Encrypted Three-dimensional Dynamic Imaging using Snapshot Time-of-flight Compressed Ultrafast Photography

Abstract

Compressed ultrafast photography (CUP), a computational imaging technique, is synchronized with short-pulsed laser illumination to enable dynamic three-dimensional (3D) imaging. By leveraging the time-of-flight (ToF) information of pulsed light backscattered by the object, ToF-CUP can reconstruct a volumetric image from a single camera snapshot. In addition, the approach unites the encryption of depth data with the compressed acquisition of 3D data in a single snapshot measurement, thereby allowing efficient and secure data storage and transmission. We demonstrated high-speed 3D videography of moving objects at up to 75 volumes per second. The ToF-CUP camera was applied to track the 3D position of a live comet goldfish. We have also imaged a moving object obscured by a scattering medium.

Figure 1. ToF-CUP system configuration.

CCD, charge-coupled device; DMD, digital micromirror device; ED, engineered diffuser; V, sweeping voltage; , time-of-flight. Equipment details: camera lens, Nikon, f = 18–55 mm; CCD 1, Point Grey, FMVU-03MTM-CS; CCD 2, Hamamatsu, ORCA-R2; DMD, Texas Instruments, DLP LightCrafter 3000; engineered diffuser, Thorlabs ED1-S20-MD; laser, Attodyne, APL-4000; microscope objective, Olympus UPLSAPO 4×; streak camera, Hamamatsu C7700; tube lens, Thorlabs AC254-150-A.

Figure 2. Quantification of ToF-CUP’s depth resolution.

(a) Side view and photograph of the height-varying fin-pattern target used in the experiment. The ToF-CUP system was placed perpendicularly to the target base and collected backscattered photons from the target surface. (b) Reconstructed x, y, datacube representing the backscattered laser pulse intensity from the fins with different depths. (c) Representative x-y frames at and 280 ps. Supplementary Video 1 shows the ToF snapshots of the x-y light distributions. Scale bar: 10 mm.

Figure 3. Depth-encoded ToF-CUP images of static objects.

(a) letters “W” and “U” with a depth separation of 40 mm, (b) a wooden mannequin, and (c) a human hand. Scale bar: 10 mm.

Figure 4. Security of the ToF-CUP camera against two types of decryption attacks.

(a) Cross-correlation coefficients between the image decrypted using the correct decryption key and images using 50 brute force attacks with wrong pseudo-random binary masks. (b) 3D datacube of letters “W” and “U” decrypted using the correct decryption key. (c) As in (b), but using an invalid decryption key in the brute force attack (trial #26). (d) Cross-correlation coefficients between the reconstructed image decrypted using the correct decryption key and each image using a part of the correct decryption key with a different horizontal shift. Positive shift pixel numbers mean the key was shifted to the right of the acquired image, while negative numbers mean a shift to the left. The inset shows the relative positions of the letters “W” and “U” and the full decryption key (green). The part of the decryption key used in the shift security test is marked in purple. (e) As in (b) and (c), but using a part of the correct decryption key right shifted by one encoded pixel.

Figure 5. ToF-CUP of moving objects.

(a) Experimental setup of two rotating balls. (b) Representative depth-encoded 3D images at six different slow-time points showing the relative depth positions of these two balls. The corresponding movie is in Supplementary Video 2. We also applied ToF-CUP imaging to a live comet goldfish. (c) Representative depth-encoded 3D images at six different slow-time points. (d) Trace of the 3D position of the fish. The corresponding movie is Supplementary Video 3. Scale bar: 10 mm.

Figure 6. ToF-CUP imaging of an object moving behind a scattering medium.

(a) Experimental setup. BS, beam sampler; PD, photodiode detector. The airplane moves from the lower left to the upper right, as well as toward the ToF-CUP camera in the depth direction. The trajectory of the airplane-model target is marked by the magenta dashed line with an arrow. (b) Projected images of the airplane-model target acquired at various scattering thicknesses of the scattering medium, where the projection was achieved by summing over the x, y, z datacube voxels along the z axis. (c) Comparison of the normalized fluence profiles of the airplane wing along the green dotted lines in (b). (d) Representative depth-encoded ToF-CUP images of an airplane target moving behind a scattering medium with an equivalent scattering thickness of 1.0l_t. (e) As in (d), but with an equivalent scattering thickness of 2.1l_t. The corresponding movie is Supplementary Video 4. Scale bar: 10 mm.