Fig. 1. Coaxial focusing measurement system

Fig. 2. Analytical results of modulation based on two dimensional Fourier transform. (a) Analog diagram of acquired image; (b) spectrum distribution; (c) modulation distribution

Fig. 3. Corresponding relationship between the value of modulation and the height. (a) Longitudinal response curves of modulation; (b) linear mapping relationship between differential modulation and height

Fig. 4. Simulation results. (a) Images captured by CCD1; (b) images captured by CCD2; (c) linear distribution region of longitudinal response of differential modulation; (d) error distribution of defocusing

Fig. 5. Distribution of focusing error with noise

Fig. 6. Simulation curves of modulation under different differential quantities. (a) Response curves of modulation; (b) linear mapping relationship of differential modulation

Fig. 7. Experimental system

Fig. 8. Experimental curves of modulation under different differential quantities. (a) Response curves of modulation; (b) linear mapping relationship of differential modulation