Using active thermography to inspect pin-hole defects in anti-reflective coating with k-mean clustering

•Thermography is used to overcome illumination issues in visual inspection of film.•De-trend filter is designed based on heat conduction and residual analysis.•Pinholes as small as 0.03 mm can be detected effectively with uneven heat source.•k-means clustering provides nearly noise-free edge detection.•Pinholes from 4 mm to 0.03 mm can be detected cost-effectively.

The study here demonstrates the capability of thermography to detect and characterize pinhole defects in a visually transparent anti-reflection (AR) film. The diameter of the pin-holes varies from 0.03 mm to 4 mm. Each inspected area was unevenly heated for 65 s. Thermal images were processed by the following steps: de-trend processing, primary edge detection based on Sobel approximation, and further edge separation based on k-means clustering. Then, the diameter of pinholes was directly measured from binary images. The proposed image processing enables thermography to detect 86 of 90 inspected areas correctly in position.