Surface inspection and characterization system and process

• US 5,406,082 A
• Filed: 11/27/1992
• Issued: 04/11/1995
• Est. Priority Date: 04/24/1992
• Status: Expired due to Fees

• Current Assignee: Northrop Grumman Innovation Systems, Inc. (Northrop Grumman Corporation), Schmitt Industries, Inc.
• Original Assignee: Thiokol Corporation (Howmet Aerospace, Inc.)
• Inventors: Pearson, Lee H., Knighton, Mary, Stover, John, Swimley, Brett
• Primary Examiner(s): Hannaher, Constantine
• Application Number:
  US07/982,293
• Time in Patent Office: 865 Days
• Field of Search: 250/341, 250/339, 250/301
• US Class Current: 250/339.11
• CPC Class Codes: G01B 11/0625 with measurement of absorption or reflection
• CPC Class Codes: G01J 3/1256 using acousto-optic tunable filter; acousto-optic elements or systems G02F1/11, G02F1/33
• CPC Class Codes: G01N 2021/3133 with selection of wavelengths before the sample
• CPC Class Codes: G01N 2021/4709 Backscatter
• CPC Class Codes: G01N 2021/8427 Coatings
• CPC Class Codes: G01N 2021/945 Liquid or solid deposits of macroscopic size on surfaces, e.g. drops, films, or clustered contaminants
• CPC Class Codes: G01N 21/17 Systems in which incident light is modified in accordance with the properties of the material investigated where the material investigated is optically excited causing a change in wavelength of the incident light G01N21/63
• CPC Class Codes: G01N 21/21 Polarisation-affecting properties G01N21/19 takes precedence
• CPC Class Codes: G01N 21/314 with comparison of measurements at specific and non-specific wavelengths dual wavelength spectrometry G01J3/427
• CPC Class Codes: G01N 21/4738 Diffuse reflection precedence is given to G01N21/55 - G01N21/57 if specular component is taken into consideration, e.g. also for testing fluids, fibrous materials
• CPC Class Codes: G01N 21/474 Details of optical heads therefor, e.g. using optical fibres
• CPC Class Codes: G01N 21/5911 of the scanning type scanning per se G02B
• CPC Class Codes: G01N 21/6456 Spatial resolved fluorescence measurements; Imaging
• CPC Class Codes: G01N 21/8422 Investigating thin films, e.g. matrix isolation method
• CPC Class Codes: G01N 21/8806 Specially adapted optical and illumination features
• CPC Class Codes: G01N 21/93 Detection standards; Calibrating baseline adjustment, drift correction
• CPC Class Codes: G01N 21/94 Investigating contamination, e.g. dust G01N21/85 takes precedence

Abstract

A system and process for obtaining near real time, non-destructive inspection and characterization of surfaces. The system includes an infrared light source which is directed on a surface to be inspected. A portion of the reflected light is gathered and directed through an optical filter arrangement which separates the light into a plurality of sets of wavelengths which correspond to particular physical properties of the thin film, such as absorbance. The intensity of each set of wavelengths is detected by optical detectors and the resulting signals analyzed to characterize the surface.

Claims

1. A system for characterizing a rough, metallic surface, comprising:

a light source capable of generating an incident beam of light;

means for directing the incident beam of light onto the surface to produce reflected light;

an optical filter arrangement configured to separate light into at least a first set of wavelengths which includes a target wavelength band, a second set of wavelengths which includes a first reference wavelength band outside the target band and a third set of wavelengths which includes a second reference band outside the target band and on the opposite side of the target band as the first reference band;

means for directing a component of the reflected light into said optical filter arrangement;

a first detector capable of detecting the intensity of light at the target band and of generating a signal corresponding to the detected intensity;

means for directing the first set of wavelengths of received light into said first detector;

a second detector capable of detecting the intensity of light at the first reference band and of generating a signal corresponding to the detected intensity;

a third detector capable of detecting the intensity of light at the second reference band and of generating a signal corresponding to the detected intensity;

means for directing the second set of wavelengths of received light into said second detector;

means for directing the third set of wavelengths of received light into said third detector, and

a signal processor in communication with said first and said second detectors for processing the signal generated by said first and said second detectors.

2. A system for characterizing a rough, metallic surface as defined in claim 1, further comprising a source detector capable of detecting the intensity of the incident beam of light and generating a signal corresponding to the detected intensity.

3. A system for characterizing a rough, metallic surface as defined in claim 2, further comprising means for modulating the incident beam such that the effect of any ambient light is substantially eliminated.

4. A system for characterizing a rough, metallic surface as defined in claim 1, wherein said means for directing the incident beam of light onto the surface to produce reflected light comprises a lens capable of focusing the incident beam at a discrete location on the surface.

5. A system for characterizing a rough, metallic surface as defined in claim 1, wherein said means for directing a component of the reflected light into said optical filter arrangement is configured to direct at least a portion of the back-scatter component of the reflected light.

6. A system for characterizing a rough, metallic surface as defined in claim 1, further comprising a scanning apparatus configured to change the point on the surface at which the incident beam of light is directed, thereby permitting the inspection of various discrete locations on the surface.

7. A system for characterizing a rough, metallic surface as defined in claim 6, further comprising communication means connecting said signal processor to said scanning apparatus such that the change of the point on the surface at which the beam of light is directed at the surface is capable of being synchronized with the processing of the signal generated by said detectors.

8. A system for characterizing a rough, metallic surface as defined in claim 7, further comprising an output device in communication with said signal processor for displaying data processed by said signal processor.

9. A system for characterizing a rough, metallic surface as defined in claim 1, further comprising at least one calibration plate having at least one contaminant of known thickness on its surface.

10. A system for characterizing a rough, metallic surface, comprising:

a light source capable of generating an incident beam of light;

means for directing the incident beam of light onto the surface to produce reflected light;

an optical filter arrangement comprising;

a first optical filter configured to reflect light corresponding to a first set of wavelengths while passing all remaining wavelengths of light; and

a second optical filter configured to reflect light corresponding to a second set of wavelengths while passing all remaining received light, said optical filter arrangement further configured such that the light received by said optical filter arrangement is separated into three beams which each correspond to a different set of wavelengths, wherein one of said beams includes a target wavelength band, wherein the second of said beams includes a first wavelength reference band outside of the target band and wherein the third of said beams includes a second wavelength reference band outside of the target band and on the opposite side of the target band as the first reference band;

means for directing a component of the reflected light into said optical filter arrangement;

a first detector capable of detecting the intensity of light at the target band and of generating a signal corresponding to the detected intensity;

a second detector capable of detecting the intensity of light at the first reference band and of generating a signal corresponding to the detected intensity;

a third detector capable of detecting the intensity of light at the second reference band and of generating a signal corresponding to the detected intensity;

means for directing the three beams of light separated by said optical filter arrangement into the detector corresponding to the set of wavelengths of the beam; and

a signal processor in communication with said detectors for processing the signal generated by said detectors.

11. A system for characterizing a rough, metallic surface as defined in claim 10, further comprising a source detector capable of detecting the intensity of the incident beam of light and generating a signal corresponding to the detected intensity.

12. A system for characterizing a rough, metallic surface as defined in claim 11, further comprising a chopper wheel for modulating the incident beam such that the effect of any ambient light is substantially eliminated.

13. A system for characterizing a rough, metallic surface as defined in claim 10, wherein said means for directing a component of the reflected light into said optical filter arrangement is configured to direct at least a portion of the back-scatter component of the reflected light.

14. A system for characterizing a rough, metallic surface as defined in claim 10, further comprising a scanning apparatus configured to change the point on the surface at which the incident beam of light is directed, thereby permitting the inspection of various discrete locations on the surface.

15. A system for characterizing a rough, metallic surface as defined in claim 14, further comprising communication means connecting said signal processor to said scanning apparatus such that the change of the point on the surface at which the beam of light is directed at the surface is capable of being synchronized with the processing of the signal generated by said detectors.

16. A system for characterizing a rough, metallic surface as defined in claim 15, further comprising an output device in communication with said signal processor for displaying data processed by said signal processor.

17. A system for characterizing a rough, metallic surface as defined in claim 10, further comprising a temperature sensor in communication with at least one of said detectors.

18. A system for inspecting a rough, metallic surface for the presence of a hydrocarbon contaminant, comprising:

an infrared light source capable of generating an incident beam of light;

a lens for directing the incident beam of light onto the surface, said lens positioned such that the incident beam is directed at the surface at an angle of incidence of about 45 degrees, thereby producing reflected light;

an optical filter arrangement configured for receiving a portion of the back-scatter component of the reflected light, said optical filter arrangement comprising;

a first high-pass filter configured to reflect light having a wavelength less than about 3.3 microns while passing all remaining wavelengths of light;

a first band-pass filter positioned to receive the light passed by said first high-pass filter, said first band-pass filter configured to pass light having a wavelength of from about 3.65 microns to about 3.75 microns while reflecting all remaining wavelengths of light;

a second band-pass filter positioned to receive the light reflected by said first high-pass filter, said second band-pass filter configured to pass light having a wavelength of from about 3.15 microns to about 3.25 microns; and

a third band-pass filter positioned to receive the light reflected by said first band-pass filter, said third band-pass filter configured to pass light having a wavelength of from about 3.35 microns to about 3.45 microns;

a first detector positioned to receive the light reflected from said first high-pass filter and through said second band-pass filter, said first detector capable of detecting the intensity of light and of generating a signal corresponding to the intensity of the received light;

a second detector positioned to receive the light reflected from said first band-pass filter and through said third band-pass filter, said second detector capable of detecting the intensity of light and of generating a signal corresponding to the intensity of the received light;

a third detector positioned to receive the light passed by said first band-pass filter, said third detector capable of detecting the intensity of light and of generating a signal corresponding to the intensity of the received light;

a signal processor in communication with said detectors for processing the signal generated by said detectors;

a scanning apparatus configured to change the point on the surface at which the incident beam of light is directed, thereby permitting the inspection of various discrete locations on the surface; and

an output device in communication with said signal processor for displaying data processed by said signal processor.

19. A system for inspecting a rough, metallic surface as defined in claim 18, further comprising a source detector capable of detecting the intensity of the incident beam of light and generating a signal corresponding to the intensity of the incident beam.

20. A system for inspecting a rough, metallic surface as defined in claim 19, further comprising a chopper wheel for modulating the incident beam such that the effect of any ambient light is substantially eliminated.

21. A system for inspecting a rough, metallic surface as defined in claim 18, further comprising a temperature sensor in communication with at least one of said detectors.

22. A process for characterizing a rough, metallic surface, comprising the steps of:

directing an incident beam of light onto a discrete location on the surface to be inspected;

gathering at least a portion of the light reflected off the surface;

introducing the gathered portion of the reflected light into an optical filter arrangement configured to separate the gathered portion of the reflected light into at least a first set of wavelengths which includes a target wavelength band, a second set of wavelengths which includes a first reference wavelength band outside the target band and a third set of wavelengths which includes a second reference wavelength band outside the target band and on the opposite side of the target band as the first reference band;

monitoring the intensity of the gathered portion of the reflected light at the target band and at the first and second reference bands by directing the first set of wavelengths into a first detector capable of detecting the intensity of light at the target band and of producing a signal corresponding to the detected intensity, directing the second set of wavelengths into a second detector capable of detecting the intensity of light at the first reference band and of producing a signal corresponding to the detected intensity and directing the third set of wavelengths into a third detector capable of detecting the intensity of light at the second reference band and of producing a signal corresponding to the detected intensity; and

analyzing the signals produced by the detectors to characterize the surface.

23. A process for characterizing a rough, metallic surface as defined in claim 22, wherein said introducing step comprises introducing the gathered portion of the reflected light into a first optical filter configured to reflect light generally corresponding to the second set of wavelengths while passing all remaining wavelengths of light and introducing the light passed by the first optical filter into a second optical filter configured to reflect light corresponding to the first set of wavelengths while passing all remaining received light.

24. A process for characterizing a rough, metallic surface as defined in claim 22, wherein the step of directing an incident beam of light onto a discrete location on the surface to be inspected comprises directing the incident beam of light at an approximate 45 degree angle to the surface.

25. A process for characterizing a rough, metallic surface as defined in claim 22, wherein said analyzing step is followed by the step of redirecting the beam of light onto a different discrete location on the surface to be inspected and repeating the preceding steps, whereby representative locations on the surface are inspected.

26. A process for characterizing a rough, metallic surface as defined in claim 22, wherein said gathering step includes gathering at least a portion of the back-scatter component of the reflected light.

27. A process for characterizing a rough, metallic surface as defined in claim 26, wherein the surface is a one-dimensionally rough surface and said directing step includes directing the beam onto the surface at a direction perpendicular to the lines which comprise the surface roughness.

28. A process for characterizing a rough, metallic surface as defined in claim 22, further comprising the step of detecting the intensity of the incident beam of light with a source detector, generating a signal corresponding to the intensity of the incident beam and normalizing the signals produced by the first, second and third detectors to reduce the effect of fluctuations in the intensity of the incident beam of light.

29. A process for characterizing a rough, metallic surface as defined in claim 28, further comprising the step of substantially eliminating the effect of ambient light by modulating the incident beam with a chopper wheel.

30. A process for characterizing a rough, metallic surface as defined in claim 22, further comprising the step of monitoring the temperature of at least one of the detectors and adjusting the signals produced by the detectors to reduce the effect of fluctuations in the temperature of the detectors.

31. A process for characterizing a rough, metallic surface as defined in claim 22, wherein said analyzing step includes processing the signal produced by the detectors with a computer.

32. A process for characterizing a rough, metallic surface as defined in claim 31, wherein the step of processing the signal produced by the detectors is synchronized with the step of redirecting the beam of light onto a different location on the surface to be inspected.

33. A process for characterizing a rough, metallic surface as defined in claim 22, wherein said monitoring step includes monitoring the intensity of light at a target band of about eight microns to thereby characterize a silicone release agent.

34. A process for characterizing a rough, metallic surface as defined in claim 22, wherein said monitoring step includes monitoring the intensity of light at a target band of from about three to about four microns to thereby characterize a hydrocarbon.

35. A process for characterizing a rough, metallic surface as defined in claim 22, wherein said directing step includes focusing the beam onto a discrete location on the surface with a lens.

36. A process for characterizing a rough, metallic surface as defined in claim 22, wherein said directing step is preceded by calibrating the system with at least one calibration plate having at least one contaminant of known thickness on its surface.

37. A process for characterizing a rough, metallic surface as defined in claim 36, wherein said analyzing step further includes determining the thickness of the contamination.

38. A process for characterizing a rough, metallic surface as defined in claim 37, wherein said identifying step further includes displaying a color scale image corresponding to the thickness of the contamination.

39. A process for characterizing a rough, metallic surface as defined in claim 37, wherein said identifying step further includes displaying a surface map of the contamination.