All-optical subtractive convolution in photochromic absorbing media

Optical image recognition represents a promising alternative to traditional digital methods. In this paper, we present a fully optical system that performs subtraction convolution, meaning that common features between two light maps, a reference and an unknown to be recognized, are eliminated. This is due to the use of a material with induced absorption at the heart of the beam interaction: a thin film of TII photochromic molecules in epoxy resin. The increased absorption in the illuminated regions of the photochromic material causes the optical convolution between two light beams to eliminate the regions with common features, leaving only the dissimilar features in transmission. Both theoretical and experimental results demonstrate high selectivity and resolution, highlighting the great potential of photochromic materials for reconfigurable optical processors.