Visual Color (Tonal) Change
Based on the tonal color changes determined in the automated analysis it is apparent that cracks do interfere with the visual color judging capability of humans. Because the cracks entrap light, the entire surface appears visually darker than the color of the unspoiled surface. Image analysis reveals that the PU1 and PU2 specimens have the least visual color change compared to other specimens. Contradictory to the visual ratings, where whitening was detected, digital images show that these specimens had darkened slightly. Specimens SRI and SR2 were rated visually as no change in color; however, digital image analysis shows that these specimens had darkened quite a bit. This is probably due to dirt pick-up on the specimen surface. While it is possible to subtract the effect of dirt particles on color change, if the surface is not completely covered with dirt, an effective removal of dirt particles on the sealant surface prior to digital image capturing is not feasible. Microscopic examination of sealant surfaces typically reveals that dirt particles are embedded in the sealant surface. Because of this incorporation of dirt particles into the sealant polymer matrix, it is generally not possible to clean samples after exposure even by extensive brushing. While the authors of the earlier study of sealant surface degradation using the VIEEW™ system  were able to subtract the dirt particles from the images, due to a high difference between their color and the color of the sample background, this option did not prove feasible in the current study. The reason may be that in the earlier study, sealant samples were weathered outdoors for a maximum of three months, thus, displayed only a limited amount of dirt pick-up, while in the current study, sealant samples had been exposed for 6.8 years and in general showed much stronger dirt pick-up.
The limitation in the capability of the system of separating dirt pick-up and color change is especially
Uniformity of surface detrioration
FIG. 10—Uniformity of surface deterioration of weathered sealant samples.
obvious in the analysis of dark colored sealants, especially those pigmented in black or dark bronze. Another limitation comes from the fact that dirt particles are normally not all black but composed of white and gray particles. Some of these particles, depending on the color of sealant, will not be recognized (e. g., white particles on a white sealant). This limitation does not affect comparative studies where relative performance is important and dirt composition is standard for all specimens.