207   Further discussion require s some understanding of how lasers mark stainless steel. The heating of the  surface in an oxygenated  environment causes growth in the chromium oxide layer. The oxide layer is  natural ly transparent, but as its thickness increases , thin film effects begin to occur, and color develops.  Thick enough layers absorb much of t he light passing through, and appear black.     Even if no material is removed, material properties could change. The oxide layer (the bright white layer  on the cross section in Figure 1) essentially behaves like a ceramic, with substantially reduced ductility  compared to the surrounding metal. More critically, the laser could heat the underlying material sufficiently to alter or remove heat -treating  effects  in the bulk of the material. In fact, this laser marking  method that avoids etching or ablation is often called ‘anneal marking’ or ‘anneal mode’. Computer simulations predicted  minimal change to part properties outside of the marked area. Metallurgical  samples show an annealed zone beneath the oxide layer in the range of 1.8- 2.5 µm. This is a substantial  improvement from initial marking settings, which annealed up to nine times as deep. However, as a precaution, surfaces which contact other parts of the mechanism should be shielded from the laser beam  to maintain material integrity. Extensive testing has been performed with marked parts without showing  adverse effects.  

The second concern stems from the thin film nature of the marking. Thin films are viewing- angle  dependent , and can change color and contrast depending on camera and lighting angle. Laser settings  that generate surface melt can also create diffraction gratings depending on line spacing. To mitigate these concerns, settings target  dark gray to black mark s with close line spacing and high spot overlap.  Laser vendors do not generally provide all the information needed to calculate settings appropriate for a  desired result, so some experimentation is necessary for optimizing settings, particularly when dealing  with miniature components.   Regardless of marking method, speckle patterns must take into account the tracking algorithm and camera setup that will be used, and should be optimized to provide the best chances of success.  

   Figure 1.  An example of laser marked surface (left) and a cross- section (right).  

Pattern Design   Gradient -based DIC algorithms (like the one used in VIC -2D) perform optimally for  speckles with ‘soft’  edges that gradually blend into the background, rather than images with sharp contrast edges which  introduce high frequency content in the image. To assist with this, most gradient methods will interpolate pixel values to smooth out t ransitions  or use an image filter . As a result, the pattern must be sized to  prevent aliasing. Standard rules -of-thumb have been developed that relate the speckle to pixel size.    For X -Y motion with rotation, speckle should be at least 2•2 1/2 pixels in each dimension.  Speckle s larger  than this size by at least one interpolation step would provide a wider peak, which is preferred. The maximum speckle size is limited by the region of interest ( ROI) size. Ignoring strain, f or m inimum -size