Method and apparatus for measuring shape of objects

• US 6,208,416 B1
• Filed: 03/10/1999
• Issued: 03/27/2001
• Est. Priority Date: 03/22/1996
• Status: Expired due to Term

• Current Assignee: Level 3 Inspection, LLC
• Original Assignee: Loughborough University Innovations Limited
• Inventors: Huntley, Jonathan Mark, Saldner, Henrik
• Primary Examiner(s): Kim, Robert
• Assistant Examiner(s): Lee, Andrew H.
• Application Number:
  US09/142,707
• Time in Patent Office: 748 Days
• Field of Search: 356/374, 356/512, 356/511
• US Class Current: 356/606
• CPC Class Codes: G01B 11/2536 using several gratings with variable grating pitch, projected on the object with the same angle of incidence

Abstract

There is disclosed a method for measuring the shape of objects which might have surface discontinuities using projected fringes, in which method the pitch of the fringes is varied over time and a sequence of phase-stepped images is recorded from which a three-dimensional complex intensity is calculated and either the phase of the distribution is unwrapped along the time axis or the Fourier transform of the distribution is calculated along the time axis.

Claims

1. A method for measuring the shape of objects which might have surface discontinuities using projected fringes, in which method the pitch of the fringes is varied over time and a sequence of phase-stepped images is recorded from which a three-dimensional complex intensity distribution is calculated.

2. A method according to claim 1, in which the phase of the distribution is unwrapped along the time axis.

3. A method according to claim 1, in which the Fourier transform of the distribution is calculated along the time axis.

4. A method according to claim 1, in which the phase maps are recorded as pixel images.

5. A method according to claim 1, in which the pitch of the fringes is varied by piezo-electric device means.

6. A method according to claim 1, in which the fringes are produced by Michelson interferometer means.

7. A method according to claim 6, in which phase and spatial frequency of the fringes are varied by plane mirrors.

8. A method according to claim 1, in which the fringes are generated by interference between laser light traversing dissimilar paths of which at least one is variable for changing phase and spatial frequency.

9. A method according to claim 1, in which the fringes are generated from incoherent light by a spatial light modulator.

10. Apparatus for measuring the shape of objects which might have surface discontinuities using projected fringes comprising fringe generating means projecting fringes on the object to be measured, phase stepping means to vary the phase of the fringes and fringe pitch varying means adapted to vary the pitch of the generated fringes over time, recording means adapted to record a sequence of phase-stepped images and computing means programmed to calculate a three-dimensional complex intensity distribution.

11. Apparatus according to claim 10, in which the computing means are programmed to unwrap the phase of the distribution along the time axis.

12. Apparatus according to claim 10, in which the programming means are programmed to calculate the Fourier transform along the time axis of the distribution.

13. Apparatus according to claim 12, in which the fringe generating means comprise Michelson interferometer means.

14. Apparatus according to claim 13, in which the fringe phase and pitch varying means comprise plane mirror means.

15. Apparatus according to claim 14, in which the interferometer means comprise a beamsplitter cube with orthogonal adjacent faces and adjustable mirrors in juxtaposition to said adjacent faces.

16. Apparatus according to claim 15, in which one mirror is adjustable towards and away from its associated face and at least one of the mirrors is adjustable as to tilt.

17. Apparatus according to claim 15, in which at least one mirror is adjustable by means of a piezo-electric device.

18. Apparatus according to claim 10, in which the fringe generating means comprise laser illuminated optical fibre means providing two dissimilar light paths.

19. Apparatus according to claim 18, in which the phase varying means comprise optical fibre length varying means.

20. Apparatus according to claim 19, in which said length varying means comprise a piezo-electric cylinder around which the fibre is wrapped and which is electrically actuated to vary its diameter.

21. Apparatus according to claim 10, in which the fringes are generated from incoherent light by a spatial light modulator.

22. Apparatus according to claim 21, in which the fringe generating means comprise a white light source, a condenser lens system, a spatial light modulator and a projection lens projecting the fringes on to the object to be measured.

23. Apparatus according to claim 10, in which the recording means comprise a pixel image recorder such as a video system.

24. Apparatus according to claim 10, in which the computing means comprise a processor arranged to operate in real time by carrying out parallel simple arithmetic operations on an entire pixel image frame simultaneously.