Digital holography is employed in applications including 3D displays, non-destructive testing, cryptography and homeland security. However, the quality of digital holograms is usually degraded by both laser speckle, due to the coherent nature of the laser sources used, and incoherent additive noise. Vittorio Bianco and co-workers in Italy and the USA have now proposed a scheme to achieve quasi-noise-free digital holography reconstructions by employing a combination of noise-reduction techniques (Light Sci. Appl. 5, e16142; 2016). The result is the generation of high-quality holographic images with greatly reduced noise (pictured).

Credit: NATURE PUBLISHING GROUP

The noise reduction consists of two steps. The first is an enhanced grouping algorithm that groups image fragments similar to a given reference and provides noisy groups as an output. The second step is sparsity enhancement filtering by which each image fragment is used to filter the entire group.

Holographic images of a matryoshka doll (40 mm high, 22 mm wide) were processed by the proposed method (right). The doll was illuminated by two lasers emitting at the wavelengths of 532 nm and 632.8 nm to capture the green and red components, respectively, of its appearance. Both the laser beams were superposed and took the same optical paths for both the reference and the object beams. The object beams were scattered by a rotating diffuser in front of the doll.

The holograms corresponding to the two-colour components were separately processed and synthesized by a spatial correlation coefficient maximization approach. When the holographic reconstruction was processed by conventional means, the coherent speckle noise was still apparent (left), whereas the noise-reduced version (right) is of greatly improved quality and contrast with comparable resolution.