where \(i\) is the incidence angle of the observation. This, of course, requires knowledge of \(i\) or alternatively a simplifying assumption (e.g., modeling the martian surface as a flat plane). R* is defined as the reflectance relative to that of a perfect Lambertian scatterer illuminated in the same geometry as the observation.
Methods
Selection of the Regions of Interest on the Calibration Targets
In the practical implementation of the IOF calibration, the radiance values of the eight primary clean spots (expected to be the least dusty surfaces) were extracted. The process of selection of these regions was part of a more general context, which aimed at collecting the average radiance values of all the main areas, named Regions of Interest (ROIs), of the cal-targets. This process was similar to that implemented for MER (Bell et al. , 2003, 2007) and MSL (Bell et al. , 2017) calibration targets. The selection of ROIs was made automatically by an algorithm that, by recognizing the pattern of the cal-targets, overlaid two model templates (one for each eye) on the cal-target images. Each template consisted in a graphic mask (over transparent background) containing all the ROI selections in different colors (Figure 4). This operation required regular human intervention, usually in order to manually edit the regions when the algorithm did not recognize shadows.