A mixture of poly(vinyl methyl ether) (PVME) and a polystyrene derivative bearing cinnamate groups (PSC) was chemically designed so that its phase separation can be tunable by visible light for computer-assisted irradiation (CAI) experiments. This PSC/PVME blend exhibits a lower critical solution temperature (LCST) and undergoes phase separation upon irradiation with 405 nm visible light. The phase separation was induced by photodimerization of the cinnamate moieties in the presence of 5-nitroacenaphthene used as a photosensitizer. It was found that for visible light with high intensity, phase separation process was almost frozen by photodimerization of the cinnamate groups which act as a photo-cross-linker for the PSC component. It is demonstrated in this work that by using this PSC/PVME blend, phase separation restricted to the micrometer scales can be induced and manipulated by irradiation using a computer-controlled digital projector. These preliminary results open a new route for spatio-temporal manipulation of phase separation in photo-reactive polymer blends.