## 6 Surface Characteristics Assessment
### Surface Texture Assessment
#### 6.2.3 Microtexture Assessment Using Non-Contact Methods  
Microtexture assessment can be performed manually using a microscopic picture of the surface, with the shadow from a straightedge projected onto it at an angle. By measuring the profile of the shadow, it is possible to isolate and evaluate the microtexture manually. This approach has been used to assess the correlation between wear rate and microtexture of pavement surfaces [134; 135]. The optical method with a straightedge shadow allows a precise microtexture assessment but is not suitable for large areas due to the manual evaluation, which requires time.  
Microtexture assessment from photometry or 3D scanning can be challenging, due to the irregularity of the reference surface because of the macrotexture. Therefore, during the assessment, it is important to isolate the microtexture from the macrotextures. In a study performed by the Dutch Royal National Aerospace Centre [25], data obtained by a high-resolution surface texture scanner were used for the assessment of both microtexture and macrotexture. Consequently, researchers have developed algorithms for the separation of microtexture from the overall data. An example of separating the raw laser trace is shown in Figure 28. The surface laser scanner had a vertical resolution of 0.003 mm, a maximum length resolution of 0.00635 mm, and a maximum width resolution of 0.0247 mm. However, the drawback of this technique is the low scanning speed, due to the high resolution of the scanner.  
At the same time, the microtexture assessment using the texture profile is overestimating the microtexture in the case of surfaces being worn [134]. Due to the wear of the texture in the contact area, microtexture depth can be significantly reduced. However, average texture depth can still show good results. For example, the average microtexture depth for every section of a profile was calculated for the data shown in Figure 28. The results (Figure 29) shown an average microtexture depth based on the microtexture profile. It can be seen that the lower microtexture depth occurred at the lowest point of macrotexture. Therefore, the average microtexture depth cannot provide an appropriate microtexture assessment. For this purpose, automatic or manual evaluation should be performed only in areas of contact between the tire and the pavement surface, where surface erosion has occurred. Otherwise, the result will be affected by the higher microtexture in the other areas.  
In another study [136], white paint on the tire was used to evaluate the microtexture on the contact areas as well as the contact area itself. The results showed that the correlation coefficient between the British Pendulum Test results and the microtexture shape factor increases from 0.45 to 0.56 after restricting the measurements to only the contact parts of the tire footprint. In a similar study [137], the same problem was solved automatically by combining six different profiles from the same place. The resulting profile, which was referred to as the \"summit profile,\" was used for the friction assessment (Figure 30).  
Figure 28: Example of separating the micro- and macrotextures from the high-resolution laser texture scanner [25].  
Figure 29: Average microtexture on the different sections of the profile.