Optical inspection

• US 4,191,476 A
• Filed: 03/13/1978
• Issued: 03/04/1980
• Est. Priority Date: 03/15/1977
• Status: Expired due to Term

• Current Assignee: British Technology Group Ltd.
• Original Assignee: National Research Development Corporation
• Inventors: Pollard, Harry C.
• Primary Examiner(s): Corbin, John K.
• Assistant Examiner(s): Koren, Matthew W.
• Application Number:
  US05/886,130
• Time in Patent Office: 722 Days
• Field of Search: 356/347, 356/349, 356/359, 356/360
• US Class Current: 356/489
• CPC Class Codes: G01B 11/2441 using interferometry
• CPC Class Codes: G01B 9/02007 Two or more frequencies or sources used for interferometric measurement using only beat G01B9/02003
• CPC Class Codes: G01B 9/02094 Speckle interferometers, i.e. for detecting changes in speckle pattern

Abstract

In a method of optical inspection of a surface, two patterns of illumination are formed independently on a photosensitive screen respectively by means of light of two different wavelengths. The patterns are formed in the same manner and each results from irradiation of the screen with light derived from a coherent source and consisting of two interfering beams, one of which is constituted by light scattered from the surface under inspection and imaged on to the screen. A video signal is derived from the screen representing the spatial variations in the sum of the intensities in the two patterns.

Claims

1. A method of optical inspection of a surface, the method comprising:

forming two patterns of illumination independently on a photosensitive screen respectively by means of light of two different wavelengths, said patterns being formed in the same manner and each of them resulting from the irradiation of said screen with light derived from a coherent source and consisting of first and second interfering beams, said first beam for each of said patterns being constituted by light scattered from said surface and imaged on to said screen; and

deriving from said screen a video signal representing the spatial variations in the sum of the intensities in said two patterns of illumination.

2. A method according to claim 1, further comprising processing said video signal to remove its d.c. component and then rectifying the resultant to provide a unidirectional signal.

3. A method according to claim 2, further comprising utilising said unidirectional signal as a picture signal to generate a television picture.

4. A method according to claim 1, in which said second beam for each of said patterns has a smooth wavefront.

5. A method according to claim 1, in which said second beam for each of said patterns is constituted by light scattered from a surface different from, but having a similar form to, said surface and imaged on to said screen in the same manner as for said first beam.

6. A method according to claim 1, in which said two patterns of illumination are formed successively on said screen, said screen having an integrating characteristic in respect of incident light.

7. A method according to claim 1, in which said two patterns of illumination are formed simultaneously on said screen.

8. An apparatus for use in optical inspection, the apparatus comprising:

a television camera device incorporating a photosensitive screen;

an optical system for imaging on to said screen light from surfaces viewed by said camera device;

means for causing, in respect of a surface viewed by said camera device, the formation independently on said screen, respectively by means of light of two different wavelengths, of two patterns of illumination which are formed in the same manner and each of which results from the irradiation of said screen with light derived from a coherent source and consisting of first and second interfering beams, said first beam for each of said patterns being constituted by light scattered from said surface and imaged on to said screen by said optical system; and

means for operating said camera device to derive a video signal representing the spatial variations in the sum of the intensities in said two patterns of illumination.

9. An apparatus according to claim 8, further comprising means for processing said video signal to remove its d.c. component, and means for rectifying the resultant to provide a unidirectional signal.

10. An apparatus according to claim 9, further comprising means for utilising said unidirectional signal as a picture signal to generate a television picture.

11. An apparatus according to claim 8, in which said means for causing the formation of two patterns of illumination comprises means for causing said second beam for each of said patterns to have a smooth wavefront.

12. An apparatus according to claim 8, in which there are provided means for enabling said camera device to view two surfaces having a similar form simultaneously in a similar manner, and in which said means for causing the formation of two patterns of illumination comprises means for causing said interfering beams for each of said patterns to be constituted by light respectively scattered from said two surfaces and imaged on to said screen by said optical system.

13. An apparatus according to claim 8, in which said means for causing the formation of two patterns of illumination comprises means for causing said patterns to be formed successively, said screen having an integrating characteristic in respect of incident light.

14. An apparatus according to claim 8, in which said means for causing the formation of two patterns of illuminations comprises means for causing said patterns to be formed simultaneously.