Figure 1
Principles of time/warped stretch imaging. (a) Time/warped stretch imaging employs frequency-to-time and frequency-to-space mapping of an ultrashort optical pulse to perform a line scan. Here, to clarify the method, we use a one-dimensional spatial disperser, which leads to a line scan per pulse. A 4 × 4 ray-pulse matrix computation of vector (space (x), slope (θ), time (t) and frequency (f)) specifies the optical mapping relationship. Colored circles represent uniformly spaced frequency components (beam-pulselets) and rainbow gradient lines represent their continuous distribution in both space and time domains. (b) Uniform temporal sampling under this space-to-time relationship determines the spatial sampling position on the image space. Yellow dots on the fingerprint sample image account for the warped pixel distribution of a given imaging system. (c) The corresponding temporal scan signal from the sampled points is acquired by a single-pixel photodetector and an analog-to-digital converter (ADC). The three pronounced peaks in the scan signal correspond to the fingerprint ridges in the center of the line scan. The scan signal is reconstructed by remapping the temporal data-stream back to the original image space. Regions with higher temporal dispersion are effectively assigned more samples (central region), while the part of the waveform that is not highly time-stretched corresponds to fewer imaging pixels (peripheral regions).
