Figure 3. Distributions of incidence and azimuth angles of all the 197 cal-target image sequences. The plot on the left shows the incidence-azimuth combinations of the sequences, while the histograms on the right illustrate how those sequences are distributed in terms of azimuth and incidence. Each data point in the plot on the left, corresponding to an entry in the two histograms on the right, is a single cal-target sequence, which includes at least one image for each eye of Mastcam-Z in the same observation (in most cases it is either in all filters or only in L0/R0).
The Radiance-to-Reflectance Calibration
The primary objective of the cal-targets is the generation of reflectance-calibrated images. As a last step of the radiometric calibration, all Mastcam-Z images in units of radiance (i.e.,\(W/(m^{2}\bullet\ \text{nm}\bullet\text{sr})\)), (Hayes et al. , 2021), are calibrated to radiance factor I/F (or IOF), where \(I\)is the radiance from the scene and \(F\) is the instantaneous solar irradiance. IOF is defined as the reflectance relative to that of a perfect Lambertian scatterer illuminated from the zenith. While the radiance can be measured, the local irradiance \(F\) is challenging to model at the martian surface due to its dependency on illumination geometries and atmospheric conditions that change over short timescales. To address this directly, frequent imaging of the cal-targets in all filters is performed (in general cal-target images accompanied any multispectral observation of surface targets).
The following equation is used to convert radiance images into IOF (Kinch et al ., 2020):