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Table 2 Comparison colour variations between clean and tarnished coupons, measured with the spectrophotometer and calculated from pictures using the two different calibration procedures

From: Design and validation of tailored colour reference charts for monitoring cultural heritage degradation

Copper Measured Calculated (raw) Linear calibration Spatial calibration Silver Measured Calculated (raw) Linear calibration Spatial calibration
ΔL* − 14.74 − 3.21 − 10.65 − 11.81 ΔL* − 18.49 − 4.68 − 14.00 −12.62
Δa* 10.53 12.27 8.13 11.58 Δa* 4.54 4.14 − 1.70 6.76
Δb* 26.93 31.27 22.82 30.05 Δb* 22.42 29.36 27.44 25.64