We evidence experimentally the convective and absolute instabilities in an optical system. In our system, the presence of noise and spatial non‐uniformities (as in most of the “real systems”) add additional complexities. First we derive analytical expressions of both thresholds for the ideal noiseless and spatially uniform situation. Then numerical simulations allow to determine signatures of those instabilities. The corresponding noise sustained and dynamical patterns are observed experimentally in a liquid crystal layer subjected to a laser beam with tilted feedback. © 2004 American Institute of Physics