Moire interferometry system and method with extended imaging depth

• US 5,835,218 A
• Filed: 07/22/1997
• Issued: 11/10/1998
• Est. Priority Date: 07/18/1995
• Status: Expired due to Fees

• Current Assignee: Dynamic Measurement and Inspection, LLC
• Original Assignee: Insutrial Technology Institute
• Inventors: Harding, Kevin G.
• Primary Examiner(s): Nelms, David C.
• Assistant Examiner(s): KIM, ROBERT H
• Application Number:
  US08/898,647
• Time in Patent Office: 476 Days
• Field of Search: 356/354, 356/359, 356/360, 356/374, 356/376, 250/237 G
• US Class Current: 356/521
• CPC Class Codes: G01B 11/2441 using interferometry
• CPC Class Codes: G01B 11/254 Projection of a pattern, viewing through a pattern, e.g. moiré
• CPC Class Codes: G01N 21/8806 Specially adapted optical and illumination features

Abstract

A moire interferometry system and method are provided for achieving full field surface contouring with an extended depth of view of image. The moire interferometry system includes a projection system generally made up of a light source, imaging lens and a square wave grating pattern. The imaging lens is configured to filter higher order light rays passing through the square wave grating pattern so as to project a sine wave like pattern onto a desired surface. The moire interferometry system also includes a viewing system generally made up of an imaging lens, a submaster grating and a camera. The submaster grating is preferably a customized grating that may be produced by recording a grating pattern in relation to a reference surface. The camera is able to view an image anywhere within the extended depth of image and analyze the moire fringes. A determination of deviation between a test part and a reference surface provides a part inspection system.

Claims

1. A moire interferometry system for measuring the shape of a contoured surface, said system comprising:

a projection system including a light source and a first periodic diffraction grating for projecting grating lines onto the contoured surface, said first periodic grating having a first array of square wave grating lines for producing square wave lines, and said projection system further including an imaging lens having a selected modulation transfer function, said imaging lens disposed between the first diffraction grating and the surface for filtering higher diffraction orders of said square wave lines so as to produce an array of sine wave like grating lines on said surface and provide an extended depth of view of image; and

a viewing system including a second periodic diffraction grating having a second array of grating lines, said viewing system further having a means for viewing the grating lines projected on said surface through said second array of grating lines within the extended depth of view of image, the intersection of the projected lines on the surface and the lines on the second periodic diffraction grating producing moire fringes.

2. The moire interferometry system as defined in claim 1 wherein said means for viewing comprises a camera focused within the extended depth of view of image.

3. The moire interferometry system as defined in claim 1 wherein said imaging lens is tilted with respect to the contoured surface so as to permit viewing at an angle in excess of a field angle of the imaging lens.

4. The moire interferometry system as defined in claim 1 wherein said second diffraction grating is custom produced in relation to a reference surface.

5. The moire interferometry system as defined in claim 1 further comprising a condenser lens optically coupled to the light source for providing directional light rays.

6. The moire interferometry system as defined in claim 1 wherein said light source produces white light rays.

7. A moire interferometry system for measuring the shape of a contoured surface, said system comprising:

a projection system including a light source, a first diffraction grating and an imaging lens for projecting grating lines onto the contoured surface, said imaging lens being tilted with respect to the contoured surface; and

a viewing system including a second diffraction grating having an array of grating lines and means for viewing the projected grating lines on said surface through said second diffraction grating within an extended depth of view of image, wherein said second diffraction grating is a customized phase grating that is formed with respect to a reference surface and compensates for variations caused by the imaging lens being tilted, the intersection of the projected lines on the surface and the lines on the submaster grating producing moire fringes.

8. The moire interferometry system as defined in claim 7 wherein said first periodic grating comprises an array of square wave grating lines.

9. The moire interferometry system as defined in claim 8 wherein said imaging lens filters out higher diffracted orders of light passing through the first periodic grating so as to produce an array of sine wave lines for projection onto the surface.

10. The moire interferometry system as defined in claim 7 wherein said means for viewing includes a camera focused within the extended depth of view of image.

11. The moire interferometry system as defined in claim 7 wherein said projection system further comprises a condenser lens optically coupled to the light source for providing directional light rays.

12. A method for measuring shape of a contoured surface using moire interferometry, said method comprising the steps of:

projecting a directional light beam through a first diffraction grating having an array of square wave grating lines so as to produce square wave lines;

filtering higher order diffraction orders from said square wave lines with an imaging lens having a selected modulation transfer function so as to pass a sine wave like pattern;

focusing said sine wave like pattern onto the contoured surface; and

viewing the sine wave like pattern on the contoured surface through a second diffraction grating within an extended depth of view of image so that the intersection of the projected lines on the surface and lines of the second diffraction grating produce moire fringes.

13. The method as defined in claim 12 further comprising the step of focusing a camera within the extended depth of view of image that is produced by an interference pattern as seen through the second diffracted grating.

14. The method as defined in claim 12 further comprising the step of tilting the imaging lens with respect to the contoured surface so as to permit viewing at an angle in excess of a field angle of the imaging lens.

15. The method as defined in claim 12 further comprising the step of custom recording the second diffraction grating while viewing the grating projection on a reference surface.

16. The method as defined in claim 12 further comprising the step of collecting light rays emitted by a light source to produce the directional light rays.

17. A method for measuring shape of a contoured surface using moire interferometry, said method comprising the steps of:

projecting a directional light beam through a first diffraction grating so as to produce grating lines of light;

focusing said grating lines of light through an imaging lens and onto the contoured surface, said imaging lens being tilted with respect to the contoured surface;

viewing the projection of grating lines on the contoured surface through a second diffraction grating so that the intersection of the projected lines on the surface and lines of the second diffraction grating produce moire fringes, wherein said second diffraction grating is a phase grating that is custom produced in relation to a reference surface and compensates for variations caused by the imaging lens being tilted.

18. The method as defined in claim 17 wherein said step of projecting directional light through the first diffraction grating produces an array of square wave grating lines.

19. The method as defined in claim 18 further comprising the step of filtering higher diffracted orders of said square wave grating lines so as to produce an array of sine wave like lines.