Note: Descriptions are shown in the official language in which they were submitted.
;: 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
~ . ..
``i`' ,
i. -4-
~ ~i
t.;,
,;
~ 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~-
-6-
; .
,
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.
-8-
: ;~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-
!¦ ,
--1 0--
.,
,, ~
'~
': 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.
-13-
' 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
-14-
-~ 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,
.~ .
-16-
.,
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.
-18-
,:`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
-20-
~ ;
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:
:
-22-
