Fig. 1. Principle illustration of parallel object-side differential axial measurement method

Fig. 2. Experimental setup of the parallel object-side differential axial measurement method

Fig. 3. Calibration curve of axial measurement

Fig. 4. Coin surface morphology is affected by disturbance error. (a) Uneven illumination; (b) effect of coin surface reduction by uneven illumination

Fig. 5. Logarithmic differential confocal microscopy calibration curve of axial measurement

Fig. 6. Experimental sample STEP-Si-5-1 and measurement position

Fig. 7. Photo of a dime as the second experimental sample

Fig. 8. Measurement images of coin surface morphology by the proposed method. (a) M_F; (b) M_A; (c) M_S; (d) M_X; (e) M_JF; (f) M_JA

Fig. 9. Reduction result of coin surface morphology after error compensation

Fig. 10. Reduction of coin surface morphology. (a) Before error compensation; (b) after error compensation

Fig. 11. Height measurement curve of optical surface profilometer. (a) Two-dimensional section view of sample height; (b) top view of three-dimensional height profile of the sample