CA2133307A1

CA2133307A1 - Acousto-optic tunable filter-based surface scanning system and process

Info

Publication number: CA2133307A1
Application number: CA002133307A
Authority: CA
Inventors: Lee H. Pearson; Kendall B. Johnson; Timothy E. Doyle
Original assignee: Individual
Current assignee: ATK Launch Systems LLC
Priority date: 1992-04-24
Filing date: 1993-04-23
Publication date: 1993-11-11
Also published as: WO1993022655A1; EP0637375A1; EP0637375A4; JPH08500432A; AU4521893A

Abstract

2133307 9322655 PCTABScor01 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

;: w~ g3,22655 ~ 1 3 3 ~ ~ ~ PCT/US93/03831 ACO~8T~rOP~IC ~NABLE FI~TER~BA8~D
~RFACE 8CA~NING_8Y~T~M AND PR~C~88 ! AÇKGR_UND
1. The Field of the_Invention The present invention is related to a ~ystem and process ~or inspecting surfaces. More particularly, the present inven-tion is related to a system ~or obtaining near real time, non-destructive detection and evaluation o~ various materials on surf~ces by directing light at the surface and analyzing the in-tensity and polarity o~ the light emanating from the surface at a wavelength corresponding to a known optical property of a pre-determined material.

~__359bl~u~ L_und A typical manufacturing process utilized in many applica-tions is the ~onding of two materials. The criticality of the strength of the bond will vary depending on the particular ap-plication for which the bond~d material is ~wo be used. For ~xample, in the manu~actur~ of solid rocket motors, bond strength is particularly critical.
: 20 ?he 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 s~ress riser~ 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 sxam-ple, some of the bonds found in a typical solid rocket motor are the bond between ~he 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 ~ozzle housing. A
weak bond or debond in any of these bonds could re~ult in catas-trophic failure of the rocket motor.
When two materials axe ~onded together, contaminants on the surface of either of the materials can weaken the bQnd and, in s~me instances, cause areas of debonding. Organic materials ~' W093/22655 PcT/us93/o3831i , ;, ~1 such as greases, hydraulic fluids and mold release agents are ;( the primary source of contamination of bonding surfaces in solid rocket motors. Other contaminants incl~de particulates such as sand or dust. Oil vapors are often present in environments where hydraulic systems and electric motors are present. These vapors can conde~se on surfaces to be bonded. Even small le~els o~ these contaminants, not visible to the human eye, can degrade ~ bond stren~th.
'~' The extent to which a surface can be cleaned prior to bond-10 ing and the method to be utilized in cleaning the surface vary , according to the nature o~ 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-~5 tom of which boi}ing me.thylchloro~orm is located. The methyl-. .
.' chloroform evaporates and condenses on the rocket case. As it , runs of~ the rocket case, it.dissolves any grease in its path.
,~, While this process works well in cleaning small amounts of i grease from the rocket case, if there are areas o~ localiz~d 0 buildup of grease, not all of the grease may be removed by the cleaning process.
~;~t,fl Using a solvent such as methylchloroform to clean a bonding ' surface may not be viable if the ~ondinq surface is a phenolic material. In a solid rocket motor the noææle is typically made f~ 25 of a phenolic material. The n~zzle is m~de by wrapping uncured r~ tape onto a mandrel, permitting the tape to cure and then ma-~" chining the part into the desired shape.
Phenolic materials will absorb ~irtually any type of clean-ing solvent with which ~hey come into contact. These solvents can alter th~ surface chemistry and/or carry dissolved contami-nants into the phenolic. In applications such as tho~e discu~-s~d herein, the surface properties of the phenolic~ must remain : unchanged.
Presently, the preferred method of cleaning a phenolic material is to place it on the mill and machine a n~w surface, thereby removing the cont~minated surface. However, this can-only be done if the tol.erances of the part permit a portion of ~he surface to be removed. Otherwise, a contaminated part may have to be replaced.

-2-~,. ..

; iWOg3/22655 PCT/US93J03831 ~i 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 ~i 5 acceptable limits.
i~ A crude method of conduct.ing a surface in~pection i~ to place s~me 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 ;i~ 10 verify the exiætence of contaminants on the wipe and determine their identity.
A principal obstacle to the success~ul use of this method is that it can only be used as a check method. It cannok be u~ed as an inspection method on the entire bondlng sur~ace.
And, while the method may provide information about the exis-~'~ tence of a contaminant and its identity, it cannot be used to determine the thic~ness of the contamination. It i5 a qualita~
tive method and there~ore does not provide a quantitative meas-urement of the contamination. Additionally, this method cannot ~:: 20 be used with phenolic matarials because the surface chemistry of the phenolics would be alt~red 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 ~or rust protection, fluoresce under ultraviolet light. Thus, by visual-ly i~specting the surface under ultraviolet light, any contami-nants which fluore~ce u~der the light can readily be de~ected.
A disadvantage of thi~ method is that the method cannot be reliably used to detect low levels of contamination as it is limited by what can be ~een with the human eye r 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 ¢ontaminated area. As with many manual methods, the possibility of human error renders t~is method inadequate for many applica-tions O
Automated inspection methods include an optically stimulat-ed electron emission ("OSEE") method~ This method is based on

3-~1 W093/22655 7 PCT/US93/03831 J'' the photoelectric effect. By shining ultr~vlolet light on the - surface to be inspected, electrons are emitted from the surface.
l By placing an electrode near the surface and raising the elec-.' trode to a predetermined voltage, an electric field is generat-.:. 5 ed, drawing an electron current from the surface whose strength : can ~e 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 ralevant to the determiination 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 ~,1 15 system for the in~p~ction of bonding surfaces which would detect j~, the presence of ~hin ~ilms, including low-level contamination or ~,~, æurface coa~ings, which may not be detectible with prior-art visual inspection method~i.
Indeed, it would also be an advancement in the art if such 20 a surface inspection system could work effectively to detect contamination on a variety of surfaces and with differen~ levels o~ roughness, including metal, phenolic and rubber surfaces.
It would be yet a further advancement in th~ art to provide ~uch a system that could work efficiently and e~fectively in inspecting large surface areas.
Such a ~ystem for inspecting surfaces is disclosed and claimed herein.

. ~ CTS OF THE INVENTION
The present invention is directed to a novel system for inspecting surfaces to detect and characterize thin ~ilms, in-cluding contaminants. The system includes a light source ca pable of generating a beam of light and an:optical interface for re~iving the beam of light from the light source. The optical ~, inter~ace directs the beam of light along.a predetermined path extending to and from the~surface. An acousto-optic tunable ~ilter is positioned within the path of light and is tuned to pass light having a wavelength corresponding to a ~nown optical property of the material for which inspéction is sought. Such ~ . ..
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~ wo 93/22655 ~ o 7 PCT/US93/03831 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.
; 5 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-erate~ a signal corresponding to the intensity of ea~h wave-length being monitored. The signal generated by the detector is f`~' lO fed ~nto a signal processor which processes the signal and gen-",~
~, erates data concernin~ the characteristics of the sur~ace.
The system also includes means ~or moving the system relative to the ~urface such that the surface may be scanned ~: with th~ beam of light.
In one embodiment, the system may be used to detect and mea5ure 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 ab~orption band of one predetermined material and at least one reference band outside the absorption band.
An optical inter~ace 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 sur~ace and direct i~ 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 ~ranslated 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 m~tal surfaces, includ ng 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~
~ ter~d beam. For smooth surfaces or roug~ n~n-metallic surfaces, `.~ it i~ presently preferred to adjust the optical interface to ! ~ _ 5 _ S`i '`~' ` .

J ~ .J ~ / JUL I~Y4 ~, gather a portion of ~he 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 ~ilter. 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 inter~ace includes a partition positioned to block one of the components of polarized light from being directed onto the sur~ace. It is currently preferred that the incident , beam b~ vertically polarized, i.e., that component of th~ inci-;~ deht beam which is polarized parallel to the incident plane of light.
When utilizing a polarized incident beam~ the gathered 1 15 portion of the scattered beam is pre~erably passed through an I analyzing polarizer. The orientation of the analyzing polarizer with respect to the incident polarized beam may be adjusted to ma~imize the ability to detect absorbance. When inspectlng rough metal surfaces, it is preferred to orient the analyzing polarizer to pass the 90 degree depolarized portion of the beam.
In a pre~erred embodiment, a scanning apparatus is employed ~1; to rapidly change the point on the surface at which the beam of Pl light is directed, thereby permitting the inspection of various ll locations on th~ sur~ace or of large sur~ace areas. 3y synchro-~ 25 nizing the signal processing and the scanning of the surface, '~ data concerning materials on the surface is generated. In one embodiment of th~ invention, successful scannin~ for contamina-tion has been accomplished by directing the beam of liyht at discrete locations on the surface which are spaced about 0.10 inches (0.22 cm) apart and changing the point on the surface at which the beam o~ light is directed about every 0.01 seconds.
To obtain data concerning the thicknes of a material on the sur~ace as well as the existence o~ the material, an em-~! bodiment of the invention measuring absorbance of the incident beam of light is utilized in combination with calibrationplate~. Such calibration plates may include one plate with no contamination and on2 plate with a known amount of contamina-tion. By scanning calibration p~ates prior to inspecting a sur~ace, the linear relationship between ab~or~ance and thic~-; .
, A~ENDEO S~lEF~
, .

i ~ w~ g3/226s5 ~ 0 7 P~T/US93/03~31 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 o~ 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 ultr~violet range, i.e. generally from about lS0 nm to about 1~ 400 nm.
The incident beam is preferably polarized wikh a polarizer b~fore being directed onto the surface. Also, it i5 preferred to modulate the incident beam with a chopper wheel so that the effects of ambient light may be eliminated.
The polariz~d incident beam of ultra~iole.t light is direct-ed onko the ~urface by the optical inter~ace. Upon striking the sur~ace, the ultra~iolet light including light in the fluore~-cence inducing wavelength of the surface causes excitation of valence'electrons inducing them to tem~orarily jump ~o a higher enexgy state. ~he fluorescence inducing wavelength is that wavelength oE light which causes the mate~ial for which inspec-tion is sought to f luore~ce. Upon dropping to an intermediate energy s-ate, photons in the visible spectrum corresponding to the fluorescent wav~length o~ the material are emitted from the surface. B~cause the wa~elength of the emitted fluorescent I light generated by this phenomeno~ is characteristic of the materia1 producing it, the existence of a particular material on the surface can be ascer ained by monitoring for light at a I fluorescent wavelength of khat material.
j 30 In ~his embodiment which utilixes an ultraviolet incident beam of light, the optical interface is also configured to j gather at least a portion of the light emitted from the sur~ace.
The acousto-optic kunable ~ilter is positioned to receive the ~ ~athered portion of the ~luorescent bea~ and is tuned to pass ; 35 light corresponding to the fluorescent wavelength of the mate-il, rial for which inspection is sought.
Because of ~he positioning of the acous~o-optic tunable filter, it acts as an analyzin~ polarizer. T~us, the acousto-optic tunable filter polarizes the gath~red fluore~cent beam and ':
-7- :
. . .

h 1 e3 ~ Z~ U J --WOg3/2265~ PCT/US93/03~31 separates it into two orthogonal components of linearly polar-iæed light which exit the filter at two different angles. Det~c-tors are positioned to receive each component of polarized light transmitted by the acousto-optic tunable filter an~ generate a signal corresponding to the intensity of the detected light~
In accordance with the teachings o~ the present invention, the light source, optical interface and acousto-optic tunable filter 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 inspect~d. So configured, the system of the present invention may be utilized to provide near real-time data concerning the charact~ristics of a surface.

BRIEF DESCRIPTION OF THE DRAWINGS
Figure l.is a schematic of ~he 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 interfac~ of the system of Figure ~ 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 l, illustrating how a portion of 1 the back-scatter component of the scattered beam is gathPred by Z the mirror.
,i 25 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 o~ absor~ance measured on a rough metal surface as a function of angle of orientation of the analyzinq polarizer.
~igure 7 is a schematic illustrating an additio~al alterna-tive embodiment of the present invention.

: ;~O 93/226~ i J ~ ~ 0 7 PCI'/US93/03831 DETAILED DESCRIPTION OF THE P~EFERRED 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 sur~ace for contamination in accordance with the present inven~
~ion is generally designated at 10. The system of the present invention may be used to inspect for a variety of materials for ~i which certain optical properties are known or can be ascer-: tained.
~Indeed, because of the use of the acousto-optic tunable filter in the sy~t~m of the pres~nt invention, near real-time analysis may be conducted for a variety of materials ha~ing 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 properti~s may al~o be utilized within the scope ~ o~ the present inventionO
;it The present inventio~ is particularly useful when the ~ material for which inspection is sought is known or suspeGted to J 20 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 syskems or :~ 25 electric motors frequ~ntly leads to the presence of oil vapors in the ambient air which condense on bonding surfaces. By utilizin~ the pres~nt in~ention, whether these vapors have con-~` ~ densed on bondin~ surfaces can be ascertained. Indeed, the ~, pres~nt invention has been used successfully to inspect for oil 3l 3 0 and grease, suc:h a HD2 grease commonly used f or rust pro~ec-tion .
i In Qne embodiment, the system 10 of the present invention includes a spectrometer having an acou6to-optic tunable filter i 12 t ometimes referred to herein as an '~AOTF spectrometer." It j 35 has been found that an AOTF spectrometer is capable of providing an optimal combination of-fast processing time and ~pectral res-,~i olutionO In a presently preferred embodiment o~ the invention, spectrometer 12 is a solid state spectrometer based on the ~j , _ 9 _ ,i U ~
W093/226~5 PCT/US93/03831 i,i.;
acousto-optic tunable filter., such as is ~arketed by Infrared Fiber Systems, Inc~ of Silver Spring, Maryland.
In communication with the spectrometer 12 is an optical interface 14. As explained below in ~reater detail, the optical interface directs a beam of light from ~he 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~
515 .
In one embodiment of the present invention, the surface or ~,lO substrate 16 being inspected is supported by a scan table ~8.
i The scan table is controlled by a ~can controller 20. Scan tabl~ 18 and sc~n contr~ller 20 may be any of those controller~
and tables which are commercially available, such as the 4000 i Series controller and the HM-1212 table, both of which are sold il5 by Design Components, Inc. of Franklin, Massachusetts.
I~ accordance with the embodiment of ~he present invention illustrated in Figure 1, the spectrometer 12 and optical in-terface 14 are held in a stationary po ition while the surface , 16 being scanned is moved by the scan table 18. While such an ,l 20 embodiment îs presently preferred for a laboratory scale model of the invention wherein small surfaces are being sc~nned, it is not the preferred embodiment if the surface to be inspected is a I large ~ur~ace, such as the bonding surfaces in a large solid-rocket motor.
Thus, it will b~ appreciated by one of skill in the art , that the spe trometer 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 ~pectrometer and optical interface are moved relative to the surface to obtain data from various discrete locations on the ` surface.
signal processor ~uch as a computer 22 is provided to control the motion of the scan controller 20 and proc~ss the ~5 signal produced by the spectrometer 12. Use of computer 22 permits khe synchronizatio~ of the motion of the scan controller ~ 20 with the processing of data acquired from the spectrom~ter `; 12, thereby providing information concerning the location of an~
~ cantamination detected on the surface 16 during scanning. Com-!¦ , ., ,, ~

'~

': W~93/~2655 ~ 0 7 P~T/US93/03~31 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 ~, 5 AOTF spectrometer 12 and the computer 22 for converting the ~ analog signal generated by the spectrometer into a digital sig-j`! nal which can be processed by the computer 22. It will b~ 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 e~uipped with such a con~erter. Alternatively, the converter 24 may be a separate component of the system l0.
An oukput device 26 i5 provided in communication with the computer 22 for providing a display of the data generated during '~. 15 the examination of surfac~ ~6. The output device 26 may include ;.: any device known among those skilled in the art for displaying -i data, including a video monitor or.plotter. It may provide the data in either human readable or machine~readable ~arm. In one embodiment of the present invention, an EGA color graphics '~ 20 syste.m has been ~ound to provlde ~atls~actory output.
,: The display of data may be accomplished in either graphical or numerical form. In a presently preferred embodiment of the `~i invention, the data is displayed formatted in a manner to illus-.`` trate a surface map or a color scale image of the contamination.
~:~ 25 For graphical output~ a color monitor may be used to display ; contour correeponding to various preassigned colorsO Alterna-ti~ely, a similarly ~ormatted output may be illustrated in .1 : shades of gray.
~i As illustrated in Figure 2, the AOTF spectrometer 12 in-.' 30 cludes a light source 30 which g~nerates a beam of light 32. In this embodiment, light source 30 is preferably a quartz, halogen lamp suc~ as that made by Gilway Technical Lamp of Woburn, Mass~
achusetts. Such a light source 30 is optlmized for neax to mid infrared wavelengths. In most commercially available AOTF spec-trometers, light source 30 will be housed within the spectromet-er. ~he spectr~meter 12~is configured such that the beam of ` light 32 passes through the AOTF crystal 34 within the ~pectro~
meter. The crystal 34 acts to filter out all wavelengths of ~ ` ' ' ' -11-~..
., W093/226~5 ~ 3 ~ PCT/US93/03~31 .
light from the beam 32 except those to be monitored by the sys-tem 10 during the surface inspe.ction.
Before the beam 32 exits khe AOTF spectrometer 12, the beam is trans~ormed into a collimated beam. Upon its exit from the S spectrometer 12, the collimated beam of light 32, including only . those wavelength~ of light ko be monitored during the surface ,l in5pection, comes into contact with a ~irst paraboloid mirror 36. First mirror 36 focuses the beam onto the discrete location on the surface 16 to be inspected. In this embodiment o~ the invention, first mirror 36 act5 both to focus the incident beam ~ on the surface and to gather a portion o~ the scattered compo-i nent of the beam.
If the surface 16 to be inspecte~ is a rough surface, such ,, ~s is the case with most metal -~urfaces, first paraboloid mirror : 15 36 is preferably positioned with r~spect to the sur~ace such that it will gather a portion of the back-scatter component of the scatt~red beam, as is illustrated in Figures 2 and 3. As us2d herein, a surface is considered to be "rough" if its RMS
' (root mean square) roughn~ss is on the order of a wavelength or '~l 20 greater than the wavelength of the light beiny 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 sur~ace tha~ has been machined, ' firs-t paraboloid mirror 36 is preferably positioned with respect to the 5urface such that the incident beam is perpendicular to the parall~l lines which comprise the roughness. One of the principal advantag~s of the present invention is that even if the surface is randomly rough, such as a grit~blasted metal sur-face, ~y positioning the paraboloid mirror ~6 to collect a por-; 30 tion of the diffuse r~flectance of the incident beam, meaningful qata 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.
Importantly, in accordance with the teachings of ~he pres-ent inventlQn, surface roughness actually enhances the ability of the ~ystem of the present invention to detect and quantita-tlvely me~sure surface contamination. Generally, the sensiti vity of the present invention in detecting and mea~uring conta-~i , -12-i~

;NO93~226~5 ~ 3 3 o 7 PCT/US93/03$31 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 curface which results in 5 increased intensity in the electric field at the surface.
Because of this ability to successfully inspect rough sur-faces, the present invention may ~e used to inspect surfaces of phenolic materiaL - materials which have proved particularly dif~icult to inspect by othe~ methods. Carbon phenolics, for example, which have a surface which is generally treated as ran-domly rough ~ven when machin~dt can be efficiently and effec-tively inspected by practicing the teachings of the present in-: vention.
For 2 rough metallic surface, such as that illustrated in Figures 2 and 3, it is presently preferred to direct the beam atthe surface at an incident angle in the range of from abo~t 30 degrees to about 40 degrees.
The present invention may also be used on smooth surfaces, de~ined as sur~aces having a RMS roughne~s less than the wave-length of light being used by the inspection met~od. For smoothsurf aces, or rou~h sur~aces of non-metallic materials, the f irst paraboloicl mirror 3 6 is pref erably positioned with r~spect to the ~urface l~ such that the mirror 36 will gather a portion oE
the specular component of the scattered beam, as illustrated in Fiqure 4. Th~ angle of incidence a of the beam is at or near ~ the Brews~ex angle. I~ is at the Brewster angle that the elec-j tric field ~ntensity near the surface is the strongest for the normal ~omponent of the electric field. For a typical polymer, th2 Brewster angle would be approximately 45 to 50 degrees a~
in~rared wavelengths.
The g~thered portion of the scattered beam, whether it be taken from the back-scatter component (mirror 36 of Figures ~
and 3) or the specular component of the beam (mirror ~8 of Fig-ure 4), is converted ba~k into a collimated beam and directed 3~ 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 mi~ror 40. The detector signal is~digitized by the analog-to-digital converter 24 and received by the computer ~2 for analysis.

' W093t226~5 PCT/US93/03~31 t 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 horizonkally into the detector ~ 5 to avoid spilling the li~uid nitrogen used to cool the detector.
;;ij It will be appreciated by one of skill in the art, however, that i 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 ~cattered 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 . 15 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 :l~ 20 embodiment of th~ invention, the AOTF spectrometer 12 is ~et to ~ inspect for a single material. However, if it is desired to .~, simultaneously inspect for a variety of materials, the AOTF
spectrom~ter could be set to monitor the peak absorbance o~
., each. Simultaneously monitoring two or more materials may be ` 25 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 ref erence bands be monitored, one on each side e~f 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 materi2l or ~ue to external factors such as fluc~
tuations or variations in surface roughness. For example, if h~
the surface i5 being inspec~ed for the presence of a hydrorarbon having an absorption band of 3.4 microns, preferred reference -~ W093/2265~ ~3 3 3 3 ~ 7 PCT/US93/03831 ;: bands are 3.24 microns and 3.6 microns. If it is desired to in-pect a surface for silicone release agents, an absorption j' band of about eight microns may be monitored. When inspecting "~ ~or silicone release agionts it is presently preferred to monitor s 5 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 I between the thickness o~ the material on the surface and the amount o~ absorbance is approxima~ely 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 ma~erial and having the ~ame roughness as the , substrate to be in~pected. In a preferred embodiment, five pre~
determined thicknesses o~ contamination are applied to approx-imately ~i~e different locations on the plate, thereby pro~iding a suf~icient number of data points that the relationship between absarpti~n and thickness can readily be determined. The calib~
ration plate should be repre~entative of both the material type ~;l and the roughness level of the surface to be inspected.
l'he.system 10 should be calibrated each time the substrate to be inspected is changed. Also, each time the mirrors are adju~ted or the angle of incidence of the beam is altered, the system should be calibrated to regenerate the calibration curve.
With th2 system calibrated, it is rsady to be used to ~3 inspect surface 16. In use, as illustrated in Figures 1 through ~ 4, the bi~am of light 32 is focused onto a discrete location on `r~ 30 the surface 16 by the optical interface 14. The optical inter-face 14 thein 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 ~i rough and metallic, it is preferred that a portion of the back-,3; 5 scatter component of the scattered beam be analyzed; if the sur-~ace is smooth, or if it is rough and non-metallic, a portion of , i the specu~ar component of the scattered beam is preferred.
The detector 42 of the AOTF spectrometer 12 analyzes the ~., i~; absorbanca of the bands being monitored by generating a signal ,~ ~
~ -15-~1 wo g3~2265s ~ 1 3 ~ ~ o ~ PCT/US93/03831 '~.
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 ~or which inspection is sought is present and provides in-formation concerning its thickness and location on the surface.
An alternati~e embodiment o~ the present invention is illustrated in Figure 5. As with the previously discussed ~m-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 o~ light from the light . ~ource 30 and direct it into the acousto-optic kunable ~ilter .~ 15 34. Another lens ~2 receives the light exiting ~rom the fil er 3~ .
1 The acousto-optic tun~ble filter 34 is tuned to pa~s light ;.` corresponding tu the absorption band of the material for which inspection is sought and at least one reference band outside the absorption hand, as discussed above. The ~ilter 34 is inherent-ly configured to linearly pslariæe the incident beam to prsduce ~ two orthogonal components of polarized light, a vertical compo-3 nent 64 and a horizontal component 66, exiting the filter 34 at dif~erent angles. The "~ertical" 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 S. In this embodiment, the two com-ponents of light exiting the filter are separated by an angle of about 12 d~grees.
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 inkerface, positioned to block the horizontal component 66 from being directed onto the surface l~.
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~
;~ti ting mirror 74 is included in the optical interface for gath ~ ering a portion of the scattered beam 7 6 . As described above, .~ .

., U I
3/226~5 PCT/US93/~3831 ~; the roughness of ~he surface will generally dictate how the col-lecting mirror 74 is positioned.to gather a particular portion of the scattered ~ight.
,, ~
`'. The polarization o~ the incident beam is modified upon in-teraction with the sur~ace 16. Thus, by passing the gathered portion of the scattered beam 76 through a polarizing analy~er, .: the amount the incide~t beam has been depolarized by the surface :~ can be analyzed. Thus, an analyzing polariæer 78 is positioned ~I to rec ive the gathered portion of the scattered ~eam 76. Anal-5~' l0 yzing polarizer 78 may include virtually an~ polarizers, such as those which are co~mercially available.
~ A detector 80 is positioned to receive the gathered portion '~; of the scattered beam 76 as it exits the analyz.ing polarizer 78.
A5 with the detector in the previously discussed embodiment, de-~ 15 tector 80 gen~rates a signal c~rresponding to the intensity of y~ light it detects. As will be appreciated by one of skill in the art, the processing o~ the data and the hardware necessary ~or such processing is substantially the same as that outlined in connection with the previously described embodimentO
~:20 It has been found in some applications that by varying the angular orientation of the analyzing polarizer 78, the ability : of the ~ystem to measure absorbance data varles. In particular, when scanning rough metal æurfacas, 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 ~o 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 inventio~ to inspect rough metal surfaces, the analyzing polar-izer 78 is preferably positioned to pass the 90 degree depolar-iæed portion of the beam 76. This is generally achieved by rotating the analyzing polarizer 90 degrees with respect to the incident polariz~tion (in this embodi~ent, provided by the ~` acousto-optic tunable filter 34). This is illustrated in ~igure S with the analyzing polarizer 78 positioned to pass the horiz-ontal component of the gathered portion o~ the scattered b~am.
~`,.3 ~ -17-, . .
, . ~

, W0~3/226~5 PCT/U~93/~3~31 ~,;
.~ An additional alternative embodiment of the present inven-~ion is illustrated in Figure 7~ This zmbodiment of the present invention is illustrated with the light source, optical inter-face and acousto-optic tunable filter mounted on a scan board S 90. When attached to such a scan board, the present invention 'i may easily be included as part of the end effector of a robotic a~n or other apparatus to accomplish scannin~ of the surface to ,; be inspected.
When positioned on a scan board, a source optics train 92 and ~ receiving optics train 94 are g~nerally defi~ed. The æource optics train 9~ generates the incident beam, prepares it or appllcation to the sur~ace and directs it to the surface.
i The recei~ing optics train 94 is configured to gather a portion ~; o~ the li~ht emanatin~ from the ~urface, process the gathered light and generate a signal corresponding to det~cted intensity.
~: The scan board pref~rably encloses the source and receiving optics trains 92 and 94. An enclosed sca~ board would, o~
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 sur~ace may be gather~d 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 syst~m.
One of skill in the art will appreciate that the utiliæa-tion o~ optics trains to configure various embodiments of the present invention on a scan board or other hardware to facili-tate use o~ the invention in scanning may be readily accom-: plished. Ind~ed, for particular applications it may be desir~able to conf igure an apparatus incl~ding a plurality of source 30: and recei~ing optic trains designed to simultaneously inspect for various materials. Alternatively, such a conflguration 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 ge~er-ates an incident beam of light including wavelengths in the ultravi~let range, i.e. generally from about 150 nm to about 400 nm. Such a light source may include any of those commerci~lly ~, available ultraviolet lights, such as a mercury vapor lamp.

,:`i ., . ~

093/226~5 ~ ~ J~ U l PCT/US93~3~31 The optical inter~ace includes a lens 98 which focuses the light into a parallel beam and directs it into an optical filter arrangement lO0. In this embodiment, the optical filter ar-rangement preferably comprises a band-pass filter configured to ,` 5 pass light ~t 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 con~igured with a series of blades which inter-j~; lO cept,the incident beam as it is emitted ~rom the light source ~ 96. The chopper wheel is configured to rotake at a predeter~
'~ mined xat2 ~uch that the light emitted from the light source 96 is modulated~
The ef f ects of any ambient light entering the system are substantially eliminated by mod~lating the incident beam with the chopper wheel 102. Any ambient light which does penetxat~
the system is not detasted by any o~ the detectors as having a modulated amplitude. Because the system is designed to detect only the modulated component o~ the detected ~ignal, the pres-~:~ 20 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. Anather lens 106 focuses the incident beam onto the sur~ace 16.
The receiving optics train 94 includes a lens 108 which gathers a portion o~ the light emanating from the sur~ace 16 and dire~ts the gathered po~tion of light into the acousto-optic tunable filter 34. The acou~to-optic tunabla filter 34 is tuned to pass liqht corresponding to the fluorescent wavelength of the ~at~rial 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 1~2 and 114 which generate a ~'~ 35 ` 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 ~:` connectisn with other embodiments of the invention.

, --19--.

;

W093/226~ PCT/US93/03831 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 l00 is also selected to pass light having the fluorescence inducing wavelength of the material for which inspection is sought.
As the surface l6 is scanned, presence of the material for which inæpe~tion is sought will result in the emission of a fluorescent beam ha~ing a fluorescent wavelength characteristic ~1 f that material. Hence, the acousto-optic tunable filter 34 is ,~, lO ~uned 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 prop~rty of fluorescence to inspect for a material on a ~urface provides the invention with an expanded group of ma~erials for which inspection may be conducted. This embod-¦ iment may b~ effectively utilized in identifying the presence and location of organic materials such as grease, many oils and silicone bas.ed 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 ~luoresce at the fluorescent wavelength to be utilized in the system. Such a ` reading provideq a baseline, or zero signal level, against which fluoresc~nce from the sur~ace to be inspected may be measured~
While the present invention may be used to inspect a single portion of a surface, it is pr~ferably used to inspect an entire surface by inspecting discrete locations on the 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 ~` 35 variety of discrete locatiDns of a sur~ace to be co~ducted quick~y, thereby enabling~the system of the present invention to be efficiently used in analyzing large surface areasO Once data has b~en obtained fr~m one location of ~he surface, the syst~m may be utilized to inspect an adjacent location of the surface ~ ;
CJUVJ 03 PKec~ `i r ~ 2 7 JUl ~994 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 thiGkness.
It is presently contemplated that the surface scanning system 10 be configured to permit surface scanniny rates on the order of inches (centimeters) 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 ~or some applications may be less stringent than for others, thereby permitting measurements to be taken farther apart and permitting faster scanning.
In one embodiment of the present invention, for each pixel on a graphic imâge representing O.lO inches (0.22 cm) 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 surfac~ measurements. So con~igured~ the system ~0 provides a good signal-to-noise ratio and generates sufficiently reliabl~ 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 pre~ently pre-ferred embodiment of the in~ention, such a color scale image is pre~erred.
Alt~rnatively, a surface image could be generated which appears as a thr~e 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 hid~en 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 urface being inspected. The synchronization of these two functions enables the computer to generate output correlating the measured data , W093/22655 .41 ~ ~ ~ 0 7 P~T/US93/03831 i .
with ~he 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 ~aterials which are generally not accurately de~e~t :~
ible by visual inspection methods. The present invention may be utili.zed to detect contamination on a variety of surfaces, in-cluding rough and smooth surf aces and surf aces made of met l, rubber and phenolics. Importantly, the present invention pro-vides an ef~icient and effective system ~or inspecting large surface areas for contamination.
lS It should be appreciated that the apparatus and methods of the present invention are capable of being incorporated in the ~orm of a variety of embodiments, only a few of which have been ~.
illustrat~d 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 xestrictive and the cope of the invention is, therefore, indicated by the appended `~
cIaims rather than by the foregoing description. All Ghanges :~
which come within the meaning and range of e~uivalency of the claims are to be embraced within their scope.
: What is ~laimed and desired to be secured by patent is:

:

Claims

1. A system for scanning a rough 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 surface, the optical interface including means for directing the beam into 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 absorption band of a predetermined material and at least one reference band outside the absorption band;
a polarizer positioned within the path of light for polarizing the beam of light before it is directed onto the surface;
an analyzing polarizer positioned within the path of light for analyzing the polarization of light scattering off the surface;
a detector positioned to receive light passing through the analyzing polarizer, the 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 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, wherein the analyzing polarizer is oriented to pass the 90 degree depolarized portion of the beam when the surface being scanned is a metallic surface.

3. 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.

4. A system for scanning a rough 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, the acousto-optic tunable filter being inherently configured to linearly polarize the incident beam to produce two orthogonal components of polarized light exiting the filter at different angles, the optical interface further including a partition positioned to block one of the components of polarized light from being directed onto the surface;
an analyzing polarizer positioned to receive the gathered portion of the scattered beam;
a detector positioned to receive the gathered portion of the scattered beam from the analyzing polarizer, the 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 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 optical interface relative to the surface such that the surface may be scanned with the beam of light.

5. A system for scanning a surface as defined in claim 4, 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 metallic surface.

6. A system for scanning a surface as defined in claim 4, 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 non-metallic surface.

7. A system for scanning a surface as defined in claim 4, wherein the light source emits light in the near to mid infrared range.

8. A system for scanning a surface as defined in claim 4, 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.

9. A system for scanning a surface as defined in claim 4, wherein the analyzing polarizer is oriented to pass the 90 degree depolarized portion of the beam when the surface being scanned is a metallic surface.

10. A system for scanning a surface as defined in claim 4, wherein the means for moving the optical interface 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.

11. 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;
a polarizer positioned to polarize the incident beam of light;
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.

12. A system for scanning a surface as defined in claim 11, further comprising a modulator for modulating the incident beam such that the effect of any ambient light at the fluorescent wavelength of the predetermined material is substan-tially eliminated.

13. A system for scanning a surface as defined in claim 11, 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.

14. A system for scanning a surface as defined in claim 13, wherein the optical filter arrangement includes a band-pass filter.

15. A system for scanning a surface as defined in claim 11, 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.

16. A process for scanning a rough surface to obtain near real-time data concerning characteristics of the surface, com-prising 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;
polarizing the incident beam;
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;
directing the gathered portion of the scattered beam through an analyzing polarizer;
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.

17. A process for scanning a surface as defined in claim 16, wherein the step of gathering at least a portion of the beam scattered of the surface includes gathering at least a portion of the back-scatter component of the scattered beam when the beam is scattered off a metallic surface.

18. A process for scanning a surface as defined in claim 16, 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 non-metallic surface.

19. A process for scanning a surface as defined in claim 16, wherein the step of polarizing the incident beam comprises 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.

20. A process for scanning a surface as defined in claim 16, wherein 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.

21. A process for scanning a surface as defined in claim 20, 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 metallic surface.

22. 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;
passing the incident beam through a polarizer to polarize the incident beam of light;

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 analyzing polarizer;
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.

23. A process for scanning a surface as defined in claim 22, 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.

24. A process for scanning a surface as defined in claim 22, 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.

25. A process for scanning a surface as defined in claim 22, 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.

26. A process for scanning a surface as defined in claim 25, 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.