# A Methodological Approach to the Study of Retroreflective Pavements
## 2 Materials and Methods
### Methods
#### 2.2.1 Spectrophotometer with Integrating Sphere to Evaluate Total Reflectance  
To evaluate the total reflectance, an AvaSpec-3648 spectrophotometer (Avantes, Oberursel, Germany, E.U.) and the AvaSphere-30 (Avantes, Germany, E.U.) integrating sphere were used, the spherical diameter of which was 30 mm and the sample port diameter was 6 mm (Figure 7). The measurements were carried out using an Avantes AvaLight-DH-S (Avantes, Germany, E.U.) light source that consisted of a deuterium light source (190-400 nm) and a halogen light source (360-2500 nm).  
To ensure accurate measurements, the procedure was conducted in a dark room with only minimal light from the computer screen and any light that managed to penetrate through the window covers. Before starting the measurement, the light source needed to be turned on and allowed to warm up for approximately 20 min.  
After the initialization of the equipment, the first step involved taking a blank measurement. This measurement utilised a reference white material, BaSO4, to obtain a low baseline that allows for the comprehensive evaluation of a solid diffusion spectrum. A small amount of BaSO4 was placed under a glass microscope slide to prevent any powder contamination in the integrating sphere. The blank reference measurement was then taken. Subsequently, a dark reference measurement was taken without removing the BaSO4; from under the sphere. This was achieved by utilising the TTL shutter (Avantes, Germany, E.U.), which closes the beam from the source to the sphere. The spectrophotometer then took a measurement with any residual light present in the sphere, which would have entered through the sphere's port.  
Once the reference measurements were completed, the bituminous samples were analysed. For each sample, up to five measurements were taken from different positions on its surface, and the wavelength range of the measurements was from 190 to 1100 nm. Due to the non-homogeneous nature of the sample's surface, a geometric pattern resembling a cross was followed for the measurements. The four points at the end vertices of the cross and the central point of the cross were measured. To maintain a stable horizontal position above the sample, a glass microscope slide was placed between the sample and the integrating sphere (see Figure 8).  
Figure 8: Diagram of the measurement points taken of a sample and sample during measurements in the centre point.  
Figure 7: Spectrophotometer set-up: (1) AvaSphere-30 includes an integrating sphere; (2) Avantes AvaLight-DH-S; (3) AvaSpec-3648; (4) laptop to register the results; and (5) sample under test.  
The different reflectance spectra obtained from these measurements were calculated following the E903-20 (2020) ASTM Standard [44].  
\[\rho(\lambda)=\frac{(S_{\lambda}-Z_{\lambda})}{(100_{\lambda}-Z_{\lambda})} \tag{1}\]  
where \(S_{\lambda}\) is the recorded specimen reading, \(Z_{\lambda}\) is the zero line reading (dark reference), and \(100_{\lambda}\) is the 100% correction obtained with the specimen port replaced by a sample having a coating and a curvature identical to the sphere wall (BaSO4), all at wavelength \(\lambda\). Based on this spectrum, the average for the five measured points was calculated.