Optimum Paint Sequence for Speckle Patterns in Digital Image Correlation

This note answers whether it is better to use black-on-white or white-on-black painted speckle patterns, and provides recommendations for optimum painted patterns for digital image correlation (DIC). Although DIC algorithms have no preference between tracking the patterns of either bright features or dark features on contrasting backgrounds, we show that paint sequence can be important due to fundamental differences in paint pigments. If the sample and experiment conditions call for two painting steps (basecoat and speckles), then applying a basecoat of white paint followed by black speckles is better than the converse. Black speckles are preferred because they increase contrast and provide a higher mean intensity gradient, lower correlation confidence interval, and lower displacement error. The primary reason for the increased contrast is the greater hiding power of black paint versus white paint. A secondary effect that also reduces contrast in white speckles is undertone, or a slight blue hue shift from Rayleigh scattering.