Thermography and k-means Clustering Methods for Anti-reflective Coating Film Inspection - Scratch and Bubble Defects
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2016 SPIE. Anti-reflective coating is widely used on telescopes, eyeglasses and screens to effectively enhance the transmission of light. However, the presence of defects such as bubbles or scratches lowers the usability and functionality of optical film. Optical cameras are often used for coating inspection, but their accuracy relies heavily on the illumination source, camera viewing angles and defect location. This paper describes an active thermography approach that can potentially overcome this issue. Eighteen scratch and bubble defects were located on AR film with dimensions ranging from 0.03mm to 4.4 mm. An infrared camera was used to capture thermal images of those defects over 65 seconds of heating. After the thermal images were acquired, time-domain analysis and space-domain analysis were conducted and k-means clustering methodology was used to highlight the defective area. Results suggest active thermography can be used to detect scratch defects with widths of 0.03mm to 4.40 mm and bubble defects with diameters ranging from 0.08 to 4 mm. For defects with dimensions larger than 0.4 mm, our algorithm can estimate the dimension with less than 15% bias. However, for defects with dimensions less than 0.4mm, the algorithm estimation error ranged from 68% to 900% due to camera resolution limitations. It should be noted that our algorithm can still distinguish a scratch defect with a width of less than one pixel. This study also suggests active thermography can detect scratch and bubble defects regardless of the location of the illumination source.