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.