Sunday, March 27, 2022

Lensless Imaging with Fresnel Zone Plates

Although the idea of Fresnel zone plates is not new and can be traced back several decades to X-ray imaging and perhaps to Fresnel's original paper from 1818*, there is renewed interest in this idea for visible light imaging due to the need for compact form-factor cameras.

This 2020 article in the journal Light: Science and Applications by a team from Tsinghua University and MIT describes a lensless image sensor with a compressed-sensing style inverse reconstruction algorithm for high resolution color imaging.

Lensless imaging eliminates the need for geometric isomorphism between a scene and an image while allowing the construction of compact, lightweight imaging systems. However, a challenging inverse problem remains due to the low reconstructed signal-to-noise ratio. Current implementations require multiple masks or multiple shots to denoise the reconstruction. We propose single-shot lensless imaging with a Fresnel zone aperture and incoherent illumination. By using the Fresnel zone aperture to encode the incoherent rays in wavefront-like form, the captured pattern has the same form as the inline hologram. Since conventional backpropagation reconstruction is troubled by the twin-image problem, we show that the compressive sensing algorithm is effective in removing this twin-image artifact due to the sparsity in natural scenes. The reconstruction with a significantly improved signal-to-noise ratio from a single-shot image promotes a camera architecture that is flat and reliable in its structure and free of the need for strict calibration.

Full article is available here:

* "Calcul de l'intensité de la lumière au centre de l'ombre d'un ecran et d'une ouverture circulaires eclairés par un point radieux," in Œuvres Complètes d'Augustin Fresnel 1866-1870.

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