Assessing Total Differences For Effective Samples Having Variations In Color, Coarseness And Glint

Metallic coatings have been generally adopted in industries recently, such as the finishing of vehicles, printing, cell phones, and etc. The products with metallic coatings have unique property, i.e. large change of appearance (color, glint and coarseness) under different viewing conditions. The traditional methods of characterizing this kind of coatings can only assess their color or the gloss level based on the standard illumination and observation conditions recommended by CIE or ISO without judging the glint or the coarseness effect of them, which is quite far from the fact and cannot meet the demand of on-line quality control in industries. Hereby further studies have become necessary and urgent in the light of the growing percentage of products with metallic coatings and the increasing amount of exotic metallic coatings produced by pigment manufacturers.Based on the various instrumental measurement and massive visual assessment data under different viewing conditions, the proper illumination and observation geometries were first found under which observers could precisely implement the visual evaluation of samples with metallic coatings. Then the relationship among the color difference, coarseness difference, glint difference and the total visual difference was analysed.Under the diffused illumination the glint effect of samples with metallic coatings was not obvious and could hardly perceived by the observers, while the coarseness difference and color difference took great part in the total difference of sample pairs and could be easily detected by observers. The viewing condition that observers could perceive the maximum total difference was almost the same with the one that observers could perceive the maximum coarseness difference. Under directional illumination, observers could assess the color difference and glint difference of sample pairs precisely, while the coarseness difference was very small and too difficult to estimate, so the coarseness difference could be ignored in the total difference. Three total difference models were established based on the instrumentally measured color difference, coarseness difference and glint difference. The first two models were under diffused illumination, of which one was under the viewing condition that observers could perceive the maximum total difference and the other was under the geometry that observers could perceive the maximum coarseness difference. The third model was under directional illumination. All the three models could precisely predict the total difference of sample pairs perceived by observers under the corresponding viewing conditions.With the method of analysing the images of samples taken by a digital camera under certain viewing condition, the color, coarseness and glint could be quantized separately, and the color difference, coarseness difference and glint difference of sample pairs could be calculated, so that two camera-based models were established to predict the total visual difference of sample pairs under diffused and directional illumination respectively. The input images of the two models were taken under the viewing conditions that were identical to those for the corresponding visual assessments such that the appearances of the samples could be accurately represented in the images. The testing results indicated that the two camera-based models both could precisely predict the total difference between samples with metallic coatings with satisfactory consistency to the visual data.Finally, on the basis of summarizing the research work of this dissertation, the perspectives of the future study were also forecasted.