## 3 Results  
### Sensor Calibrations and Validation  
For each of the SRS-PRI sensor pairs (532 nm and 570 nm), cross-calibration ratios showed a strong linear relationship to illumination but varied in response to solar elevation. At low elevations,functions had steeper slopes and, therefore, showed higher sensitivity to illumination changes (Figure 2a). Illumination sensitivity decreased as solar elevation increased and functions were near horizontal at solar elevations of \(>\)52\({}^{\circ}\), which represented peak solar noon elevations at our site. For both the 532 nm and 570 nm pairs, the difference in signal (radiance \(-\) irradiance) between the hemispherical and field stop fore-optics over the white balance (Figure 2b) decreased with diffused light observed cloudy conditions. This effect leads to calibration functions with negative slopes where cross-calibration ratios were higher during low illumination than high illumination. Boxplot distributions of cross-calibration functions (Figure 3) show an overall decrease in variability as sun elevation increased towards higher solar elevations. This was expected as low sun angles cause higher specular reflectance. However, generally, cross-calibration ratios were within range of theoretical radiance/irradiance ratio value of 0.318 ([PERSON] _et al._, 2018). The diurnal corrections were applied by first matching the empirical cross-calibrations functions with the solar elevations of each SRS-PRI data points. Then, the derived percent illumination of each point was used to automatically determine the cross-calibration multipliers and derive new corrected reflectance SRS-PRI values. Scaled photochemical reflectance index (sPRI) values were derived from corrected SRS-PRI. A similar process was applied for the midday correction procedure; however, instead of accounting for continuous changes in solar elevation, only daily solar noon elevations were used to select the appropriate empirical cross-calibration response. Solar noon elevations at our site varied from 24.28\({}^{\circ}\) to 56.36\({}^{\circ}\) (fall and summer, respectively). Both diurnal and midday cross-calibration procedures (Figure 4a) had a significant effect on the sPRI diurnal pattern. Each of the corrections caused a downward shift in midday sPRI values as well as a sharper recovery during the start and end  
Figure 2: (a) Example of midday and diurnal cross-calibration functions for the 532 nm SRS-PRI fore-optic. Cross-calibration ratios are shown as a function of sun elevation (binned every 3\({}^{\circ}\)). Illumination of 100% represents clear and sunny sky conditions. (b) Example of the observed signal difference (radiance \(-\) irradiance), in machine units, as a function of illumination percentage for the 41\({}^{\circ}\)–43\({}^{\circ}\) solar elevation bin. Radiance and irradiance measurements were collected from the 532 nm SRS-PRI sensor pair. of the day. These patterns more closely represented the spectrometer derived sPRI diurnal pattern than that formed by the uncorrected SRS-sPRI values. In particular, the dynamic pattern of the diurnal cross-calibration best represented the reference spectrometer shape. Regression analysis (Figure 4b) of diurnal corrected SRS-sPRI values and spectrometer sPRI showed a strong linear correlation (\(R^{2}\) = 0.78, \(p\) < 0.05). Diurnally corrected and uncorrected values were also significantly correlated to spectrometer sPRI (\(R^{2}\) = 0.72, \(p\) < 0.05 and \(R^{2}\) = 0.62, respectively). Figure 4: (**a**) Comparison of uncorrected and corrected (diurnal and midday) PRI diurnal patterns. (**b**) Comparison of spectrometer PRI measurements with uncorrected and corrected SRS-PRI measurements. The diurnal cross-calibration procedure shows the strongest and closest correlation to spectrometer readings and 1:1 (gray) line. Figure 3: Boxplot distribution of diurnal cross-calibration ratios for the 532 nm SRS-PRI fore-optic. Each boxplot represents a 3\({}^{\circ}\) solar elevation bin.