Application of a He3Ne infrared laser source for detection of geometrical dimensions of cracks and scratches on finished surfaces of metals

• An optical method for detecting surface defects using He–Ne laser source is proposed.
• Geometrical dimensions of scratches and cracks were detected using laser imaging.
• Centroid of image allowed the calculation of the depth of scratches and cracks.
• Extreme points of image allowed the measurement of the scratch's and crack's width.
• Experiments show good correlation between theory and experimental results.

In this study, the authors addressed a new method of measurement for microscratches and cracks on finished metallic surfaces. The technology proposed here can be used in aviation, automobile, machine-tools, bearings, electrical and thermal devices (detection of electrical and thermal contact resistance), and many other industrial fields where finished surfaces have to be inspected for quality before use. The technology is based on the application of He3Ne laser optics. This measurement method allows for the inspection of geometrical parameters such as depth and width of scratches and cracks and consists of a low-energy He3Ne red laser beam (wavelength 633 nm) with a cross-section of 0.48 mm, beam expander with special designed slit, multi-axis stage with sample holder and CCD (charge-coupled device) visual detection system. Using experimental data, a mathematical algorithm was created in order to calculate the depths and widths of the scratches and cracks where the method of center of gravity (centroid) of the imaged area was used. The results showed a good correlation between the experimental and calculated values of the dimensions of the scratches. Future work will deal with the determination of the geometrical dimensions of microcracks.