A Radiometric Correction Method and Performance Characteristics for PIXL’s Multispectral Analysis Using LEDs

The Planetary Instrument for X-ray Lithochemistry (PIXL) onboard the Perseverance rover, part of NASA’s Mars 2020 mission, has the first camera system that utilizes active light sources to generate multispectral data directly on a planetary surface. PIXL collects the multispectral data using three different components in the Optical Fiducial System (OFS): Micro Context Camera (MCC), Floodlight Illuminator (FLI), and Structure light illuminator (SLI). MCC captures images illuminated at different wavelengths by FLI while topography information is obtained by synchronously operating the MCC and SLI. A radiometric calibration for such a system has not been attempted before. Here we present a novel radiometric correction process and verify the output to a mean error of 0.4% by comparing it to calibrated spectral data from the Three Axis N-sample Automated Goniometer for Evaluation Reflectance (TANAGER). We demonstrate that the radiometrically corrected data can clearly discern different features in natural rock and mineral samples. We also conclude that the same radiometric correction process can be used on Mars as the optical system is designed to autonomously compensates for the effects of the Martian environment on the instrument. Having multispectral capabilities has proven to be very valuable for extrapolating the detailed mineral and crystallographic information produced by X-ray spectroscopy from the X-ray system of PIXL.