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):