Mars 2020 Rover 
Calibration Targets
Lab assembly of the Mastcam-Z Flight Calibration Targets -
Now installed on NASA´s Mars 2020 Rover
for more info, please read the research article published in Space Science Reviews, with BUILT 4 Space Explorations owner Ole Busborg Jensen as the co-author: 

Radiometric Calibration Targets for the Mastcam-Z cameras on NASA's Mars 2020 Mission

Also read NASA's official reveal of the Calibration Target Design here: 

5 Hidden Gems Are Riding Aboard NASA's Perseverance Rover

Or the officiel article from the Mastcam-Z Team:

Figure 1. (a) The Mastcam-Z primary radiometric calibration target. The base of the target is ~9.8 x 9.8 cm (3.9 x 3.9 in); (b) The Mastcam-Z secondary radiometric calibration target. The length of the target is ~8 cm (3.1 in) and the “shelf” is ~1.6 cm (0.6 in) wide; (c) Inspection photo. Photo credits: (a) Niels Bohr Institute, Copenhagen, Denmark; (b, c) NASA/JPL-Caltech/ASU

Figure 3: Assembly drawings showing the components of the Mastcam-Z primary and secondary radiometric calibration targets.  The shadow post or gnomon is made of aluminum and painted with an ultralow reflectance black paint. The grayscale rings are made of sintered alumina or glazed and matted aluminum-silicates, and they have the average visible-wavelength reflectances shown here. The color and grayscale patches are also made from the same materials, and are called (going clockwise): Red, Blue, Yellow, Green, White, Light Grey, Dark Grey, and Black. The eight Sm2Co17 magnets are each 11 mm diameter and 7 mm tall and are bonded 0.4 mm beneath each color patch. The base plate of the target is made of aluminum, coated with thin layers of silver then gold plating. The inset at upper right shows the components of the Mastcam-Z secondary calibration target.  The color and grayscale patches are identical to those in the primary target, except the yellow is missing. Each patch repeats twice, once on the vertical face and once on the horizontal face. The base bracket of the target is made of aluminum. Photo credit: Niels Bohr Institute, Copenhagen, Denmark/NASA/JPL-Caltech/ASU.