New Fourier CGH coding using DMD generated masks for grayscale and color images

Computer Generated Holograms (CGHs) are powerful optical elements used in many fields, such as wavefront shaping, quality testing of complex optics and anti-counterfeiting devices. Lee algorithm is the most used to generate binary amplitude Fourier holograms. Grayscale CGHs are known to give a higher reconstruction quality than binary holograms, but they usually require a cumbersome production process. A very simple and straightforward method of manufacturing rewritable grayscale CGHs is here proposed by taking advantage of two key components: a Digital Micro-mirror Device (DMDs) and a photochromic plate. An innovative algorithm, named Island algorithm, able to generate grayscale amplitude Fourier CGHs, is reported and compared with the standard Lee approach, based on 9 levels. A crucial advantage lies on the fact that the increase or decrease of the quantification does not affect the spatial resolution. In other words, the new coding leads to a higher spatial resolution (for a given CGH size) and a reconstructed image with an order of magnitude higher contrast with respect to the classical Lee-coded hologram. In order to show the large potential of our approach, we have investigated the coding of a grayscale image with a 201 levels Island hologram: it shows a much better reconstruction (resolution, fidelity, contrast) thanks to the quantification of the transparency than the Lee algorithm commonly used. By combining 3 grayscale images with RGB filters and adding them, we show the reconstruction of a full color image with high fidelity, high contrast and low noise.