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. 2020 Feb 14;7(5):849–862. doi: 10.1093/nsr/nwaa022

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© The Author(s) 2020. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

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Figure 3. — Principle of the optical difference in the frequency domain. (A) Processing steps of the optical difference in the time domain (FFT, fast Fourier transform; IFFT, inverse fast Fourier transform). (B) Two examples of the waveform corresponding to the main steps in optical difference processing, including the stable signals (top) and the signals affected by motion artifact (bottom). (C) PWSP and PWE on G–R (top) and G–IR (bottom) signals obtained by optical difference and bandpass filter. (D) PTT calculated by the systolic peaks in (B). (E) Bar graph of the quantities of peak and valley counts in (B). (F) Period time calculated by PWSE, PWE and their average. Error bars: standard deviation, 20 sets.