An optical method for assessing scratch damage in bulk materials and coatings

For some applications, the most important property of a scratch in a surface or coating is the visual perception of that scratch by a human observer. A method is proposed to predict the visual impact of scratch damage from measurements of geometrical dimensions and changes in grey level. A ‘logarithmic instrumental visibility’, Vi, is defined by Vi=log∫ΔG2.5dA where ΔG is the difference in grey level between the scratched region and the surrounding background, and A is the area of the scratch. This objective measure is shown to correlate well with more subjective estimates of visibility, as perceived by human observers, for a wide range of simulated scratch images and also for real scratches in smooth-surfaced filled polyolefin sheets.