Mura detection apparatus and method

• US 5,917,935 A
• Filed: 05/22/1996
• Issued: 06/29/1999
• Est. Priority Date: 06/13/1995
• Status: Expired due to Fees

• Current Assignee: Photon Dynamics Incorporated (KLA Corporation)
• Original Assignee: Photon Dynamics Incorporated (KLA Corporation)
• Inventors: Hawthorne, Jeffrey A., Setzer, Joseph
• Primary Examiner(s): Kelley, Christopher S
• Assistant Examiner(s): CHAWAN, SHEELA C
• Application Number:
  US08/651,417
• Time in Patent Office: 1,133 Days
• Field of Search: 382/141, 382/143, 382/149, 382/145, 382/176, 382/203, 382/260, 382/279, 382/266, 348/125, 348/126, 345/189, 345/191, 345/904
• US Class Current: 382/149
• CPC Class Codes: G02F 1/1303 Apparatus specially adapted to the manufacture of LCDs
• CPC Class Codes: G02F 1/1309 Repairing; Testing
• CPC Class Codes: G02F 1/136254 Checking; Testing
• CPC Class Codes: G02F 1/136259 Repairing; Defects
• CPC Class Codes: G09G 3/006 Electronic inspection or testing of displays and display drivers, e.g. of LED or LCD displays testing individual LED's G01R31/2635; testing lamps G01R31/44; testing of optical features of LCD displays G02F1/1309

Abstract

A method and apparatus for identifying and classifying pixel defects, and in particular Mura defects using digital processing techniques. The present method includes steps of acquiring an image with a Mura defect, and performing a Laplacian convolution on the image to enhance the Mura defect against background illumination. A step of thresholding the Mura defect against the background illumination is also provided. The thresholded Mura defect is compared against the original Mura defect to define statistical parameters of the original Mura defect. An annular region is defined around the periphery of the Mura defect. Statistics of the annular region defines statistics for background illumination as compared to the original Mura defect. The statistics from the Mura defect are then compared to the background illumination statistics for Mura defect classification and analysis.

Claims

1. A method for detecting Mura defects on a substrate for a flat panel display, the method comprising the steps of:

a) acquiring an image of a first plurality of pixels in at least a portion of the substrate, the image including values of pixels from the first plurality of pixels;

b) enhancing differences in the values of pixels in the first plurality of pixels to form an enhanced image, the enhanced image including values of pixels from the first plurality of pixels;

c) thresholding the enhanced values of pixels from the first plurality of pixels to form a thresholded image, the thresholded image including thresholded values of pixels from the first plurality of pixels;

d) identifying a second plurality of pixels from the first plurality of pixels corresponding to at least one blob within the thresholded image;

e) identifying a third plurality of pixels from the first plurality of pixels corresponding to an annular region around at least one blob within the thresholded image;

f) comparing values of the second plurality of pixels to values of the third plurality of pixels;

g) determining a Mura defect in response to the comparison step.

2. The method of claim 1 wherein the enhancing step comprises the step of performing filters on the image to form the enhanced image.

3. The method of claim 1 wherein the enhancing step comprises performing an edge enhancement convolution.

4. The method of claim 2 wherein the filters are directional Laplacian kernels having directional orientation from 1 degree to 90 degrees.

5. The method of claim 2 wherein the filters are directional Laplacian kernels having directional orientation every 15 degrees beginning from 0 degree.

6. The method of claim 1 wherein the enhancing step comprises the steps of:

performing a plurality of filters individually on the image to form a plurality of filtered images; and

combining the plurality of filtered images to form the enhanced image.

7. The method of claim 2 wherein the filters are defect specific filters.

8. The method of claim 1 further comprising the steps of:

after step g), subsampling the image to form a subsampled image; and

repeating steps b)-g) using the subsampled image.

9. The method of claim 1 wherein the step of thresholding further comprises the step of:

determining a mean pixel value for the enhancement values of the pixels in the first plurality of pixels;

determining a standard deviation pixel value for the enhanced values of the pixels in the first plurality of pixels;

determining a threshold pixel value for the enhanced values of the pixels in the first plurality of pixels in response to the mean pixel value and the standard deviation pixel value; and

applying the threshold pixel value to the enhanced values of the pixels in the first plurality of pixels to form the thresholded image.

10. The method of claim 1 wherein the step of thresholding further comprises the step of:

forming a histogram from the enhanced values of pixels in the first plurality of pixels;

determining a range of potentially defective pixel values in response to the histogram; and

thresholding pixels in the first plurality of pixels in response to the range of potentially defective pixel values to form the thresholded image.

11. The method of claim 1 further comprising the step of:

calculating an average pixel value for the second plurality of pixels;

wherein the step of identifying a third plurality of pixels comprises the step of;

dilating the second plurality of pixels corresponding to at least one blob to form a fourth plurality of pixels, the fourth plurality of pixels including the second plurality of pixels;

forming the annular region around the at least one blob by inhibiting the third plurality of pixels from the fourth plurality of pixels;

calculating an average background value by averaging pixel values for the third plurality of pixels; and

wherein the step of comparing comprises the step of;

comparing the average pixel value to the average background value.

12. The method of claim 11 wherein the step of calculating an average pixel value further comprises the step of calculating a standard deviation value for the second plurality of pixels.

13. The method of claim 11 wherein the step of calculating an average background value further comprises the step calculating a standard deviation for the third plurality of pixels.

14. The method of claim 13 wherein the step of comparing the average pixel value further comprises the step of comparing the standard deviation for the second plurality pixels to the standard deviation for the third plurality of pixels.

15. The method of claim 1 further comprising the step of characterizing specific attributes of the Mura defect.

16. The method of claim 1 further comprising the step of determining the cause of the Mura defect in response to the specific attributes of the Mura defect.

44. A liquid crystal display substrate inspected according to the method described in claim 1.

45. A display device including a liquid crystal display substrate inspected according to the method described in claim 1.

17. A method identifying a Mura defect on a substrate, comprising steps of:

acquiring an image comprising a blob defined onto a background image and storing the image as frame data, the blob being defined by first pixels and the background image being defined by second pixels;

thresholding the image to create a binary representation of the blob and the background image, the binary representation being defined by the first pixels at a first value and the second pixels at a second value;

calculating an average blob pixel value of the blob from pixels in the image corresponding to the blob;

dilating the binary representation of the blob by converting a portion of the second pixels at the second value adjacent to the first pixels at the first value to second pixels at the first value, the dilating step defining a dilated binary representation of the blob;

performing an XOR of the binary representation of the blob with the dilated binary representation of the blob to define an annular region comprising the second pixels at the first value;

calculating an average background image pixel value of the background image from pixels in the image corresponding to the annular region; and

comparing the average background image pixel value with the average blob pixel value to identify a Mura defect.

18. A computer system for detecting Mura defects on a substrate for a flat panel display, the substrate including a first plurality of pixels, the computer system comprising:

an image acquisition device for acquiring an image of at least a portion of the substrate, the portion including a second plurality of pixels, the image including values of pixels in the second plurality of pixels;

an enhancer for enhancing differences in the values of pixels in the second plurality of pixels to form an enhanced image, the enhanced image including enhanced values of pixels in the second plurality of pixels;

a thresholder for thresholding the enhanced values of pixels in the second plurality of pixels to form a thresholded image, the thresholded image including thresholded values of pixels in the second plurality of pixels;

an identifier for identifying a third plurality of pixels forming at least one blob within the portion of the substrate in response to the thresholded image;

a comparator for comparing values of the third plurality of pixels to values of pixels corresponding to an annular region around the at least one blob; and

a determiner coupled to the comparator for determining a Mura defect.

19. The computer system of claim 18 wherein the enhancer further comprises a convolver for performing an edge enhancement convolution.

20. The computer system of claim 18 wherein the comparator further comprises:

a first averager for calculating an average pixel value for the third plurality of pixels;

a dilator for dilating the at least one blob to form a fourth plurality of pixels, the fourth plurality of pixels including the third plurality of pixels;

a neighborhood determiner for forming the annular region around the at least one blob by inhibiting the third plurality of pixels from the fourth plurality of pixels;

a second averager for calculating an average background value by averaging pixel values for pixels within the annular region; and

a comparator for comparing the average pixel value to the average background value.

21. A computer system for detecting Mura defects on a substrate for a flat panel display, the substrate including a first plurality of pixels, the computer system comprising:

a computer-readable memory including;

code that directs an image acquisition device to acquire an image of at least a portion of the substrate, the portion including a second plurality of pixels, the image including values of pixels in the second plurality of pixels;

code that enhances differences in the values of pixels in the second plurality of pixels to form an enhanced image, the enhanced image including enhanced values of pixels in the second plurality of pixels;

code that thresholds the enhanced values of pixels in the second plurality of pixels to form a thresholded image, the thresholded image including thresholded values of pixels in the second plurality of pixels;

code that identifies a third plurality of pixels forming at least one blob within the portion of the substrate in response to the thresholded image;

code that compares values of the third plurality of pixels to values of pixels corresponding to an annular region around the at least one blob; and

code that determines a Mura defect in response to the comparison step.

22. The computer system of claim 21 wherein the computer-readable memory also includes code that performs an edge enhancement convolution.

23. The computer system of claim 21 wherein the computer-readable memory also includes:

code that calculates an average pixel value for the third plurality of pixels;

code that dilates the at least one blob to form a fourth plurality of pixels, the fourth plurality of pixels including the third plurality of pixels;

code that forms the annular region around the at least one blob by inhibiting the third plurality of pixels from the fourth plurality of pixels;

code that calculates an average background value by averaging pixel values for pixels within the annular region; and

code that compares the average pixel value to the average background value.

24. A method for detecting Mura defects in a substrate for a liquid crystal display, the substrate having a plurality of pixels, the method comprising the steps of:

acquiring an image of the substrate;

creating a plurality of subsampled images from the image;

determining locations of potential Mura defects in the substrate in response to locations of Mura defects in each of the plurality of subsampled images; and

determining Mura defects in the substrate in response to the locations of potential Mura defects in each of the plurality of subsampled images.

25. The method of claim 24 further comprising the steps of:

applying a filter to each of the plurality of subsampled images to form a plurality of filtered images;

thresholding each of the plurality of filtered images to form a plurality of thresholded images; and

wherein the step of determining locations further comprises determining locations of potential Mura defects in the substrate in response to locations of pixels in each of the plurality of thresholded images exceeding a threshold value;

wherein the step of detecting Mura defects further comprises detecting Mura defects in the substrate in response to the locations of the potentially defective pixels in the substrate.

26. The method of claim 24 further comprising the steps of:

inhibiting offset components from pixels in the image; and

inhibiting gain components from the pixels in the image.

27. The method of claim 25 wherein the filter comprises an edge-detect filter.

28. The method of claim 25 wherein the filter is a convolution.

29. The method of claim 25 wherein the filter is a Laplacian filter.

30. The method of claim 25 wherein the step of thresholding each of the plurality of filtered images comprises:

determining a plurality of mean pixel values, one mean pixel value for each of the plurality of filtered images;

determining a plurality of standard deviations, one standard deviation for each of the plurality of filtered images;

determining a plurality of threshold pixel values, one threshold pixel value for each of the plurality of images in response to the mean pixel value and the standard deviation for the image; and

applying the plurality of threshold pixel values to each respective image in the plurality of images to form the plurality of thresholded images.

31. The method of claim 24 wherein the step of determining locations of potentially defective pixels comprises the steps of:

determining locations of a first plurality of potentially defective pixels in the substrate in response to locations of pixels in a first subsampled image having anomalous values;

determining locations of a second plurality of potentially defective pixels in the substrate in response to locations of pixels in a second subsampled image having anomalous values; and

wherein the step of determining Mura defects comprises the steps of;

comparing locations of the first plurality of potentially defective pixels in the substrate to locations of the second plurality of potentially defective pixels in the substrate, to form a third plurality of potentially defective pixels in the substrate; and

determining Mura defects in the substrate in response to the third plurality of potentially defective pixels in the substrate.

41. A liquid crystal display substrate inspected according to the method described in claim 24.

42. A computer system manufactured with a liquid crystal display substrate inspected according to the method described in claim 24.

43. A display device including a liquid crystal display substrate inspected according to the method described in claim 24.

32. A method for detecting Mura defects in a substrate for a liquid crystal display, the substrate having a plurality of pixels, the method comprising the steps of:

acquiring an image of the substrate;

applying a plurality of filters individually to the image to form a plurality of filtered images, the plurality of filters having different frequency cut-offs;

thresholding each of the plurality of filtered images to form a plurality of thresholded images; and

determining locations of potentially defective pixels in the substrate in response to locations of pixels in each of the plurality of thresholded images exceeding a threshold value; and

detecting Mura defects in the substrate in response to the locations of the potentially defective pixels in the substrate.

33. The method of claim 32 further comprising the steps of:

inhibiting offset components from pixels in the image; and

inhibiting gain components from the pixels in the image.

34. The method of claim 32 wherein the plurality of filters are convolutions.

35. The method of claim 32 wherein the plurality of filters are Laplacian filters.

36. The method of claim 32 wherein the step of thresholding each of the plurality of filtered images comprises:

determining a plurality of mean pixel values, one mean pixel value for each of the plurality of filtered images;

determining a plurality of standard deviations, one standard deviation for each of the plurality of filtered images;

determining a plurality of threshold pixel values, one threshold pixel value for each of the plurality of images in response to the mean pixel value and the standard deviation for the image; and

applying the plurality of threshold pixel values to each respective image in the plurality of images to form the plurality of thresholded images.

37. The method of claim 32 wherein the step of determining locations of potentially defective pixels comprises the steps of:

determining locations of a first plurality of potentially defective pixels in the substrate in response to locations of pixels in a first filtered image having anomalous values;

determining locations of a second plurality of potentially defective pixels in the substrate in response to locations of pixels in a second filtered image having anomalous values; and

wherein the step of determining Mura defects comprises the steps of;

comparing locations of the first plurality of potentially defective pixels in the substrate to locations of the second plurality of potentially defective pixels in the substrate, to form a third plurality of potentially defective pixels in the substrate; and

determining Mura defects in the substrate in response to the third plurality of potentially defective pixels in the substrate.

38. A computer system for detecting Mura defects in a substrate for a liquid crystal display, the substrate having a plurality of pixels, the computer system comprising:

an image acquiring device for acquiring an image of the substrate;

a sampler for creating a plurality of subsampled images from the image;

a pixel determiner for determining locations of potentially defective pixels in the substrate in response to locations of pixels in each of the plurality of subsampled images having anomalous values; and

a Mura determiner for determining Mura defects in the substrate in response to the locations of potentially defective pixels determined in the substrate.

39. The computer system of claim 38 further comprising:

a filter device for applying a filter to each of the plurality of subsampled images to form a plurality of filtered images;

a thresholding device for thresholding each of the plurality of filtered images to form a plurality of thresholded images;

wherein the pixel determiner comprises a potential pixel determiner for determining locations of potentially defective pixels in the substrate in response to locations of pixels in each of the plurality of thresholded images exceeding a threshold value; and

the Mura determiner for determining Mura defects in the substrate in response to the locations of the potentially defective pixels in the substrate.

40. A computer system including a computer program for detecting Mura defects in a substrate for a liquid crystal display, the substrate having a plurality of pixels, the computer system comprising:

a computer-readable memory including;

code that directs an image acquisition device to acquire an image of the substrate;

code that creates a plurality of subsampled images from the image;

code that determines locations of potentially defective pixels in the substrate in response to locations of pixels in each of the plurality of subsampled images having anomalous values; and

code that determines Mura defects in the substrate in response to the locations of potentially defective pixels determined in the substrate.