Holographic reflection gratings in azobenzene polymers.

Writing and reading of reflection gratings in films of an azo side-group polymer are reported. The gratings were induced holographically by use of an argon-ion laser at 488 nm. The measured diffraction efficiency was in the range 2-10%. To estimate the scattering of the reflected light within the material, we further characterized the gratings by calculating a characteristic transmission. A distinct minimum for this transmission was observed, which was redshifted farther from the writing wavelength for gratings written in 50-microm polymer samples.

Development of feature detectors by self-organization. A network model.

We present a two-layered network of linear neurons that organizes itself as to extract the complete information contained in a set of presented patterns. The weights between layers obey a Hebbian rule. We propose a local anti-Hebbian rule for lateral, hierarchically organized weights within the output layer. This rule forces the activities of the output units to become uncorrelated and the lateral weights to vanish. The weights between layers converge to the eigenvectors of the covariance matrix of input patterns, i.e., the network performs a principal component analysis, yielding all principal components. As a consequence of the proposed learning scheme, the output units become detectors of orthogonal features, similar to ones found in the brain of mammals.

A regularized approach to color constancy.

Color constancy is the ability of color perception independent of the spectrum of the ambient illumination. We present an algorithm that makes use of biologically plausible assumptions concerning the spectra of illumination and surface reflectance in order to solve the undetermined problem of color constancy. We test the proposed algorithm by means of computer simulations and examine its range of performance.