intensity. The scan board preferably encloses the source and receiving optics trains 92 and 94. An enclosed scan board would, of course, be configured with an opening through which light may be directed onto the surface to be inspected and through which light emanating from the surface may be gathered for analysis. Enclosing the optics trains would facilitate cooling of the hardware, reduce the exposure of the optics to dust and reduce the amount of ambient light which enters into the system. One of skill in the art will appreciate that the utiliza- tion of optics trains to configure various embodiments of the present invention on a scan board or other hardware to facili¬ tate use of the invention in scanning may be readily accom¬ plished. Indeed, for particular applications it may be desir¬ able to configure an apparatus including a plurality of source and receiving optic trains designed to simultaneously inspect for various materials. Alternatively, such a configuration may be desirable merely to provide a single apparatus having the capacity of inspecting for one of a variety of materials, as the application might require. In the embodiment of Figure 7, the light source 96 gener¬ ates an incident beam of light including wavelengths in the ultraviolet range, i.e. generally from about 150 nm to about 400 nm. Such a light source may include any of those commercially available ultraviolet lights, such as a mercury vapor lamp. The optical interface includes a lens 98 which focuses the light into a parallel beam and directs it into an optical filter arrangement 100. In this embodiment, the optical filter ar¬ rangement preferably comprises a band-pass filter configured to pass light at the fluorescence inducing wavelength of the mate¬ rial for which inspection is sought, as is explained in greater detail below. A chopper wheel 102 is positioned in the source optics train 92 and is configured with a series of blades which inter- cept the incident beam as it is emitted from the light source 96. The chopper wheel is configured to rotate at a predeter¬ mined rate such that the light emitted from the light source 96 is modulated. The effects of any ambient light entering the system are substantially eliminated by modulating the incident beam with the chopper wheel 102. Any ambient light which does penetrate the system is not detected by any of the detectors as having a modulated amplitude. Because the system is designed to detect only the modulated component of the detected signal, the pres- ence of ambient light does not affect the measurement of the system. The source optics train 92 also preferably includes a polarizer 104 for polarizing the incident beam. Another lens 106 focuses the incident beam onto the surface 16. The receiving optics train 94 includes a lens 108 which gathers a portion of the light emanating from the surface 16 and directs the gathered portion of light into the acousto-optic tunable filter 34. The acousto-optic tunable filter 34 is tuned to pass light corresponding to the fluorescent wavelength of the material for which inspection is sought. Positioned in the receiving optics train 94, the filter 34 acts as an analyzing polarizer, producing two orthogonal compo¬ nents of polarized light. A lens 110 directs these two compo¬ nents of light into detectors 112 and 114 which generate a signal corresponding to the intensity of the detected light. Processing of that signal proceeds utilizing substantially the same hardware and following the same processes as outlined in connection with other embodiments of the invention. In operation, the light source 96 is selected to include the fluorescence inducing wavelength of the material for which inspection is sought. The optical filter arrangement 100 is also selected to pass light having the fluorescence inducing wavelength of the material for which inspection is sought. As the surface 16 is scanned, presence of the material for which inspection is sought will result in the emission of a fluorescent beam having a fluorescent wavelength characteristic of that material. Hence, the acousto-optic tunable filter 34 is tuned to pass light at the fluorescent wavelength of the materi¬ al for which inspection is sought. Advantageously, the utilization by the present invention of the optical property of luorescence to inspect for a material on a surface provides the invention with an expanded group of materials for which inspection may be conducted. This embod¬ iment may be effectively utilized in identifying the presence and location of organic materials such as grease, many oils and silicone based materials. Additionally, inorganic materials, such as zirconium silicate particulates and cloth or dust par- ticulates, may also be identified with this embodiment. This embodiment of the present invention is easily cali¬ brated by inspecting a surface known not to fluoresce at the fluorescent wavelength to be utilized in the system. Such a reading provides a baseline, or zero signal level, against which fluorescence from the surface to be inspected may be measured. While the present invention may be used to inspect a single portion of a surface, it is preferably used to inspect an entire surface by inspecting discrete locations on he surface. For large surfaces, such as the bonding surfaces of solid rocket motors, a robotics system may be utilized. Alternatively, the system may be used in combination with scan table 18 to inspect smaller surfaces which are capable of being placed on the scan table. Use of the AOTF spectrometer 12 permits the analysis of a variety of discrete locations of a surface to be conducted quickly, thereby enabling the system of the present invention to be efficiently used in analyzing large surface areas. Once data has been obtained from one location of the surface, the system may be utilized to inspect an adjacent location of the surface and the process repeated until representative samples of the entire surface have been inspected. With data from representa¬ tive samples of the entire surface, the computer 22 can generate an output on output device 26 indicating both the location of any contamination as well as its thickness. It is presently contemplated that the surface scanning system 10 be configured to permit surface scanning rates on the order of inches per second. However, one skilled in the art will appreciate that the surface scanning rate may be adjusted according to the requirements of the particular application. For example, tolerance for contaminants for some applications may be less stringent than for others, thereby permitting mea¬ surements to be taken farther apart and permitting faster scan¬ ning. In one embodiment of the present invention, for each pixel on a graphic image representing 0.10 inches of a surface scan, a system built and operated in accordance with the teachings of the present invention is capable of averaging tens to hundreds of surface measurements. So configured, the system provides a good signal-to-noise ratio and generates sufficiently reliable data for most purposes. As previously discussed, this data may be output in either graphical, numerical or machine-readable form. In graphical form, the data may be displayed as an image in which a different color or shade of gray is designated as corresponding to a pre¬ determined thickness of the contamination. In a presently pre¬ ferred embodiment of the invention, such a color scale image is preferred. Alternatively, a surface image could be generated which appears as a three dimensional image on the screen. A surface image is advantageous for graphically illustrating relative thickness of the contamination as compared to background noise level. A disadvantage to surface images is that some of the information is hidden by the peaks generated. The computer 22 is ideally programmed to synchronize the processing of the signal received from the detector with the movement of the beam of light with respect to the surface being inspected. The synchronization of these two functions enables the computer to generate output correlating the measured data with the precise location on the surface to which it corres¬ ponds. One of ordinary skill in the art will appreciate that there are a variety of ways to program a computer to accomplish this stated objective. From the foregoing it will be appreciated that the present invention provides a system for the inspecting of surfaces to detect the presence of materials on a surface, including low levels of materials which are generally not accurately detect- ible by visual inspection methods. The present invention may be utilized to detect contamination on a variety of surfaces, in¬ cluding rough and smooth surfaces and surfaces made of metal, rubber and phenolics. Importantly, the present invention pro¬ vides an efficient and effective system for inspecting large surface areas for contamination. It should be appreciated that the apparatus and methods of the present invention are capable of being incorporated in the form of a variety of embodiments, only a few of which have been illustrated and described above. The invention may be embodied in other forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive and the scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. What is claimed and desired to be secured by patent is:

Claims: 1. A system for scanning a surface to obtain near real¬ time data concerning characteristics of the surface, comprising: a light source capable of generating a beam of light; an optical interface configured to receive the beam of light from the light source and direct the beam of light along a predetermined path extending to and from the sur¬ face, the optical interface including means for directing the beam onto a discrete location on the surface; an acousto-optic tunable filter positioned within the path of light, the filter tuned to pass light having a wavelength corresponding to a known optical property of a predetermined material; a detector positioned to receive light emanating from the surface, the detector capable of monitoring the inten- sity of light at at least one predetermined wavelength, the detector capable of generating a signal corresponding to the intensity of each wavelength being monitored; a signal processor in communication with the detector for processing the signal generated by the detector; and means for moving the directing means relative to the surface such that the surface may be scanned with the beam of light. 2. A system for scanning a surface as defined in claim 1, further comprising a polarizer positioned within the path of light for polarizing the beam of light before it is directed onto the surface. 3. A system for scanning a surface as defined in claim 2, further comprising an analyzing polarizer positioned within the path of light for analyzing the polarization of light emanating from the surface. 4. A system for scanning a surface as defined in claim 1, wherein the means for moving the directing means relative to the surface comprises a scan board to which the light source, the optical interface, the acousto-optic tunable filter and the detector are attached. 5. A system for scanning a surface to obtain near real¬ time data concerning characteristics of the surface, comprising: a light source capable of generating an incident beam of light; an optical interface configured to receive the inci¬ dent beam of light from the light source and direct the incident beam onto a discrete location on the surface, the optical interface further configured to gather at least a portion of the beam which is scattered off the surface; an acousto-optic tunable filter positioned to receive the incident beam, the filter tuned to pass light corre¬ sponding to the absorption band of a predetermined material and at least one reference band outside the absorption band; a detector positioned to receive the gathered portion of the scattered beam, the detector capable of monitoring the intensity of light at the absorption band of the prede¬ termined material and at the reference band, the detector capable of generating a signal corresponding to the inten- sity of each wavelength being monitored; a signal processor in communication with the detector for processing the signal generated by the detector; and means or moving the optical interface relative to the surface such that the surface may be scanned with the beam of light. 6. A system for scanning a surface as defined in claim 5, wherein the optical interface is further configured to gather at least a portion of the back-scatter component of the scattered beam when the beam is scattered off a rough metallic surface. 7. A system for scanning a surface as defined in claim 5, wherein said optical interface is further configured to gather a portion of the specular component of the scattered beam when the beam is scattered off a smooth surface. 8. A system for scanning a surface as defined in claim 5, wherein the optical interface is further configured to gather a portion of the specular component of the scattered beam when the beam is scattered off a rough, non-metallic surface. 9. A system for scanning a surface as defined in claim 5, wherein the light source emits light in the near to mid infrared range. 10. A system for scanning a surface as defined in claim 5, wherein the acousto-optic tunable filter is inherently config¬ ured to linearly polarize the incident beam to produce two orthogonal components of polarized light exiting the filter at different angles and wherein the optical interface further includes a partition positioned to block one of the components of polarized light from being directed onto the surface. 11. A system for scanning a surface as defined in claim 10, further comprising an analyzing polarizer positioned to receive the gathered portion of the scattered beam. 12. A system for scanning a surface as defined in claim 11, wherein the acousto-optic tunable filter and the optical interface are positioned relative to the surface such that the component of the incident beam directed onto the surface is vertically polarized. 13. A system for scanning a surface as defined in claim 11, wherein the analyzing polarizer is oriented to pass the 90 degree depolarized portion of the beam when the surface being scanned is a rough metallic surface. 14. A system for scanning a surface as defined in claim 5, wherein the means for moving the directing means relative to the surface comprises a scan board to which the light source, the optical interface, the acousto-optic tunable filter and the detector are attached. 15. A system for scanning a surface to obtain near real¬ time data concerning characteristics of the surface, comprising: a light source capable of generating an incident beam of light including wavelengths in the ultraviolet range; an optical interface configured to receive the inci¬ dent beam of light from the light source and direct the incident beam onto a discrete location on the surface, the optical interface further configured to gather at least a portion of the fluorescent beam emitted from the surface; an acousto-optic tunable filter positioned to receive the gathered portion of the fluorescent beam, the filter tuned to pass light corresponding to the fluorescent wave¬ length of a predetermined material; a detector positioned to receive the fluorescent beam emitted from the surface, the detector capable of monitor¬ ing the intensity of light at the fluorescent wavelength of the predetermined material, the detector capable of gener¬ ating a signal corresponding to the intensity of the wave¬ length being monitored; a signal processor in communication with the detector for processing the signal generated by the detector; and means for moving the optical interface relative to the surface such that the surface may be scanned with the beam of light. 16. A system for scanning a surface as defined in claim 15, further comprising a chopper wheel for modulating the inci¬ dent beam such that the effect of any ambient light at the fluorescent wavelength of the predetermined material is substan¬ tially eliminated. 17. A system for scanning a surface as defined in claim 15, further comprising an optical filter arrangement positioned to filter the incident beam of light and configured to pass light having wavelengths corresponding to the fluorescence inducing wavelength of the predetermined material. 18. A system for scanning a surface as defined in claim 17, wherein the optical filter arrangement includes a band-pass filter. 19. A system for scanning a surface as defined in claim 15, further comprising a polarizer positioned to polarize the incident beam of light. 20. A system for scanning a surface as defined in claim 19, wherein the acousto-optic tunable filter is inherently configured to linearly polarize the gathered portion of the fluorescent beam to produce two orthogonal components of polar¬ ized light exiting the filter at different angles and wherein the detector includes a first detector positioned to receive one component of the polarized light exiting the filter and a second detector positioned to receive the other component of polarized light exiting the filter. 21. A process for scanning a surface to obtain near real¬ time data concerning characteristics of the surface, comprising the steps of: generating an incident beam of light with a light source; passing the incident beam of light through an acousto- optic tunable filter tuned to pass light corresponding to the absorption band of a predetermined material and at least one reference band outside the absorption band; directing the incident beam of light passed through the acousto-optic tunable filter onto a discrete location on the surface; gathering at least a portion of the beam scattered off the surface; introducing the gathered portion of the scattered beam into a detector capable of monitoring the intensity of light at the absorption band of the predetermined material and at the reference band, the detector capable of generat¬ ing a signal corresponding to the intensity of each wave- length being monitored; analyzing the intensity of the gathered portion of the scattered beam at the absorption band of the predetermined material and at the reference band; and selecting a different discrete location on the surface and repeating the preceding steps. 22. A process for scanning a surface as defined in claim 21, wherein the step of gathering at least a portion of the beam scattered off the surface includes gathering at least a portion of the back-scatter component of the scattered beam when the beam is scattered off a rough metallic surface. 23. A process for scanning a surface as defined in claim 21, wherein the step of gathering at least a portion of the beam scattered off the surface includes gathering at least a portion of the specular component of the scattered beam when the beam is scattered off a smooth surface. 24. A process for scanning a surface as defined in claim 21, wherein the step of gathering at least a portion of the beam scattered off the surface includes gathering at least a portion of the specular component of the scattered beam when the beam is scattered off a rough, non-metallic surface. 25. A process for scanning a surface as defined in claim 21, further comprising the step of polarizing the incident beam with the acousto-optic tunable filter to produce two orthogonal components of polarized light exiting the filter at different angles and blocking one of the components of polarized light from being directed onto the surface. 26. A process for scanning a surface as defined in claim 25, further comprising the step of directing the gathered por¬ tion of the scattered beam through an analyzing polarizer. 27. A process for scanning a surface as defined in claim 26, further comprising the step of polarizing the incident beam includes producing a vertically polarized beam and the step of directing the incident beam onto a discrete location on the surface includes directing the vertically polarized beam onto a discrete location on the surface. 28. A process for scanning a surface as defined in claims 26 and 27, wherein the step of directing the gathered portion of the scattered beam through an analyzing polarizer comprises directing the gathered portion of the scattered beam through an analyzing polarizer oriented to pass the 90 degree depolarized portion of the beam when the surface being scanned is a rough metallic surface. 29. A process for scanning a surface to obtain near real¬ time data concerning characteristics of the surface, comprising the steps of: generating an incident beam of light including wave¬ lengths in the ultraviolet range; directing the incident beam onto a discrete location on the surface; gathering at least a portion of the fluorescent beam emitted from the surface; passing the gathered portion of the fluorescent beam through an acousto-optic tunable filter tuned to pass light corresponding to the fluorescent wavelength of a predeter¬ mined material; introducing the light passed through the acousto-optic tunable filter into a detector capable of monitoring the intensity of light at the fluorescent wavelength of the predetermined material, the detector capable of generating a signal corresponding to the intensity of the wavelength being monitored; analyzing the intensity of the gathered light at the fluorescent wavelength of the predetermined material; and selecting a different discrete location on the surface and repeating the preceding steps. 30. A process for scanning a surface as defined in claim 29, further comprising the step of substantially eliminating the effect of ambient light at the fluorescent wavelength of the predetermined material by modulating the incident beam with a chopper wheel. 31. A process for scanning a surface as defined in claim 29, wherein the step of directing the incident beam onto a discrete location on the surface includes passing the incident beam of light through an optical filter arrangement configured to pass light having wavelengths corresponding to the fluores¬ cence inducing wavelength of the predetermined material. 32. A process for scanning a surface as defined in claim 29, wherein the step of directing the incident beam onto a discrete location on the surface includes passing the incident beam through a polarizer to polarize the incident beam of light. 33. A process for scanning a surface as defined in claim 32, further comprising the step of passing the gathered portion of the fluorescent beam through an analyzing polarizer. 34. A process for scanning a surface as defined in claim 33, wherein the step of passing the gathered portion of the fluorescent beam through an analyzing polarizer includes passing the gathered portion of the fluorescent beam through the acous- to-optic tunable filter to produce two orthogonal components of polarized light exiting the filter at different angles. 35. A process for scanning a surface as defined in claim 34, wherein the step of introducing the light passed through the acousto-optic tunable filter into a detector includes introduc- ing one of the orthogonal components of the polarized light exiting the filter into a first detector and introducing the remaining orthogonal component of the polarized light exiting the filter into a second detector.

Inventors: DOYLE TIMOTHY E, JOHNSON KENDALL B, PEARSON LEE H, DOYLE, TIMOTHY, E., JOHNSON, KENDALL, B., PEARSON, LEE, H.
Applicants: DOYLE TIMOTHY E, JOHNSON KENDALL B, PEARSON LEE H, THIOKOL CORP, THIOKOL CORPORATION
Application year: 1993
Main classification: G01N21
IPC codes: F02K9, G01B11, G01J3, G01N21