Non-invasive imaging through opaque scattering layers

The image of a fluorescent object hidden behind an opaque layer can be retrieved non-invasively by exploiting the correlation properties of the speckle pattern produced by illuminating the object through the layer using laser light. Imaging through opaque, light-scattering layers is an important capability in many fields, including nanotechnology and the biosciences. Several promising methods are being developed, but typically involve invasive procedures such a placing a detector or nonlinear material behind the scattering layer. Jacopo Bertolotti et al. now demonstrate a non-invasive imaging procedure that makes use of the correlations in the speckled intensity pattern that is produced when laser light passes through a scattering medium. Fluorescent micrometre-sized objects obscured by scattering layers can be imaged by measuring total fluorescence at several different angles of laser incidence and by using an iterative algorithm that disentangles the spatial information of the object and the speckle pattern. The authors successfully construct detailed images of cell-sized fluorescent objects hidden six millimetres behind scattering layers, and a complex biological sample sandwiched between two opaque screens. Non-invasive optical imaging techniques, such as optical coherence tomography1,2,3, are essential diagnostic tools in many disciplines, from the life sciences to nanotechnology. However, present methods are not able to image through opaque layers that scatter all the incident light4,5. Even a very thin layer of a scattering material can appear opaque and hide any objects behind it6. Although great progress has been made recently with methods such as ghost imaging7,8 and wavefront shaping9,10,11, present procedures are still invasive because they require either a detector12 or a nonlinear material13 to be placed behind the scattering layer. Here we report an optical method that allows non-invasive imaging of a fluorescent object that is completely hidden behind an opaque scattering layer. We illuminate the object with laser light that has passed through the scattering layer. We scan the angle of incidence of the laser beam and detect the total fluorescence of the object from the front. From the detected signal, we obtain the image of the hidden object using an iterative algorithm14,15. As a proof of concept, we retrieve a detailed image of a fluorescent object, comparable in size (50 micrometres) to a typical human cell, hidden 6 millimetres behind an opaque optical diffuser, and an image of a complex biological sample enclosed between two opaque screens. This approach to non-invasive imaging through strongly scattering media can be generalized to other contrast mechanisms and geometries.