In order to use color as a reproducible and repeatable quantitative proxy for degradation, color calibration needs to be applied because the illumination conditions in the aging test chamber may vary over time. At the beginning of the degradation study, an image of a reference color chart (X-Rite Colour Checker Passport; 28 reference color patches), I R , is taken under the same illumination conditions as the perovskite semiconductor samples. Images at each time step, Ω(∆t), are transformed into L*a*b and subsequently to a stable reference color space (standard observer CIE 1931 2 degrees, standard illuminant D50) by applying a 3D-thin plate spline distortion matrix D defined by I R and known colors of the reference color chart: