Abstract

Novel coherent optical defect detecting methods, which can distinguish between opaque defects and transparent defects, are proposed. One of these novel methods makes use of a holographylike polarity reference signal and is suited to automatic detection systems. The other employs two independent light sources, and suitable for visual detection systems. Realizing these novel methods using omnidirectional spatial filters, simple but effective periodic pattern defect-detecting systems are formed, which can be easily accommodated with defect touching-up stages.

© 1981 Optical Society of America

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References

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  1. L. S. Watkins, Proc. IEEE, 57, 1634 (1969).
    [CrossRef]
  2. A. Iwamoto, H. Sekizawa, Appl. Opt. 19, 1196 (1980).
    [CrossRef] [PubMed]
  3. R. J. Collier, C. B. Burckhardt, L. H. Lin, Optical Holography (Academic, New York, 1971).

1980 (1)

1969 (1)

L. S. Watkins, Proc. IEEE, 57, 1634 (1969).
[CrossRef]

Burckhardt, C. B.

R. J. Collier, C. B. Burckhardt, L. H. Lin, Optical Holography (Academic, New York, 1971).

Collier, R. J.

R. J. Collier, C. B. Burckhardt, L. H. Lin, Optical Holography (Academic, New York, 1971).

Iwamoto, A.

Lin, L. H.

R. J. Collier, C. B. Burckhardt, L. H. Lin, Optical Holography (Academic, New York, 1971).

Sekizawa, H.

Watkins, L. S.

L. S. Watkins, Proc. IEEE, 57, 1634 (1969).
[CrossRef]

Appl. Opt. (1)

Proc. IEEE (1)

L. S. Watkins, Proc. IEEE, 57, 1634 (1969).
[CrossRef]

Other (1)

R. J. Collier, C. B. Burckhardt, L. H. Lin, Optical Holography (Academic, New York, 1971).

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Figures (6)

Fig. 1
Fig. 1

Defect detecting optical system. An object illuminated by He-Ne laser light and placed at the front focal plane of a Fourier transform lens makes its Fourier pattern at the back focal plane of the lens. A specially prepared spatial filter inserted at that plane detects unwanted patterns. The inverse Fourier-transform lens, whose front focal plane coincides with the filter plane, makes the filtered unwanted (defects) pattern at the output plane, which is a conjugate plane to the input. Filtered light from an incandescent lamp is introduced coaxially to the optical system via a half-mirror. The object pattern illuminated by this incoherent light is also imaged on the output plane.

Fig. 2
Fig. 2

Defects pattern in a periodic mesh pattern. An opaque (or intrusive defect) is situated at the left and a transparent (or extrusive) defect at the right.

Fig. 3
Fig. 3

Experimental results with object’s dc method. Signal levels of detected defects are approximately proportional to defect sizes, including the polarity, and the defect-free area is imaged at medium level (a). When incoherent green light is added to coherent red light, the color of the defect-free area turns yellow proportionally to the amount of the incoherent light (b).

Fig. 4
Fig. 4

Scanned electric data from the object’s dc method optics. An electronic detector is placed at the output plane and scanned over the detected area. The medium level corresponds to the defect-free area and pulses of both polarities correspond to defects. Scanning aperture diameter for electronic converter is ~1 mmϕ.

Fig. 5
Fig. 5

Experimental results with the color mixing method. In this optical system without incoherent light, all the defects make bright red spots against a black background at the output plane (a). When green tinted incoherent light of adequate level is added, bright red spots turn yellow or remain red according to defect types and the black background turn green (b).

Fig. 6
Fig. 6

Defect colors on a CIE chromaticity diagram. Defect colors are plotted on a CIE chromaticity diagram, where points according to color mixing method (bandpass filter scheme) are denoted by Δ and the object’s dc method (low pass filter scheme) is denoted by ○. From the visual inspection standpoint, color contrast in a bandpass scheme is superior to that in a low pass scheme. (▲ and ● stand for transparent defect colors, and Δ and ○ denote opaque defect colors.

Equations (4)

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± d = o n .
( o d ± d ) 2 = ± 2 o d · d + ( o d 2 + d 2 ) ,
ξ c λ f p ( 1 1 N p ) ,
SNR = 2 o d · d d 2 + o d 2 = 2 d / o d 1 + ( d / o d ) 2 2 d / o d

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