A new method for the simultaneous measurement of surface topography and ink distribution on prints

A fast photometric stereo method for the determination of surface topograph y and reflectance is described. First it is shown how the irradiance in two images of a surface, illuminated from the left and right respectively, can be used to calculate a partial derivative of the surface height. The parti al derivative is integrated to give a surface height function with the help of a Wiener filter, which suppresses frequencies with an expected poor sig nal-to-noise ratio. Comparisons between surface profiles measured with the described method and profiles scanned with optical or mechanical methods show a good agreement, r(2)=0.95. Studies on gravure-printed surfaces show that areas with a greater probabil ity for missing dots can be obtained through bandpass filtering and thresho lding of the surface height function. The threshold level, 1 mu m below the local average level, can be seen as a critical value. A prediction of wher e missing dots would occur if printing were attempted in non-printed areas is also given. A flexographic print has also been studied. In this case, a full-tone conta ins small white spots, or 'skips'. The simultaneous distribution of reflect ance (ink) and surface height in a highpass filtered version of the topogra phy is used to construct a model for the coupling between this surface heig ht and the ink distribution. The model is then used to predict the print re sult on an unprinted area. The measurements show the value of the method as a tool for studies of the coupling between print result and surface topography. Compared to other met hods, it is fast and very variable in scale.