The image of an observed object hidden behind a cloudy medium becomes visible when absorption is introduced into the cloudy medium. The underlying physical principle is that multiple scattered light (which effectively becomes noise) travels over a longer path length than the ballistic signal. The average distance the multiple scattered light travels in the medium is z2/It, which is longer than z, the distance the ballistic light transverses through a slab of thickness z, and It is the photon transport mean free path. Light traveling over a longer path length is more highly attenuated. Thus, an introduction of an absorbing dye into the random medium will preferentially absorb the multiple scattered light over the ballistic light. This absorption can substantially reduce the detected noise below the desired ballistic signal intensity and hence improve image quality. This is an important point which had not been recognized previously.
© 1991 Optical Society of AmericaPDF Article