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