where \(\text{Image}_{\text{RAD}}\) and \(\text{CT}_{\text{RAD}}\) are the pixel-by-pixel measured radiances of the image to calibrate and the cal-targets, respectively, and \(\text{CT}_{\text{IOF}}\) is the reflectance of the cal-target materials, which is known from laboratory spectra. The ratio between \(\text{CT}_{\text{IOF}}\) and\(\text{CT}_{\text{RAD}}\) in equation (1 ) is equal to the inverse of the irradiance \(F\). The IOF calibration allows the extraction of reflectance spectra of interesting terrain units and specific geologic targets that can be consistently compared and for which the nature can be assessed in detail (e.g., from this issue: Garczynski et al. , Horgan et al. , Núñez et al. , Rice et al. , Vaughan et al. for Mastcam-Z reflectance data, and Royer et al. , Mandon et al. , for SuperCam data). In addition, frequent estimates of the solar irradiance \(F\) constrain atmospheric conditions and dust displacement over time.
IOF values can be converted to units of reflectance factor R* through the equation: