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

• US 5,541,413 A
• Filed: 09/26/1994
• Issued: 07/30/1996
• Est. Priority Date: 04/24/1992
• Status: Expired due to Fees

• Current Assignee: Northrop Grumman Innovation Systems, Inc. (Northrop Grumman Corporation)
• Original Assignee: Thiokol Corporation (Howmet Aerospace, Inc.)
• Inventors: Pearson, Lee H., Doyle, Timothy E., Johnson, Kendall B.
• Primary Examiner(s): Hannaher, Constantine
• Application Number:
  US08/307,734
• Time in Patent Office: 673 Days
• Field of Search: 356/445, 356/446, 250/301, 250/339.11, 250/341.8, 250/461.1
• 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 dust particles and microscopic contaminants in G01N21/94
• 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 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).

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 onto a discrete location on the surface;

an acousto-optic tunable filter positioned within the path of light, the filter tuned to pass light having a wavelength corresponding to a known 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 incident 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 corresponding 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 incident 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 wavelength of a predetermined material;

a detector positioned to receive the fluorescent beam emitted from the surface, the 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;

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 substantially 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 polarized 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, comprising the steps of:

generating an incident beam of light with a light source;

passing the incident beam of light through an acousto-optic tunable filter tuned to pass light corresponding to the absorption band of a predetermined material and at least one reference band outside the absorption band;

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 generating a signal corresponding to the intensity of each wavelength 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 off the surface includes gathering at least a portion of the back-scatter component of the scattered beam when the beam is scattered off a 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 wavelengths 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 predetermined 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 fluorescence 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 acousto-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 introducing 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.