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Method for Inspection and Detection of Defects on Surfaces of Disc-Shaped Objects and Computer System with a Software Product for Carrying out the Method

  • US 20120163698A1
  • Filed: 11/10/2011
  • Published: 06/28/2012
  • Est. Priority Date: 12/22/2010
  • Status: Active Grant
First Claim
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1. A method for inspecting and detecting defects on surfaces of disc-shaped objects, comprising the following steps:

  • capturing an image of the respective disc-shaped object, wherein the entire surface is captured with a plurality of fields;

    deriving a defect-free reference of a pre-defined size from plural fields equal in size on the surface of the disc-shaped object;

    generating a difference image of the surface of the disc-shaped object, wherein the derived reference is subtracted from each field on the surface of the disc-shaped object;

    applying a color transformation to the difference image, wherein, by a suitable choice of transformation, signals in one channel are maximized, while undesired variations of the fields, caused by the production, are moved to a different channel;

    evaluating the signals, wherein a mean value, a variance, a maximum value and a minimum value are determined for each field;

    determining a distribution of the signals for each pixel of a field and a distribution of the signals for each defect to be examined, wherein the distribution is determined for each combination of transformation and detection channel used; and

    selecting that combination of transformation and detection channel, for which the largest number of pixels of a field with the defect to be found lie outside the spread of the pixels of the respective field on the surface of the respective disc-shaped object from production, and which results in the largest signals for the defect.

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