The complex-valued speckle fields E( λ 1) and E( λ 2) can be recorded in many different ways. Details and internals of the light engine are specified in the Supplementary Fig. The VD is imaged by the camera, meaning that the synthetic hologram is captured at the VD surface. A small fraction of the light incident on the object is scattered back to the wall/scatterer where it hits the Virtual Detector (VD).
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b and c Schematic setups for NLoS imaging around corners ( b) and NLoS imaging through scatterers ( c) with the SWH principle: The sample beam illuminates a spot on the wall/scatterer (the Virtual Source VS), which scatters light towards the obscured object. The object is reconstructed by backpropagating E(Λ) with the SWL Λ. Our SWH approach draws inspiration from multi-wavelength interferometry on rough surfaces 40, 41, 42, 43, 44, 45, 46: We exploit spectral correlations in scattered light at optical wavelengths λ 1, λ 2 to assemble a hologram of the obscured objects at the SWL \(\). We conclude this paper by discussing benefits of applying the SWH principle in a diverse set of application areas. The idea of utilizing wavelength diversity to alleviate the effects of unwanted aberrations in the detection of electromagnetic signals has potential applications that go far beyond the original scope of NLoS imaging. However, the SWH idea is by no means restricted to the applications discussed above. The mathematical principles underlying the proposed imager concept expand the understanding of light transport in any scattering geometry. We establish that the optimal choice of the SWL scales with increasing scatter, and the synthetic phase computed at the distal end of the scattering medium encodes a holographic representation of the obscured objects. The images of obscured objects recovered using these techniques feature the highest lateral resolution ( > λ 1, λ 2 40, 41. The second class of techniques for imaging around corners exploits spatial or angular correlations in scattered light 23, 32, 33, 34, 35, 36, 37. In many cases the approach, however, is limited by the need for raster-scanning large areas on the intermediary VS/VD surface whose dimensions are comparable to the obscured volume. Recent work in the area 16, 18, 19, 20, 21, 22 has demonstrated results with cm-scale lateral resolution over a 1 m × 1 m × 1 m working volume, and in select cases providing near real-time reconstructions. The process is repeated across multiple spatial locations (so-called virtual sources and detectors - VS and VD) of an intermediary surface such as a wall or floor that is simultaneously visible to the obscured objects and the NLoS sensor unit. ToF based NLoS Imaging techniques recover a surface representation of the hidden scene by probing the scene with a temporally modulated source, and recording the response using fast detectors.
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Existing active schemes for imaging around corners attempt to recover the obscured scene by either exploiting the finite speed of light (time of flight (ToF) based techniques 16, 20, 21, 22) or spatial correlations in scattered light (memory effect (ME) based techniques 23, 32, 33, 34, 35, 36). We do not consider passive approaches 15, 28, 29, 30, 31 and restrict our attention to active NLoS imaging schemes. We use this task to motivate the proposed Synthetic Wavelength Holography (SWH) approach and provide a basis for the comparative assessment with competing approaches. A specific embodiment of the discrete scattering problem in NLoS imaging is the challenge of looking around corners.
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Imaging through deep turbulence or fog, face identification around corners, or imaging through optically opaque barriers like skulls are just a few of the potential application scenarios.īroadly speaking, current approaches to NLoS imaging circumvent the effect of scatter in one of two geometries: continuous scattering within a volume such as fog or tissue, and discrete scattering events distributed across multiple interfaces such as walls. The problem is enjoying renewed attention due to potential applications in autonomous navigation, planetary exploration, industrial inspection, and early warning systems for first-responders 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27. These techniques are collectively referred to as Non Line-of-Sight Imaging (NLoS) in our work. Over the years, many attempts 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 have been made to non-invasively recover images of objects obscured from direct view. There are numerous instances of imaging within the physical sciences wherein an opaque barrier (such as a wall) or a scattering medium (such as fog or tissue) impedes direct view of the object.