Abstract

We have extended the use of shadow Moiré technique to be implemented in simple curved surfaces by using a flexible grating. Dynamic visual inspection of surface micro-damages is significantly favored by the use of well adapted pliable gratings compared to the use of flat reference gratings. The experimental set-up consists of a plastic foil with a printed Ronchi grating stretched between three points which adapts to any cylindrical or conical convex surface independently of the relative orientation grating/surface. Static quantification of defects profiles is also possible with an attached CCD camera. Visual detection of defects in the range of ~30 µm in depth is obtainable.

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References

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  1. O. Kafri, I. Glatt, The Physics of Moiré Metrology, (J. Wiley and Sons, New York, 1990)
  2. K. Patorski, Handbook of the Moiré fringe technique, (Elsevier, Amsterdam, 1993).
  3. Jae-Sun-Lim, Jongsu-Kim, Myung-Sai-Chung, "Automatic shadow moire topography: a moving-light-source method," Opt. Lett. 14, 1252-1253 (1989).
    [CrossRef]
  4. J. M. Burg, "The metrological applications of diffraction gratings," Prog. Opt. 2, 75-107 (1961).
  5. H. Takasaki, "Moire topography", Appl. Opt. 9, 1467-1472 (1970).
    [CrossRef] [PubMed]
  6. J. Shamir, "Moire gauging by projected interference fringes," Opt. Las. Tech. 5, 78-86 (1973).
    [CrossRef]
  7. S. H. Rowe, W. T. Weldford, "Surface topography of non-optical surfaces by projected interference fringes," Nature 216, 786-787 (1967).
    [CrossRef]
  8. X. Xinjun, J. T. Atkinson, M. J. Lalor, D. R. Burton, "Three-map absolute moire contouring," Appl. Opt. 35, 6990-6995 (1996).
    [CrossRef]
  9. G. Mauvoisin, F. Bremand, A. Lagarde, "Three-dimensional shape reconstruction by phase-shifting shadow moire," Appl. Opt. 33, 2163-2169 (1994).
    [CrossRef] [PubMed]
  10. L. Pirodda, "Shadow and projection moire techniques for absolute or relative mapping of surface shapes," Opt. Eng. 21, 640-649 (1982)
  11. A. S. Redner, "Shadow-Moiré surface inspection," Materials Evaluation 48, 873-878 (1990).
  12. D. R. Andrews, "Shadow moire contouring of impact craters," Opt. Eng. 21, 650-654 (1982).
  13. J. Marasco, "Use of a curved grating in shadow Moiré," Exp. Mech. 15, 464-470 (1975).
    [CrossRef]
  14. A. M. F. Wegdam, O. Podzimek, H. T. Bosing, "Simulation of shadow moire systems containing a curved grating surface," Appl. Opt. 31, 5952-5955 (1992).
    [CrossRef] [PubMed]
  15. E. A. Patterson, M. Heredia, internal communication, European project "INDUCE" ref. BRPR-CT98-0805.

Other (15)

O. Kafri, I. Glatt, The Physics of Moiré Metrology, (J. Wiley and Sons, New York, 1990)

K. Patorski, Handbook of the Moiré fringe technique, (Elsevier, Amsterdam, 1993).

Jae-Sun-Lim, Jongsu-Kim, Myung-Sai-Chung, "Automatic shadow moire topography: a moving-light-source method," Opt. Lett. 14, 1252-1253 (1989).
[CrossRef]

J. M. Burg, "The metrological applications of diffraction gratings," Prog. Opt. 2, 75-107 (1961).

H. Takasaki, "Moire topography", Appl. Opt. 9, 1467-1472 (1970).
[CrossRef] [PubMed]

J. Shamir, "Moire gauging by projected interference fringes," Opt. Las. Tech. 5, 78-86 (1973).
[CrossRef]

S. H. Rowe, W. T. Weldford, "Surface topography of non-optical surfaces by projected interference fringes," Nature 216, 786-787 (1967).
[CrossRef]

X. Xinjun, J. T. Atkinson, M. J. Lalor, D. R. Burton, "Three-map absolute moire contouring," Appl. Opt. 35, 6990-6995 (1996).
[CrossRef]

G. Mauvoisin, F. Bremand, A. Lagarde, "Three-dimensional shape reconstruction by phase-shifting shadow moire," Appl. Opt. 33, 2163-2169 (1994).
[CrossRef] [PubMed]

L. Pirodda, "Shadow and projection moire techniques for absolute or relative mapping of surface shapes," Opt. Eng. 21, 640-649 (1982)

A. S. Redner, "Shadow-Moiré surface inspection," Materials Evaluation 48, 873-878 (1990).

D. R. Andrews, "Shadow moire contouring of impact craters," Opt. Eng. 21, 650-654 (1982).

J. Marasco, "Use of a curved grating in shadow Moiré," Exp. Mech. 15, 464-470 (1975).
[CrossRef]

A. M. F. Wegdam, O. Podzimek, H. T. Bosing, "Simulation of shadow moire systems containing a curved grating surface," Appl. Opt. 31, 5952-5955 (1992).
[CrossRef] [PubMed]

E. A. Patterson, M. Heredia, internal communication, European project "INDUCE" ref. BRPR-CT98-0805.

Supplementary Material (1)

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Figures (8)

Fig. 1
Fig. 1

Geometrical scheme of the classical shadow Moiré technique

Fig. 2
Fig. 2

a) Direct observation of a flat surface test with an indentation of 350µm in depth practically invisible to the naked-eye. b) The same with an interposed Ronchi grating of 10 lines/mm making clearly apparent an indentation and which illustrates the enhancement of visual inspection aided by shadow Moiré. c) When the grating is tilted few millimeters over the test it appears several fringes with decreasing contrast due to diffraction. Curved surfaces produce similar patterns with low contrast which finally influences the detectability of defects and reduces the field of view drastically.

Fig. 3:
Fig. 3:

Curved panel used to make the shadow Moiré tests (radius of curvature r is 2 m). It is made of composite material and typically used in aeronautic applications (r>1-2m).

Fig. 4:
Fig. 4:

First prototype in use over a curved panel. In this case the adaptation needs a right alignment and the tensile stress on the foil is somewhat inhomogeneous. It gives poorer results that the tripod approach. We may appreciate moiré fringes in the central area due to a non-perfect adaptation disturbing a reliable detection of surface deviations.

Fig. 5:
Fig. 5:

Tripod solution for the adaptation of flexible gratings to curved surfaces. The grating is stretched between three contacting points. Tensile load is provided by a spring. The uniform gray appearance of the grating is a signal of good adaptation to the curved surface. Local profile deviations produce a stain like appearance. The source is a commercial 50 W halogen lamp with a dichroic reflector and a rectangle narrow aperture aligned parallel to the grating.

Fig. 6
Fig. 6

Left: shadow Moiré with a curved panel and a flat grating (11 lines/mm). Right: shadow Moiré over the same area (left) with a flexible grating (10 lines/mm) mounted on the tripod support. Indentations (like a hoof track) are detected left to the numeric labels. A protrusion can be appreciated right between the labels due to a particle originated in the painting. Notice the different appearance (a white small point in the middle of the Moiré pattern).

Fig. 7.
Fig. 7.

Underlying structures pointed out by red arrows are readily visible through the use of the flexible grating shadow Moiré system. In the center of the image there is an area damaged by scratches.

Fig. 8.
Fig. 8.

(1.07 MB Movie). When moving the grating it is easier to distinguish real surface deviations from external inclusions (dirt and air bubbles) which have different behaviors. The tissue texture appreciated in the picture is due to the structural nature of the composite panel. There are two screws on the plastic grating as a reference.

Equations (3)

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z = d ( tan α S + tan α O ) ,
I = A + B cos [ 2 π z ( tan α S + tan α O ) d ] ,
δ z = d 50 ,

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