Seung Ah Lee's Imaging Intelligence Laboratory at Seoul National University develops rolling-shutter speckle imaging (RSSI), which measures blood flow from a single frame of a standard CMOS sensor (C. Yi et al., Optica 2022; C. Yi et al., Biomed. Opt. Express 2024), with applications to fingertip pulse waveforms (Y. Lee et al., Biomed. Opt. Express 2024) and real-time analysis by physics-guided neural networks (S. Byun et al., J. Phys. Photonics 2026).
We analyze speckle produced by dynamic scattering to measure the dynamics of a sample. When laser light enters living tissue, moving red blood cells make the speckle pattern fluctuate, at a rate set by the flow speed. Measuring the fluctuation directly usually requires cameras running at more than ten thousand frames per second. Rolling-shutter speckle imaging (RSSI) uses the row-by-row exposure of CMOS sensors, in which successive rows are exposed microseconds apart, so that a single frame records the speckle dynamics. With elongated speckles and a physical model, one frame gives both the flow speed and the fraction of moving scatterers. Applications include cerebral blood-flow mapping, pulse waveforms at the fingertip and real-time analysis with physics-guided neural networks.
6 projects
Laser speckle imaging maps blood flow without dyes, but quantitative measurements require following how the speckle decorrelates over microseconds, which normally means costly cameras running above ten thousand frames per second. RSSI instead uses the row-by-row exposure of CMOS sensors, in which successive rows are exposed microseconds apart.
A single RSSI frame becomes a measurement only once we know how the row-by-row correlation should behave for a given flow, in tissue where moving blood is mixed with static scatterers. Our first model assumed an instantaneous exposure, but real sensors integrate light for longer than the delay between rows.
Photoplethysmography (PPG), the green light on the back of a smartwatch, tracks how much blood lies under the sensor but not how fast it moves. RSSPG applies rolling-shutter speckle imaging to a sequence of frames of a fingertip and extracts, frame by frame, both the decorrelation time, which follows flow speed, and the dynamic scattering fraction, which follows blood volume.
Single-shot RSSI trades statistics for speed: each map is estimated from relatively few speckle grains, so pixel-wise curve fitting is both noisy and slow. Averaging many frames reduces the noise but gives up the time resolution of single-shot imaging.
Fast eye movements, from saccades to the tiny microsaccades made while fixating, carry information about attention and neurological health, yet capturing them usually calls for high-speed cameras. We are extending rolling-shutter speckle imaging to the eye: laser light scattered from the white of the eye (the sclera) forms elongated speckles on a rolling-shutter sensor.
Speckle fluctuations also reveal blood flow far below the surface. Diffuse correlation spectroscopy (DCS) measures how laser light flickers after diffusing through centimeters of tissue, but in adults the light that reaches the brain, beneath scalp and skull, returns faint and noisy.
5 publications
Physics-guided neural network for single-shot real-time dynamic speckle imaging
Byun, S., Yi, C., Bae, D., Kang, Y.G., Kim, J., Chang, H., & Lee, S.A.
Journal of Physics: Photonics, 8(4), 045002 (2026) doi:10.1088/2515-7647/ae99ba
Kreiss, L., Wu, M., Wayne, M., Xu, S., McKee, P., Dwamena, D., Kim, K., Lee, K.C., Cowdrick, K.R., Liu, W., Ülkü, A., Harfouche, M., Yang, X., Cook, C., Lee, S.A., Buckley, E., Bruschini, C., Charbon, E., Huettel, S., & Horstmeyer, R.
Neurophotonics, 12(2), 025007 (2025) doi:10.1117/1.NPh.12.2.025007
Rolling shutter speckle plethysmography for quantitative cardiovascular monitoring
Lee, Y., Byun, S., Yi, C., Jung, J., & Lee, S.A.
Biomedical Optics Express, 15(3), 1540–1552 (2024) doi:10.1364/BOE.511755
Yi, C., Byun, S., Lee, Y., & Lee, S.A.
Biomedical Optics Express, 15(2), 1253–1267 (2024) doi:10.1364/BOE.514497
Single-shot temporal speckle correlation imaging using rolling shutter image sensors
Yi, C., Jung, J., Im, J., Lee, K.C., Chung, E., & Lee, S.A.
Optica, 9(11), 1227–1237 (2022) doi:10.1364/OPTICA.465361